关于iOS上图图片压缩问题解决办法

我是用这个函数对图片进行压缩的

NSData *fData = UIImageJPEGRepresentation(self.photo, 1.0);

这样，图片是7MB这样。假如压缩级别是0.5，如：

NSData *fData = UIImageJPEGRepresentation(self.photo, 0.5);

图片压缩之后，大小是 1MB左右。。现在问题来了。

一张图片，压缩级别是0.5的话，大小不会变成原来的0.5倍。我算了一下，大概是0.14.所以压缩级别和这个大小好像不能形成一定的关联。

比如我一张 500KB的图片，0.5的压缩级别。大小可能才100多KB。对我来说，正好。但是假如像上面那样，7MB的大小，按照这样的压缩率，就是1MB.所以图片时大时小。对用户来说，体验肯定比较差。

我想动态的压缩图片，让没有wifi的情况下，无论多少大小的图片，压缩之后大小最大 200KB。有wifi，压缩大小最大 700KB。

目前有比较好的算法吗?或者开源的函数。

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贴一个相关函数

//图片压缩到指定大小

- (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 theimage

       if (widthFactor > heightFactor)

       {

           thumbnailPoint.y =(targetHeight - scaledHeight) * 0.5;

       }

       elseif (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 tothe default

   UIGraphicsEndImageContext();

    return newImage;

}纳

第二个参数是压缩系数，他的设置并不能保证图片的大小，因为压缩之后的大小和图片的内容也有关系，比如你的图片的的颜色相似的话压缩之后图片的大小就会小点。你所在在意的wifi情况压缩图片是不是指需要上传呢，如果是需要上传的话你可以压缩两种格式的图片，高质量的和低质量的图片，有wifi的情况下上传高质量的图片，没有wifi的情况下上传低质量的图片

UIImageJPEGRepresentation(self.photo, 0.5);

对于每张图片进行压缩，其实有一个最小值，此后无论再怎么改小压缩系数都无济于事。

如果还是过大，只能裁剪图片了。

- (NSData *)imageWithImage:(UIImage*)image

         scaledToSize:(CGSize)newSize;

{

   UIGraphicsBeginImageContext(newSize);

   [image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];

    UIImage* newImage =UIGraphicsGetImageFromCurrentImageContext();

   UIGraphicsEndImageContext();

    return UIImageJPEGRepresentation(newImage, 0.8);

}

你可以弄一个 for 循环，不断的逼近你要的大小

- (UIImage *)compressImage:(UIImage *)image toMaxFileSize:(NSInteger)maxFileSize {

    CGFloat compression = 0.9f;

    CGFloat maxCompression = 0.1f;

    NSData *imageData =UIImageJPEGRepresentation(image, compression);

    while ([imageData length] > maxFileSize&& compression > maxCompression) {

       compression -= 0.1;

       imageData = UIImageJPEGRepresentation(image, compression);

    }

   

    UIImage *compressedImage = [UIImage imageWithData:imageData];

    return compressedImage;

}

  

图片的压缩其实是俩概念，

1、是 “压” 文件体积变小，但是像素数不变，长宽尺寸不变，那么质量可能下降，

2、是 “缩” 文件的尺寸变小，也就是像素数减少。长宽尺寸变小，文件体积同样会减小。

这个 UIImageJPEGRepresentation(image,0.0)，是1的功能。

这个 [sourceImagedrawInRect:CGRectMake(0,0,targetWidth, targetHeight)] 是2的功能。

所以，这俩你得结合使用来满足需求，不然你一味的用1，导致，图片模糊的不行，但是尺寸还是很大。

 

我有个图片，可能是14M或者更多，想把它压缩到500K以内，UIImageJPEGRepresentation(image,0.0)，就算这个压缩系数设置为最小，压缩后仍然比500k大；那有什么方法能把任意大小的图片压缩到指定的字节大小呢，忽略图片清晰度的要求！楼主解决此问题没，求解！复制放进笔记

-(UIImage *) imageCompressForWidth:(UIImage*)sourceImage targetWidth:(CGFloat)defineWidth

{

    CGSize imageSize = sourceImage.size;

    CGFloat width = imageSize.width;

    CGFloat height = imageSize.height;

    CGFloat targetWidth = defineWidth;

    CGFloat targetHeight = (targetWidth / width) *height;

   UIGraphicsBeginImageContext(CGSizeMake(targetWidth, targetHeight));

   [sourceImage drawInRect:CGRectMake(0,0,targetWidth,  targetHeight)];

    UIImage* newImage =UIGraphicsGetImageFromCurrentImageContext();

   UIGraphicsEndImageContext();

    return newImage;

}

最近做论坛功能，发帖的时候需要用到从相册中选取图片然后上传，由于每次上传图片的最大数量为9张，所以需要对图片进行压缩。开始时用了以前经常用的压缩的方法：

 //压缩图片质量
 +(UIImage *)reduceImage:(UIImage *)image percent:(float)percent
 {
     NSData *imageData = UIImageJPEGRepresentation(image, percent);
     UIImage *newImage = [UIImage imageWithData:imageData];
     return newImage;
 }
 //压缩图片尺寸
 + (UIImage*)imageWithImageSimple:(UIImage*)image scaledToSize:(CGSize)newSize
 {
     // Create a graphics image context
     UIGraphicsBeginImageContext(newSize);
     // new size
     [image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
     // Get the new image from the context
     UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();

     // End the context
     UIGraphicsEndImageContext();
     // Return the new image.
     return newImage;
 }

上面的方法比较常见，可是需要加载到内存中来处理图片，当图片数量多了的时候就会收到内存警告，程序崩溃。研究半天终于在一篇博客中找到了解决方法：

 static size_t getAssetBytesCallback(voidvoid *info, voidvoid *buffer, off_t position, size_t count) {
     ALAssetRepresentation *rep = (__bridge id)info;

     NSError *error = nil;
     size_t countRead = [rep getBytes:(uint8_t *)buffer fromOffset:position length:count error:&error];

     if (countRead == 0 && error) {
         // We have no way of passing this info back to the caller, so we log it, at least.
         NDDebug(@"thumbnailForAsset:maxPixelSize: got an error reading an asset: %@", error);
     }

     return countRead;
 }

 static void releaseAssetCallback(voidvoid *info) {
     // The info here is an ALAssetRepresentation which we CFRetain in thumbnailForAsset:maxPixelSize:.
     // This release balances that retain.
     CFRelease(info);
 }

 // Returns a UIImage for the given asset, with size length at most the passed size.
 // The resulting UIImage will be already rotated to UIImageOrientationUp, so its CGImageRef
 // can be used directly without additional rotation handling.
 // This is done synchronously, so you should call this method on a background queue/thread.
 - (UIImage *)thumbnailForAsset:(ALAsset *)asset maxPixelSize:(NSUInteger)size {
     NSParameterAssert(asset != nil);
     NSParameterAssert(size > 0);

     ALAssetRepresentation *rep = [asset defaultRepresentation];

     CGDataProviderDirectCallbacks callbacks = {
         .version = 0,
         .getBytePointer = NULL,
         .releaseBytePointer = NULL,
         .getBytesAtPosition = getAssetBytesCallback,
         .releaseInfo = releaseAssetCallback,
     };

     CGDataProviderRef provider = CGDataProviderCreateDirect((voidvoid *)CFBridgingRetain(rep), [rep size], &callbacks);
     CGImageSourceRef source = CGImageSourceCreateWithDataProvider(provider, NULL);

     CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(source, 0, (__bridge CFDictionaryRef) @{
                                                                                                       (NSString *)kCGImageSourceCreateThumbnailFromImageAlways : @YES,
                                                                                                       (NSString *)kCGImageSourceThumbnailMaxPixelSize : [NSNumber numberWithInt:size],
                                                                                                       (NSString *)kCGImageSourceCreateThumbnailWithTransform : @YES,
                                                                                                       });
     CFRelease(source);
     CFRelease(provider);

     if (!imageRef) {
         return nil;
     }

     UIImage *toReturn = [UIImage imageWithCGImage:imageRef];

     CFRelease(imageRef);

     return toReturn;
 }

采用上面的方法之后内存占用率很低！