ios - 调用 imageWithData :UIImageJPEGRepresentation() multiple times only compresses image the first time

Question

为了防止我的应用程序滞后，我尝试压缩大于 1 MB 的图像(主要用于从 iphone 的普通相机拍摄的照片。

UIImage *image = [info objectForKey:UIImagePickerControllerOriginalImage];
NSData *imageSize = UIImageJPEGRepresentation(image, 1);
NSLog(@"original size %u", [imageSize length]);

 UIImage *image2 = [UIImage imageWithData:UIImageJPEGRepresentation(image, 0)];
    NSData *newImageSize = UIImageJPEGRepresentation(image2, 1);
    NSLog(@"new size %u", [newImageSize length]);
    UIImage *image3 = [UIImage imageWithData:UIImageJPEGRepresentation(image2, 0)];
    NSData *newImageSize2 = UIImageJPEGRepresentation(image3, 1);
    NSLog(@"new size %u", [newImageSize2 length]);

picView = [[UIImageView alloc] initWithImage:image3] ;

但是，我得到的 NSLog 输出了类似

的内容

 original size 3649058
 new size 1835251
 new size 1834884

第一次和第二次压缩之间的差异几乎可以忽略不计。我的目标是使图像大小小于 1 MB。我是否忽略了什么/是否有替代方法来实现这一目标？

编辑:如果可能，我想避免缩放图像的高度和宽度。

Answer 1

几个想法:

1. UIImageJPEGRepresentation 函数不返回“原始”图像。例如，如果您使用 1.0 的 compressionQuality，从技术上讲，它不会返回“原始”图像，而是返回带有 compressionQuality 处于最大值。这实际上可以产生比原始 Assets 更大的对象(至少如果原始图像是 JPEG)。您还将在此过程中丢弃所有元数据(有关图像拍摄位置、相机设置等的信息)。

2. 如果你想要原始资源，你应该使用 PHImageManager:

   NSURL *url = [info objectForKey:UIImagePickerControllerReferenceURL];
   
   PHFetchResult *result = [PHAsset fetchAssetsWithALAssetURLs:@[url] options:nil];
   PHAsset *asset = [result firstObject];
   
   PHImageManager *manager = [PHImageManager defaultManager];
   [manager requestImageDataForAsset:asset options:nil resultHandler:^(NSData *imageData, NSString *dataUTI, UIImageOrientation orientation, NSDictionary *info) {
       NSString *filename = [(NSURL *)info[@"PHImageFileURLKey"] lastPathComponent];
       // do what you want with the `imageData`
   }];
   

   在 iOS 8 之前的版本中，您必须使用 ALAssetsLibrary 类的 assetForURL:

   NSURL *url = [info objectForKey:UIImagePickerControllerReferenceURL];
   ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
   [library assetForURL:url resultBlock:^(ALAsset *asset) {
       ALAssetRepresentation *representation = [asset defaultRepresentation];
   
       NSLog(@"size of original asset %llu", [representation size]);
   
       // I generally would write directly to a `NSOutputStream`, but if you want it in a
       // NSData, it would be something like:
   
       NSMutableData *data = [NSMutableData data];
   
       // now loop, reading data into buffer and writing that to our data strea
       NSError *error;
       long long bufferOffset = 0ll;
       NSInteger bufferSize = 10000;
       long long bytesRemaining = [representation size];
       uint8_t buffer[bufferSize];
       NSUInteger bytesRead;
       while (bytesRemaining > 0) {
           bytesRead = [representation getBytes:buffer fromOffset:bufferOffset length:bufferSize error:&error];
           if (bytesRead == 0) {
               NSLog(@"error reading asset representation: %@", error);
               return;
           }
           bytesRemaining -= bytesRead;
           bufferOffset   += bytesRead;
           [data appendBytes:buffer length:bytesRead];
       }
   
       // ok, successfully read original asset; 
       // do whatever you want with it here
   
   } failureBlock:^(NSError *error) {
       NSLog(@"error=%@", error);
   }];
   

   请注意，此 assetForURL 异步运行。

3. 如果您想要压缩的NSData，您可以使用UIImageJPEGRepresentation，compressionQuality小于1.0。您的代码实际上使用 0.0 的 compressionQuality 执行此操作，这应该提供最大压缩。但是你不保存那个 NSData，而是用它来创建一个 UIImage 然后你得到一个新的 UIImageJPEGRepresentation 和一个 1.0 的 compressionQuality，因此失去了您最初实现的大部分压缩。

   考虑以下代码:

   // a UIImage of the original asset (discarding meta data)
   
   UIImage *image = [info objectForKey:UIImagePickerControllerOriginalImage];
   
   // this may well be larger than the original asset
   
   NSData *jpgDataHighestCompressionQuality = UIImageJPEGRepresentation(image, 1.0);
   [jpgDataHighestCompressionQuality writeToFile:[docsPath stringByAppendingPathComponent:@"imageDataFromJpeg.jpg"] atomically:YES];
   NSLog(@"compressionQuality = 1.0; length = %u", [jpgDataHighestCompressionQuality length]);
   
   // this will be smaller, but with some loss of data
   
   NSData *jpgDataLowestCompressionQuality = UIImageJPEGRepresentation(image, 0.0);
   NSLog(@"compressionQuality = 0.0; length = %u", [jpgDataLowestCompressionQuality length]);
   UIImage *image2 = [UIImage imageWithData:jpgDataLowestCompressionQuality];
   
   // ironically, this will be larger than jpgDataLowestCompressionQuality
   
   NSData *newImageSize = UIImageJPEGRepresentation(image2, 1.0);
   NSLog(@"new size %u", [newImageSize length]);
   

4. 除了前面提到的 JPEG 压缩质量外，您还可以 resize the image .您也可以将其与 JPEG compressionQuality 相结合。