This short tutorial has been written for 3ds max and vray users, but most probably you can easily “translate” the workflow in your favorite 3d software programs.
Everybody knows that when using indirect illumination (GI), the color of the materials in a scene affects other objects as well, due to the light that “borrows” color information from a surface after having “bounced” from it.
Although it is normal and happens in real life as well, sometimes the bounced light is affecting the scene too much and it doesn’t look right.
The most common situations when this happens is when you have a wooden floor and the color ceiling and the walls becomes too red/orange (like in the example bellow).
If you use “linear workflow” that is less likely to happen, but if the deadline is knocking on your door, you need a quick way to adjust it and not lose time trying to find ways to do it directly from the render ( and completely change your workflow).
Here is a quick solution that can prove useful in situations when time is a major factor:
Make the material of the wall and ceiling a vraymtl wrapper, keeping the base material as it was.
Under “alpha contribution” type “-1”, hit render and save the resulting rendering as a tiff with alpha channel.
Open the rendering in photoshop, click on “select”, choose “load selection” and choose “alpha 1”. Now invert the selection, and you will end up with the selection of the walls and ceiling (which are the elements that are affected too much by the light bounced from the wooden floor).
With the selection active, use the hue/saturation slider from “image”, “adjustments” drop down menu until you obtain a desirable result. You can also use brightness and contrasts or level adjustments if you feel necessary.
Bellow is the result I achieved following the steps shown above.
Although this tutorial is aimed particularly at de-saturating colors on some elements of the scene, the same method can be applied in order to color correction adjustments on any particular materials you need, by making separate alpha channels for those specific materials and adjust the color and saturation from photoshop.