UIImage的高质量缩放
我需要在iPhone应用程序中缩放来自视图图层的图像的分辨率。 显而易见的方法是在UIGraphicsBeginImageContextWithOptions中指定一个比例因子，但是任何时候比例因子不是1.0，图像的质量就会下降，远远超过像素丢失的预期。
我已经尝试了其他几种缩放技术，但它们似乎都围绕着CGContext的东西，而且都显得是一样的。
简单地改变图像的大小（不改变点的分辨率）是不够的，主要是因为这些信息似乎被pipe道中的其他人非常快地丢弃（图像将被转换成JPG和电子邮件）。
有没有其他的方式来缩放iPhone上的图像？
关于UIImageresize的问题， 这篇文章给出了很多处理UIImage对象的方法。 UIImage有一些定位问题需要修复。 这和另一篇文章将解决它。
-(UIImage*)resizedImageToSize:(CGSize)dstSize { CGImageRef imgRef = self.CGImage; // the below values are regardless of orientation : for UIImages from Camera, width>height (landscape) CGSize srcSize = CGSizeMake(CGImageGetWidth(imgRef), CGImageGetHeight(imgRef)); // not equivalent to self.size (which is dependant on the imageOrientation)! /* Don't resize if we already meet the required destination size. */ if (CGSizeEqualToSize(srcSize, dstSize)) { return self; } CGFloat scaleRatio = dstSize.width / srcSize.width; // Handle orientation problem of UIImage UIImageOrientation orient = self.imageOrientation; CGAffineTransform transform = CGAffineTransformIdentity; switch(orient) { case UIImageOrientationUp: //EXIF = 1 transform = CGAffineTransformIdentity; break; case UIImageOrientationUpMirrored: //EXIF = 2 transform = CGAffineTransformMakeTranslation(srcSize.width, 0.0); transform = CGAffineTransformScale(transform, -1.0, 1.0); break; case UIImageOrientationDown: //EXIF = 3 transform = CGAffineTransformMakeTranslation(srcSize.width, srcSize.height); transform = CGAffineTransformRotate(transform, M_PI); break; case UIImageOrientationDownMirrored: //EXIF = 4 transform = CGAffineTransformMakeTranslation(0.0, srcSize.height); transform = CGAffineTransformScale(transform, 1.0, -1.0); break; case UIImageOrientationLeftMirrored: //EXIF = 5 dstSize = CGSizeMake(dstSize.height, dstSize.width); transform = CGAffineTransformMakeTranslation(srcSize.height, srcSize.width); transform = CGAffineTransformScale(transform, -1.0, 1.0); transform = CGAffineTransformRotate(transform, 3.0 * M_PI_2); break; case UIImageOrientationLeft: //EXIF = 6 dstSize = CGSizeMake(dstSize.height, dstSize.width); transform = CGAffineTransformMakeTranslation(0.0, srcSize.width); transform = CGAffineTransformRotate(transform, 3.0 * M_PI_2); break; case UIImageOrientationRightMirrored: //EXIF = 7 dstSize = CGSizeMake(dstSize.height, dstSize.width); transform = CGAffineTransformMakeScale(-1.0, 1.0); transform = CGAffineTransformRotate(transform, M_PI_2); break; case UIImageOrientationRight: //EXIF = 8 dstSize = CGSizeMake(dstSize.height, dstSize.width); transform = CGAffineTransformMakeTranslation(srcSize.height, 0.0); transform = CGAffineTransformRotate(transform, M_PI_2); break; default: [NSException raise:NSInternalInconsistencyException format:@"Invalid image orientation"]; } ///////////////////////////////////////////////////////////////////////////// // The actual resize: draw the image on a new context, applying a transform matrix UIGraphicsBeginImageContextWithOptions(dstSize, NO, self.scale); CGContextRef context = UIGraphicsGetCurrentContext(); if (!context) { return nil; } if (orient == UIImageOrientationRight || orient == UIImageOrientationLeft) { CGContextScaleCTM(context, -scaleRatio, scaleRatio); CGContextTranslateCTM(context, -srcSize.height, 0); } else { CGContextScaleCTM(context, scaleRatio, -scaleRatio); CGContextTranslateCTM(context, 0, -srcSize.height); } CGContextConcatCTM(context, transform); // we use srcSize (and not dstSize) as the size to specify is in user space (and we use the CTM to apply a scaleRatio) CGContextDrawImage(UIGraphicsGetCurrentContext(), CGRectMake(0, 0, srcSize.width, srcSize.height), imgRef); UIImage* resizedImage = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); return resizedImage; } 快速扩展：
extension UIImage{ // returns a scaled version of the image func imageScaledToSize(size : CGSize, isOpaque : Bool) -> UIImage{ // begin a context of the desired size UIGraphicsBeginImageContextWithOptions(size, isOpaque, 0.0) // draw image in the rect with zero origin and size of the context let imageRect = CGRect(origin: CGPointZero, size: size) self.drawInRect(imageRect) // get the scaled image, close the context and return the image let scaledImage = UIGraphicsGetImageFromCurrentImageContext() UIGraphicsEndImageContext() return scaledImage } }
例：
aUIImageView.image = aUIImage.imageScaledToSize(aUIImageView.bounds.size, isOpaque : false)
如果图像没有alpha，则将isOpaque设置为true：绘制将具有更好的性能。
我想出了这个algorithm来创build一个半尺寸的图像：
- (UIImage*) halveImage:(UIImage*)sourceImage { // Compute the target size CGSize sourceSize = sourceImage.size; CGSize targetSize; targetSize.width = (int) (sourceSize.width / 2); targetSize.height = (int) (sourceSize.height / 2); // Access the source data bytes NSData* sourceData = (NSData*) CGDataProviderCopyData(CGImageGetDataProvider(sourceImage.CGImage)); unsigned char* sourceBytes = (unsigned char *)[sourceData bytes]; // Some info we'll need later CGBitmapInfo bitmapInfo = CGImageGetBitmapInfo(sourceImage.CGImage); int bitsPerComponent = CGImageGetBitsPerComponent(sourceImage.CGImage); int bitsPerPixel = CGImageGetBitsPerPixel(sourceImage.CGImage); int __attribute__((unused)) bytesPerPixel = bitsPerPixel / 8; int sourceBytesPerRow = CGImageGetBytesPerRow(sourceImage.CGImage); CGColorSpaceRef colorSpace = CGImageGetColorSpace(sourceImage.CGImage); assert(bytesPerPixel == 4); assert(bitsPerComponent == 8); // Bytes per row is (apparently) rounded to some boundary assert(sourceBytesPerRow >= ((int) sourceSize.width) * 4); assert([sourceData length] == ((int) sourceSize.height) * sourceBytesPerRow); // Allocate target data bytes int targetBytesPerRow = ((int) targetSize.width) * 4; // Algorigthm happier if bytes/row a multiple of 16 targetBytesPerRow = (targetBytesPerRow + 15) & 0xFFFFFFF0; int targetBytesSize = ((int) targetSize.height) * targetBytesPerRow; unsigned char* targetBytes = (unsigned char*) malloc(targetBytesSize); UIImage* targetImage = nil; // Copy source to target, averaging 4 pixels into 1 for (int row = 0; row < targetSize.height; row++) { unsigned char* sourceRowStart = sourceBytes + (2 * row * sourceBytesPerRow); unsigned char* targetRowStart = targetBytes + (row * targetBytesPerRow); for (int column = 0; column < targetSize.width; column++) { int sourceColumnOffset = 2 * column * 4; int targetColumnOffset = column * 4; unsigned char* sourcePixel = sourceRowStart + sourceColumnOffset; unsigned char* nextRowSourcePixel = sourcePixel + sourceBytesPerRow; unsigned char* targetPixel = targetRowStart + targetColumnOffset; uint32_t* sourceWord = (uint32_t*) sourcePixel; uint32_t* nextRowSourceWord = (uint32_t*) nextRowSourcePixel; uint32_t* targetWord = (uint32_t*) targetPixel; uint32_t sourceWord0 = sourceWord[0]; uint32_t sourceWord1 = sourceWord[1]; uint32_t sourceWord2 = nextRowSourceWord[0]; uint32_t sourceWord3 = nextRowSourceWord[1]; // This apparently bizarre sequence scales the data bytes by 4 so that when added together we'll get an average. We do lose the least significant bits this way, and thus about half a bit of resolution. sourceWord0 = (sourceWord0 & 0xFCFCFCFC) >> 2; sourceWord1 = (sourceWord1 & 0xFCFCFCFC) >> 2; sourceWord2 = (sourceWord2 & 0xFCFCFCFC) >> 2; sourceWord3 = (sourceWord3 & 0xFCFCFCFC) >> 2; uint32_t resultWord = sourceWord0 + sourceWord1 + sourceWord2 + sourceWord3; targetWord[0] = resultWord; } } // Convert the bits to an image. Supposedly CGCreateImage will dispose of the target bytes buffer. CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, targetBytes, targetBytesSize, NULL); CGImageRef targetRef = CGImageCreate(targetSize.width, targetSize.height, bitsPerComponent, bitsPerPixel, targetBytesPerRow, colorSpace, bitmapInfo, provider, NULL, FALSE, kCGRenderingIntentDefault); targetImage = [UIImage imageWithCGImage:targetRef]; // Clean up CGColorSpaceRelease(colorSpace); // Return result return targetImage; }
我试图把每隔一行的其他像素，而不是平均，但它导致了像默认algorithm一样糟糕的图像。
我想你可以使用像imagemagick的东西。 显然，它已经成功移植到iPhone： http : //www.imagemagick.org/discourse-server/viewtopic.php? t =14089
我一直对这个图书馆的图像质量感到满意，所以我觉得你会对结果感到满意。