Thanks for your time and input on the issue... I've filed a
radar on this issue -- hopefully, this issue will get some visibilty
since the only workaround is to either ditch QuickTime/ImageIO or to
use a deprecated API (GWorlds).
Well, it may be that what you want to do is beyond the scope of
what Apple sortof intends ImageIO to be used for. I'm not sure if
using ImageIO + CoreGraphics was ever really targetted at being
used for an image loading scheme for OpenGL applications.
I assumed that the only way to read an image from somewhere, have
access to the source RGBA data for application-level editing, and
upload it as a texture was via GraphicsImportCreateCGImage() and
CGBitmapContextCreate(). Is there any way to this that isn't
deprecated?
In any case, thanks again.
Regards,
Andy
On May 15, 2007, at 2:16 AM, Kenneth Dyke wrote:
On May 14, 2007, at 6:34 PM, Andy O'Meara wrote:
Ken,
Thanks for the suggestion, but unfortunately, glBlendFunc(GL_ONE,
GL_ONE_MINUS_SRC_ALPHA) isn't a substitute for two reasons...
First, we use CGBitmapContextCreate() in the first place is so
that we can perform some arbitrary drawing or processing on the
image that we just read in. In our case we perform some custom
drawing operations (which require the original RGB values).
Second, we can't use glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA)
because we need to use a different blend mode.
As indicated by the other posters here, the frustration seems to
be running pretty high on this issue. I mean, with something as
powerful as QuickTime, it just seems asinine that you can't get at
the original RGBA data.
So is Apple's policy then that we have to use a 3rd party solution
(and not QuickTime!) to import an image if we want the actual RGB
values? It just seems like it'd be trivial to implement
kCGImageAlphaFirst since CGContextDrawImage() multiplies the RGB
values by the alpha values--kCGImageAlphaFirst just wouldn't do
that multiplication.
Honestly, I wouldn't expect CGBitmapContextCreate() to support non-
premultiplied alpha any time soon. There are various sound
mathematical reasons why CG does everything in premultiplied space
and wants to leave it that way. That being said, I hear you.
It's probably worth filing a Radar against ImageIO at the very
least that says you need to be able to get at the original
unpremultiplied alpha values when they are present in an image.
If you can explain in as much detail as possible what it is you
need to achieve and why premultiplied alpha will not work in your
case, that would probably help things greatly.
If you have any other suggestions, we're all ears. It's just hard
to believe that on Mac OS X, we need to use libpng over QuickTime
to get the job done -- is this really what Apple wants its
developers to do?
Well, it may be that what you want to do is beyond the scope of
what Apple sortof intends ImageIO to be used for. I'm not sure if
using ImageIO + CoreGraphics was ever really targetted at being
used for an image loading scheme for OpenGL applications.
Certainly ImageIO also uses libpng internally AFAIK, and is also
passing non-premultiplied alpha into CoreGraphics.
I think fundamentally CoreGraphics is designed to accept
premultiplied alpha on *input*, it just never outputs it for
various reasons. I believe it has something to do with the
compositing mathematics. Someone from the CG team could probably
explain the rationale much better than I could. I found this page
which probably is as good as anything, and my guess is that CG is
the way it is because it boils down to the issues surrounding the
compositing mathematics and the speed you get from storing things
in premultiplied format: http://en.wikipedia.org/wiki/
User:Lkesteloot/premultiplied_alpha
I'd be more than willing to spend a tech support incident on this
if it will get someone to look into this.
That's a judgement call you'll have to make on your own. I can't
promise anything one way or another. Again, when it doubt,
*always* file a Radar.
-Ken
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