iOS图片压缩

项目中常会遇到，上传图片的操作，由于iPhone手机直接拍照的图片往往比较大，一般3-4M,如果直接上传不做处理会浪费用户很多流量，再者有很多场景并不需要高清图片，所以在上传图片前对图片进行压缩，是很有必要的。

1.OC中的UIKit中提供了现成的压缩函数  UIImageJPEGRepresentation(UIImage * __nonnull image, CGFloat compressionQuality) ，但压缩比率只能是0.1到0.9，如果图片过大，还是无法达到我们想要的效果。

2.对于大图片（10M以上），我们可以先对图片进行裁剪，然后再压缩。这个方法能大大压缩图片的大小。以我的项目中需求为例，需求是：不管多大图片，都要压缩至50kb左右才可以上传到服务器，而且像素不能过度失真。

 + (NSData *)compressWithOrgImg:(UIImage *)img
 {

     NSData *imageData = UIImageJPEGRepresentation(img, );
     float length = imageData.length;
     length = length/;
     NSLog(@"压缩前的大小：%fKB",length);
     // 裁剪比例
     CGFloat cout = 0.5;

     // 压缩比例
     CGFloat imgCout = 0.1;
     if(length > ){ // 25M以上的图片
         cout = 0.1;
         imgCout = ;
     }else if(length > ){ // 10M以上的图片
         cout = 0.2;
         imgCout = ;
     }else if (length > ) { // 5M以上的图片
         cout = 0.3;
         imgCout = ;
     }else if (length > ) { // 如果原图大于1.5M就换一个压缩级别
         cout = 0.7;
         imgCout = 0.1;
     }else if (length > ) {
         cout = 0.8;
         imgCout = 0.2;
     }else if (length > ) {
         cout = 0.8;
         imgCout = 0.3;
     }else if (length >){ // 小于500k的不用裁剪

         imageData = UIImageJPEGRepresentation(img,  / imageData.length);
         float length = imageData.length;
         length = length/;
         NSLog(@"压缩后的大小：%fKB",length);
         return imageData;
     }else{

         imageData = UIImageJPEGRepresentation(img, 0.5);
         float length = imageData.length;
         length = length/;
         NSLog(@"压缩后的大小：%fKB",length);
         return imageData;
     }

     // 按裁剪比例裁剪
     UIImage *compressImage =  [img imageByScalingAndCroppingForSize:CGSizeMake(img.size.width * cout, img.size.height *cout)];

     // 那压缩比例压缩
     imageData = UIImageJPEGRepresentation(compressImage, imgCout);

     length= imageData.length / ;
     NSLog(@"裁剪比例：%f，压缩比例：%f,压缩后的大小：%fKB",cout,imgCout,length);
     return imageData;
 }

 // 裁剪
 - (UIImage*)imageByScalingAndCroppingForSize:(CGSize)targetSize
 {
     UIImage *sourceImage = self;
     UIImage *newImage = nil;
     CGSize imageSize = sourceImage.size;
     CGFloat width = imageSize.width;
     CGFloat height = imageSize.height;
     CGFloat targetWidth = targetSize.width;
     CGFloat targetHeight = targetSize.height;
     CGFloat scaleFactor = 0.0;
     CGFloat scaledWidth = targetWidth;
     CGFloat scaledHeight = targetHeight;
     CGPoint thumbnailPoint = CGPointMake(0.0,0.0);

     if (CGSizeEqualToSize(imageSize, targetSize) == NO)
     {
         CGFloat widthFactor = targetWidth / width;
         CGFloat heightFactor = targetHeight / height;

         if (widthFactor > heightFactor)
             scaleFactor = widthFactor; // scale to fit height
         else
             scaleFactor = heightFactor; // scale to fit width
         scaledWidth= width * scaleFactor;
         scaledHeight = height * scaleFactor;

         // center the image
         if (widthFactor > heightFactor)
         {
             thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
         }
         else if (widthFactor < heightFactor)
         {
             thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
         }
     }

     UIGraphicsBeginImageContext(targetSize); // this will crop

     CGRect thumbnailRect = CGRectZero;
     thumbnailRect.origin = thumbnailPoint;
     thumbnailRect.size.width= scaledWidth;
     thumbnailRect.size.height = scaledHeight;

     [sourceImage drawInRect:thumbnailRect];

     newImage = UIGraphicsGetImageFromCurrentImageContext();
     if(newImage == nil)
         NSLog(@"could not scale image");

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

代码中针对不同大小图片，给出了不同的压缩比率，以保证压缩后的图片大小都在50k左右。此处的“裁剪”是将图片的宽高等比例缩小到指定的比率