gistfile1.m

- (UIImage*)upsideDownBunny {
	__block CGImageRef cgImg;
	__block CGSize imgSize;
	__block UIImageOrientation orientation;
	dispatch_block_t createStartImgBlock = ^(void) {
		// UIImages should only be accessed from the main thread
		UIImage *img = [UIImage imageNamed:@"bunny.png"];
		imgSize = [img size]; // this size will be pre rotated
		orientation = [img imageOrientation];
		cgImg = CGImageRetain([img CGImage]); // this data is not rotated
	};
	if([NSThread isMainThread]) {
		createStartImgBlock();
	} else {
		dispatch_sync(dispatch_get_main_queue(), createStartImgBlock);
	}
	CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
	// in iOS4+ you can let the context allocate memory by passing NULL
	CGContextRef context = CGBitmapContextCreate( NULL,
		imgSize.width,
		imgSize.height,
		8,
		imgSize.width * 4,
		colorspace,
		kCGImageAlphaPremultipliedLast);
	// rotate so the image respects the original UIImage's orientation
	switch (orientation) {
		case UIImageOrientationDown:
		CGContextTranslateCTM(context, imgSize.width, imgSize.height);
		CGContextRotateCTM(context, M_PI);
		break;
		case UIImageOrientationLeft:
		CGContextTranslateCTM(context, 0.0, imgSize.height);
		CGContextRotateCTM(context, 3.0 * M_PI / 2.0);
		break;
		case UIImageOrientationRight:
		CGContextTranslateCTM(context,imgSize.width, 0.0);
		CGContextRotateCTM(context, M_PI / 2.0);
		break;
		default:
		// there are mirrored modes possible
		// but they aren't generated by the iPhone's camera
		break;
	}
	// rotate the image upside down
	CGContextRotateCTM(context, M_PI);
	CGContextTranslateCTM(context, -imgSize.width, -imgSize.height);
	CGContextDrawImage( context, CGRectMake(0.0, 0.0, imgSize.width, imgSize.height), cgImg );
	// grab the new rotated image
	CGContextFlush(context);
	CGImageRef newCgImg = CGBitmapContextCreateImage(context);
	__block UIImage *newImage;
	dispatch_block_t createRotatedImgBlock = ^(void) {
		// UIImages should only be accessed from the main thread
		newImage = [UIImage imageWithCGImage:newCgImg];
	};
	if([NSThread isMainThread]) {
		createRotatedImgBlock();
	} else {
		dispatch_sync(dispatch_get_main_queue(), createRotatedImgBlock);
	}
	CGColorSpaceRelease(colorspace);
	CGImageRelease(newCgImg);
	CGContextRelease(context);
	return newImage;
}