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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 ...

It seemed to be behaving oddly for me with a gworld - and I was only doing a
FillRect and a FillOval, no copybits. It worked intermittantly.

>> (...) 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.

I ended up just converting the calls to CoreGraphics instead. It worked
beautifully, and the shapes ended up antialiased as well.

Of course, those two lines of QuickDraw code ended up being more like 10
lines of code in Quartz. :-)

Oh well - thanks for your help!

-- Bobby

---------------------------------------------------------------------
Bobby Thomale
Senior Software Developer
Inoveon Corporation
http://www.inoveon.com/
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References: 
 >Re: Alpha Channel and QuickDraw? (From: Joseph Maurer <email@hidden>)



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