RE: [ft] Accuracy of generated glyphs in smaller bitmaps

[Jordan Lehmiller]

Thanks for the quick response!

We are using Freetype version 2.3.8.  I get the same result whether or
not I enable the bytecode interpreter. I've also tried
FT_LOAD_FORCE_AUTOHINT to see if using the autohinter would help. I
tried all combinations of gridfitting and grayscale rendering in the
gasp table (and doing neither).  Also if it means anything, we have
FT_RASTER_OPTION_ANTI_ALIASING defined.  All of these combinations
ultimately produced identical results for the 26x26 glpyh.

Anyways, I just looked at the characters giving poor results in a TTF
editor and added 2-4 helper points per character in problematic places.
That ended up giving us much better looking letters at lower
resolutions, so this is more than likely just a bad font setup.  I don't
know enough about TTF files to know what kind of overall impact that
would have, adding extra hinters to most characters.  Let me know if
it's a bad idea to start doing that here and there, or if there is
something else in Freetype I could be missing that would improve the
render quality :)

Thanks,
Jordan


-----Original Message-----
From: Werner LEMBERG [mailto:address@hidden 
Sent: Wednesday, April 29, 2009 11:30 PM
To: Jordan Lehmiller
Cc: address@hidden
Subject: Re: [ft] Accuracy of generated glyphs in smaller bitmaps


First of all, which FreeType version?

> We are having an issue with one of our TTF fonts, where the
> generated bitmaps look radically different between smaller and
> medium resolutions.

Normally, this *always* happens.  This is exactly what TrueType
instructions are designed for.

Do you use the native TrueType hinter, with bytecode handling enabled?
If the latter is disabled you might get inferior results since certain
patented instructions are replaced with workarounds.

> I attached a screenshot of some bitmaps Freetype generates from our
> Assuan TTF font at 26x26, 36x36, and 52x52 (values plugged into
> FT_Set_Pixel_Sizes).

This font looks like not being suited to small resolutions at all --
it is too fat for that purpose.  To tell anything more I need to
investigate the font itself.  In particular, the `gasp' table values
are of interest.

> The bitmaps generated at 36x36 and above are acceptable, but at
> 26x26 and below, the font looks pretty bad (Note the 'RE'
> combination in the image).

This is caused basacally by the missing serifs.  It's easily possible
that this is a badly hinted font which disables serifs below a certain
threshold without adjusting the advance width properly.

> It is very important for us to keep all font sizes below 32x32 since
> we have very limited amounts of memory to work with.

Why not using anti-aliased bitmap fonts directly?