Quote - transparency set to 80% for a block, white ball behind it. Expected: light passes 2 surfaces front and back, that's a 80% x 80% = 64% brightness reduction by the block. Rendered with default gamma, result measured in Photoshop: 82% brightness. Which matches the 64% reduction corrected with GC. It should not, it should remain at 64%.

Wrong! You are making the totally incorrect assumption that 64% brightness is 64% of 255. You are totally ignoring the sRGB color space.

Your monitor displays the 8-bit integer 255 at the brightest it can deal with. Now you're making the incorrect assumption that a pixel showing 64% of that brightness would be .64 * 255, or 163. That is incorrect. The number system used by monitors (sRGB) has a gamma of 2.2. That means the output brightness is equal to the fractional value you supply raised to the 2.2 power!

A brightness 64% of the maximum is not 64% of 255. It is (.64 ** (1/2.2)) * 255 = 208. The number 208, in sRGB integers, stands for the value 64%. Any other interpretation results in errors and that is why the linear workflow is so, so important.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)