Question

I’m attempting to gauge the percentage difference between two images.

Having done a lot of reading I seem to have a number of options but I’m not sure what the best method to follow for:

1. Ease of coding
2. Performance.

The methods I’ve seen are:

Non language specific – academic Image comparison – fast algorithm and Mac specific direct pixel access http://www.markj.net/iphone-uiimage-pixel-color/

Does anyone have any advice about what solutions make most sense for the above two cases and have code samples to show how to apply them?

Answer 1

I’ve had success calculating the difference between two images using the histogram technique mentioned here. redmoskito’s answer in the SO question you linked to was actually my inspiration!

The following is an overview of the algorithm I used:

1. Convert the images to grayscale—compare one channel instead of three.
2. Divide each image into an n * n grid of “subimages”. Then, for subimage pair:
   1. Calculate their colour composition histograms.
   2. Calculate the absolute difference between the two histograms.
3. The maximum difference found between two subimages is a measure of the two images’ difference. Other metrics could also be used (e.g. the average difference betwen subimages).

As tskuzzy noted in his answer, if your ultimate goal is a binary “yes, these two images are (roughly) the same” or “no, they’re not”, you need some meaningful threshold value. You could produce such a value by passing images into the algorithm and tweaking the threshold based on its output and how similar you think the images are. A form of machine learning, I suppose.

I recently wrote a blog post on this very topic, albeit as part of a larger goal. I also created a simple iPhone app to demonstrate the algorithm. You can find the source on GitHub; perhaps it will help?