psychopy.org | Reference | Downloads | Github

GratingStim Contrast - is it defined as the Michelson Contrast?

Hello,

Is the contrast of the GratingStim defined according to the Michelson Contrast?

I have looked at a similar question in this forum here - but it doesn’t quite answer my question.

And the description of contrast I found in the docs do not mention it either:

A value that is simply multiplied by the color
Value should be: a float between -1 (negative) and 1 (unchanged).
Set the contrast of the stimulus, i.e. scales how far the stimulus deviates from the middle grey. You can also use the stimulus opacity to control contrast, but that cannot be negative.

This website however described it as:

the difference between the maximum and minimum luminance divided by the sum of the maximum and minimum luminances

which is basically the Michelson Contrast.

Is that correct?

Thank you!

The first of those options is the one that’s technically correct. Contrast, in PsychoPy, is a scale factor, at a value of 1 it leaves the image unchanged, at a value of zero it forces everything to grey and there’s a linear slope between them.

In the specific case that your texture includes white and black pixels (e.g. you select a sin texture and your sitmulus contains at least one full cycle) then the second description is also true - the contrast stated is the Michelson Contrast.

Just keep in mind that it depends on your texture.

Jon

Hi Jon,

Thank you for clarifying that. May I just check that I am understanding you correctly?

I have used a Sine Texture with a Gaussian Mask to create arrays of Gabors, but because I have adjusted the phase so that it is a sine wave cycle that shows at minimum at least one full cycle - it technically is a Michelson Contrast.

But the max and min luminance is actually the contrast value (e.g. 0.5) multiplied by 1 and -1.

Is that right?