Drawing inner glow with Core Graphics

gistfile1.m

- (CGImageRef)createMaskFromAlphaChannel:(UIImage *)image
{
	size_t width = image.size.width;
	size_t height = image.size.height; 

	NSMutableData *data = [NSMutableData dataWithLength:width*height];

	CGContextRef context = CGBitmapContextCreate(
		[data mutableBytes], width, height, 8, width, NULL, kCGImageAlphaOnly);

	// Set the blend mode to copy to avoid any alteration of the source data
	CGContextSetBlendMode(context, kCGBlendModeCopy);

	// Draw the image to extract the alpha channel
	CGContextDrawImage(context, CGRectMake(0.0, 0.0, width, height), image.CGImage);
	CGContextRelease(context);

	CGDataProviderRef dataProvider = CGDataProviderCreateWithCFData((__bridge CFMutableDataRef)data);

	CGImageRef maskingImage = CGImageMaskCreate(width, height, 8, 8, width, dataProvider, NULL, TRUE);
	CGDataProviderRelease(dataProvider);

	return maskingImage;
}

- (void)drawRect:(CGRect)rect
{
	UIImage *image = [UIImage imageNamed:@"TheImage"];
	CGRect imageRect = CGRectMake(10, 10, image.size.width, image.size.height);

	CGContextRef context = UIGraphicsGetCurrentContext();

	// Draw black background
	CGContextSetRGBFillColor(context, 0.0f, 0.0f, 0.0f, 1.0f);
	CGContextFillRect(context, rect);

	// Adjust for different coordinate systems from UIKit and Core Graphics
	CGContextTranslateCTM(context, 0.0f, self.bounds.size.height);
	CGContextScaleCTM(context, 1.0f, -1.0f);

	// Draw the original image
	CGContextDrawImage(context, imageRect, image.CGImage);

	CGContextSaveGState(context);

	// Clip to the original image, so that we only draw the shadows on the
	// inside of the image but nothing outside.
	CGContextClipToMask(context, imageRect, image.CGImage);

	// Set up the shadow
	UIColor *innerGlowColor = [UIColor colorWithRed:1.0f green:1.0f blue:1.0f alpha:1.0f];
	CGContextSetShadowWithColor(context, CGSizeZero, 2, innerGlowColor.CGColor);

	// Create a mask with an inverted alpha channel and draw it. This will
	// cause the shadow to appear on the inside of our original image.
	CGImageRef mask = [self createMaskFromAlphaChannel:image];
	CGContextDrawImage(context, imageRect, mask);
	CGImageRelease(mask);

	CGContextRestoreGState(context);
}