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

[David Turner]

Hi Guilano,

Giuliano Pochini a écrit :
> 
> 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.

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..

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 !!)
..

It's very technical but I'll summarize that the FreeType monochrome
renderer, as well as the AA one used in the betas, perform dropout
control. the current renderer doesn't..


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

Regards,

- David