Generating custom thumbnail from ALAssetRepresentation

Question

My main problem is i need to obtain a thumbnail for an ALAsset object.

I tried a lot of solutions and searched stack overflow for days, all the solutions i found are not working for me due to these constraint:

• I can't use the default thumbnail because it's too little;
• I can't use the fullScreen or fullResolution image because i have a lot of images on screen;
• I can't use UIImage or UIImageView for resizing because those loads the fullResolution image
• I can't load the image in memory, i'm working with 20Mpx images;
• I need to create a 200x200 px version of the original asset to load on screen;

this is the last iteration of the code i came with:

#import <AssetsLibrary/ALAsset.h>
#import <ImageIO/ImageIO.h>   

// ...

ALAsset *asset;

// ...

ALAssetRepresentation *assetRepresentation = [asset defaultRepresentation];

NSDictionary *thumbnailOptions = [NSDictionary dictionaryWithObjectsAndKeys:
    (id)kCFBooleanTrue, kCGImageSourceCreateThumbnailWithTransform,
    (id)kCFBooleanTrue, kCGImageSourceCreateThumbnailFromImageAlways,
    (id)[NSNumber numberWithFloat:200], kCGImageSourceThumbnailMaxPixelSize,
    nil];

CGImageRef generatedThumbnail = [assetRepresentation CGImageWithOptions:thumbnailOptions];

UIImage *thumbnailImage = [UIImage imageWithCGImage:generatedThumbnail];

problem is, the resulting CGImageRef is neither transformed by orientation, nor of the specified max pixel size;

I also tried to find a way of resizing using CGImageSource, but:

• the asset url can't be used in the CGImageSourceCreateWithURL:;
• i can't extract from ALAsset or ALAssetRepresentation a CGDataProviderRef to use with CGImageSourceCreateWithDataProvider:;
• CGImageSourceCreateWithData: requires me to store the fullResolution or fullscreen asset in memory in order to work.

Am i missing something?

Is there another way of obtaining a custom thumbnail from ALAsset or ALAssetRepresentation that i'm missing?

Thanks in advance.

Answer 1

You can use CGImageSourceCreateThumbnailAtIndex to create a small image from a potentially-large image source. You can load your image from disk using the ALAssetRepresentation's getBytes:fromOffset:length:error: method, and use that to create a CGImageSourceRef.

Then you just need to pass the kCGImageSourceThumbnailMaxPixelSize and kCGImageSourceCreateThumbnailFromImageAlways options to CGImageSourceCreateThumbnailAtIndex with the image source you've created, and it will create a smaller version for you without loading the huge version into memory.

I've written a blog post and gist with this technique fleshed out in full.