Fighting Compression in Blender

German version

Especially while working with selective color correction, you may quickly strech the heavily compressed material to it's limits.

So, because of the low bit-depth and bitrates you will encounter annoying compression Blocks, primarily in the color components.

Yet, Blender's compositor provides a little workaround to save the trouble: The Bilateral Blur Node.
Though that name sounds quite complicated, the way it works is rather simple. It blurs the picture while trying to preserve important details. The benefit for us is the ability to blur the subtile compression artefacts without losing important features in the image.

It is not that easy though, because Blender's internal computing in a linear, set-related color space will lead to the darker areas getting blurred much more than the rest of the picture and that's not what we want!
A quick corrective is to use two gamma nodes: One we put before the Blur with a Value of 1/2.2 while the other one is placed behind with a value of 2.2.
We set the Bilateral's Color Sigma to a relatively small value of approx. 0.03 and leave the other parameters as they are.

Look, the traces of compression have vanished!

Unfortunately this is also true for many details.
To solve our problem we will take advantage of the fact that the really evil compression artefacts occur in the color channels only. So we will use a Mix node which is set to "Color" to limit the effect to the color components.

Finally, we get a clean picture (Aside from the nasty web compression here ;) )