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iPhone拍出来的照片一般大小在1-10M,我们在上传照片时，不可能上传如此大的图片到服务器，一般我们会对照片进行压缩。
常用的做法是，使用这个函数对图片压缩 UIImageJPEGRepresentation(UIImage * __nonnull image, CGFloat compressionQuality); 但这个函数有一个临界值，不能对图片无限制压缩，一般到当压缩比传入0.1已达到临界值了。这时如果我想把5M的图片压缩到50K是行不通的。
如果图太大，我们要求压缩的比例太大就不能使用以上的方法来压缩，这时就要先对图片裁剪，裁剪后再使用这个函数压缩，这样才能达到想要的大小。为了不使图片变形，裁剪时要按原图的宽高比来裁剪。可以写成UIImage的分类方法代码如下：
1 - (UIImage*)imageByScalingAndCroppingForSize:(CGSize)targetSize 2 { 3 UIImage *sourceImage = self; 4 UIImage *newImage = nil; 5 CGSize imageSize = sourceImage.size; 6 CGFloat width = imageSize.width; 7 CGFloat height = imageSize.height; 8 CGFloat targetWidth = targetSize.width; 9 CGFloat targetHeight = targetSize.height; 10 CGFloat scaleFactor = 0.0; 11 CGFloat scaledWidth = targetWidth; 12 CGFloat scaledHeight = targetHeight; 13 CGPoint thumbnailPoint = CGPointMake(0.0,0.0); 14 15 if (CGSizeEqualToSize(imageSize, targetSize) == NO) 16 { 17 CGFloat widthFactor = targetWidth / width; 18 CGFloat heightFactor = targetHeight / height; 19 20 if (widthFactor > heightFactor) 21 scaleFactor = widthFactor; // scale to fit height 22 else 23 scaleFactor = heightFactor; // scale to fit width 24 scaledWidth= width * scaleFactor; 25 scaledHeight = height * scaleFactor; 26 27 // center the image 28 if (widthFactor > heightFactor) 29 { 30 thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5; 31 } 32 else if (widthFactor < heightFactor) 33 { 34 thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5; 35 } 36 } 37 38 UIGraphicsBeginImageContext(targetSize); // this will crop 39 40 CGRect thumbnailRect = CGRectZero; 41 thumbnailRect.origin = thumbnailPoint; 42 thumbnailRect.size.width= scaledWidth; 43 thumbnailRect.size.height = scaledHeight; 44 45 [sourceImage drawInRect:thumbnailRect]; 46 47 newImage = UIGraphicsGetImageFromCurrentImageContext(); 48 if(newImage == nil) 49 NSLog(@"could not scale image"); 50 51 //pop the context to get back to the default 52 UIGraphicsEndImageContext(); 53 return newImage; 54 }
然后给系统工具类写一个压缩图片的类方法，根据不同大小的图片采用不同大小的裁剪比和压缩比，将压缩后的图片保证在50K左右的大小。
1 + (NSData *)compressWithOrgImg:(UIImage *)img 2 { 3 4 NSData *imageData = UIImageJPEGRepresentation(img, 1); 5 float length = imageData.length; 6 length = length/1024; 7 NSLog(@"压缩前的大小：%fKB",length); 8 // 裁剪比例 9 CGFloat cout = 0.5; 10 11 // 压缩比例 12 CGFloat imgCout = 0.1; 13 if(length > 25000){ // 25M以上的图片 14 cout = 0.1; 15 imgCout = 0; 16 }else if(length > 10000){ // 10M以上的图片 17 cout = 0.2; 18 imgCout = 0; 19 }else if (length > 5000) { // 5M以上的图片 20 cout = 0.3; 21 imgCout = 0; 22 }else if (length > 1500) { // 如果原图大于1.5M就换一个压缩级别 23 cout = 0.7; 24 imgCout = 0.1; 25 }else if (length > 1000) { 26 cout = 0.8; 27 imgCout = 0.2; 28 }else if (length > 500) { 29 cout = 0.8; 30 imgCout = 0.3; 31 }else{ 32 cout = 0.9; 33 imgCout = 50/length; 34 } 35 36 37 // 按裁剪比例裁剪 38 UIImage *compressImage = [img imageByScalingAndCroppingForSize:CGSizeMake(img.size.width * cout, img.size.height *cout)]; 39 40 41 // 那压缩比例压缩 42 imageData = UIImageJPEGRepresentation(compressImage, imgCout); 43 44 length= imageData.length / 1024; 45 NSLog(@"裁剪比例：%f，压缩比例：%f,压缩后的大小：%fKB",cout,imgCout,length); 46 return imageData; 47 }
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原文地址：http://www.cnblogs.com/heyode/p/5913803.html
