Originally Posted by zoyd
If by APL you mean constant luminance then yes, you can't do it with that geometry but that is not the intent. They are designed to be 22% constant stimulus.
I didn't make it clear, but I did mean constant stimulus. I don't think a 22% constant stimulus is possible for patterns containing only two different pixel values (for window and surround) where the window area is one ninth of the total pattern area.
E.g., consider the pattern with the 100% stimulus window. For every one 100% stimulus pixel in the window area there are eight of stimulus level S in the surround. For the average stimulus of the whole pattern to be 22%, the following must hold:
(8S + 100)/9 = 22
We can solve for S with this result:
S = 98/8
So each pixel in the surround needs to have stimulus level 98/8 %. The
nearest 8-bit greyscale value is the nearest value to 255 * 98/(800) = 31.2375. That is, it is 31. But then the APL is:
(8S + 100)/9
= (8 * (100 * 31/255) + 100)/9
= 21.92 %
I know that's only a small error, but it does illustrate that these pattern types are problematic. And in fact you can't choose any APL value that fixes the problem.