Procedural Compositing with Batch > Batch FX >

# Generating an Alpha from a Clip Node

You can generate an alpha from a clip inside a BFX level. When you output the alpha result, the clip node appears with an explicit alpha channel. You can then work with both the RGB and alpha outputs directly from Batch FX. You can also generate an alpha for the left eye and right eye from a clip containing a stereo track.

To generate an alpha from a clip node, the clip’s primary track must contain only one layer. The layer can contain either a matte container or a BFXa segment. If the clip contains more than one layer, an Axis or Sparks soft effect will be added for vertical compositing.

You can also explicitly generate a black or white alpha from a clip. Do this, for example, to have an alpha of the same resolution as the RGB clip.

To generate an alpha from a clip node:

1. Select a clip in the schematic.

   In the following example, notice that the clip’s matte output tab is muted. You will not be able to connect to the matte output tab until a matte is fed to the tab. The RGB output tab outputs the RGB of the clip.

   (a) RGB output tab (b) Inactive matte output tab

2. Enter a BFX level with the clip using the Pre or Post option.
3. Create a matte and connect it to the matte input tab of the BFX Output node.

   (a) Matte input tab

4. Exit to the previous level.

   Notice that, in addition to its RGB result, the clip node displays its alpha output. The matte output tab is blue indicating that the clip contains an alpha output. You can use both the RGB and alpha result of the clip in your process tree.

   (a) RGB output(b) Alpha output (c) RGB and alpha of clip are input to Keyer

   NoteWhen you exit to the record timeline, both the RGB and alpha results are output to the segment. An Axis soft effect is also added for vertical editing.

To explicitly generate a black or white alpha of a clip:

1. Select a clip in the schematic.
2. In the Basic menu, enable Active and then select to output a white or black alpha.
   NoteThe Active button is greyed out if the clip contains a BFXa or a matte container.

   The clip’s alpha is generated according to your selection.

   NoteFor a stereo clip, a single grey output tab is displayed and must be expanded as a group node to display its multiple output tabs. See Grouping Nodes.