Holdout Composite
Description
To add practical fire, explosions, smoke or other footage not containing an alpha channel is normally a challenge. A normal Math Composite Add function would cause areas of the background to get brighter. This is bad. Using a key of some type would most likely generate unwanted edge fringing. This is also bad. The Holdout Composite is a two-layer/track effect that effectively composites images such as fire, explosions and smoke. It creates a luminance matte of the foreground and pastes it as black (or other color) over the background. You then use one of the Blend Modes to place the foreground over the “held out background”.
Note: You can change Holdout Composite’s input order with the User Interface > Trees > Input Order preference. By default, FG, BG is selected, but you can change it to BG, FG if you’d like.
Node Group
Composite.
Controls
Blend
This selects the type of Blend mode to use for the foreground element.
Add
The pixels of the foreground are added to the background.
Screen
The foreground and background are combined using a Screen blend mode. This looks kind of like the Add blend mode, but highlights are retained.
Lighten
Compares the foreground and background and takes the pixels with the higher value.
Level
Sets the level of the foreground element.
Black Clip
Blacks are made blacker by increasing this value. As the value increases, more values are clipped to black.
Note: Often times the black portion of an image is not entirely black. This becomes an issue if something like fire is shot against a black background. If the black is not entirely black, then these impure black areas will be added to the background image as part of the Holdout Composite causing the final result to have milky blacks. Black Clip will improve these milky blacks and make them darker.
Matte
The Holdout Composite first creates a luminance matte of the foreground and then pastes it as black (or other color) over the background. You can see the extracted matte with the View set to Matte.
Black Clip
Blacks in the matte are made blacker by increasing this value. As the value increases, more values are clipped to black. This is helpful for getting rid of unwanted gray areas in what should be the black part of the matte.
White Clip
Whites in the matte are made whiter by increasing this value. As the value increases, more values are clipped to white. This is helpful for getting rid of unwanted grey areas in what should be the white part of the matte.
Holdout
The Holdout Composite takes the luminance matte of the foreground and pastes it as black (or other color) over the background. You can see the matte pasted onto the background with the View set to Holdout.
Color
The Color parameter sets the color of the Holdout matte through the use of a standard eyedropper or color picker. The default color is black.
Note: There may be times when you want to select a color from the background as the Holdout color. This will appear to give the foreground some atmosphere.
Opacity
The opacity of the color.