Using image created with CGBitmapContextCreate as an opengl texture - iphone

Question

I'm generating an image using quartz2d and I want to use it as an opengl texture.
The tricky part is that I want to use as few bits per pixel as possible, so I'm creating cgContext as following:
int bitsPerComponent = 5;
int bytesPerPixel = 2;
int width = 1024;
int height = 1024;
void* imageData = malloc(width * height * bytesPerPixel);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGImageContext context = CGBitmapContextCreate(imageData, width, height, bitsPerComponent, width * bytesPerPixel, colorSpace, kCGImageAlphaNoneSkipFirst);
//draw things into context, release memory, etc.
As stated in the documentation here, this is the only supported RGB pixel format for CGBitmapContextCreate which uses 16 bits per pixel.
So now I want to upload this imageData which looks like "1 bit skipped - 5 bits red - 5 bits green - 5 bits blue" into an opengl texture. So I should do something like this:
glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_SHORT_5_5_5_1, imageData);
That won't work because in this call I've specified pixel format as 5 red - 5 green - 5 blue - 1 alpha. That is wrong, but it appears that there is no format that would match core graphics output.
There are some other options like GL_UNSIGNED_SHORT_1_5_5_5_REV, but those wont work on the iphone.
I need some way to use this imageData as a texture, but I really don't want to swap bytes around manually using memset or such, because that seems terribly inefficient.

Answer 1

You do need to swap bits around to get it into a denser format like RGBA551 or RGB565, since as you note, CGBitmapContext does not support these formats for drawing (for simplicity and efficency's sake).
memset isn't going to do the trick, but there are "fast" conversion routines in Accelerate.framework.
See vImageConvert_ARGB8888toRGB565(…) and vImageConvert_ARGB8888toARGB1555(…), available on iOS 5 and later.

Answer 2

For iOS 7.0, OS X.9 and later:
vImage_CGImageFormat fmt = {
.bitsPerComponent = 5,
.bitsPerPixel = 16,
.colorSpace = NULL, // faster with CGImageGetColorSpace(cgImage) if known to be RGB
.bitmapInfo = kCGImageAlphaNoneSkipFirst | kCGBitmapByteOrder16Little // ARGB1555 little endian
};
vImage_Buffer buf;
vImageBuffer_InitWithCGImage( &buf, &fmt, NULL, cgImage, kvImageNoFlags );
...
free(buf.data);
Data is in buf.data, along with image height, width and rowBytes info. I don't recall what GL's requirements are for whether row padding is allowed. You can control that by preallocating the buf.data and buf.rowBytes fields and passing kvImageDoNotAllocate in the flags.
565_REV is kCGImageAlphaNone | kCGBitmapByteOrder16Little.
5551_REV is kCGImageAlphaNoneSkipLast | kCGBitmapByteOrder16Little