Comment by malkia

There are several good algos used for diffing graphics for not so pixel-for-pixel rendering allowing thresholds (you are going to end up with differences between software and GPU rendering at least - it's inevitable):

https://github.com/NVlabs/flip

But also SSIM, perceptualdiff, and many others

For UI/web certain other methods are better, for video - I'm sure there are prefferences there too.