Rendering Alpha Channels

combustion automatically generates an alpha channel for composites. You can view this alpha channel at any time. You can also render it embedded into a movie or an image sequence (color and alpha channels), or render it separately (only the alpha channel). Rendering a separate alpha channel is useful for creating footage that you can use in other programs, or as a linear key in a traditional tape-based editing suite.

For more information on rendering a composite, refer to Rendering.

To render the alpha channel:

1. Choose File | Render.

The Render Queue dialog appears.

2. Under the Video Output heading, select a file format that supports alpha channels (such as TARGA Sequence) or, if you are rendering only the alpha channel, select any file format.

For more information, see Supported File Formats.

3. In the Name field, enter a name for the output alpha channel.
4. Click Browse to navigate to a destination directory for the render file(s).
5. In the Channels group, click on of the following options:
• Enable Color to render only the RGB color channels of the clip.
• Enable Alpha to render only the alpha channel.
• Enable Color+Alpha to render both the RGB color channels and the alpha channel.
6. If you enabled Color or Color+Alpha, you can access the Premultiply Color button.

Enable Premultiply Color if you want to combine the color information with the alpha channel information using black as a base. If you are outputting RGB images without alpha, you need to enable this option to get an anti-aliased edge over the background (otherwise the edges may look somewhat jaggy if you are compositing over a solid background).

7. If you enabled Alpha or Color+Alpha, you can access the Invert Alpha button.

Enable Invert Alpha if you want to reverse the transparency of the image (pixels that were opaque become transparent, and vice-versa).

8. Click Process to start rendering.

When rendering is finished, a "Render Process Finished" message appears below the status bars.

9. After reviewing the render information, click Close.