Generating custom thumbnail from ALAssetRepresentation - ios

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.

Answer 2

There is a problem with this approach mentioned by Jesse Rusak. Your app will be crashed with the following stack if asset is too large:
0 CoreGraphics 0x2f602f1c x_malloc + 16
1 libsystem_malloc.dylib 0x39fadd63 malloc + 52
2 CoreGraphics 0x2f62413f CGDataProviderCopyData + 178
3 ImageIO 0x302e27b7 CGImageReadCreateWithProvider + 156
4 ImageIO 0x302e2699 CGImageSourceCreateWithDataProvider + 180
...
Link Register Analysis:
Symbol: malloc + 52
Description: We have determined that the link register (lr) is very likely to contain the return address of frame #0's calling function, and have inserted it into the crashing thread's backtrace as frame #1 to aid in analysis. This determination was made by applying a heuristic to determine whether the crashing function was likely to have created a new stack frame at the time of the crash.
Type: 1
It is very easy to simulate the crash. Let's read data from ALAssetRepresentation in getAssetBytesCallback with a small chunks. The particular size of the chunk is not important. The only thing which matters is calling callback about 20 times.
static size_t getAssetBytesCallback(void *info, void *buffer, off_t position, size_t count) {
static int i = 0; ++i;
ALAssetRepresentation *rep = (__bridge id)info;
NSError *error = nil;
NSLog(#"%d: off:%lld len:%zu", i, position, count);
const size_t countRead = [rep getBytes:(uint8_t *)buffer fromOffset:position length:128 error:&error];
return countRead;
}
Here are tail lines of the log
2014-03-21 11:21:14.250 MRCloudApp[3461:1303] 20: off:2432 len:2156064
MRCloudApp(3461,0x701000) malloc: *** mach_vm_map(size=217636864) failed (error code=3)
*** error: can't allocate region
*** set a breakpoint in malloc_error_break to debug
I introduced a counter to prevent this crash. You can see my fix below:
typedef struct {
void *assetRepresentation;
int decodingIterationCount;
} ThumbnailDecodingContext;
static const int kThumbnailDecodingContextMaxIterationCount = 16;
static size_t getAssetBytesCallback(void *info, void *buffer, off_t position, size_t count) {
ThumbnailDecodingContext *decodingContext = (ThumbnailDecodingContext *)info;
ALAssetRepresentation *assetRepresentation = (__bridge ALAssetRepresentation *)decodingContext->assetRepresentation;
if (decodingContext->decodingIterationCount == kThumbnailDecodingContextMaxIterationCount) {
NSLog(#"WARNING: Image %# is too large for thumbnail extraction.", [assetRepresentation url]);
return 0;
}
++decodingContext->decodingIterationCount;
NSError *error = nil;
size_t countRead = [assetRepresentation getBytes:(uint8_t *)buffer fromOffset:position length:count error:&error];
if (countRead == 0 || error != nil) {
NSLog(#"ERROR: Failed to decode image %#: %#", [assetRepresentation url], error);
return 0;
}
return countRead;
}
- (UIImage *)thumbnailForAsset:(ALAsset *)asset maxPixelSize:(CGFloat)size {
NSParameterAssert(asset);
NSParameterAssert(size > 0);
ALAssetRepresentation *representation = [asset defaultRepresentation];
if (!representation) {
return nil;
}
CGDataProviderDirectCallbacks callbacks = {
.version = 0,
.getBytePointer = NULL,
.releaseBytePointer = NULL,
.getBytesAtPosition = getAssetBytesCallback,
.releaseInfo = NULL
};
ThumbnailDecodingContext decodingContext = {
.assetRepresentation = (__bridge void *)representation,
.decodingIterationCount = 0
};
CGDataProviderRef provider = CGDataProviderCreateDirect((void *)&decodingContext, [representation size], &callbacks);
NSParameterAssert(provider);
if (!provider) {
return nil;
}
CGImageSourceRef source = CGImageSourceCreateWithDataProvider(provider, NULL);
NSParameterAssert(source);
if (!source) {
CGDataProviderRelease(provider);
return nil;
}
CGImageRef imageRef = CGImageSourceCreateThumbnailAtIndex(source, 0, (__bridge CFDictionaryRef) #{(NSString *)kCGImageSourceCreateThumbnailFromImageAlways : #YES,
(NSString *)kCGImageSourceThumbnailMaxPixelSize : [NSNumber numberWithFloat:size],
(NSString *)kCGImageSourceCreateThumbnailWithTransform : #YES});
UIImage *image = nil;
if (imageRef) {
image = [UIImage imageWithCGImage:imageRef];
CGImageRelease(imageRef);
}
CFRelease(source);
CGDataProviderRelease(provider);
return image;
}