Opacity – The sum of the parts :huh:

[Krikor]

(10-22-2023, 06:50 PM)Ofnuts Wrote:

(10-22-2023, 11:56 AM)Krikor Wrote: But... wouldn't it be the opposite? In this case, wouldn't it be 1/3 px black and 2/3 px white? 

For each channel you have two pixels where the channel is 0 and one where the channel is 255. So you are sharing each channel value across 3 pixels... So that's one white for two blacks. Another way to compute that average would be to create three equal areas of Red, Green and Blue side-by-side, and average them using Filters > Blur > Pixelize. Coincidentally this also gives a Gray-156.

It took me a while to understand, although the information is clearly visible when we disable Sample merged and check each layer individually.
It really is 1 white for two blacks in all 3 colors. (255,0,0) (0,255,0) ((0,0,255).

I thought about the following, although it may be pure coincidence that the results are the same.

The bottom layer has 100% opacity, followed by 50% and 33.3% in the layers above.
This gives a sum of 183.33% (100+50+33.3).
With Sample merged enabled, this will give a value of 61.1% (183.3/3).
Knowing that 100% is equivalent to 255, then 61.1% is equivalent to Gray-156. (rounding 155.8).

But I think it's pure coincidence.

In the case of CMYK, it would be 1/3 px black and 2/3 px white, whereas for K it would be 100% black.
But for CMYK I couldn't come up with a formula that, even if by coincidence, would reach the same value. Because with CMYK the sum of colors is black and not white.