Well I’ll try to make things as simple as possible because everytime I read about it somewhere, people jump straight into weird words to explain stuff they know almost nothing about (not that I know alot…but someone thaught me the background explanation and how to work with it).
So basicly, it starts back when TV was invented. They had problem getting what was filmed to show up exactly like the reality when it was on tv. So they basicly just said, hey! lets compensate, and add a (it’s more complex I believe but this is close enought) 2,2 gamma to the screen. From then, when problems were fixed, they didn’t ask themself much so they ended up making all the camcorders/camera/whatever compensate the image with a 0,4545 gamma value, because all the screens in the world compensate the image with a 2.2 gamma afterward.
So why do it in 3d too? Because when you do color correction, a blur or glows, or any mathematical operation in comp, you actually add those on top of a compensated image and you lose alot of information. The best example is taking a nigh shot of a city. Add a blur on top of it. Then take the same image, do a 0,4545 gamma value correction on it, then do your blur, then recorrect the gamma value at 2.2. Compare the 2 images and you’ll see that the highlights aren’t lost in the gamma corrected version.
The only ‘problem’ with this is that you have to render 16bits files, so you don’t ‘clip’ in black or in the white when you gamma correct it. The good point is that 16bits are generally ALREADY gamma corrected, because mental ray (and other renderers) calculate the images in a linear way. So when you render an image and you save it as .exr it will appear darker then what you rendered on screen. So in Comp, you just have to add whatever correction you want on it, and at the end add a 2.2 gamma correct because saving in another format.
The pro of using this? Your lighting will look much better in the end, your reflections, highlights, glow and all the passes your merge together in comp will work in a much more realistic way.
Did that make any sense? If not, just try the example I said about the city night shot. You’ll see the difference, and that’s all you need really 