In the last Blender tip I stated that I would write something about frequency extraction. Here is one useful technique which depends on that.
Although I am not a fan of skin retouching in general because I believe a human's natural and individual appearance is most appealing and candid, I am sure that some of you might like to soften an actress' or actor's skin sometimes. The main difficulty doing this is that you want to obtain a smooth looking skin without making it look like wax and to get an idea how achieve this, it is useful to understand what we are really trying to do here:
No, we don't. We want the skin to look gentle and clean, not smooth! So it is not our Intention to remove every feature, but only the ones that are annoying, while keeping the detail which make the skin look natural.
Here we can benefit from the fact that the bad features like little flecks are bigger than the skinpores that should be kept - which leads us to frequency extraction.
We want to create a little node setup that blurs the image while preserving the edges, but keeps the small scaled details. This is surprisingly simple:
Let us go through this step by step:
As described in an earlier post, we use the two Gamma nodes to work in a gamma corrected color space. After the Gamma node, the tree is split into two branches: The lower one blurs the the image but keeps the edges. Using the Color Sig value we can control which details will be blurred and the Space Sig controls the blur amount. This branch alone creates this:
To recover the little details, the upper branch compares the raw image with a version that is less blurred than the one from the lower branch by subtraction. The output is an Image containing only the features that were blurred and the Space and Color Sig values enable us to control which details will be there. In the end, the two branches are added up and we get the final result.
I have exaggerated the rather subtle effect a bit to show it more clearly.