Frequency Separation

Let's explain a basic retouching technique, the Frequency Separation. This allow us to split our image into two layers, one will have the colour/light and the other will have the high frequency, the texture/details. Mostly know for cleaning skins in fashion photography, but also powerful for retouching our cg backplates or to prepare our textures of fabrics, leathers, walls or even remove moire. We will also show how to use it to add some sharpness.

Let's use this leather texture as a base image. First thing to do is duplicate it and apply a gaussian blur to the lower layer. You should apply as less blur as possible but enough to remove the high frequency (leather texture). Name this layer "low frequency".

Now select the top layer (not the blurred). Name it "high frequency". Go to Image/Apply Image, select the "low frequency" layer, invert the RGB channels, use "add" blending mode with a scale of 2 and an offset of 0. Apply it and use this layer in Linear Light. Done! The result of these two layers is exactly the same as the base image, but now we can work on low and high frequency.

Easily replace the low frequency layer by a flat colour and stamp some wrinkles to change you're texture from an old leather to a brand new.

You can also add some contrast to the high frequency layer to improve the sharpness of the image.