iphone - (iphone)快速 UIImage 合成(合并)？

Question

我正在使用下面的代码来合并两个 UIImage，
不知道是否有更快的方法。

- (UIImage*) combineImage: (UIImage*) aImage
{
    UIGraphicsBeginImageContext(self.size);
    [self drawInRect: CGRectMake(0, 0, self.size.width, self.size.height)];  
    [aImage drawInRect: CGRectMake(0, 0, self.size.width, self.size.height)];


    UIImage* combinedImage = UIGraphicsGetImageFromCurrentImageContext(); //                                                                                                                                                                                                  
    UIGraphicsEndImageContext();
    return combinedImage;
}

Answer 1

使用 UIKit 要求您在主线程上执行此操作(最新版本的 iOS 声称可以在后台线程上工作，但在我的测试中似乎并非如此)。

一种更快(或者更灵敏)的方法是使用 Quartz 在后台线程中执行此操作，如上所述。这样做的问题是该过程实际上变成了异步。您将需要某种形式的回调。方法名称的示例可能是

- (void)generateMergedImageWith:(UIImage *)aImage callback:(mergeCallbackBlock)callback

(如果您不想使用 block 回调，它可以是符合某些协议(protocol)的委托(delegate)；如果您确实想使用 block ，则需要适本地 typedef 'mergeCallbackBlock')

示例(未经测试，从工作应用程序中提取)可能如下所示:

CGImageRef firstCGImage = [self CGImage];
CGImageRef secondCGImage = [aImage CGImage];
// we presume the two images are equal size, or that the two images should be the size of self
CGSize size = [self size];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0), ^{
    // below context presumes a number of things about the images and may need to be more aware
    CGContextRef ctx = CGBitmapContextCreate(NULL, size.width, size.height, 
                           8, size.width * 4, CGImageGetColorSpace(firstCGImage),
                           kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little);
    // we presume ctx succeeded; it may be NULL if it did not
    CGContextDrawImage(ctx, (CGRect){CGPointZero, size}, firstCGImage);
    CGContextDrawImage(ctx, (CGRect){CGPointZero, size}, secondCGImage);
    CGImageRef result = CGBitmapContextCreateImage(ctx);
    CGContextRelease(ctx);
    // we presume result succeeded; it is NULL if it did not succeed for some reason
    dispatc_async(dispatch_get_main_queue(), ^{
        // back on main thread we create the resulting UIImage
        UIImage *image = [[UIImage alloc] initWithCGImage:result];
        CGImageRelease(result);
        callback(image); // if a protocol, e.g. [thatObject didGenerateMergedImage:image];
        [image release];
    });
});