Finally I connect the Tile_2 to a Blender. This Blender is mixing 50% of GRAY 127 and 50% of GRAY 128.  The Blending value is .5, which means it is calculating .5 * 127 + .5 * 128 = 127.5.

Observe that the preview is now smooth - there are no squares anymore.

This shows us that color arithmetic in Poser is actually performed using floating point numbers, not 8-bit integers. If this was not true, then WHITE / 2 would have to be either 127 or 128. Since it is not equal to either of those, it must be something else.

Half of 255 is indeed 127.5 so by definition mid-gray is 127.5.

This also shows us that you cannot directly enter the color mid-gray into a color editor in Poser, but you can construct it in several ways using nodes.

Using matmatic, you can type it directly as .5.

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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)