UIImage/CGImage changing my pixel color

I have an image that is totally white in its RGB components, with varying alpha -- so, for example, 0xFFFFFF09 in RGBA format. But when I load this image with either UIImage or CGImage APIs, and then draw it in a CGBitmapContext, it comes out grayscale, with the RGB components set to the开发者_如何学Python value of the alpha -- so in my example above, the pixel would come out 0x09090909 instead of 0xFFFFFF09. So an image that is supposed to be white, with varying transparency, comes out essentially black with transparency instead. There's nothing wrong with the PNG file I'm loading -- various graphics programs all display it correctly.

I wondered whether this might have something to do with my use of kCGImageAlphaPremultipliedFirst, but I can't experiment with it because CGBitmapContextCreate fails with other values.

The ultimate purpose here is to get pixel data that I can upload to a texture with glTexImage2D. I could use libPNG to bypass iOS APIs entirely, but any other suggestions? Many thanks.

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White on a black background with an alpha of x IS a grey value corresponding to x in all the components. Thats how multiplicative blending works.