Effects Painter Tool
This tool allows you to paint photo effects onto your photo using a nib. It’s based on the Shape Painter Tool, so it shares a lot of the same controls such as those for choosing the size of the nib, smoothing the painted shape and softening (feathering).
Selecting and applying an effect
Choose the effect you want to apply from the list on the left of the infobar. For example the Desaturate effect will reduce the saturation (color) on the areas of the photo you paint onto.
Choose a nib size using the Size control. Note that by default, as in the Shape Painter tool, the on-screen nib size stays the same as you change the zoom, which makes it easy to zoom in and work on intricate parts of the photo without also having to change the nib size. However if you prefer you can switch to an absolute nib size by clicking the Size toggle button.
Use the Softness control to give your strokes some feathering, so they blend into the photo, instead of having a hard edge (softness 0).
Start painting onto your photo to apply the effect. This creates a new region shape on top of your photo.