What is the most memory-efficient way of downscaling images on iOS?

Question

In background thread, my application needs to read images from disk, downscale them to the size of screen (1024x768 or 2048x1536) and save them back to disk. Original images are

Answer 1

What you said on twitter does not match your question.

If you are having memory spikes, look at Instruments to figure out what is consuming the memory. Just the data alone for your high resolution image is 10 megs, and your resulting images are going to be about 750k, if they contain no alpha channel.

The first issue is keeping the memory usage low, for that, make sure that all of the images that you load are disposed as soon as you are done using them, that will ensure that the underlying C/Objective-C API disposes the memory immediately, instead of waiting for the GC to run, so something like:

 using (var img = UIImage.FromFile ("..."){
     using (var scaled = Scaler (img)){
          scaled.Save (...);
     }
 }

As for the scaling, there are a variety of ways of scaling the images. The simplest way is to create a context, then draw on it, and then get the image out of the context. This is how MonoTouch's UIImage.Scale method is implemented:

public UIImage Scale (SizeF newSize)
{
    UIGraphics.BeginImageContext (newSize);

    Draw (new RectangleF (0, 0, newSize.Width, newSize.Height));

    var scaledImage = UIGraphics.GetImageFromCurrentImageContext();
    UIGraphics.EndImageContext();

    return scaledImage;            
}

The performance will be governed by the context features that you enable. For example, a higher-quality scaling would require changing the interpolation quality:

 context.InterpolationQuality = CGInterpolationQuality.High

The other option is to run your scaling not on the CPU, but on the GPU. To do that, you would use the CoreImage API and use the CIAffineTransform filter.

As to which one is faster, it is something left for someone else to benchmark

CGImage Scale (string file)
{
    var ciimage = CIImage.FromCGImage (UIImage.FromFile (file));

    // Create an AffineTransform that makes the image 1/5th of the size
    var transform = CGAffineTransform.MakeScale (0.5f, 0.5f);

    var affineTransform = new CIAffineTransform () { 
        Image = ciimage,
        Transform = transform
    };
    var output = affineTransform.OutputImage;
    var context = CIContext.FromOptions (null);
    return context.CreateCGImage (output, output.Extent);
}