GratingStim.opacity same as Michelson contrast?

Coding
6

In most cases, yes, you can treat opacity as a way to set Michelson contrast but two caveats:

  • it’s really a multiplier: the contrast of your stimulus is ultimately governed by a multiplication of the color, the contrast value, the opacity and the inherent contrast of the texture (although if you’re using the built-in sin texture then that is 1)
  • opacity (unlike the contrast parameter itself) is making a weighted average of the stimulus and whatever is behind it. If you set opacity = 0.2 then each pixel will be 0.2xStimulus and 0.8xbackground color. So the contrast also now depends on what is behind the stimulus. That’s assuming your window has blend mode = ‘avg’
  • if you need to combine multiple gratings then understanding the weighted average for the pixels gets especially complicated, so then I’d really recommend you use a blend mode = ‘add’

I hope that helps

1 Like