从 CGImageRef 创建的 UIImage 失败并显示 UIImagePNGRepresentation

Question

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：没有将 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？

Answer 1

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/ 两者都适用，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

Answer 2

在 PNG 中存在各种块，一些包含调色板信息，一些实际图像数据和一些其他信息，这是一个非常有趣的标准。 IDAT块是实际包含图像数据的位。如果没有“IDAT 写入文件”，则 libpng 从输入数据创建 PNG 时遇到一些问题。

我不知道你的 stillView.image 到底是什么，但是当你向你的代码传递一个肯定有效的 CGImageRef 时会发生什么？ theRect 中的实际值是多少？如果你的 theRect 超出了图像的边界，那么你试图用来制作 UIImage 的 cgCrop 很容易为零 - 或不为零，但不包含图像或宽度和高度为 0 的图像，使 libpng 无法工作和。

Answer 3

看来您正在尝试的解决方案应该有效，但我建议使用这个：

CGImageRef image = [stillView.image CGImage];
CGRect cropZone;

size_t cWitdh = cropZone.size.width;
size_t cHeight = cropZone.size.height;
size_t bitsPerComponent = CGImageGetBitsPerComponent(image);
size_t bytesPerRow = CGImageGetBytesPerRow(image) / CGImageGetWidth(image) * cWidth;
        
//Now we build a Context with those dimensions.
CGContextRef context = CGBitmapContextCreate(nil, cWitdh, cHeight, bitsPerComponent, bytesPerRow, CGColorSpaceCreateDeviceRGB(), CGImageGetBitmapInfo(image));

CGContextDrawImage(context, cropZone, image);

CGImageRef result  = CGBitmapContextCreateImage(context);
UIImage * cropUIImage = [[UIImage alloc] initWithCGImage:tmp];

CGContextRelease(context);
CGImageRelease(mergeResult);
NSData * imgData = UIImagePNGRepresentation ( cropUIImage);

Answer 4

UIImage *croppedImage = [self imageByCropping:yourImageView.image toRect:heredefineyourRect];
 
    CGSize size = CGSizeMake(croppedImage.size.height, croppedImage.size.width);
    UIGraphicsBeginImageContext(size);
    
    CGPoint pointImg1 = CGPointMake(0,0);
    [croppedImage drawAtPoint:pointImg1 ];

    [[UIImage imageNamed:yourImagenameDefine] drawInRect:CGRectMake(0,532, 150,80) ];//here define your Reactangle
    
    UIImage* result = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    croppedImage = result;
    yourCropImageView.image=croppedImage;
    [yourCropImageView.image retain];