UIGraphicsBeginImageContext vs CGBitmapContextCreate

Question

I'm trying to change color of an image in a background thread.
Apple doc says UIGraphicsBeginImageContext can only be called from main thread, and I'm trying to use CGBitmapContextCreate:

context = CGBitmapContextCreate (bitmapData, pixelsWide, pixelsHigh, 8, // bits per component

                             bitmapBytesPerRow,
                             colorSpace,
                             kCGImageAlphaPremultipliedFirst);

I have two versions of "changeColor" first one using UIGraphisBeginImageContext, second one using CGBitmapContextCreate.

The first one correctly changes color, but second one doesn't.
Why is that?

- (UIImage*) changeColor: (UIColor*) aColor
{
    if(aColor == nil)
        return self;

    UIGraphicsBeginImageContext(self.size);

    CGRect bounds;
    bounds.origin = CGPointMake(0,0);
    bounds.size = self.size;
    [aColor set];

    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextTranslateCTM(context, 0, self.size.height);
    CGContextScaleCTM(context, 1.0, -1.0);

    CGContextClipToMask(context, bounds, [self CGImage]);
    CGContextFillRect(context, bounds);

    UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return img;
}

---

- (UIImage*) changeColor: (UIColor*) aColor
{
    if(aColor == nil)
        return self;

    CGContextRef context = CreateARGBBitmapContext(self.size);

    CGRect bounds;
    bounds.origin = CGPointMake(0,0);
    bounds.size = self.size;

    CGColorRef colorRef = aColor.CGColor;
    const CGFloat *components = CGColorGetComponents(colorRef);
    float red = components[0];
    float green = components[1];
    float blue = components[2];

    CGContextSetRGBFillColor(context, red, green, blue, 1);


    CGContextClipToMask(context, bounds, [self CGImage]);
    CGContextFillRect(context, bounds);

    CGImageRef imageRef = CGBitmapContextCreateImage(context);
    UIImage* img = [UIImage imageWithCGImage: imageRef];
    unsigned char* data = (unsigned char*)CGBitmapContextGetData (context);  
    CGContextRelease(context);
    free(data);

    return img;
}

---

CGContextRef CreateARGBBitmapContext(CGSize size)
{
    CGContextRef    context = NULL;
    CGColorSpaceRef colorSpace;
    void *          bitmapData;
    int             bitmapByteCount;
    int             bitmapBytesPerRow;

    // Get image width, height. We'll use the entire image.                                                                                                                                                                                 
    size_t pixelsWide = size.width;
    size_t pixelsHigh = size.height;

    // Declare the number of bytes per row. Each pixel in the bitmap in this                                                                                                                                                                
    // example is represented by 4 bytes; 8 bits each of red, green, blue, and                                                                                                                                                              
    // alpha.                                                                                                                                                                                                                               
    bitmapBytesPerRow   = (pixelsWide * 4);
    bitmapByteCount     = (bitmapBytesPerRow * pixelsHigh);

    // Use the generic RGB color space.                                                                                                                                                                                                     
    colorSpace = CGColorSpaceCreateDeviceRGB();

    if (colorSpace == NULL)
    {
        fprintf(stderr, "Error allocating color space\n");
        return NULL;
    }

    // Allocate memory for image data. This is the destination in memory                                                                                                                                                                    
    // where any drawing to the bitmap context will be rendered.                                                                                                                                                                            
    bitmapData = malloc( bitmapByteCount );
    if (bitmapData == NULL)
    {
        fprintf (stderr, "Memory not allocated!");
        CGColorSpaceRelease( colorSpace );
        return NULL;
    }
    // Create the bitmap context. We want pre-multiplied ARGB, 8-bits                                                                                                                                                                       
    // per component. Regardless of what the source image format is                                                                                                                                                                         
    // (CMYK, Grayscale, and so on) it will be converted over to the format                                                                                                                                                                 
    // specified here by CGBitmapContextCreate.                                                                                                                                                                                             
    context = CGBitmapContextCreate (bitmapData,
                                     pixelsWide,
                                     pixelsHigh,
                                     8,      // bits per component                                                                                                                                                                          
                                     bitmapBytesPerRow,
                                     colorSpace,
                                     kCGImageAlphaPremultipliedFirst);
    if (context == NULL)
    {
        free (bitmapData);
        fprintf (stderr, "Context not created!");
    }

    // Make sure and release colorspace before returning                                                                                                                                                                                    
    CGColorSpaceRelease( colorSpace );

    return context;

}

Answer 1

Your second method is doing work that the first never did. Here's an adjustment of the second method to more closely match the first one:

- (UIImage*) changeColor: (UIColor*) aColor
{
    if(aColor == nil)
        return self;

    CGContextRef context = CreateARGBBitmapContext(self.size);

    CGRect bounds;
    bounds.origin = CGPointMake(0,0);
    bounds.size = self.size;

    CGContextSetFillColorWithColor(aColor.CGColor);

    CGContextClipToMask(context, bounds, [self CGImage]);
    CGContextFillRect(context, bounds);

    CGImageRef imageRef = CGBitmapContextCreateImage(context);
    UIImage* img = [UIImage imageWithCGImage: imageRef];
    CGContextRelease(context);

    return img;
}

The two changes I made were I converted this to use CGContextSetFillColorWithColor(), and I removed the dangerous and incorrect free() of the backing data of the bitmap context. If this code snippet does not behave identically to the first one, then you will have to look at your implementation of CreateARGBBitmapContext() to verify that it is correct.

Of course, as Brad Larson mentioned in the comments, if you're targeting iOS 4.0 and above, the UIKit graphics methods are (according to the release notes) thread-safe and you should be able to use the first method just fine.