Loading 4-channel texture data in iOS

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

Answer 2

You can simply request that Xcode not 'compress' your PNG files. Click your project in the top left, select the 'Build Settings', find 'Compress PNG Files' and set the option to 'No'.

As to your other options, postdividing isn't a bad solution but obviously you'll lose overall precision and I believe both TIFF and BMP are also supported. PVRTC is PowerVR specific so it's not Apple-specific but it's also not entirely platform independent and is specifically designed to be a lossy compression that's trivial to uncompress with little input on the GPU. You'd generally increase your texture resolution to ameliorate for the low bit per pixel count.