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I would like to get a data array containing the RGB representation of a picture stored in the photo library (an ALAsset) on iOS (ios8 sdk). I already tried this method :

  • get the a CGImage from ALAsset with [ALAssetRepresentation fullScreenImage]
  • draw the CGImage to a CGContext.

That method works, I get a pointer to rgb data, but this is really slow (there are 2 conversions). The final goal is to load the image quickly in a OpenGL texture.

My code to get an image from Photo library

ALAsset* currentPhotoAsset = (ALAsset*) [self.photoAssetList objectAtIndex:_currentPhotoAssetIndex];
ALAssetRepresentation *representation = [currentPhotoAsset defaultRepresentation];
//-> REALLY SLOW
UIImage *currentPhoto = [UIImage imageWithCGImage:[representation fullScreenImage]];

My code to draw on the CGContext :

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
NSUInteger bytesPerPixel = 4;
NSUInteger bytesPerRow = bytesPerPixel * textureWidth;
NSUInteger bitsPerComponent = 8;
CGContextRef context = CGBitmapContextCreate(textureData, textureWidth, textureHeight,
                                             bitsPerComponent, bytesPerRow, colorSpace,
                                             kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);

CGColorSpaceRelease(colorSpace);
//--> THAT'S REALLY SLOW
CGContextDrawImage(context, CGRectMake(0, 0, textureWidth, textureHeight), cgimage);
CGContextRelease(context);
genpfault
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Nitenq
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1 Answers1

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There is not much you can do but I if you find a way I would be happy to hear about it.

The thing is you need to decompress the image (jpg, png...) which is usually done by creating a CGImage (UIImage is just a wrapper around it). But then you are not allowed to get the data pointer directly from the CGImage but you need to copy them (the really slow draw call). Though then again if the target size and format are the same as the source this operation should be quite fast since the data should more or less simply be copied. On the other hand if your textureWidth and textureHeight are different then the image dimensions those pixels need to be interpolated and this function can become even a few times slower.

The only way out of this I see is to get some library to directly decompress the image from file and get the data pointer of that image. But I never had a performance issue for loading image textures (use a background thread).

Anyway if you are not doing something similar already how I use this is to get the image size, then find the POT (power of two) width and height that fills the image size. Then I create an empty texture with those POT dimensions and call sub image to pass the original image data to the texture. I use a custom texture class to handle this which also contains (generates) texture coordinates so the correct part of the texture is drawn to the frame buffer. Then this class is extended to support atlasing which is generally what you want to do when dealing with many images (textures).

I hope this info helps you in any way...

Matic Oblak
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  • Yes I will use a background thread. But the other problem is the memory. I first get a UIImage from ALAsset (10mb) and then copy the UIImage to get the RGB data (10mb). That can trigger memory pressure problems. – Nitenq Nov 12 '14 at 12:59