The **volume** command controls the display of density maps and
other volume data,
*i.e.*, values associated with points on a 3D grid.
The *model-spec*
can be a specific model number or range of model numbers (preceded by #),
or the word **all** to indicate all volume models.
The *model-spec*
can be omitted for certain options that can or must apply globally,
namely **pickable** and several of the
sampling and size options.

The corresponding graphical interface,
**Volume Viewer**,
allows adjusting contour levels and some of the other settings interactively.
Contour levels can also be changed interactively with the
mouse mode for **contour level**
or **windowing**
.

Volume data can be opened with the command
**open** (which automatically starts
**Volume Viewer**)
and saved to a file with **save**.

Data set statistics and isosurface-enclosed volumes can be reported
with **measure**.
See also:
**show**,
**color**,
**transparency**,
**vseries**,
**molmap**,
**fitmap**,
**resfit**,
**surface** operations,
**sym**,
**smoothlines**,
**Map** icons,
measurements,
ChimeraX DICOM Reference

Thevolume #1 style mesh level 0.8 color red level 1.2 color 0,50,80

volume #2 level 10,0 level 100,1 level 400,1 color hot pink style image

vol all hide

vol settings #1

General Display Options

Sampling and Size Options

Dimension and Scale Options

Planes Options

Surface and Mesh Display Options

Image Display Options

Volume Operations (Map Editing)

The command **volume settings** (optionally followed by a *model-spec*)
reports the current volume display settings in the
**Log**.
See also: **info**

•show

Display the volume model.

•hide

Undisplay the volume model.

•toggle

Of the specified volume models, show those that are hidden and hide those that are shown.

•stylesurface | mesh | image

Designate the style of display: thesurfaceandmeshmodes show isosurfaces (contour surfaces), while theimagemode (synonymsolid) refers to a cloudlike display with varying intensity and opacity.

•Separate sets of level, color, brightness, and transparency information are maintained for the surface/mesh and image styles of a volume model; switching to image from surface or mesh (orpickabletrue | false

Whether a volume model should be selectable with the mouse (initial defaulttrue). This setting can be applied to individual models, but if the model specification is omitted, the setting is global and applies to all volume models, including those opened later within the same session.

•levelthreshold-level

Place afor mapping data values to the display. Can be given multiple times in the same command for multiple contour levels on the same volume data.thresholdThreshold levels can be adjusted interactively by dragging in the

- For
surfaceandmeshdisplays,threshold-levelis a single number, the contour level of the surface. This corresponds to horizontal placement on a histogram in the graphical interface. To adjust the threshold level automatically so that the contour encloses a specified spatial volume, see theencloseVolumeandfastEncloseVolumeoptions.

- For
imagedisplays,threshold-levelconsists of two numbers separated by a comma (no spaces). The first number indicates a data value and the second number indicates an intensity ranging from 0.0 to 1.0. These correspond to horizontal and vertical placement, respectively, on a histogram in the graphical interface.Volume Viewerhistogram or by using the mouse mode forcontour levelorwindowingin the graphics window.

•rmsLevelthreshold-level

•sdLevelthreshold-level

Same aslevel, except withthreshold-levelspecified in units of root-mean-square (RMS) deviations from zero or standard deviations (SD) from the mean, respectively, calculated for the current display region and step size. See also:volume scale,measure mapstats

•Multiple level and color specifications can be included in a single command. If a color is specified but no levels, the color applies to all existing levels and becomes the default color for the volume model. If levels are given but no color, the model's current default color is used for the levels, and all old levels are removed. If one color and one or more levels are given, that color applies to all levels but does not become the default color. Otherwise, if multiple levels and colors are given, there must be an equal number of each. Levels and colors are paired in the order given, but they do not need to be interleaved; only the ordering of each type of specification (levels or colors) is significant.colorcolor-spec

Assign threshold color. Can be given multiple times in the same command if multiple threshold levels are also specified. See also: initial colors

•brightnessvalue

Brightness scales the color intensity (range 0.0–1.0, default1.0).

•transparencyvalue

Transparency values range from 0.0 (fully opaque) to 1.0 (fully transparent).

- For
surfaceandmeshdisplays, transparency is the fraction of light transmitted from behind the surface or a line of mesh (default0.0). By default, these objects are dimmed as they are made more transparent (seedimTransparency.) See also:

- For
imagedisplays, this setting gives the slab thickness (as a fraction of the dataset's shortest dimension, range 0.0–1.0, default0.5) that gives the transparency indicated by threshold height on the histogram (details...). By default, more transparent voxels are made dimmer (seedimTransparentVoxels).

•encloseVolumevolume

Automatically setsurfaceormeshthreshold level to enclose the specifiedvolumein distance units cubed (e.g., Å^{3}if the grid spacing is expressed in Å). Multiple volume models can be specified in the same command to make their isosurfaces enclose the same spatial volume. The level is determined by an iterative procedure. In each iteration, the density value midway between the upper and lower bounds is tested. If the resulting enclosed volume is larger (smaller) than the target, that midpoint value becomes the new lower (upper) bound. In the first iteration, the upper and lower bounds are the maximum and minimum values in the map. Iteration stops when the actual enclosed volume differs from the target by less than 0.001 of the target or 30 iterations have been performed. ThefastEncloseVolumeoption is similar but uses a noniterative approximation. See also:vseries align,measure volume

•fastEncloseVolumevolume

Automatically setsurfaceormeshthreshold level to enclose the specifiedvolumein distance units cubed (e.g., Å^{3}if the grid spacing is expressed in Å). Multiple volume models can be specified in the same command to make their isosurfaces enclose the same spatial volume. The number of grid points to enclose is estimated by dividing the target volume by the volume of one grid cell, and then the level corresponding to that number of points is estimated by sorting the data values into 10,000 bins of equal width and identifying the value bin that attains (cumulatively) that number of points. TheencloseVolumeoption is similar but uses an iterative procedure with increased accuracy at the cost of increased computation time. See also:vseries align,measure volume

•showOutlineBoxtrue |false

Outline the bounding box of the current display region.

•outlineBoxRgbcolor-spec

Assign a color to the outline box (defaultwhite). Any transparency in the color will be ignored.

•stepN|Nx,Ny,Nz

Step values indicate sampling density; a step of 1 means all data points are used to generate the display, while 2 means every other data point is taken along each axis. Step sizes must be integers. If a single number is supplied, it is used in all three directions; if three numbers are supplied (separated by commas but not spaces), they are used in the X, Y, and Z directions, respectively. Changing a step value will change the data size limit for automatic step adjustment (seevoxelLimit). See also:

•limitVoxelCounttrue| false

Automatically adjust step size so that no more than the specified voxel limit is displayed.

•The remaining options in this section are global and apply to all volume models, regardless of which are specified:voxelLimitlimit

Set the maximum number of Mvoxels to be displayed (default1.0) whenlimitVoxelCountis set to true.

•showOnOpentrue| false

Automatically display a data set when it is opened if it does not exceed a specified size.

•voxelLimitForOpensize

Set the data size limit in Mvoxels below which data should be automatically displayed when opened (default256.0) whenshowOnOpenis set to true.

•showPlanetrue| false

Initially display just a single plane (normal to the Z axis) of a data set if it exceeds a specified size.

•voxelLimitForPlanesize

Set the data size limit in Mvoxels above which a single plane of the data should be initially displayed (default256.0) whenshowPlaneis set to true.

•dataCacheSizesize

Set how much memory in Mb should be dedicated to volume data (default512). A cache can improve performance, since accessing cached data is faster than reading it from disk. The least recently displayed data values are purged to maintain the specifiedsize. The data cache only accounts for approximately 1/3 to 1/2 of the memory used in viewing volume data, as additional memory is occupied by surfaces and color arrays.

•regionall |name|i1,j1,k1,i2,j2,k2

Show the full data set (specified withall), or the data region previously assignedname, or the data region with grid indicesi1–i2along the X axis,j1–j2along the Y axis, andk1–k2along the Z axis. Grid indices must be integers separated by commas but not spaces. See also: , thecrop volumemouse mode

•nameRegionname

Assignnameto the currently displayed region.

•originx,y,z

Place the grid origin at coordinatesx,y,z(numbers separated by commas but not spaces).

•originIndexi,j,k

Place the coordinate origin (0,0,0) at grid indicesi,j,k(numbers separated by commas but not spaces). Fractional and negative values are allowed, as the origin is not required to coincide with a grid point or even to fall within the grid.

•voxelSizeS|Sx,Sy,Sz

Voxel size indicates the scale of the data set, the spacing of points in units of distance. If a single number is supplied, it is used in all three directions; if three numbers are supplied (separated by commas but not spaces), they are used in the X, Y, and Z directions, respectively. The grid is anchored at the coordinate origin (originIndexremains unchanged).

•symmetrysym-type

Assign the specified symmetry to the volume data set. This information is retained in files saved in Chimera map format and can be used by other commands such asmolmapandfitmap. Mostsym-typeoptions have additional sub-options or parameters:

- symmetry of model
#- use “biological assembly” information from an atomic model (currently only read from PDB format, not mmCIF) or the symmetry assignment of another volume modelN

- Example:
#4- cage model (from
Cage Builder) polygon symmetry#orN,pM#- place copies at equivalent positions relative to each M-sided polygon in the cage model with ID numberN,pnMN. Thepform places one copy per M-sided polygon, whereasMpnplaces M copies per M-sided polygon using CMMsymmetry about the center of the M-sided polygon nearest the original copy.

- Examples:
#2,p6or#2,pn5- cyclic symmetry
Caround axis and centern

- Example:
C3- dihedral symmetry
Daround axis and centern

- Example:
d7- tetrahedral symmetry
T[,around centerorientation]where

- Example:
t,z3orientationcan be:

222(default) - with two-fold symmetry axes along the X, Y, and Z axes, a three-fold along axis (1,1,1)z3- a three-fold symmetry axis along Z, another three-fold axis in the YZ plane such that rotation about the X axis by ~110° is a symmetry operation (EMAN convention)- octahedral symmetry
Oaround center- icosahedral symmetry
I[,around centerorientation]where

- Example:
i,n25orientationcan be:

222(default) - with two-fold symmetry axes along the X, Y, and Z axes2n5- with two-fold symmetry along X and 5-fold along Zn25- with two-fold symmetry along Y and 5-fold along Z2n3- with two-fold symmetry along X and 3-fold along Z222r- same as 222 except rotated 90° about Z2n5r- same as 2n5 except rotated 180° about Yn25r- same as n25 except rotated 180° about X2n3r- same as 2n3 except rotated 180° about Y- helical symmetry
H,around axis and centerrise,angle,n[,offset]where

- Example:
h,43.5,21,6,-2riseis the translation along the axis per subunit,angleis the rotation in degrees per subunit, andnis how many copies total (including the original) the resulting segment of infinite helix should contain. The integeroffset(default0) allows extending the helix in both directions. The example above would given= 6 copies total, with two copies in the negative axis direction, one at the identity position, and three in the positive axis direction.- translational symmetry
shift,along axis – or –n,distanceshift,n,x,y,zwhere

- Example:
shift,3,26.7nis how many copies total (including the original) the result should contain. The translation can be expressed as adistancealong the axis or as a vectorx,y,zin the reference coordinate system.- the product of symmetry groups, each specified as described above and separated by
*to indicate multiplying each symmetry matrix of one group with each symmetry matrix of another; can be generalized to multiple symmetry groups (not just two)

- Example:
c2*h,42,21,9,-4

•axisaxis

Specify axis of symmetry (defaultz), whereaxiscan be:

x- X-axisy- Y-axisz- Z-axisx,y,z(three values separated by commas only) - an arbitrary vector in the reference coordinate system- an
atom-specof exactly two atoms (not necessarily bonded or in the same model)

•centercenter

Specify center of symmetry in physical coordinates (default0,0,0), wherecentercan be:This option is overridden by

x,y,z(three values separated by commas only) - an arbitrary point in the reference coordinate system- an
atom-specof any combination of atoms and surface models. The center of the bounding sphere of the specified items will be used.centerIndex.

•centerIndex[i|i,j,k]

Specify center of symmetry in grid coordinates, given as a single value or three values separated by commas only. Fractional and negative values can be used. If a single value is given, it is used as the grid coordinate along all three axes. This option overridescenter.

•coordinateSystemN

Specify a reference model by model numberNpreceded by #. The reference coordinate system is used for interpreting specifications of axis and center of symmetry.

•dumpHeadertrue |false

Whether to write the file header contents (if any) of a map read from MRC or CCP4 format to theLog.

•planesaxis[,start[,end[,increment[,depth]]]]

Sequentially display slabsdepthplanes thick along the specified dataaxis(x,y, orz) starting from indexstartand repositioning the slab byincrementgrid units per frame until the index at the next frame would exceedend. The indices refer to the first displayed plane along the axis. If only the axis is given, a single plane at the center of the current region will be shown. If noendis supplied, only a single slab will be shown. The defaultincrementanddepthare both1. Only planes that are multiples of the step size will be shown, and thedepthrefers to the number of planes shown rather than a grid index range. Although anincrementsmaller than the step size can be specified, at each frame the grid index will be rounded down to a multiple of the step size, resulting in the same plane being displayed in more than one frame. To avoid this, change the step size to 1 beforehand or specifystart,endandincrementvalues consistent with the current step size. Thestart,end,increment, anddepthparameters can be floating-point numbers. For example, anincrementof 0.25 with step size 1 will show each plane for 4 frames. See also: , themove planesmouse mode ,volume tilePausing the cursor over a single-plane display shows the voxel indices and data value in a pop-up balloon.

•expandSinglePlanetrue | false

Expand a single-plane display along its perpendicular axis to the full thickness of the data. This option does not apply to orthogonal planes.

•orthoplanesxyz | xy | yz | xz | off [positionPlanesi,j,k]

Display planes perpendicular to the X, Y, and/or Z data axes within the current region. If only a single plane is shown beforehand, it will be expanded to define the region. ThepositionPlanesoption specifies plane locations along the axes by integer grid indices (default0,0,0or the lowest indices in the current region). Orthogonal planes automatically use theimagedisplay style, opaque color mode, and an outline, although these can be overriden with the corresponding command options. Turning orthogonal planesoffrestores theauto8color mode. See also: , themove planesmouse mode

•boxFacestrue | false

Display orthogonal planes as the six box faces of the current region. If only a single plane is shown beforehand, it will be expanded to define the region. Box-face planes automatically use theimagedisplay style, opaque color mode, and an outline, although these can be overriden with the corresponding command options. Turning box-face planesoffrestores theauto8color mode.

The **surface** and **mesh** display styles
both depict isosurfaces.

•surfaceSmoothingtrue |false

Whether to smooth surface and mesh displays. Smoothing entails moving each vertex a specified fraction of the way toward the average position of its neighbors a specified number of times.

•smoothingIterationsN

How many iterations of smoothing to perform (default2) whensurfaceSmoothingis set to true. Each vertex is moved once per iteration.

•smoothingFactorf

How far to move each vertex whensurfaceSmoothingis set to true. In each iteration, each vertex is moved a fraction(ranging from 0.0 to 1.0, defaultf0.3) of the way toward the average position of the vertices connected to it by triangle edges.

•subdivideSurfacetrue |false

Whether to subdivide each triangle in surface and mesh displays into four smaller triangles a specified number of times. A triangle is subdivided by connecting the midpoints of its edges. Subdivision can help to produce smoother surfaces when combined with thesurfaceSmoothingoption.

•subdivisionLevelsj

How many times to subdivide triangles whensubdivideSurfaceis set to true. The number of triangles is increased by a factor of 4^{j}, wherejis a positive integer (default1).

•smoothLinestrue |false

Turn on anti-aliasing to smooth lines in mesh displays. Mesh lines with transparency > 0.0 can only be smoothed whendimTransparencyis true. A side effect of OpenGL anti-aliasing is that dense meshes look brighter from some viewpoints and darker from others, depending on the order in which the lines were drawn.

•squareMeshtrue| false

Display only a subset of the lines in the triangular mesh. Lines in the square mesh show the intersection of the XY, YZ, and XZ grid planes with the contour surface.

•dimTransparencytrue| false

Decrease the brightness of surface and mesh displays as their transparency is increased. When dimming is on, OpenGL (alpha,1–alpha) blending is used instead of (1,1–alpha) blending.

•meshLightingtrue| false

Make the inside of a mesh-enclosed volume dimmer than the outside by varying the brightness according to the angle between each surface point normal and the line of sight. Brightness is maximal when the outward-facing normal is parallel to the line of sight and pointing at the user (see more on the definition of “outward” underflipNormals). When this option is off, brightness is uniform regardless of the angle between the normal and the line of sight.

•twoSidedLightingtrue| false

Light both sides of surface displays. Otherwise, only the outside of a surface-enclosed volume will be lit (see more on the definition of “outside” underflipNormals). The brightness of each lit side varies according to the angle between a surface point normal and the line of sight; brightness is maximal when the normal is parallel to the line of sight.

•flipNormalstrue| false

Affects surface displays whentwoSidedLightingis set to false, mesh displays whenmeshLightingis set to true. WhenflipNormalsis true, the side toward larger or more positive values is treated as the outside for negative thresholds and the side toward smaller or more negative values is treated as the outside for positive thresholds (appropriate for data in which the sign is meaningful, such as electrostatic potential). WhenflipNormalsis false, the side toward smaller or more negative data values is always treated as the outside.

•capFacestrue| false

Cover the faces of the volume data box where high values would be exposed.

The **image** display style
shows data as a semitransparent solid.

•colorModecmode

Specify color pixel format (OpenGL texture format). Possible values ofcmodecombine a string describing the types of information:... with a number of bits:

rgba- multiple colors and transparencyrgb- multiple colors, opaquela- luminance (single color) and transparencyl- luminance (single color), opaqueauto- set the mode based on the current display4,8,12,16. For example, the defaultcmodeisauto8. See also:graphics colorDepth

•projectionModepmode

Specify projection mode for memory-efficient display of large data sets such as tomograms. Possible values ofpmode: Displaying just the planes perpendicular to one data axis (2d-x, 2d-y, or 2d-z) uses less memory than automatically switching to those along the data axis most perpendicular to the screen at a given time (2d-xyz). The auto setting uses 2d-z for volumes with X or Y dimensions at least 4 times greater than Z, otherwise 2d-xyz. The 3d mode avoids jumps in brightness during rotation by continuously generating planes perpendicular to the line of sight, spaced according to theplaneSpacingoption. See also:

•planeSpacingspacing

Whatspacingto use withprojectionMode 3d, with possible values:

min(default) - smallest spacing of the data along X, Y, or Zmax- largest spacing of the data along X, Y, or Zmean- mean of the data spacing along X, Y, and Zd- a specific distance in physical units of length

•dimTransparentVoxelstrue| false

Iftrue(default), scale voxel brightness in image displays by a factor of (1–transparency). Iffalse, increasing the transparency makes a volume appear brighter because less light is blocked. Theappearanceoption also adjusts this setting.

•colormapOnGputrue | false

Whether the mapping of volume data values to colors should be done on the GPU (if true) or CPU (if false). The default is true for DICOM, otherwise false. Colormapping with the GPU is faster but done every time the scene is drawn, whereas colormapping with the CPU is only done when the colors change. Colormapping with the GPU may also give a better, less noisy appearance because the data values rather than the resulting color values are interpolated.

•colormapSizeN

Specify the size of the color lookup table (default256) for colormapping on the GPU (any data type) or on the CPU with floating-point data. For color mapping on the CPU with 8-bit or 16-bit map data, this value is ignored and sizes 256 and 65536 are used instead so that each data value has its own color.

•fullRegionOnGputrue | false

Whether to keep the data (or colors, ifcolormapOnGpuis false) for the full volume region on the GPU to allow fast cropping. The default is true for DICOM, otherwise false. It is best to use this option withcolormapOnGputrue; otherwise, updating colors for the full region even when only a smaller region is shown may slow performance significantly.

•blendOnGputrue |false

Whether image-display color blending of multichannel data should be done on the GPU (if true) or CPU (iffalse; default). Color blending on the GPU is about an order of magnitude faster.

•maximumIntensityProjectiontrue |false

At each pixel, display the the most intense color value underlying the pixel along the line of sight. The maximum intensities of the red, green, and blue color components are determined separately, and transparency is ignored. This option can be useful for enhancing detail. Unphysical effects can result, but are usually not very noticeable; examples include the disappearance of a dim spot when it passes in front of a brighter spot and the simulation of a single spot when the maximal values of different color components under the same pixel actually come from different spots.

•btCorrectiontrue |false

Correct brightness and transparency for the viewing angle. Without this correction, the apparent brightness and transparency of image displays (in projection modes other than 3d) will depend on the viewing angle relative to the data axes. For a cube-shaped volume with equal resolution in the X, Y, and Z dimensions, the brightness drops and the transparency increases by a factor of 3^{1/2}(approximately 1.7) as the viewing angle is changed from along any axis to along the cube diagonal. The brightness correction remedies this, but doubles rendering time.

•minimalTextureMemorytrue |false

Reuse a single 2D texture for image displays (in projection modes other than 3d) instead of allocating separate textures for every plane of the data. This is useful for viewing large data sets that would otherwise fail to display, but can degrade interactive response.

•linearInterpolationtrue| false

Linearly interpolate brightness and transparency between voxels in image displays. Turning interpolation off may yield a pixelated appearance but speed up rendering, depending on the graphics hardware.

•appearancepreset-name

•appearanceinitial

Apply a Horos 3D preset (withdimTransparentVoxelsfalse) or MRIcroGL color lookup table (CLUT) (withdimTransparentVoxelstrue andtransparency0.05) to the image display. Horos and MRIcroGL are programs for medical image analysis and visualization. The Horos 3D presets are listed on github, and the available MRIcroGL CLUTs areCT_Bones,CT_Kidneys,CT_Liver,CT_Lungs,CT_Muscles,CT_Skin,CT_Soft_Tissue,CT_Vessels, andCT_w_Contrast. Thepreset-nameshould be enclosed in quotation marks if it contains any spaces.Implementation of these presets in ChimeraX is only approximate, as there may not be a direct correspondence between settings in different programs, and parts of the definition files may have been ignored. The Horos 3D presets are generally intended for 3D rendering (not planes).

Besides a preset name,

initialcan be used to (re)set to the threshold levels and colors chosen automatically for the data when first shown as image, withdimTransparentVoxelstrue. DICOM data are initially shown as image with thresholds: (-1000,0),(300,0.9),(3000,1). Theinitialsettings may be useful for viewing planes or thin slabs. See also:Mapicons , ChimeraX DICOM Reference

A volume operation edits density maps or other
volume data
to create a new volume data set.
The original map is undisplayed and the new map
is displayed with the same threshold and color as the original.
See also:
**measure**,
**surface** operations,
**smoothlines**,
**Map** icons

Examples:

volume add #2-25 onGrid #1

vol add #1,2,5 onGrid #5 inPlace true

vol add #1,2 boundingGrid false

vol gaussian #3 sd 5

vol subtract #2 #4 modelId #5

vol unbend #1 path #2 yaxis z xsize 200 ysize 200

Several operations are available:

**volume add**- add two or more maps**volume bin**- reduce data size by averaging over cells of multiple grid points**volume boxes**- extract cubic regions centered on markers or atoms**volume cover**- extend map to cover specified atoms or box**volume falloff**- smooth the boundaries of masked density**volume flatten**- scale values to flatten map baseline**volume flip**- reverse data planes along a specified axis**volume fourier**- Fourier transform**volume gaussian**- Gaussian smoothing or sharpening**volume laplacian**- Laplacian filtering**volume localCorrelation**- calculate map-map correlation over a sliding box**volume mask**- mask a map to a surface; see also**volume onesmask****volume maximum**- take the maximum values pointwise from two or more maps**volume median**- set each value to the median of values in a surrounding box**volume minimum**- take the minimum values pointwise from two or more maps**volume morph**- morph (interpolate) between two or more maps**volume multiply**- multiply values in two or more maps**volume new**- create empty (zero-valued) map; see also**volume onesmask****volume ~octant**- erase positive octant**volume octant**- erase all but the positive octant**volume onesmask**- create map with values of 1 bounded by a surface**volume permuteAxes**- permute axes**volume resample**- resample on the grid of another map**volume ridges**- skeletonize; emphasize ridges or filaments in the density**volume scale**- scale, shift, normalize, and/or cast to a different data value type**volume splitbyzone**- split map by zones previously colored with**color zone****volume subtract**- subtract another map from the first**volume threshold**- reassign values that are below a specified minimum and/or above a specified maximum**volume tile**- make a single-plane volume from tiled slices of another volume**volume unbend**- unbend a map near a path formed by markers/links or atoms/bonds**volume unroll**- unroll a cylindrical slab into a flat slab**volume unzone**- show the full extent of a map previously limited to a zone with**volume zone****volume zone**- zero the values at grid points within or beyond a cutoff distance from specified atoms, or limit the display to a zone without making a new map

Add two or more maps to create a new map. Option keywords are the same as for•volume minimum,volume maximum, andvolume multiply:The

scaleFactorskeyword specifies a multiplier for each map (default1.0); as many values as input maps must be supplied, separated by commas but not spaces.The new map can be created on the grid of another map specified with

onGrid, wheregridmapis a model number preceded by #. Ifgridmapis not specified, it defaults to the first involume-spec(the first of the maps being added). The input maps are resampled on the grid by trilinear interpolation, and the resulting values summed for each grid point. Note resampling causes some loss in resolution (details...). Further options related togridmap:The

boundingGrid- whether to adjust (extend or shrink) the grid ofgridmapto bound the input maps (defaulttruewhen adding maps without specifying agridmap, otherwisefalse)gridStep- whether to use the full resolution ofgridmap(step size1, default) or a specified subsample (step size > 1). Step sizes must be integers. If a single number is supplied, it is used along all three axes; if three numbers are supplied (separated by commas but not spaces), they are used along the X, Y, and Z axes, respectively.gridSubregion- whether to use the full extents ofgridmap(all, default) or a specified subregion. A subregion can be specified by:

namepreviously assigned with thenameRegionoption ofvolume- grid indices
i1–i2along the X axis,j1–j2along the Y axis, andk1–k2along the Z axis. Grid indices must be integers separated by commas but not spaces.value-typedefaults to the current type of theonGridmap (if any), otherwise the first involume-spec, and can be 8-, 16-, or 32-bit signed integer (int8,int16, orint32), 8-, 16-, or 32-bit unsigned integer (uint8,uint16, oruint32), or 32- or 64-bit floating-point (float32orfloat64).If the new map is large, for example a whole tomogram, the command may fail for lack of memory. The whole new map must fit in memory.

Average over cells of multiple grid points in the original map to produce a smaller map. Supplying a single integer N (default•2) indicates partitioning the map into bins of NxNxN grid points and averaging the N^{3}values per bin to produce a new map with 1/N as many points in each dimension. Cells with different numbers of grid points in each dimension can be specified by supplying three integersNseparated by commas only._{x},N_{y},N_{z}

For each marker or atom in•atom-spec, extract a surrounding cube of data. IfuseMarkerSizeis false (default), the edge length of each cube must be specified in physical units of length withsizeor in grid units withisize. IfuseMarkerSizeis true, the diameter of its central marker or atom is used as the edge length or added to thesizevalue if also given (defaultd=0.0). Theisizeoption facilitates getting the same-sized cubes for each marker, except where a marker is too close to the edge of the density map to obtain a cube of the requested size. Ifsizeis used instead, rounding may yield edges that differ slightly in grid dimensions. Marker/atom size can be changed with thesizecommand.

Extend a map to cover specified atoms or to fill a rectangular box, using map symmetries and periodicity. The output dimensions can be specified as:•Unspecified dimensions will be kept the same as the input map. The output grid will have the same spacing and alignment as the grid of the input map. The

atomBoxspanning the specified atoms plus any extrapadin each dimension (dis in units of physical distance, default5.0)boxor just individual dimensionsx,y, and/orzin the X,Y,Z coordinate system of the input mapfboxor just individual dimensionsfxetc.in fractional coordinates where 0.0-1.0 spans each dimension of the input mapiboxor just individual dimensionsixetc.in grid indices of the input map. The input map's grid indices start at 0.cellSizeoption specifies unit cell dimensions in grid units along the X, Y, and Z axes. The default unit cell dimensions correspond to the full size of the map, or for CCP4 and MRC maps, are taken from the header. TheuseSymmetryoption indicates whether to use any symmetries associated with the map (defaulttrue); if false, only unit cell periodicity will be used. Map symmetries are read from the CCP4 or MRC file header, or can be assigned manually with thesymmetryoption ofvolume.Values from symmetry copies are determined by trilinear interpolation (with potential loss in resolution, see details). Where symmetries and periodicity give multiple copies of the input map overlapping a grid point, the average value will be assigned. The maximum difference between values from different copies at a grid point will be reported in the

Log. If there are grid points not covered by symmetry or unit cell periodicity, a message will be sent to theLog, and the points will be assigned values of 0.0. To ensure complete coverage by symmetry copies, the asymmetric unit of the map should extend far enough that its symmetry copies overlap by a non-zero amount. For example, if the unit cell is 100 grid points wide and there is two-fold symmetry along the X axis, the asymmetric unit would need to contain at least 52 grid points along the X axis. If it contains only 50, the two symmetric copies will not overlap, and values in the space between the copies cannnot be determined because the current code cannot interpolate between different copies of the map. If it contains 51, the two copies will have a single plane of grid points in common. Although that would be sufficient with exact arithmetic, the copies still might not overlap given the rounding errors inherent in computer calculations.

Smooth the boundaries of a masked map by replacing the value at each grid point outside the boundary with the average of the values of its six nearest neighbors, for•Miterations (default10). All grid points with values of zero before the first iteration are taken to be outside the boundary, thus assigned a new value at each iteration. Thanks to Greg Pintilie for the initial implementation.

If the•methodismultiply(default), scale data values by factor (a*i + b*j + c*k + d) where i,j,k are the grid indices and a,b,c,d are calculated to zero out the first moments of the resulting map (make its mass balance at the center of the grid). If themethodisdivide, data values are divided by the factor (a*i + b*j + c*k + d), which is a least-squares fit to the map data values. For both methods, the a,b,c,d coefficients are scaled to make (a*i + b*j +c*k + d) equal to 1 at the center of the map. If afitregionis specified, the calculation of the a,b,c,d coefficients uses only the data values in the specified region, while the scaling operation applies to the entire map or the part specified withsubregion. Thefitregioncan be the full extents of the data (all, default) or a subregion specified by:

namepreviously assigned with thenameRegionoption ofvolume- grid indices
i1–i2along the X axis,j1–j2along the Y axis, andk1–k2along the Z axis. Grid indices must be integers separated by commas but not spaces.

Reverse data planes along a specified•axis(defaultz, reversing the order of the Z planes).

Calculate the 3D Fourier transform. If•phaseis false (default), generate a magnitude map; ifphaseis true, generate a map of phase values (–π to π) instead.

Perform Gaussian filtering, where•σis one standard deviation of the 3D Gaussian function in physical units such as Å (default1.0). Different standard deviations along X,Y,Z can be specified as three values separated by commas only. Gaussian filtering attenuates high frequencies to smooth the map; however, the opposite behavior (amplifying the high frequencies to sharpen the map) can be specified withinvert true. Sharpening withσvalues smaller than the pixel size is most useful; larger values overemphasize high frequencies, in effect amplifying noise. Thevalue-typedefaults to the current type and can be 8-, 16-, or 32-bit signed integer (int8,int16, orint32), 8-, 16-, or 32-bit unsigned integer (uint8,uint16, oruint32), or 32- or 64-bit floating-point (float32orfloat64).Gaussian smoothing improves the ratio of signal to noise but reduces resolution. It is fastest for data sizes that are powers of 2, and can be very slow when insufficient memory is available. It may be helpful to limit the input to just a subsample or subregion of the original data. Although it uses a fast Fourier transform calculation method, it does not use map periodicity. Values outside the map boundaries are treated as zero. See also:

Perform Laplacian filtering. The Laplacian operation is a sum of second derivatives. Laplacian filtering is useful for edge detection but amplifies noise, so it may be necessary to perform smoothing such as Gaussian filtering beforehand. Finite differences v(i-1)-2*v(i)+v(i+1) along each axis are used, and voxels at the edge of the box are set to zero.•

Calculate the correlation between two maps over a sliding box of NxNxN grid points, generating a new map by assigning the correlation value to the box center. The sliding box is based on the grid of the first map, and•N=5grid units by default. If the grids of the two input maps do not coincide, the values of the second map will be interpolated (with potential loss in resolution, see details). ThesubtractMeanoption specifies whether to subtract the mean of the values in the window from each value in the window before calculating the correlation. The output map will beN–1 smaller in each dimension than the first map.

Mask a map to a surface (details...). The•fullmapoption indicates making the new map dimensions the same as the full dimensions of the original map, even if only a subregion is being displayed; otherwise (default), the new map will be made as small as possible to enclose the surface. Thepadoption allows extracting a larger or smaller region by moving the surface a positive or negativedistance(in the distance units of the data, typically Å or nm) along surface normals before creating the masked volume. However, if the resulting surface intersects itself, the masked volume will not include the intersection. For larger-region extraction, this problem can be avoided by instead using theextendoption to move the surface outward byNvoxels before creating the masked volume. In other words,extendincludes grid points that are withinNgrid units (along the grid X, Y, and Z axes) of the original surface.The

slaboption allows instead extracting a slab of data around a surface layer. Two additional surfaces, displaced as specified from the existing surface and joined at their edges (if any), are computed but not displayed. Data for voxels between the computed surfaces are retained. If a single value (width) is supplied, the two computed surfaces are offset along the normals of the original surface by ±½(width). Alternatively, two values separated by a comma but no spaces can be used to specify the offsets of the two surfaces independently. Positive or negative values can be used.The

invertMaskoption allows getting the opposite result (spatial complement of values and zeros) of what would otherwise be obtained.The region between surface layers is computed along the projection

axis(default is along the data Y axis:0,1,0). The axis direction is relevant when the surface has holes. Vector coordinatesx,y,zare in the map coordinate system; the vector can point in any direction and need not be of unit length. Thesandwichoption specifies including only volume voxels between paired surface layers; if false, the volume projected along the axis beyond a single surface layer will also be included. When intersecting or nested surfaces are involved, thefillOverlapoption indicates retaining the union of the values from masking to each surface separately. For example, if the surface(s) include two concentric spheres,fillOverlaptrue will return values for all grid points within the larger sphere, whereasfillOverlapfalse will return values for only those points in the shell between the two surfaces.The related command

volume onesmaskcreates a new map with values of 1 bounded by a surface. See also:volume multiply

Set each value to the maximum at that point in two or more input maps. See•volume addfor descriptions of the options.

Smooth the data by setting each value to the median of the values in a box centered at that point. Values at points for which the surrounding box extends outside the data are simply set to zero. Box dimensions are specified in grid units with•binSizeand must be odd integers. Supplying a single integer N (default3) indicates a box size of NxNxN grid points. Boxes with different numbers of grid points in each dimension can be specified by supplying three integersNseparated by commas only. The_{x},N_{y},N_{z}iterationsoption indicates how many cycles of smoothing to perform (default1).

Set each value to the minimum at that point in two or more input maps. See•volume addfor descriptions of the options.

Morph between two or more maps. For a reasonable result, the input maps should have the same grids: dimensions, spacing, and numbers of points. Note•volume resamplecan be used to make a copy of one map that has the same grid as another. A morphing fraction of 0.0 corresponds to the first map and a fraction of 1.0 corresponds to the last, with intermediate maps evenly spaced within that range. There is smooth interpolation between each adjacent pair of maps.The morph display will proceed from

start-fraction(default0.0) in steps ofincrement(default0.04) forNframes(default25). By default (playDirection 1), the initial direction of play is from low to high fractions. If the number of frames and step increment are more than needed to reach theplayRangebounds (default is the entire range:0.0,1.0), the morph display will “bounce” back and forth. ThescaleFactorskeyword specifies a multiplier for each map (default1.0); as many values as input maps must be supplied. TheconstantVolumeoption specifies adjusting the threshold (contour level) automatically to keep the enclosed volume constant. TheaddModeoption specifies treating the second map as a delta to be added to the first instead of linearly interpolating between the two. It is not recommended for inputs of >2 maps. ThehideOriginalMapsoption specifies hiding the input maps. TheinterpolateColorsoption only applies when the maps have the same number of thresholds (contour levels for surface/mesh display, coloring control nodes for image display).The morph is created as a new map (volume) model. However, if the

modelIdof an existing morph map is given, the existing morph will be used instead of a new one being calculated.See also:

vseries, making movies

Multiply the values pointwise in two or more maps. This is used to apply a mask (values 0,1) to a map. See•volume addfor descriptions of the options. See also:volume mask

Create an “empty” zero-valued map named•map-name(defaultnew) with the specifiedsize(number of grid points along each axis, default100),gridSpacingin physical distance units (default1.0along each axis),origincoordinates (default0.0,0.0,0.0), andcellAngles(default90,90,90°; a single value can be supplied if α = β = γ). Grid size and spacing can each be given as a single value to apply to all three axes or as three values separated by commas only. Ifmap-nameincludes spaces, it must be enclosed in quotation marks. Thevalue-typedefaults tofloat32and can be 8-, 16-, or 32-bit signed integer (int8,int16, orint32), 8-, 16-, or 32-bit unsigned integer (uint8,uint16, oruint32), or 32- or 64-bit floating-point (float32orfloat64). See also:volume onesmask

Erase values inside the positive octant (all grid points with X,Y,Z coordinates greater than the center). The center can be specified in physical units (such as Å) with•centeror in grid units withiCenter. The default is the center of the volume data box. The coordinates should be separated by commas but not spaces, and the values can be fractional.iCenteroverridescenterif both are given. The values in the erased regions will be set tovalue(default0). A different value may improve contour surface appearance; for example, large negative values produce flatter surfaces where an octant has been cut away from a map of positive values.

Erase values outside the positive octant. Options are as described for•volume ~octantabove.

Create a map with values of 1 bounded by a surface (details...). The•borderoption indicates how far out from the bounding surface in all six directions (±X, ±Y, ±Z) to place the edge of the output map. Thespacingoption gives the grid spacing for the output map in physical units of length, typically Å (default is 1% of the maximum of the X, Y, and Z dimensions of the bounding surfaces). If a single number is supplied, it is used in all three directions; if three numbers are supplied (separated by commas but not spaces), they are used in the X, Y, and Z directions, respectively. The remaining options are the same as described forvolume mask. See also:volume new

Permute grid axes to the specified•axis-order, which can be any of the 6 ordered combinations ofx,y, andz. The original order isxyz.

Skeletonize map(s) by tracing along high-density grid points to identify ridges or filamentous structures in the density. At each grid point, the value is compared to the values of all the points in the surrounding 3x3x3 box, and the count of how many directions (up to 13) along which the value is a local maximum is assigned as that point's value in the new map. The•levelkeyword indicates aminimumvalue in the original map below which to automatically set the new value to 0, essentially ignoring those points in the skeletonization. The defaultminimumis the lowest display threshold (contour level) in the original map. Viewing the new skeleton map with a threshold of 6-10 highlights ridgelike features in the original map.

Resample values on the grid of another map specified with•onGrid, wheregridmapis a model number preceded by #. Values on the grid are obtained by trilinear interpolation of the input map. The other arguments are as described above forvolume add. Note resampling causes some loss in resolution (details...).

Shift values by adding a•constant(default0.0), scale values by a multiplicative factorf(default1.0), and/or cast them to a different data value type. When values are both shifted and scaled, the shift is applied first. Two normalization options calculate a scaling factor from the data:If a

rms- scale values to makenew-rms= ((∑x^{2})/N)^{½}wherexis each value and N is the total number of valuessd- first shift values so that the mean is 0.0, then scale values to makenew-std-dev= ((∑x^{2})/N)^{½}factorfis also specified, it is applied last. Thevalue-typedefaults to the current type and can be 8-, 16-, or 32-bit signed integer (int8,int16, orint32), 8-, 16-, or 32-bit unsigned integeruint8,uint16, oruint32), or 32- or 64-bit floating-point (float32orfloat64).See also:

rmsLevel, sdLeveloptions ofvolume,measure mapstats

Split a map by zones previously colored with•color zone. A new map is created for each distinct color and for the “uncolored” zone (any parts of the original map not within a color zone). Values outside the respective zones are set to zero. The new data sets are created as submodels of a new model named by appending “split” to the name of the original map.

Subtract the values of•othermapfrommap, both specified by model number preceded by #. ThescaleFactorsoption specifies multipliersf1andf2formapandothermap, respectively; two values must be supplied, separated by a comma but not spaces. Alternatively, ifminRmsis true,othermapwill be scaled automatically to minimize the root-mean-square sum of the resulting (subtracted) values at grid points within the lowest contour ofothermap.The new map can be created on the grid of another map specified with

onGrid, wheregridmapis a model number preceded by #. Ifgridmapis not specified, it defaults tomap. The input maps are resampled on the grid by trilinear interpolation, and the resulting values subtracted for each grid point. Note resampling causes some loss in resolution (details...).The remaining arguments are as described above for

volume add, except thatboundingGridalways defaults tofalse. When subtraction from an unsigned-integer map could give negative numbers, thevalueTypeoption should be used to specify a signed data type for the result.See also: , the

subtractMapsoption offitmap

Replace all values that are below a•mininumvalue (min) withnewmin(default equal tomin), and/or replace all values that are above amaximumvalue (max) withnewmax(default equal tomax).

Create a single-plane volume by tiling slices of a specified volume perpendicular to the specified•axis(defaultz). The spacing of slices in grid units (default1) is given with thepstepkeyword. Thetrimkeyword indicates each slice should be trimmed on all four edges byigrid units (default0). The slices are arranged into a single plane with number ofrowsrand number ofcolumnsc. If neither the number of rows nor the number of columns is supplied, they are computed to produce as near a square tiling as possible. If one or the other is supplied, the remaining parameter is adjusted to accommodate the total number of slices. ThefillOrdersetting (defaultulh) specifies the tiling pattern, including the starting corner, the tiling direction (horizontal or vertical), and whether to reverse the order of slices. The first two characters specify the corner for the first tile, the first character beingufor upper orlfor lower and the second beinglfor left orrfor right. These directions are defined with the specified axis pointing at the viewer and the remaining two axes pointing up and right. The third character ishfor horizontal tiling orvfor vertical tiling. The optional fourth characterrindicates that the order of the slices should be reversed. The resulting volume data set has the same origin and orientation of axes as the original volume, and grid size 1 along the specified axis.

Unbend a map near a path formed by markers/links or (equivalently) atoms/bonds. The•path-specshould be anatom-specthat specifies a single chain of atoms (markers) connected by bonds (links). The path will be mapped to the Z axis of the result. Theyaxissetting indicates which axis in the existing volume should be mapped to the Y axis of the result, and can be given as:The

x- X-axisy(default) - Y-axisz- Z-axisx,y,z(three values separated by commas only) - an arbitrary vector- an
atom-specof exactly two atoms (not necessarily bonded or in the same model)gridSpacingsis the separation between grid points in the new map (default is the minimum spacing along the three axes of the input map). Thexsizexsandysizeysare the X and Y dimensions of the new map in physical units, typically Å (default 10 times the new grid spacing). A cubic spline is placed through the path points and the input volume is interpolated on planes perpendicular to the splined path.

Unroll a hollow cylindrical section of the map into a flat slab. The cylinder•axiscan be given as:Cylinder

x- X-axisy- Y-axisz(default) - Z-axisx,y,z(three values separated by commas only) - an arbitrary vector- an
atom-specof exactly two atoms (not necessarily bonded or in the same model)axisandcenter(default0,0,0) coordinates are interpreted in the coordinate system of the input map, unless another reference model is specified withcoordinateSystem. The dimensions of the cylindrical slab are given in physical units of length, typically Å:lengthd(default is the map extent parallel to the cylinder axis) and inner and outer radiir1andr2(defaults are 90% of the smallest radius and 110% of the largest radius of the displayed isosurface, respectively, given the cylinder center and axis direction). The flattening is done by interpolating values from the original map on a cylindrical grid of points, then unwrapping the cylindrical grid into a rectangular grid. The cylinder radial direction becomes the X-axis of the new map, circumference the Y-axis, and cylinder axis direction the Z-axis. ThegridSpacingsis the requested separation of grid points along each axis in the new map (default is the minimum spacing along the three axes of the input map). The actual spacing may be slightly different because the dimensions of the new map may not be an exact multiple of the requested value; the number of grid divisions along each axis is chosen to give spacing as close as possible to the requested value without being smaller.

Show the full extent of a map that was previously limited to a zone using•volume zonewith optionnewMapfalse.

IfnewMapistrue(default), create a new map in which the values of grid points farther thanrÅ from any atom inatom-spec(those beyond the zone) is set to zero, or ifinvertis true, set the values of grid points within the zone to zero. IfminimalBoundsis true, make the resulting map as small as possible while enclosing the zone; otherwise, the dimensions will be the same as for the input map. IfbondPointSpacingis specified, use points spacedsÅ apart along bonds in addition to the atoms to define the zone.If

newMapis false, instead of creating a new map, zone the surface display and restrict the current display region of the map to the box containing the zoned surface. The full extent of the map can be restored withvolume unzone. Theinvertoption is not available withnewMap false. See also:surface zone,zone, thezonemouse mode

modelIdN

Open the new data set as model numberN(an integer, optionally preceded by #). The default is the lowest unused number.

stepN|Nx,Ny,Nz

Whether to use the full resolution of the data (step size1, default) or a specified subsample (step size > 1). Step sizes must be integers. A step size of 1 indicates all data points, 2 indicates every other data point, 3 every third point,etc.If a single number is supplied, it is used along all three axes; if three numbers are supplied (separated by commas but not spaces), they are used along the X, Y, and Z axes, respectively.

subregionname|i1,j1,k1,i2,j2,k2|all

Whether to use the full extents of the data (all, default) or a specified subregion. A subregion can be specified by:

namepreviously assigned with thenameRegionoption ofvolume- grid indices
i1–i2along the X axis,j1–j2along the Y axis, andk1–k2along the Z axis. Grid indices must be integers separated by commas but not spaces.

inPlacetrue |false

Whether to overwrite the existing data set instead of creating a new one. *Not all volume operations accept this option.* Regardless of this setting, the existing data will only be overwritten if it was created in ChimeraX (for example, with a previous volume operation) rather than read from a file. In the case of map addition, the model to overwrite is thegridmap(the model whose grid will be used for the result).

**Transfer function**.
The thresholds and connecting lines on each histogram define a
** transfer function**
that maps data values to colors and intensities.
For each voxel, the data value is compared to the thresholds on the histogram.
The colors and intensities of the closest threshold at a lower data value
(to the left) and the closest threshold at a higher data value (to the right)
are linearly interpolated. Voxels with data values greater than the rightmost
threshold or less than the leftmost threshold are colorless and completely
transparent. Color is defined by red, green, blue and opacity components.
The intensity at a threshold is further scaled by its vertical position,
where 0 is the bottom of the histogram and 1 is the top. Rendering time
does not depend on the positions of the thresholds, but increases with
greater numbers of thresholds.

**Image transparency details**.
For **image** displays,
the transfer function maps a
voxel's data value to a vertical position (ranging from 0 to 1)
on the histogram. A lower position indicates more transparency:

TheT= 1 – vertical position_{histogram}

T=_{pixel}T_{histogram}^{(1/p)}

**Byte order**.
Different computers store the bytes of a single numeric value in
different orders. PowerPC processors use one order
(called big-endian), while Intel/AMD x86 family processors use the
opposite order (called little-endian).
If a binary data file is created on one machine and read on
another having the opposite byte order, byte-swapping is required
to get the correct numeric values.
The DSN6 file reader assumes that the file was written in big-endian byte
order. DSN6 files can be read on big- or little-endian machines.
All of the other file readers detect file byte order and machine byte order,
then perform byte-swapping if necessary.

**Resampling and loss of resolution**.
There is some loss of resolution with resampling a map on a different grid
with trilinear interpolation. The effect is similar to smoothing.
The amount of reduction in resolution depends on
the initial resolution, the initial and final grid spacing,
the “bumpiness” of the data, and the shift between the grids.
For example, starting with a 6-Å resolution map with 2-Å
spacing from **molmap** and resampling on
a grid (with the same spacing) shifted by 1 Å reduced resolution
to approximately 7.7 Å. Starting with a 6-Å resolution map with
1-Å spacing and resampling on a grid shifted by 0.5 Å reduced
resolution to approximately 6.4 Å.

**Fourier transform**.
Only the magnitudes of the complex Fourier components are included in the
new data set; the phases are discarded and the constant component is set to
zero. The box containing the Fourier transform
(with axes in units of reciprocal space) is centered on the original data
and scaled to have the same total volume.
Some properties of the original data are evident from the Fourier transform.
High-frequency components are near the edges of the box,
low-freqency components near the center.
Volume data is typically oversampled (voxel size two to three times smaller
than the actual data resolution) and this causes the Fourier transform
to have nonzero values only in the middle half or third of its bounding box.
The missing wedge in electron microscope tomograms can also be seen.
Spikes radiating along the principal axes in the Fourier transform are
caused by nonperiodicity of the original data.

**Mask algorithm**.
The masked volume is computed by finding where lines
parallel to the projection **axis** intersect the surface(s).
For a given line, the volume data between the 1st and 2nd,
3rd and 4th, 5th and 6th, ... points of intersection are included
in the masked volume,
while those between the 2nd and 3rd, 4th and 5th, ... are excluded
(unless there are intersecting or nested surfaces
and **fillOverlap** is set to true).
The calculation uses a set of parallel lines that pass through points
on a rectangular grid perpendicular to the projection axis.
If the projection is along a data axis (data X, Y, or Z), the
lines will pass through the grid points of the data; otherwise,
lines along the projection axis with spacing equal to the minimum
grid plane spacing of the data will be used.
For each volume voxel, the intersections of the closest grid line
are used to determine inclusion in the masked volume.
If there is an odd number of intersections, then points beyond
the final one are not included in the masked volume
unless the **sandwich** option is set to false.
In the new data set, values outside the masked region are set to zero
and those inside are set to the original volume values
(for **volume mask**) or 1
(for **volume onesmask**).
The grid points of the calculated volume align exactly with those of
the original volume, if any.

UCSF Resource for Biocomputing, Visualization, and Informatics / May 2019