Generating a Movie with an Embedded Alpha Channel
August 22nd, 2009 | Posted by in Movie StreamThis tip will discuss how to create an embedded alpha channel in a movie file you are generating with Studio Artist. Embedded alpha channels could be associated with movie stream or action processing movie file output.
What is an embedded alpha channel?
An embedded alpha channel can be though of as a selection mask that is associated with each frame of a movie file. Each movie frame consists of 3 RGB image channels and 1 alpha channel for the mask associated with the frame. Each pixel in the alpha channel is used to define percent coverage for the associated RGB pixels values.
An alpha value of 0 means 0% coverage, so the pixel RGB values are totally masked. An alpha value of 255 means 100% coverage, so the pixel RGB values are fully displayed. In between alpha values will define partial transparency for the pixel.
The ARGB frame and it’s associated image channels will probably be additionally compressed by whatever compression type (referred to as a codec) you have chosen for your movie file output. Keep in mind that not all compression types support embedded alpha channels. So if you are interested in including one in your generated movie file you need to choose a codec type that supports embedded alpha. This is discussed in more detail below.
How to generate an embedded alpha channel
Previous versions of Studio Artist had separate movie settings dialogs for movie stream and for action processing movie output. In version 4 all of the various movie output preferences are located in the movie tab of the main preferences dialog.
The Embedded Alpha preference option determines what the generated movie file’s alpha channel will be set to. If you don’t care about embedded alpha channels set this option to full on. If you want the movie alpha to match the current layer alpha channel you would set it to layer alpha as shown in the screen snapshot below.
The movie compression codec settings are accessed via the File : Movie Codec Settings menu dialog shown below.
Note that not all compression codec types provide support for embedded alpha channels. If you don’t choose a compression type that supports embedded alpha then the embedded alpha movie preference option is ignored.
For example, the Animation codec with ‘millions of colors’ depth does not support alpha. But the Animation codec with ‘millions of colors +’ depth does support embedded alpha. So this compression type is a good one to use for testing when working with generating embedded alpha channels if you get confused.
Movie Stream vs Action Processing Movie Output
The embedded alpha setting is used for all of the action menu movie processing output options. However, if you are outputting a movie stream as opposed to running an action movie generation menu there’s an additional menu flag you need to set if you want the embedded alpha movie preference setting to be used for your generated movie stream. The menu flag is the File : Stream : Flags : Use Alpha Gen Movie Pref menu flag shown below.
Using the embedded alpha preference options for movie stream generation can slow down a fast auto write movie stream, so this is the reason why there’s a menu flag to turn it on or off for movie stream output.
For more information
Here’s a tip that discusses wortking with alpha channels in layers.
Here’s a tip that discusses processing a movie file with a PASeq.
Here’s a tip that discusses working with Movie and Image Streams.
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If you want to pass through your source movie’s embedded alpha channel to the Studio Artist rendered output movie file, this is very straightforward to do. All you need to do is to set the Embedded Alpha movie preference popup (as shown in a screen shot above) to the Source Alpha setting. This will pass through the source movie’s embedded alpha channel to the output movie’s embedded alpha channel.
This is literally all you need to do. You don’t have to enable alpha views, or worry about what is in the canvas layer’s alpha channel, or anything else.
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Also, remember that if you want to output a movie with an embedded alpha channel, you need to make sure you are using a movie compression codec that actually support alpha channels. Not all of them do.
And it’s not enough to just select a codec that supports an embeded alpha channel, you also need to configure it to make sure it is writing an alpha channel. usually this involves selecting the millions of colors+ option (as opposed to just millions of color). This will be an option in the Movie Codec Settings dialog when you select the codec type.
You can access Studio Artist’s Movie Codec Settings dialog by using the File : Movie Codec Settings menu.
If you are working with the ProRes codec and you want to output an embedded alpha channel, you need to make sure you use the ProRes 4444 codec. As of the writing of this comment, it’s the only ProRes codec that supports an embedded alpha channel. You can always do some google searching if you want to verify this at a later date.
Once you have selected the ProRes 4444 codec, you need to make sure you select the millions of colors+ option. If it’s set to just millions of colors (without the +), then it will not contain an embedded alpha channel.