User Manual

How Channels Propagate During Compositing
Images are combined, or composited together, using the Merge node. The Merge node takes
two RGBA inputs labeled “Foreground” (green) and “Background” (orange) and combines them
into a single RGB output (or RGBA if both the foreground and background input images have
alpha), where the foreground image is in front (or on top, depending on what you’re working on),
and the background image is, you guessed it, in back.
A simple Merge node composite.
Auxiliary channels, on the other hand, are handled in a much more specific way. When you
composite two image layers using the Merge node, auxiliary channels only propagate through
the image that’s connected to the background input. The rationale for this is that in most CGI
composites, the background is most often the CG layer that contains auxiliary channels, and the
foreground is a live-action green-screen plate.
Since most compositions use multiple Merge nodes, it pays to be careful about how you
connect the background and foreground inputs of each Merge node to make sure that the
correct channels flow properly.
TIP: Merge nodes are also capable of combining the foreground and background
inputs using Z-Depth channels using the Perform Depth Merge checkbox, in which
case all pairs of pixels are compared. Which one is in front depends on its Z-Depth and
not the connected input.
Rearranging or Combining Channels
Last, but certainly not least, it’s also possible to rearrange and recombine channels in any way
you need, using one of three different node operations. For example, you might want to
combine the red channel from one image with the blue and green channels of a second image
to create a completely different channel mix. Alternatively, you might want to take the alpha
channel from one image and merge it with the alpha channel of a second image in different
ways—adding, subtracting, or using other intersection operations to create a very specific
blend of the two.
Chapter – 69 Understanding Image Channels 1394