Saving to/getting JPEG from user gallery without recompression

问题

I am trying to find a way to read and write JPEG images to user gallery (camera roll) without iOS re-compressing them. UIImage seems to be the bottleneck here. The only method for saving to user gallery I've found is UIImageWriteToSavedPhotosAlbum(). Is there a way around this?

For now my routine looks like this

–Ask UIImagePickerController for a photo. And when it didFinishPickingMediaWithInfo, do:

NSData *imgdata = [NSData dataWithData:UIImageJPEGRepresentation([info objectForKey:@"UIImagePickerControllerOriginalImage"], 1)];
[imgdata writeToFile:filePath atomically:NO];

–Process JPEG losslessly on disk.

–Then save it back:

UIImageWriteToSavedPhotosAlbum([UIImage imageWithContentsOfFile:[self getImagePath]], self, @selector(image:didFinishSavingWithError:contextInfo:), nil);

Here is a tiny animation of what quality degradation looks like after 3 passes:

It obviously gets worse each time I do this, but I couldn't automate the image picking part in order to fully test it for 50/100/1000 cycles.

回答1:

UIImage decodes the image data so it can be edited and displayed, so

UIImageWriteToSavedPhotosAlbum([UIImage imageWithContentsOfFile:[NSData dataWithContentsOfFile:[self getImagePath]]], self, @selector(image:didFinishSavingWithError:contextInfo:), nil);

would first decode the image, and than encode it back by the UIImageWriteToSavedPhotosAlbum method.

Instead, you should use ALAssetsLibrary/writeImageDataToSavedPhotosAlbum:metadata:completionBlock:, something like this:

ALAssetsLibrary *assetLib = [[[ALAssetsLibrary alloc] init] autorelease];
[assetLib writeImageDataToSavedPhotosAlbum:[self getImagePath] metadata:nil completionBlock:nil];

You may also pass metadata and the completion block to the call.

EDIT:

For getting the image:

[info objectForKey:@"UIImagePickerControllerOriginalImage"] contains the decoded UIImage selected from UIImagePickerController. You should instead use

NSURL *assetURL = [info objectForKey:UIImagePickerControllerReferenceURL];

Using the assetURL you can now get the ALAsset for it using the ALAssetsLibrary/assetForURL:resultBlock:failureBlock: method:

ALAssetsLibrary *assetLib = [[[ALAssetsLibrary alloc] init] autorelease];
[assetLib assetForURL:assetURL resultBlock:resultBlock failureBlock:failureBlock];

You can now get the unaltered NSData of that image:

ALAssetsLibraryAssetForURLResultBlock resultblock = ^(ALAsset *asset){
  ALAssetRepresentation *assetRep = [asset defaultRepresentation];
  long long imageDataSize = [assetRepresentation size];
  uint8_t* imageDataBytes = malloc(imageDataSize);
  [assetRepresentation getBytes:imageDataBytes fromOffset:0 length:imageDataSize error:nil];
  NSData *imageData = [NSData dataWithBytesNoCopy:imageDataBytes length:imageDataSize freeWhenDone:YES]; // you could for instance read data in smaller buffers and append them to your file instead of reading it all at once
  // save it
  [imgdata writeToFile:filePath atomically:NO];
};
ALAssetsLibraryAccessFailureBlock failureblock  = ^(NSError *myerror){
  NSLog(@"Cannot get image - %@",[myerror localizedDescription]);
  //
};

I may have made some mistakes in the code, but the steps are as listed above. In case of something not working right, or if you want to make this a little more efficient, there are plenty of examples of doing stuff like reading NSData from an ALAsset on stackoverflow or other sites.

来源：https://stackoverflow.com/questions/13150017/saving-to-getting-jpeg-from-user-gallery-without-recompression

标签

ios

uiimage

ios6

uiimagepickercontroller

jpeg