User Manual

Setting the Premultiplied Status of Nodes That Need It
When you select a Loader node, the Import panel in the Inspector includes a group of
checkboxes that let you determine how an alpha channel embedded with that image should be
handled. There are checkboxes to make the alpha channel solid (to eliminate transparency), to
invert the alpha channel, and to post-multiply the RGB channels with the alpha channel, should
that be necessary.
NOTE: This functionality was not yet available for MediaIn nodes in DaVinci Resolve
16.2 as of this writing.
Nodes That Affect Premultiplication
Most nodes that require you to explicitly deal with the state of premultiplication of RGBA image
input have a Pre-Divide/Post-Multiply checkbox. This includes simple color correction nodes
such as Brightness Contrast and Color Curves, as well as the Color Correct node, which has the
Pre-Divide/Post-Multiply checkbox in the Options panel of its Inspector settings.
The Pre-Divide/Post-Multiply checkbox of the
Color Curves node, seen in the Inspector.
Control Premultiplication with
Alpha Divide and Alpha Multiply
The Alpha Divide and Alpha Multiply nodes, found in the Matte category of the Effects Library,
are available when multiple operations in a row expect a “straight” alpha channel. Instead of
performing repetitive Pre-Divide/Post Multiply operations on each node, you can use these two
nodes to bookend the other nodes. Simply add the Alpha Divide node when you want the
RGBA image data not to be premultiplied, and add the Alpha Multiply node when you want the
image data to be premultiplied again. For example, if you’re using third-party OFX nodes that
make color adjustments, you may need to manually control premultiplication before and after
such an adjustment.
A node tree with explicit Alpha Divide and Alpha Multiply nodes.
Chapter – 69 Understanding Image Channels 1397