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.
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?
I'd be more than willing to spend a tech support incident on this if
it will get someone to look into this.
Thanks,
Andy
On May 14, 2007, at 6:08 PM, Kenneth Dyke wrote:
On May 14, 2007, at 2:47 PM, James Walker wrote:
Andy O'Meara wrote:
As OGLers here know, one regularly needs to upload textures that
have an alpha channel (with non-multiplied RGB).
Pardon my ignorance, but why? If you just want to apply an RGBA
texture, you can use premultiplied alpha.
Indeed. The WindowServer uses pre-multiplied ARGB data from window
backing stores all the time:
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA) is your friend. ;)
-Ken
On May 14, 2007, at 6:24 PM, James Milne wrote:
Andy O'Meara wrote:
As OGLers here know, one regularly needs to upload textures that
have an alpha channel (with non-multiplied RGB). Meanwhile,
CGBitmapContextCreate() doesn't support kCGImageAlphaFirst (see
Apple QA 1037), which makes CGBitmapContextCreate() utterly
useless for getting true image RGBA data from a CGImage (see
QA1509). Am I going nuts or is this kind of ridiculous for anyone
loading images in to be used as textures (with alpha) in OpenGL?
Given this, what's the best-performance way to upload images from
arbitrary image files? High performance and low overhead is
critical since we can't keep all the textures that we use memory
resident.
I don't appear to be the only one running into this...
I got into this mess because I wanted to move away from all the
deprecated (GWorld) stuff. Now it looks GraphicsImportSetGWorld()
is the only way to get the non-premultiplied image data. So much
for leaving QuickDraw...
Any recommendations would be kindly appreciated.
You could simply upload the textures with pre-multiplied alpha then
blend them using glBlendFunc( GL_ONE, GL_ONE_MINUS_SRC_ALPHA );
Alternatively, if you're doing something that doesn't lend itself
to using the textures in their pre-multiplied form, you can upload
the texture to OpenGL as-is, lying about the true ordering of the
components (saying its ARGB when it's really RGBA), and unswizzle
the components in a fragment program.
real_texture.rgba = texture.argb;
At least, I think this should be the correct swizzling...
--
James Milne
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