8054203: add regression tests for JDK vs ICU layout

[Behdad Esfahbod]

On Fri, Aug 15, 2014 at 6:10 PM, Steven R. Loomis <srl at icu-project.org>
wrote:

>
> Behdad, what have you been doing for comparisons w/ Uniscribe?


We have only been comparing unscaled results.  Ie, we shape at font's upem
size (2048, etc), to avoid subpixel rounding / precision issues.

I haven't read the rest of the thread yet, will reply with more details
later.  In the mean time, recently I bumped icu-le-hb's fixed-point format
from 24.8 to 20.12, to hopefully be closer to ICU's float results.


>
> >
> >
> >>> 5) HarfBuzz does its computations in integer device units, with
> >>> rounding to 16.16 or 24.8 or 26.6 values (though iculehb does some in
> >>> floating point). ICU makes more use of native float units.  I've not
> >>> been able to track down what exactly happens, but it does seem that
> >>> advances might differ between ICU and HB even if kerning is not
> >>> applied. The main place I've seen suggestions of this is with scaling
> >>> based on common fractions (e.g. 1/10, etc.), native float units can
> >>> represent common fractions much better than fixed point power-of-two
> >>> units can, and small differences can accumulate over the course of a
> >>> line of text. Occasionally this trips over a pixel and glyph images
> >>> change.
> >>>
> >>> So I guess I think we need to first figure out what degree of
> >>> compatibility is achievable, and what we want, and then design our
> >>> regression/metrics tests around that.
> >> OK.
> >>
> >> Maybe I should rephrase this particular ticket - it is for very basic
> >> compatibility, to first verify if  embedded-ICU vs external-ICU is
> >> compatible, and then secondly to compare embedded-ICU with
> >> external-ICU-really-HarfBuzz.
> >>
> > Sure, for different ways to access ICU this is of course fine, and that's
> > a perfectly good place to start.  I was just focusing on the switch to
> HB.
> I think you are looking at perhaps a more agressive internal change than
> I was trying to do at first.  Plus I am over optimistic about the kinds
> of changes needed.
>
> OK.  So I'll open another ticket for follow on conformance testing.
>
> Will you sign off on this test as approved for now then? :)
>
> -s
>
> --
>
> IBMer but all opinions are mine.
> https://www.ohloh.net/accounts/srl295 // fingerprint @
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>
>
>