= Generate mattes based on image colors (keying) =

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

table>>
    tr>>
        th>> To do this...
        th>> Use this operator
    tr>>
        td>> Remove color and produce alpha mattes.
        td>>
            [Chromakey|Node:cop2/chromakey]
    tr>>
        td>> Copy the luminance to the alpha channel.
        td>>
            [Luma Matte|Node:cop2/lumamatte]
    tr>>
        td>> Replace one color with another.
        td>>
            [Color Replace|Node:cop2/colorreplace]
    tr>>
        td>> Mask luminance in/outside a certain range.
        td>>
            [Lumakey|Node:cop2/lumakey]

TIP:
    The [Chromakey|Node:cop2/chromakey], [Color Replace|Node:cop2/colorreplace], and [Lumakey|Node:cop2/lumamatte]
    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 ((RMB))
  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).