Video quality comparission

I recently think out a method for numerical estimation of video quality comparing to source material. I have the source SOURCE.avs (or SOURCE.avi, it is not very important) I encode it to DEST1.AVI, DEST2.AVI, DEST3.AVI with different codecs or different codec settings, it is unimportant. The I write some .avs scripts like this: cmp1.avs subtract(avisource("SOURCE.avs"),avisource("DEST1.avi")) cmp2.avs subtract(avisource("SOURCE.avs"),avisource("DEST2.avi")) cmp3.avs subtract(avisource("SOURCE.avs"),avisource("DEST3.avi")) (Also there can be added somithing like SelectRangeEvery(140,14) to reduce processing time) Then I make first pass (with high bitrate and quantizer=2 for DivX) for each of this avs scripts. (Nandub, XviD or DivX5, it is unimportant too, I think) At last I compare estimated file sizes. Most noise - largest size, less noise - lesser size). What do you think about such comparision ?

What does the subtract command exactly do?

interesting 

so does this actually correlate to quality?

Re: interesting 

Yeah what he said. All in all, it's a pretty smart way to estimate quality without "quality measuring tools". which happen to also succomb to the same psycovisual deficiencies.