C# Checking if an image exists within another image

Question

Hi,

I'm not sure where to start with this so some guidance would be good. What I need to achieve is, examine a large image (say 1280x1024) and check to see if another smaller image exists within it or not (maybe a 50x50 pixel image).

I tried doing this by comparing every pixel which is really slow and I may need to do it 100+ times so it doesn't seem suitable. I'm just wondering if there is a better way?

Thanks

Answer 1

I was just working on something similar and the quick and dirty result I came up with is to use AForge.Net's implementation of "ExhaustiveTemplateMatching" with images 1/4 of their size. 720p images at full size took a couple minutes but at 1/4 size it's about a second on my puny computer.

public static class BitmapExtensions
{
    /// <summary>
    /// See if bmp is contained in template with a small margin of error.
    /// </summary>
    /// <param name="template">The Bitmap that might contain.</param>
    /// <param name="bmp">The Bitmap that might be contained in.</param>        
    /// <returns>You guess!</returns>
    public static bool Contains(this Bitmap template, Bitmap bmp)
    {
        const Int32 divisor = 4;
        const Int32 epsilon = 10;

        ExhaustiveTemplateMatching etm = new ExhaustiveTemplateMatching(0.9f);                      

        TemplateMatch[] tm = etm.ProcessImage(
            new ResizeNearestNeighbor(template.Width / divisor, template.Height / divisor).Apply(template),
            new ResizeNearestNeighbor(bmp.Width / divisor, bmp.Height / divisor).Apply(bmp)
            );

        if (tm.Length == 1)
        {
            Rectangle tempRect = tm[0].Rectangle;

            if (Math.Abs(bmp.Width / divisor - tempRect.Width) < epsilon
                &&
                Math.Abs(bmp.Height / divisor - tempRect.Height) < epsilon)
            {
                return true;
            }                
        }

        return false;
    }
}

You could also of course just check for tm.length > 0 and yes there are some unnecessary divides in there :P

Answer 2

I have tried using ExhaustiveTemplateMatching, with your method and the default from the AForge documentation and I always get at least 1 match with the similarity rating set to 1, whether or not the image exists in the bigger image. This applies if I set the match parameter to anything (e.g 0f or 1f).

Any idea why that is? I only need to check if the image definitely exists

Thanks