New to PTB - Gamma correction with gratings stimuli

hyma2194 · May 28, 2020, 1:02pm

Hello team,
I apologize for the trivial question but I cannot find a definitive answer otherwise. I am trying to understand how to use gamma correction in the context of having constant luminance when I shift from a black and white gratings stimuli to a grey background and back for a visual experiment. The idea is to ensure a linear gradient and constant total luminance when shifting between the gratings and grey background.
I have coded the paradigm already but I am not sure how to apply gamma correction with the available function options in PTB.

Thanks
Hyma

xiangrui · May 28, 2020, 1:28pm

You could compute the correction by yourself, but I found the easy way is to use PsychImaing pipeline:

PsychImaging('AddTask', 'FinalFormatting', 'DisplayColorCorrection', 'SimpleGamma');
...
PsychColorCorrection('SetEncodingGamma', win, 1/gamma);

For details, you may check one of the demos like BitsPlusCSFDemo.m

hyma2194 · May 28, 2020, 4:58pm

Thank you so much! Worked well for me!

Ian-Max-Andolina · May 29, 2020, 12:56am

Note if you want more precise correction, you need to measure the exact gamma curve with a light meter. You can then fit a curve more precisely to the data, and then test that the result is fully linear with the same light meter. If you have access to a SpectroCal / ColorCal / i1Pro then I have a class to automate this here:

github.com

iandol/opticka/blob/master/calibration/calibrateLuminance.m

% ========================================================================
%> @brief calibrateLuminance manaul / automatic luminance calibration
%>
%> calibrateLuminance manaul / automatic luminance calibration
%> To enter settings use a structure:
%>
%> >> mycal = calibrateLuminance(struct('nMeasures',25,'useCCal2',true))
%>
%> calibrateLuminance will ask if you need to zero calibrate, you only need
%> to do this after first plugging in the ColorCal. Then simply place the
%> ColorCalII in front of the monitor and follow instructions. After doing
%> nMeasures of luminance steps, it will fit the raw luminance values using a
%> variety of methods and then plot these out to a figure (it will ask you for
%> comments to enter for the calibration, you should enter monitor type,
%> lighting conditions etc). You can then save mycal to disk for later use by
%> your programs. To use in PTB, choose the preffered fit (1 is the gamma
%> function and the rest are the various model options listed in analysisMethods).
%> you may need to expand the selected model fit to 3 columns before passing to
%> LoadNormalizedGammaTable:
%>

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New to PTB - Gamma correction with gratings stimuli

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