C++ :: Comparing Images In Current Directory And Sub-directories

I am currently working on a program that will take an image and compare it with all other images in the current directory and sub-directories. I decided to use Qt for this as Qt has a QImage class with an overloaded == operator. This seems to work well if there are duplicates of the same size.

However, my goal is to not only find duplicate images of the same size, but the same image which may be a different size. To do this, I shrink the larger image to the size of the smaller image using the QImage scaled function.

The concern I have with this is that the scaling isn't done the same way the smaller image may have been scaled. I tested this by shrinking a copy of a picture in Paint, that image does not get deleted. I believe that == is looking for an identical image, so when I do the shrinking (via that scaled function), even if it is of the same picture I shrunk in Paint, the two methods of shrinking are not identical, and so the images appear different.

Any better method I can try? Something that can allow for slight deviations, such as an image being 99% "the same" as another. (I admit here that I will need to define some metric for what "the same" means.) In this case, the method assumes they are "identical" and gets rid of one of them.

The main way I can think of doing this is to go through each image, one pixel at a time. If the two pixels from the different images at the same index are the same, then I can increment a same pixel counter. If this number is 99% (or some other threshold) of the total pixel count, I can make the assumption the images are the same.