by psykinematix » Thu Oct 22, 2020 6:34 am
Hi,
What you are trying to do is not clear enough: why is your goal and which rendering mode are you using (contrast- or luminance-based) ? Are you trying to set independently different contrast or luminance for the center and outer ring? Note that a contrast of 0 would not result in a black stimulus, but in a stimulus with the same luminance as the background. To obtain a pure black stimulus, you would have to set the contrast to -1. Typically the background is set as a percentage of the maximum luminance (for example 50% of maximum luminance to produce a mid-gray level), not in term of contrast. Also you have to make sure to properly create the envelope for the ring: (r>radius_c & r<radius_g), not just (r<radius_g) because this also includes the center !
Here are 2 examples to set the appearance of the center and surround:
1) In term of contrast (make sure to select the "Contrast" rendering mode below the color space with a contrast value set to 100%):
ring = contrast_g*(r>radius_c & r<radius_g)
center = contrast_c*(r<radius_c)
z = center + ring
2) In term of luminance (make sure to select the "Luminance" rendering mode below the color space with a luminance value set to 100%):
ring = 0.5*(1+contrast_g)*(r>radius_c & r<radius_g)
center = 0.5*(1+contrast_c)*(r<radius_c)
outside = 0.5*(r>radius_g)
z = center + ring + outside
Those 2 examples create the exact same stimulus. Set contrast_c to 1 to create a white center and set contrast_g to -1 to create a black ring.
Hope this helps!
The Psykinematix Support Team @ KyberVision Japan LLC