Now here is why postwork is not as good as GC in render.

I took the first image, which was not gamma corrected, into Photoshop.

Using Photoshop Levels, I adjusted the middle value from 1, to 2.2. This is the same as gamma correction, except that the incoming step of anti-gamma correction was not performed. So I had to also increase saturation, because the full linear workflow was not followed.

Notice the banding. This is because information was lost. The darker shades on the wall were recorded at very low levels in the original image. Postwork gamma correction can only adjust those individual values to their corresponding levels-adjusted values. The in-between values are not there in this version, because they did not exist in the data stored in the uncorrected image.

This is why postwork levels adjustment falls short. You are starting with less information, and the info you have is less accurate. You cannot fix this in postwork.

Notice also that the first (darkest) figure is still pretty much black. That's because in the uncorrected image, most of him was less than 1, i.e. 0. You cannot adjust levels around 0. There is no data. 0 to any power is still 0.

If you really want to do post-work levels adjustment in a scene with dark areas accurately, you must store the image in HDR or EXR format. Then the data is more than 8-bits and the low level detail can be recovered.

But from a JPEG or PNG, the data is lost forever.

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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)