Generate mattes based on image colors (keying)

Houdini includes several operators to let you isolate, select, and/or remove areas of color.

To do this…	Use this operator
Remove color and produce alpha mattes.	Chromakey
Copy the luminance to the alpha channel.	Luma Matte
Replace one color with another.	Color Replace
Mask luminance in/outside a certain range.	Lumakey

Tip

The Chromakey, Color Replace, and Lumakey operators have a supersample parameter, which gives anti-aliased edges at sharp color transitions. A supersample value of 3 is usually sufficient. Higher values will really slow down the operation. If you don’t want antialiased edges, set supersampling to 1.

To select areas of color interactively (like the Magic Wand in Photoshop), make the node active (select the node in the network editor and press Enter in the viewer) and use the following keys:

Press O, then brush-select over pixels of the colors you want to isolate, or hold Ctrl and drag out a selection box. Click to finish the selection.
To select the drop-off region interactively, press L instead of O to begin selecting.
To add to the current selection instead of replacing it, press Shift + O or Shift + L.
If you don’t want the selection to affect Alpha (normally it creates an alpha matte from the keying operation), unscope the A plane on the operator’s Mask tab (and remove the * if you have deep raster planes you don’t want affected).