Re: [Freetype] Freetype fonts not drawn as well as Windows...

[Giuliano Pochini]

>> The anti-aliasing algorithm gets in input the closed paths and
>> draws each pixel in the dest bitmat according to the coverage. So,
>> regardless to which algorithm is used, the result should be the
>> same in all cases. Otherwise I think there's something wrong.

> I think you're over-simplifying things here. First of all, there
> are several ways to compute the "coverage", and each one of them
> will give you different results, and are only approximation of
> the "real" coverage.

Yes, I know about approximations...

> the current algorithm used by FreeType is the one with best
> precision I could find (for 256 levels of gray), and it is also
> very fast. The algorithm used in the previous beta was less
> accurate, faster, but generated unpleasant results for a
> certain number of glyphs (even if it could create better
> anti-aliased "w"), which is why it's been replaced..

That case should be addressed with proper hinting. I don't think
the renderer is to blame.

> Second, some algorithms will perform drop-out control, i.e.c
> create "virtual" pixels when certain delicate conditions are
> detected or met; it deals, among other things, with "vanishing"
> diagonals (which mathematically correspond to a coverage of 0 !!)

Oh, I didn't know this.

> And finally, filter theory will tell you that a perfect coverage
> isn't necessarily the best for text display anyway..

IMHO this is what the hinter is for. The AA algo should just draw
what upper layer's give it.

>> Oh, I still have that problem with very thin AA polygons... I want
>> to try to find tha cause, but I need some help. Where can I find
>> a doc that briefly explain how the AA algorithm works ?
>>
> I have tried to reproduce your results without success, I have
> extremely small lines working correctly here !?
> 
> Could you send me the fixed-point coordinates of one of your
> lines, if possible a very small one but with noticeable artefact.
> I'll try to trace through the rasterizer to see what's happening
> when I'll get back in about 10 days..

Ok, the proggy is attached. To compile and run:

gcc -O2 -g -Wall -Wno-parentheses fttest.c -I/usr/local/include 
-L/usr/local/lib -lfreetype -lpng -lm

./a.out > /other/sources/xxyyzz.png

It draws two sets of polygons, 0.1 and 0.5 pixels thin. They have
this shape:

__
\ \
 \ \
  \_\

You can see small artifacts in the 0.5 set too.


Bye.

fttest.c
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