Subject: Sparky peak picking algorithm uses quadratic interpolation?
From: Tom Goddard
Date: Sep 18, 2013



Hi Yonggang,

Yes, Sparky peak picking mouse mode (F8) when you drag a rectangle with the mouse and it places peak markers does use quadratic interpolation -- exacly the same method that peak center (pc) does. The peak picking first finds local maxima satisfying the minimum height, linewidth and drop-off criteria. A local maximum is exactly at a grid point. But then it considers the two neighbor points along each axis from that local maximum. Using the 3 points (left neighbor, maximum, right neighbor), it fits a quadratic function (a*x*x + b*x + c) and uses the maximum of that quadratic as the position along that axis.

If you simply click a points in peak picking mode, it places the marker exactly where you clicked without quadratic interpolation, and without looking for a local maximum.

Tom



On Sep 18, 2013, at 10:11 AM, chang YG wrote:

Hi,

I have a Sparky question about its peak picking algorithm.

Question:
Does it use a three-point interpolation algorithm to find the peak position (peak picking/F8)?

I can not find the algorithm on the website online Sparky manual although it does use quadratic interpolation for the command peak center (pc).

Below are quoted sections from the site:

Picking Peaks

Use the find/add peak pointer mode (F8) to pick peaks . Clicking on a point in a spectrum view will place a peak marker there. You can move the marker to the local maxima with thepeak center command (pc). Dragging a box will place peak markers at local maxima satisfying minimum height, linewidth and drop off requirements. You can pick peaks over the whole spectrum using theregion dialog (rt). The positive and negative height thresholds equal the lowest positive and negative contour level. You canadjust the contour levels with the contour scale (vC) or the contour dialog (ct). The minimum linewidth threshold is set with the peak picking dialog (kt). The default value is zero which has the effect of no minimum linewidth. The linewidth for a prospective peak is computed as the full width at half height. The minimum drop off requirement is a way of filtering out overlapped peaks. If the data height does not drop off by a specified amount as you move along each axis from the peak marker before starting to rise then that peak is not picked. The specified amount is given as a fraction of the total peak height. The default value is zero so there is no dropoff requirement by default.

Centering peaks

To center a peak at the local maximum use peak center (pc) . This uses quadratic interpolation to position the peak to a resolution finer than one data point. If you want to type in the position select a peak and use the ornament properties dialog (ot).


Thanks for your help.


Best,

Yonggang Chang