从 CGImageRef 创建的 UIImage 失败并显示 UIImagePNGRepresentation

我使用以下代码从较大的 UIImage 中裁剪并创建一个新的 UIImage。我已将问题与函数 CGImageCreateWithImageInRect() 隔离开来，该函数似乎没有按照我想要的方式设置某些 CGImage 属性。 :-) 问题是调用函数 UIImagePNGRepresentation() 失败，返回 nil。

CGImageRef origRef = [stillView.image CGImage];
CGImageRef cgCrop = CGImageCreateWithImageInRect( origRef, theRect);
UIImage *imgCrop = [UIImage imageWithCGImage:cgCrop];

...

NSData *data = UIImagePNGRepresentation ( imgCrop);

-- libpng 错误：没有 IDAT 写入文件

知道从 UIImage 中裁剪出矩形可能会出现什么问题或替代方案吗？

我遇到了同样的问题，但只是在 iOS 3.2 上测试兼容性时。在 4.2 上它工作正常。

最后我发现了这个http://www.hive05.com/2008/11/crop-an-image-using-the-iphone-sdk/这对两者都有效，尽管有点冗长！

我将其转换为 UIImage 上的类别：

UIImage+Crop.h

@interface UIImage (Crop)
- (UIImage*) imageByCroppingToRect:(CGRect)rect;
@end

UIImage+Crop.m

@implementation UIImage (Crop)

- (UIImage*) imageByCroppingToRect:(CGRect)rect
{
    //create a context to do our clipping in
    UIGraphicsBeginImageContext(rect.size);
    CGContextRef currentContext = UIGraphicsGetCurrentContext();

    //create a rect with the size we want to crop the image to
    //the X and Y here are zero so we start at the beginning of our
    //newly created context
    CGRect clippedRect = CGRectMake(0, 0, rect.size.width, rect.size.height);
    CGContextClipToRect( currentContext, clippedRect);

    //create a rect equivalent to the full size of the image
    //offset the rect by the X and Y we want to start the crop
    //from in order to cut off anything before them
    CGRect drawRect = CGRectMake(rect.origin.x * -1,
                                 rect.origin.y * -1,
                                 self.size.width,
                                 self.size.height);

    //draw the image to our clipped context using our offset rect
    CGContextTranslateCTM(currentContext, 0.0, rect.size.height);
    CGContextScaleCTM(currentContext, 1.0, -1.0);
    CGContextDrawImage(currentContext, drawRect, self.CGImage);

    //pull the image from our cropped context
    UIImage *cropped = UIGraphicsGetImageFromCurrentImageContext();

    //pop the context to get back to the default
    UIGraphicsEndImageContext();

    //Note: this is autoreleased
    return cropped;
}


@end