comparing groff output (was: Re: [PROPOSAL] time zones and reproducible builds)

[Dave Kemper]

On 12/22/20, Deri <deri@chuzzlewit.myzen.co.uk> wrote:
> Isn't the actual build output of groff marks on the page, and it is
> this we should be comparing to detect changes between one version and
> another, not the source code produced by groff which needs to run in
> order to produce the marks. I'm sure if the postscript or pdf is rastered
> to a binary bitmap and new and old are subtracted from each other,
> if there are any black pixels left in the image then it has failed the
> reproducibility test. If the result is a completely white page there has
> been no difference between the two builds, even if the post processor
> has had changes which are intended to improve the file it produces,
> but those changes have not altered the result of "running" that file.

Has anyone developed a script or pipeline for doing this?  I'm not
even sure what tools one would use to accomplish these steps.  A
little internet digging would answer that, but if someone already does
this and has a toolchain at hand they'd be willing to share, I'd be
happy to avoid duplicating that effort.