Q Can we define our own extension format for the ImageDescription structure? In other words, can we just attach any kind of data (with the idSize field adjusted accordingly) to the end of the ImageDescription structure (which is then placed in the 'stsd' atom)? This would only be used on the Mac by our component.
A You control the destiny of the image description information as part of your codec format. Therefore, as long as the default image-description handle remains intact, you can add more information (various Movie Toolbox calls assume that the image description looks a certain way from offset 0 up to a specific point defined by the size of the image description record).