CGContextRelease(); doesn't release the context?

Question

I've got a nice and short method to load textures in my iPhone app, where I glGenTextures(13, &textures[0]);. Of course at the end of it I do glDeleteTextures(13, textures);, but the memory isn't fully released untill I comment out this line: CGContextDrawImage(textureContext, CGRectMake(0.0, 0.0, (float)texWidth, (float)texHeight), textureImage);' (which of course results in having the textures totally black) although the next line says:

 CGContextRelease(textureContext);

This is the full code for loading the textures:

- (void)loadTexture:(NSString *)name intoLocation:(GLuint)location {
    CGImageRef textureImage = [UIImage imageNamed:name].CGImage;
    if (textureImage == nil) {
        NSLog(@"Failed to load texture image");
        return;
    }

    NSInteger texWidth = CGImageGetWidth(textureImage);
    NSInteger texHeight = CGImageGetHeight(textureImage);

    GLubyte *textureData = (GLubyte *)malloc(texWidth * texHeight << 2);

    int k, l = texWidth * texHeight << 2;
    for (k=0; k<l; k++) textureData[k] = 0;

    CGContextRef textureContext = CGBitmapContextCreate(textureData, texWidth, texHeight, 8, texWidth << 2, CGImageGetColorSpace(textureImage),
                                                        kCGImageAlphaPremultipliedLast);
    CGContextTranslateCTM(textureContext, 0, texHeight);
    CGContextScaleCTM(textureContext, 1.0, -1.0);
    CGContextDrawImage(textureContext, CGRectMake(0.0, 0.0, (float)texWidth, (float)texHeight), textureImage);
    CGContextRelease(textureContext);

    glBindTexture(GL_TEXTURE_2D, location);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, texWidth, texHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, textureData);

    free(textureData);
    glEnable(GL_TEXTURE_2D);
}

Answer 1

CGImage decodes its data lazily the first time its drawn. It holds on to the decoded data.

The UIImage class caches UIImage instances by name, and the instances hold the CGImages which are holding the decoded data. The UIImage cache is purged in response to low memory warnings.

If this is causing too much memory use in your app, you want to control the creation and destruction of UIImages more carefully, which means not using +[UIImage imageNamed:].

On the other hand, usually an app doesn't have that many named images - it's a static set determined at compile time. If you were just wondering about this memory usage, there you go.