How to load PNG with alpha with Cocoa?

Question

Hey guys,

I'm developing an iPhone OpenGL application, and I need to use some textures with transparency. I have saved the images as PNGs. I already have all the code to load PNGs as OpenGL textures and render them. This is working fine for all images that don't have transparency (all alpha values are 1.0). However, now that I'm trying to load and use some PNGs that have transparency (varying alpha values), my texture is messed up, like it loaded the data incorrectly or something.

I'm pretty sure this is due to my loading code which uses some of the Cocoa APIs. I will post the relevant code here though.

What is the best way to load PNGs, or any image format which supports transparency, on OSX/iPhone? This method feels roundabout. Rendering it to a CGContext and getting the data seems weird.

* LOADING *

CGImageRef CGImageRef_load(const char *filename) {
    NSString *path = [NSString stringWithFormat:@"%@/%s",
                      [[NSBundle mainBundle] resourcePath],
                      filename];
    UIImage *img = [UIImage imageWithContentsOfFile:path];
    if(img) return [img CGImage];
    return NULL;
}

unsigned char* CGImageRef_data(CGImageRef image) {
    NSInteger width = CGImageGetWidth(image);
    NSInteger height = CGImageGetHeight(image);
    unsigned char *data = (unsigned char*)malloc(width*height*4);

    CGContextRef context = CGBitmapContextCreate(data,
                                                 width, height,
                                                 8, width * 4,
                                                 CGImageGetColorSpace(image),
                                                 kCGImageAlphaPremultipliedLast);

    CGContextDrawImage(context,
                       CGRectMake(0.0, 0.0, (float)width, (float)height),
                       image);
    CGContextRelease(context);

    return data;
}

* UPLOADING *

(define (image-opengl-upload data width height)
  (let ((tex (alloc-opengl-image)))
    (glBindTexture GL_TEXTURE_2D tex)
    (glTexEnvi GL_TEXTURE_ENV GL_TEXTURE_ENV_MODE GL_DECAL)
    (glTexImage2D GL_TEXTURE_2D
                  0
                  GL_RGBA
                  width
                  height
                  0
                  GL_RGBA
                  GL_UNSIGNED_BYTE
                  (->void-array data))
    (glTexParameteri GL_TEXTURE_2D
                     GL_TEXTURE_MIN_FILTER
                     GL_LINEAR)
    (glTexParameteri GL_TEXTURE_2D
                     GL_TEXTURE_MAG_FILTER
                     GL_LINEAR)
    (glTexParameteri GL_TEXTURE_2D
                     GL_TEXTURE_WRAP_S
                     GL_CLAMP_TO_EDGE)
    (glTexParameteri GL_TEXTURE_2D
                     GL_TEXTURE_WRAP_T
                     GL_CLAMP_TO_EDGE)    
    (glBindTexture GL_TEXTURE_2D 0)
    tex))

Answer 1

I don't know anything about OpenGL, but Cocoa abstracts this functionality with NSImage/UIImage.

Answer 2

Your Core Graphics surface should be cleared to all zeroes before you render to it, so I recommend using calloc instead of malloc, or adding a memset after the malloc.

Also, I'm not sure you want your TexEnv set to GL_DECAL. You might want to leave it set to its default (GL_MODULATE).

If you'd like to avoid Core Graphics for decoding PNG images, I recommend loading in a PVR file instead. PVR is an exceedingly simple file format. An app called PVRTexTool is included with the Imagination SDK which makes it easy to convert from PNG to PVR. The SDK also includes some sample code that shows how to parse their file format.

Answer 3

To be explicit…

The most common issue with loading textures using Core Image is that it insists on converting data to premultiplied alpha format. In the case of PNGs included in the bundle, this is actually done in a preprocessing step in the build process. Failing to take this into account results in dark banding around blended objects.

The way to take it into account is to use glBlendMode(GL_ONE, GL_ONE_MINUS_SRC_ALPHA) instead of glBlendMode(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA). If you want to use alpha channels for something other than regular blending, your only option is to switch to a different format and loader (as prideout suggested, but for different reasons).

ETA: the premultiplication issue also exists under Mac OS X, but the preprocessing is iPhone-specific.