Transparent background for Quick Look thumbnail

Question

I am writing a Quick Look plug-in and would like to generate a thumbnail with a transparent background.
If I simply create a graphics context, flush it and exit, I see a solid white square for relevant file icons and Quick Look previews. I can find no way to clear the canvas to transparent - I can paint the contents of my thumbnail onto it just fine, but everything is composited to the white background.
For example, the following implementation(ish) of GenerateThumbnailForURL results in solid white squares for thumbnails:
CGContextRef context = QLThumbnailRequestCreateContext(
thumbnail,
CGSizeMake(512, 512), /* (For example) */
false, /* Changing to 'true' for vector context makes no difference */
NULL
QLThumbnailRequestFlushContext(thumbnail, context);
CFRelease(context);
return noErr;
Various combinations of CGContextClearRect with and without wrapping the call in CGContextBeginTransparencyLayer and CGContextEndTransparencyLayer, or trying to use CGContextFillRect having set a colour which is fully transparent, have all failed. The solid white background just shines through.
Am I going about this the wrong way (e.g. should I instead try to generate an image with a mask and send this to Quick Look with, say, QLThumbnailRequestSetImage ?) or is there some Magic Call that I should be using?
Thanks...

Answer 1

This isn't nearly as intuitive as I think this should be.
You're right, video is really hard.
Looking back over your thread, it's entirely possible the movies have transparency, you're just not looking at it properly, which is, granted, confusing. But alpha channels are not fundamental to video editing, they're special types of clips and you need to learn how to create and use them.
Motion is set by default to render to the Animation codec but may not be set to millions of colors +, the + sign is the alpha.
And, no, sorry, it can't be explained in a simple step-by-step post. There are fifty or a hundred pages on alphas in the Motion documentation. Start with the explanation of the Animation codec.
bogiesan
bogiesan