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Re: Alpha Channel and QuickDraw?

On Friday, Dec 13, 2002, at 10:55 US/Pacific, Bobby Thomale wrote:

Is it supposed to work the way I am remembering - QD setting the alpha
channel where you are drawing to all opaque as it draws? Or do I need to do
that manually?

QD always makes the drawing opaque when the destination is a window buffer.
If the case of a 32-bit deep GWorld destination, QD ended up *not* overwriting the alpha channel - at least for CopyBits operations - because some existing applications already used the alpha bytes for their own purposes (and used their own blitters from there). Then again, other compatibility requirements forced to make certain other drawing operations opaque even into GWorlds. It has been a conflictful history ...

(...) and I should instead be setting the alpha channel manually,
that would be helpful to know, because then I am wasting my time looking
elsewhere for a bug in my code here.

In your situation, that's the safest approach. As indicated above, QD's behavior is mandated by compatibility requirements, and cannot be changed for the sake of a clean specification alone.

Joseph Maurer
Quickdraw
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References: 
 >Alpha Channel and QuickDraw? (From: Bobby Thomale <email@hidden>)



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