Rasterization prior to comparison is a really clever idea. Doing this with PDF spec code would be an absolute nightmare. Checking 2 bitmaps for equality is just a few lines of code.

Many times, you aren't really concerned with the specific differences being described natively. You just want to know if a human would perceive differences between 2 copies of the "same" form.

I wrote a tool to do this for circuit board artwork changes. It used gerbv to rasterize the vector (gerber) files, then used XOR on the pixels to make unchanged areas disappear. For such a simple tool it is quite useful.

> After the first few times comparing PDFs of schematics by eye, I said screw it and wrote a tool to export to PNGs and then compare them pixel-by-pixel. It saved me a ton of time and my sanity.

It saved mine! I usually work with books and sometimes it's really difficult to tell 10 people to check every single page if something has been different to a previous pdf I sent. They will never be able to find all the differences because we're humans, not bot. We make mistakes.