Masks limit a module’s effect to certain parts of the image, so that a correction lands where you want it and leaves the rest of the picture untouched.
A mask is best pictured as a grayscale image laid over the picture: each pixel holds a value between 0 and 1.0 (0% to 100%), and that value is the opacity handed to blending. An opacity of 0 leaves a pixel exactly as the module received it, 1.0 gives it the module’s full effect, and values in between apply it proportionally.
Note
A mask decides where a module applies, and how strongly. How its input and output are mixed at that strength is the job of the blend mode.
Ways to build a mask
Three methods can set that per-pixel opacity. Each works on its own, or combined with the others:
- Drawn masks — select areas by location, with shapes drawn directly on the image.
- Parametric masks — select pixels by their own color and tonal values.
- Raster masks — reuse a mask that another module already computed earlier in the pixelpipe.
Combining and refining masks
- Combining drawn & parametric masks — how polarity and the Combine masks setting merge a drawn mask with a parametric one.
- Mask contours — blur and feather a finished mask so that its edges follow the details of the image.