iOS圖片壓縮

項目中常會遇到，上傳圖片的操做，因爲iPhone手機直接拍照的圖片每每比較大，通常3-4M,若是直接上傳不作處理會浪費用戶不少流量，再者有不少場景並不須要高清圖片，因此在上傳圖片前對圖片進行壓縮，是頗有必要的。

1.OC中的UIKit中提供了現成的壓縮函數  UIImageJPEGRepresentation(UIImage * __nonnull image, CGFloat compressionQuality) ，但壓縮比率只能是0.1到0.9，若是圖片過大，仍是沒法達到咱們想要的效果。

2.對於大圖片（10M以上），咱們能夠先對圖片進行裁剪，而後再壓縮。這個方法能大大壓縮圖片的大小。以個人項目中需求爲例，需求是：無論多大圖片，都要壓縮至50kb左右才能夠上傳到服務器，並且像素不能過分失真。

 1 + (NSData *)compressWithOrgImg:(UIImage *)img
 2 {
 3     
 4     NSData *imageData = UIImageJPEGRepresentation(img, 1);
 5     float length = imageData.length;
 6     length = length/1024;
 7     NSLog(@"壓縮前的大小：%fKB",length);
 8     // 裁剪比例
 9     CGFloat cout = 0.5;
10     
11     // 壓縮比例
12     CGFloat imgCout = 0.1;
13     if(length > 25000){ // 25M以上的圖片
14         cout = 0.1;
15         imgCout = 0;
16     }else if(length > 10000){ // 10M以上的圖片
17         cout = 0.2;
18         imgCout = 0;
19     }else if (length > 5000) { // 5M以上的圖片
20         cout = 0.3;
21         imgCout = 0;
22     }else if (length > 1500) { // 若是原圖大於1.5M就換一個壓縮級別
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 if (length >100){ // 小於500k的不用裁剪
32         
33         imageData = UIImageJPEGRepresentation(img, 50 / imageData.length);
34         float length = imageData.length;
35         length = length/1024;
36         NSLog(@"壓縮後的大小：%fKB",length);
37         return imageData;
38     }else{
39         
40         imageData = UIImageJPEGRepresentation(img, 0.5);
41         float length = imageData.length;
42         length = length/1024;
43         NSLog(@"壓縮後的大小：%fKB",length);
44         return imageData;
45     }
46     
47     
48     // 按裁剪比例裁剪
49     UIImage *compressImage =  [img imageByScalingAndCroppingForSize:CGSizeMake(img.size.width * cout, img.size.height *cout)];
50     
51     
52     // 那壓縮比例壓縮
53     imageData = UIImageJPEGRepresentation(compressImage, imgCout);
54     
55     length= imageData.length / 1024;
56     NSLog(@"裁剪比例：%f，壓縮比例：%f,壓縮後的大小：%fKB",cout,imgCout,length);
57     return imageData;
58 }

 1 // 裁剪
 2 - (UIImage*)imageByScalingAndCroppingForSize:(CGSize)targetSize
 3 {
 4     UIImage *sourceImage = self;
 5     UIImage *newImage = nil;
 6     CGSize imageSize = sourceImage.size;
 7     CGFloat width = imageSize.width;
 8     CGFloat height = imageSize.height;
 9     CGFloat targetWidth = targetSize.width;
10     CGFloat targetHeight = targetSize.height;
11     CGFloat scaleFactor = 0.0;
12     CGFloat scaledWidth = targetWidth;
13     CGFloat scaledHeight = targetHeight;
14     CGPoint thumbnailPoint = CGPointMake(0.0,0.0);
15     
16     if (CGSizeEqualToSize(imageSize, targetSize) == NO)
17     {
18         CGFloat widthFactor = targetWidth / width;
19         CGFloat heightFactor = targetHeight / height;
20         
21         if (widthFactor > heightFactor)
22             scaleFactor = widthFactor; // scale to fit height
23         else
24             scaleFactor = heightFactor; // scale to fit width
25         scaledWidth= width * scaleFactor;
26         scaledHeight = height * scaleFactor;
27         
28         // center the image
29         if (widthFactor > heightFactor)
30         {
31             thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
32         }
33         else if (widthFactor < heightFactor)
34         {
35             thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
36         }
37     }
38     
39     UIGraphicsBeginImageContext(targetSize); // this will crop
40     
41     CGRect thumbnailRect = CGRectZero;
42     thumbnailRect.origin = thumbnailPoint;
43     thumbnailRect.size.width= scaledWidth;
44     thumbnailRect.size.height = scaledHeight;
45     
46     [sourceImage drawInRect:thumbnailRect];
47     
48     newImage = UIGraphicsGetImageFromCurrentImageContext();
49     if(newImage == nil)
50         NSLog(@"could not scale image");
51     
52     //pop the context to get back to the default
53     UIGraphicsEndImageContext();
54     return newImage;
55 }

代碼中針對不一樣大小圖片，給出了不一樣的壓縮比率，以保證壓縮後的圖片大小都在50k左右。此處的「裁剪」是將圖片的寬高等比例縮小到指定的比率