id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	blockedby	blocking	notify_on_close	platform	project
1762	Blend multichannel volume data on GPU	Tom Goddard	Tom Goddard	"Would like to be able to blend multichannel volume data by keeping each volume on the GPU and a colormap 1D texture for each volume.  This currently works for single channel colormapping using the volume command colormapOnGpu true option, and it gives much better appearance interpolating map values rather than colors in some cases such as lung CT scan airways.

A difficulty is that if rendering is done in a single pass then each texture needs to be bound to a texture unit.  Each channel will need the map data and colormap as textures, so 2 textures per channel.  Some texture units are needed for shadows, and only 16 texture units minimum are guaranteed by OpenGL and high-end graphics currently has 32 texture units (eg GTX 1080 on Windows).  So only a limited number of channels (maybe 6) could be handled this way.  If more channels are needed then a fallback to multipass or CPU rendering would be needed.  Handling segmentations as separate channels can easily produce more than 6 channels (although that might better be handled with one label-map segmentation).

For handling any number of channels a multipass approach might be better where each channel is blended on pass at a time into a framebuffer texture with destination alpha, and the result then rendered to the scene framebuffer.  This may be much slower as it involves shader switches for every plane.  A faster and simpler implementation could order the channels and blend directly to the scene framebuffer.  The appearance would not be the same, for instance if 3 opaque channels were overlaid in one plane, only the last channel would be visible, rather than blending the colors.  Many multichannel rendering systems only allow red/green/blue 3 channel rendering, and displaying more channels is often not useful since there are only 3 independent color components.  So maybe just supporting on GPU one pass blending up to 6 channels would be perfectly adequate.  Rendering more than 6 channels could fallback to cpu and log a warning.
"	enhancement	assigned	moderate		Volume Data								all	ChimeraX