UIImage+Alpha.m

// UIImage+Alpha.m
// Created by Trevor Harmon on 9/20/09.
// Free for personal or commercial use, with or without modification.
// No warranty is expressed or implied.

#import "UIImage+Alpha.h"

// Private helper methods
@interface UIImage ()
- (CGImageRef)newBorderMask:(NSUInteger)borderSize size:(CGSize)size;
@end

@implementation UIImage (Alpha)

// Returns true if the image has an alpha layer
- (BOOL)hasAlpha {
    CGImageAlphaInfo alpha = CGImageGetAlphaInfo(self.CGImage);
    return (alpha == kCGImageAlphaFirst ||
            alpha == kCGImageAlphaLast ||
            alpha == kCGImageAlphaPremultipliedFirst ||
            alpha == kCGImageAlphaPremultipliedLast);
}

// Returns a copy of the given image, adding an alpha channel if it doesn't already have one
- (UIImage *)imageWithAlpha {
    if ([self hasAlpha]) {
        return self;
    }
    
    CGImageRef imageRef = self.CGImage;
    size_t width = CGImageGetWidth(imageRef);
    size_t height = CGImageGetHeight(imageRef);
    
    // The bitsPerComponent and bitmapInfo values are hard-coded to prevent an "unsupported parameter combination" error
    CGContextRef offscreenContext = CGBitmapContextCreate(NULL,
                                                          width,
                                                          height,
                                                          8,
                                                          0,
                                                          CGImageGetColorSpace(imageRef),
                                                          kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedFirst);
    
    // Draw the image into the context and retrieve the new image, which will now have an alpha layer
    CGContextDrawImage(offscreenContext, CGRectMake(0, 0, width, height), imageRef);
    CGImageRef imageRefWithAlpha = CGBitmapContextCreateImage(offscreenContext);
    UIImage *imageWithAlpha = [UIImage imageWithCGImage:imageRefWithAlpha];
    
    // Clean up
    CGContextRelease(offscreenContext);
    CGImageRelease(imageRefWithAlpha);
    
    return imageWithAlpha;
}

// Returns a copy of the image with a transparent border of the given size added around its edges.
// If the image has no alpha layer, one will be added to it.
- (UIImage *)transparentBorderImage:(NSUInteger)borderSize {
    // If the image does not have an alpha layer, add one
    UIImage *image = [self imageWithAlpha];
    
    CGRect newRect = CGRectMake(0, 0, image.size.width + borderSize * 2, image.size.height + borderSize * 2);
    
    // Build a context that's the same dimensions as the new size
    CGContextRef bitmap = CGBitmapContextCreate(NULL,
                                                newRect.size.width,
                                                newRect.size.height,
                                                CGImageGetBitsPerComponent(self.CGImage),
                                                0,
                                                CGImageGetColorSpace(self.CGImage),
                                                CGImageGetBitmapInfo(self.CGImage));
    
    // Draw the image in the center of the context, leaving a gap around the edges
    CGRect imageLocation = CGRectMake(borderSize, borderSize, image.size.width, image.size.height);
    CGContextDrawImage(bitmap, imageLocation, self.CGImage);
    CGImageRef borderImageRef = CGBitmapContextCreateImage(bitmap);
    
    // Create a mask to make the border transparent, and combine it with the image
    CGImageRef maskImageRef = [self newBorderMask:borderSize size:newRect.size];
    CGImageRef transparentBorderImageRef = CGImageCreateWithMask(borderImageRef, maskImageRef);
    UIImage *transparentBorderImage = [UIImage imageWithCGImage:transparentBorderImageRef];
    
    // Clean up
    CGContextRelease(bitmap);
    CGImageRelease(borderImageRef);
    CGImageRelease(maskImageRef);
    CGImageRelease(transparentBorderImageRef);
    
    return transparentBorderImage;
}

#pragma mark -
#pragma mark Private helper methods

// Creates a mask that makes the outer edges transparent and everything else opaque
// The size must include the entire mask (opaque part + transparent border)
// The caller is responsible for releasing the returned reference by calling CGImageRelease
- (CGImageRef)newBorderMask:(NSUInteger)borderSize size:(CGSize)size {
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
    
    // Build a context that's the same dimensions as the new size
    CGContextRef maskContext = CGBitmapContextCreate(NULL,
                                                     size.width,
                                                     size.height,
                                                     8, // 8-bit grayscale
                                                     0,
                                                     colorSpace,
                                                     kCGBitmapByteOrderDefault | kCGImageAlphaNone);
    
    // Start with a mask that's entirely transparent
    CGContextSetFillColorWithColor(maskContext, [UIColor blackColor].CGColor);
    CGContextFillRect(maskContext, CGRectMake(0, 0, size.width, size.height));
    
    // Make the inner part (within the border) opaque
    CGContextSetFillColorWithColor(maskContext, [UIColor whiteColor].CGColor);
    CGContextFillRect(maskContext, CGRectMake(borderSize, borderSize, size.width - borderSize * 2, size.height - borderSize * 2));
    
    // Get an image of the context
    CGImageRef maskImageRef = CGBitmapContextCreateImage(maskContext);
    
    // Clean up
    CGContextRelease(maskContext);
    CGColorSpaceRelease(colorSpace);
    
    return maskImageRef;
}

@end