Help Needed! About Contrast of Gabor Patch in Matlab

Hi I am a student doing psychophysics studies.

Recently I used Matlab to create Gabor patches as the stimuli in my study.

Now I need to get the values of their contrast agains the background (RGB = 255,255,255);

I have the codes for creating the Gabor patches.

However, I don't know how to calculate the contrast...

Could anyone please help me with this?

Thanks a lot!

Leo