Procedural Compositing with Batch > Batch FX > Outputting from a BFX Output Node to the Timeline >

# Outputting an Alpha to the Timeline

Output alpha results in addition to the RGB result from a BFX output node. You connect the RGB result to the front input of the BFX output node, and the alpha result to the matte input of the BFX output node. Both results are fed directly to the timeline. To enable the alpha, you apply an Axis or a Sparks soft effect to the segment.

Alternatively, you can have an Axis or a Spark soft effect automatically added on output. Enable the Add Axis On Matte Output button in the Modules/Soft FX section of the Preferences menu. A soft Spark will be added for 16-bit float material; a soft Axis will be added for all other material.

To output an alpha from a BFX output node:

1. Enter a BFX level and create a matte. Connect the RGB and alpha outputs to the BFX output node.

   In the following example, a basic matte is created with a Keyer node. The blue tab on the BFX output node receives the alpha result and the red tab receives the RGB result. The background is not considered when you output the matte back to the timeline.

   (a) RGB result goes to front input tab(b) Alpha result goes to matte input tab

2. Select the BFX output node and press F4 twice to display the matte and refine as needed.

   Image courtesy of The House

3. Exit back to the timeline and process.

   The BFXa icon indicates that the segment contains a matte. In the following example, the matte is not yet enabled (the Add Axis On Matte Output button is not enabled in the Preferences menu).

   (a) BFXa matte icon

   Image courtesy of The House

4. Add a soft Spark to the matte segment if the clip is 16-bit float; otherwise, add a soft Axis. Add a background layer to the timeline.
   NoteTo have the Axis or Spark soft effect automatically added when you exit back to the timeline, you can enable the Add Axis On Matte Output button in the Preferences menu at any time.

   The matte output results from the BFX output node are composited over the background layer of the timeline (L1).

   (a) Axis soft effect indicator on background layer

   Image courtesy of The House