How do I specify track encoding when exporting a movie using QTKit?

I'm building an application that exports movies. The movies have one or more audio and video tracks. These tracks are pulled from other movies that contain media of varying compression schemes. When I export my composed movie using QTKit, I want to specify the encoding of each track. As it stands, I only see a way to specify the export format through the QTMovieExportType key passed to QTMovie writeToFile. How can I specify the encoding of each track?

Solution

Short answer

You can't!

Longer answer

As far as I know, QuickTime only ever allowed you to set the export attributes for either...

an individual track would then be exported to a movie containing nothing but this track or
the encoding scheme/compressor for the entire composition (when dealing with video).

(I don't know how this worked for multi-channel audio, as I never did anything with something that couldn't be described as mono or stereo through a mixdown.)

So if you need to define the compression/encoding schemes for your tracks individually, you'll need to create a temporary movie for each track which you can export in the format as you see fit. (Have a look at the QTMovieExportSettings-attribute and the legacy "Component Creation" documentation for additional/more advanced settings.)

You can then go on and combine all these movies into a single one.