UIGraphicsBeginImageContext vs CGBitmapContextCreate

Question

I am trying to change the color of an image in a background thread.
The Apple doc says that UIGraphicsBeginImageContext can only be called from the 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 using UIGraphisBeginImageContext, the second using CGBitmapContextCreate.

The first changes color correctly, and the second does not.
Why is this?

- (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

, . :

- (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;
}

, , , CGContextSetFillColorWithColor(), free() . , CreateARGBBitmapContext(), , .

, , iOS 4.0 , UIKit ( ) , .