Question

I want to load 4-channel texture data from a file in iOS, so I consider the texture as a (continuous) map

[0,1]x[0,1] -> [0,1]x[0,1]x[0,1]x[0,1]

If I use the fileformat .png, XCode/iOS consider the file as an image, and so multiplies each component rgb with a (premultiplied alpha), corrupting my data. How should I solve this? Examples may be

• use two textures with components rgb (3-channel)
• postdivide alpha
• use another file format

Of these, I consider the best solution to be to use another file format. The GL-compressed file format (PVRTC?) is not Apple-platform independent and seems to be of low resolution (4 bits) (reference).

EDIT:
If my own answer below is true, it is not possible to get the 4 channel data of png’s in iOS. Since OpenGL is about creating images rather than presenting images, it should be possible to load 4-channel data in some way. png is a fileformat for images (and compression depends on all 4 channels but compression of one channel is independent of the other channels), so one may argue that I should use another file format. So which other compressed file formats should I use, which is easy to read/integrated in iOS?

UPDATE: “combinatorial” mentioned a way to load 4-channel non-premultiplied textures, so I had to give him the correct answer. However, that solution had some restrictions I didn’t like. My next question is then “Access raw 4-channel data from png files in iOS” 🙂

I think it is a bad library design not making it possible to read 4 channel png data. I don’t like systems trying to be smarter than myself.

Answer 1

As you considered PVRTC then using GLKit could be an option. This includes GLKTextureLoader which allows you to load textures without pre-multiplying alpha. Using for example:

+ (GLKTextureInfo *)textureWithContentsOfFile:(NSString *)fileName options:(NSDictionary *)textureOperations error:(NSError **)outError

and passing an options dictionary containing:

GLKTextureLoaderApplyPremultiplication = NO