Frequency separation in Paint by Zaweba

Retouchers have used frequency separation for years because it solves a problem every portrait has: blotchy colour and uneven tone sit in the same pixels as pores and fine hair, so fixing one usually damages the other. Splitting the picture into two layers, one for broad colour and one for fine texture, lets you work on each without touching the other.

In many editors you build that split by hand with a blur, an extra calculation step and a blend mode, and a wrong setting leaves the recombined image slightly off. In Paint by Zaweba it is one command, Layer > Frequency Separation.

What the command makes

Frequency Separation creates a group containing two layers. The Low layer holds the large, soft information: colour, tone and shading. The High layer holds the detail: pores, hair, fabric, edges. The High layer uses the Linear Light blend mode, and the pair recombine exactly, so before you touch anything the group looks identical to the original.

The Low layer can be made with a Gaussian, surface or bilateral filter. Gaussian is the classic choice and splits cleanly by size. Surface and bilateral filters preserve edges, so strong edges such as the line of a jaw or the rim of an eyelid stay in the Low layer as edges rather than leaking into the High layer as a bright fringe. For faces against a contrasting background, try one of those.

How strongly the Low layer is smoothed decides what counts as texture. Light smoothing puts only the finest detail on the High layer; heavier smoothing moves more of the structure there. For skin, aim for a split in which pores and fine lines sit on the High layer and the blotches of colour do not.

Working on the Low layer

Colour and tone fixes happen here. Use a soft brush at low opacity, sampling nearby colour, to even out redness and patchy shading. A gentle blur on a selection of the cheek or forehead smooths tonal bumps. Because texture lives on the other layer, the skin keeps its grain however much you smooth here.

Leave the shape of the face alone. Shadows under cheekbones and along the nose are what make a face look three-dimensional, and erasing them makes skin look like plastic even with perfect texture on top.

Working on the High layer

Texture fixes happen here. Use a cloning or healing brush that samples only the current layer to cover a stray hair, a scar or a blemish with healthy texture from nearby. Because the High layer carries no colour of its own, you can move texture from the forehead onto the cheek without carrying the forehead's colour across.

Avoid painting with ordinary colours on the High layer. Anything that isn't neutral grey will change the brightness of the result.

Habits that keep it natural

Work at different zoom levels. Close up you see texture problems; zoomed out you see whether the overall tone looks real. Lower the opacity of individual fixes rather than trying to make skin perfect, since real skin is not even. When you are done, toggle the whole group off and on to compare with the original.

Frequency separation is known as a tool for skin, but it is just as good for product photography, where you might want to even the colour of a fabric without losing the weave, or for cleaning marks from a textured surface.

How it fits with other tools

Paint by Zaweba also offers Wavelet Decompose, which splits a layer into several detail scales rather than two. When two layers aren't enough, for example to treat fine pores and larger blemishes separately, that is the next step up. For quick fixes, a healing pass on a single layer may be all you need. Frequency separation sits between the two: more control than a single healing pass, simpler than a full stack of scales.

Because the result is a normal group of layers, it saves with your document and stays editable, so you can come back the next day and adjust a cheek without starting over.