Transparency/Blending issues with OpenGL ES/iPhone - iphone

Question

I have a simple 16x16 particle that goes from being opaque to transparent. Unfortunately is appears different in my iPhone port and I can't see where the differences in the code are. Most of the code is essentially the same.
I've uploaded an image to here to show the problem
The particle on the left is the incorrectly rendered iPhone version and the right is how it appears on Mac and Windows. It's just a simple RGBA .png file.
I've tried numerous blend functions and glTexEnv setting but I can't seem to make them the same.
Just to be thorough, my Texture loading code on the iPhone looks like the following
GLuint TextureLoader::LoadTexture(const char *path)
{
NSString *macPath = [NSString stringWithCString:path length:strlen(path)];
GLuint texture = 0;
CGImageRef textureImage = [UIImage imageNamed:macPath].CGImage;
if (textureImage == nil)
{
NSLog(#"Failed to load texture image");
return 0;
}
NSInteger texWidth = CGImageGetWidth(textureImage);
NSInteger texHeight = CGImageGetHeight(textureImage);
GLubyte *textureData = new GLubyte[texWidth * texHeight * 4];
memset(textureData, 0, texWidth * texHeight * 4);
CGContextRef textureContext = CGBitmapContextCreate(textureData, texWidth, texHeight, 8, texWidth * 4, CGImageGetColorSpace(textureImage), kCGImageAlphaPremultipliedLast);
CGContextDrawImage(textureContext, CGRectMake(0.0, 0.0, (float)texWidth, (float)texHeight), textureImage);
CGContextRelease(textureContext);
//Make a texture ID, bind it, create it
glGenTextures(1, &texture);
glBindTexture(GL_TEXTURE_2D, texture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, texWidth, texHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, textureData);
delete[] textureData;
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
return texture;
}
The blend function I use is glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
I'll try any ideas people throw at me, because this has been a bit of a mystery to me.
Cheers.

Answer 1

this looks like the standard "textures are converted to premultiplied alpha" problem.
you can use
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
or you can write custom loading code to avoid the premultiplication.

Answer 2

Call me naive, but seeing that premultiplying an image requires (ar, ag, a*b, a), I figured I'd just divide the rgb values by a.
Of course as soon as the alpha value is larger than the r, g, b components, the particle texture became black. Oh well. Unless I can find a different image loader to the one above, then I'll just make all the rgb components 0xff (white). This is a good temporary solution for me because I either need a white particle or just colourise it in the application. Later on I might just make raw rgba files and read them in, because this is mainly for very small 16x16 and smaller particle textures.
I can't use Premultiplied textures for the particle system because overlapping multiple particle textures saturates the colours way too much.