Re: Help with an old regression in text rendering needed

On Wed, 2016-07-20 at 08:55 +0200, Milos Sramek wrote:
> Hi,
>
> In Brno I would like to present my tools for automated detection and
> bibisection of interoperability problems between LO an MS Office. My
> method is currently of limited utility, because there is a problem
> with
> rendering of plain text, which affects most documents and overshadows
> other bugs.
>
> I managed to localized the problem by bibisection. I also wanted to
> investigate it more deeply, but was unable to compile that version.
> Thus, I would need help from you :).
>
> The problem resides in that the text layout changed (as bibisected
> using  bibisect-43all):

You don't state the platform you were working on. Was it Linux, if so
then probably "bff8fa97e16f0f06fddc5545ea36c8bd2b18a580: Enable
HarfBuzz by default" is what changed during that period.

With text layout there's the vcl level stuff, which is now backed by
harfbuzz on Linux, and different things on the other platforms.
Typically we have had "little" differences between out platforms when
it comes to text layout already. There's a GSOC underway to experiment
with using harfbuzz on all platforms. In this stack of code different
features enabled/disabled in the underlying fontconfig can cause other
differences too.

I presume you've ensured the same fonts are available to LibreOffice as
MSOffice otherwise you'd not have gotten this far anyway I guess.

There's also some extra "wiggling" of glyphs that writer does on top of
all this. I never could quite figure out what writer does there or why
it does it, but that adds some extra variability to things.

I could never get an exact pixel-to-pixel match in the past (ages ago,
before the harfbuzz rework) so I guess this was also a "slightly-fuzzy" 
approach in the close-match case.

Maybe there's some way to try disabling kerning in the harfbuzz layer
to see if that makes a difference in any way.

C.

as an aside, if I was going down the road again of investigating our
differences in layout vs msoffice I'd probably try getting harfbuzz up
and running under windows LibreOffice and then see what the differences
there were between LibreOffice/MSOffice after throwing out all the
mysterious writer adjustment code. Possibly creating some ultra-dumb
font that is plain black blocks or something like that.