Re: [ft-devel] [PATCH resend 2/2] Improve FT_Outline_Embolden for the unintended artifacts problem (#45596).

[Byeongsik Jeon]

Hi.

>> +  two coalition equation solution:
>> +    det = i.y * o.x - i.x * o.y , det != 0
>> +    shift.x = strength * ( i.x - o.x ) / det
>> +    shift.y = strength * ( i.y - o.y ) / det
>
> So your algorithm is exactly the same as the one implemented
> currently. The tricky part is how you deal with very short collapsing
> segments, the segments of zero length, and your threshold on the
> determinant. Can you please elaborate on that and explain why your
> algorithm is superior?
>

https://savannah.nongnu.org/bugs/?45596
https://savannah.nongnu.org/bugs/download.php?file_id=34479
03963_BAUHAUS.ttf  is a very bad font for real use. However, it is a suitable 
font to test the stability of the Embolden algorithm.

[0]
bauhaus_256px_0_gedit_non_patched.png:
This is a problem that appears in FreeType, which is not patched.

[1]
bauhaus_256px_1_gedit_ftsynth_patched.png:
It's only applied the fetsynth Embolden patch. The overlaped shape was found in 
its original shape, and the size of the artifact was reduced. The overall stem 
thickness or advance is correctly organized.

[2]
bauhaus_256px_2_gedit_ftsynth+shift_limit_patched.pcrushng:
Additional shift limitation patch have been applied.

shift.x = FT_MAX( FT_MIN( shift.x, strength.x ), -strength.x );
shift.y = FT_MAX( FT_MIN( shift.y, strength.y ), -strength.y );

The sharpen edge of the 'X' has been removed.

[3]
bauhaus_256px_3_gedit_all_patched.png:
The remaining patches were applied. You can see that all the artifacts are 
removed. The middle stem of the "$" character is rendered to the right thickness.

It was so difficult to modify the existing code that I rewrite it. But it's 
based on the existing code idea.

          l_out = (FT_Fixed)FT_Vector_NormLen( &out );

          if ( l_out == 0 )
            continue;

While the existing code focuses on skipping only the zero-length segment, the 
new code skips the zero-determinant segment.

        det = FT_MulFix( in.y, out.x ) - FT_MulFix( in.x, out.y );
        if ( det == 0 ) continue;

Zero-determinant segments include these things:
 - zero-length segment: |in| == 0 or |out| == 0 ( normalized vector in, out )
 - plain straight segment: in == out
 - forward-backward mixed straight segment: in + out == 0

"( det = in.y * out.x - in.x * out.y ) == 0" checks all of these.

The "forward-backward mixed straight segment" are particularly important. 
Because their shift vectors are in the opposite direction, they produce a 
prominent artifact.

The good thing about the improved code is that "det" is part of the formula for 
obtaining shift vectors. Also, since improved code is not difficult, it is easy 
to track even if new problems are found.

Unless "strength" is zero, it is inevitable that shift vectors intersect or 
sharpen edge generation. The appropriate values of "strength" and "shift limit 
code" complement this deficiencies.

[4]
bauhaus_256px_4_win7ie9_virtualbox.png:
The artifact that appears here is obviously a bug in Windows. Potentially, this 
problem can appear on any font.

What we should note is that the rendered letter shape, except for the artifacts, 
is almost identical to our results.

Thanks.


On Fri, 5 Oct 2018 11:30:46 +0900, Byeongsik Jeon <address@hidden> wrote:
> FreeType-Bugs: https://savannah.nongnu.org/bugs/?45596
> ---
>     src/base/ftoutln.c | 177 ++++++++++++++++++++++++++++++++++++++++++++-
>     1 file changed, 176 insertions(+), 1 deletion(-)
>
>
>
>
>
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bauhaus_256px_0_gedit_non_patched.png
Description: PNG image

bauhaus_256px_1_gedit_ftsynth_patched.png
Description: PNG image

bauhaus_256px_2_gedit_ftsynth+shift_limit_patched.png
Description: PNG image

bauhaus_256px_3_gedit_all_patched.png
Description: PNG image

bauhaus_256px_4_win7ie9_virtualbox.png
Description: PNG image