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// 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"
@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
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