Color correction.

Easiest way to control the mood of the picture is color correction. No serious team will go out without this feature inside their beloved engine. And there is a lot of cases you can't live without it. Every time your lead artist wants to change something globally - color correction is the first thing he will consider.

From technical point of view, color remapping is really easy to implement. But the most obvious solution, use of volume texture to simply remap colors, will suffer from banding issues until resolution of this volume will be close to 8 bit precision (like 256x256x256). This is prohibitively costly in terms of memory and performance of texture cache as we need to touch every screen pixel.

Fortunately, this can be easily solved by using signed addition instead of remap. So to transform Red (1,0,0) to Green (0,1,0) we need to use value of (-1,1,0). In this way we can use volume texture with size like 16x16x16 without any banding artifacts. Let's call this color correction matrix.

But technical solution is not enough. We need artist-friendly way of controlling this. After some consideration and communicating with artists I've choose the following solution. Artist creates some arbitrary number of screenshots of the level or area he wants to use color correction on. Then he uses any color correction tool of his choice (we've used Photoshop for prototyping) to produce derived color corrected versions of above screenshots. In this way we are not restricting him anyhow.

After creating two sets of screenshots artist feeds them into the engine tool that builds color correction matrix from them. Processor just incrementally saves difference of colors to some table and compress it into correction matrix after all images being processed. Afterward we can add some effects to this matrix.

Implementation note: remember that you need to correct image before HDR!

Shader code for applying correction is trivial:

float4 source = tex2Dlod(samplerSource, uv);
float4 corr = tex3Dlod(samplerColorCorrection, float4(source.xyz, 0));
return source + (corr * 2.0f - 1.0f); // if you use unsigned format

This might look like this in reality:

There is some obvious difference between corrected image and generated by run time. Major source of this is 16x16x16 correction matrix. HDR contributes by adjusting luminosity as well. But it looks not that bad in real situations.

You might notice unknown parameter "Color shift". It used for shifting dark colors below threshold to some arbitrary color. In this way you can make shadows blueish for example. I've seen this in CryTek sometime ago and I don't know who should be credited for this idea. Perhaps CryTek guys. ;)

It's pretty much everything. The only valuable addition to this method might be engine ability to place zones (volumes) with arbitrary color correction on level for special effects and fake global illumination. Use your imagination.