[ft] FreeType rasterization grid fitting regime?

[Dakai Liu]

Sorry If you have seen it on developer's mail list.

Hello all,

I am working on a rip project that uses FreeType 2.1.10 as font engine. 
While I render an Asian font at 6 point size at 600x600 dpi, the box 
size is 50x50. The font is a type1 font. FreeType produces un-even 
horizontal strokes. During debug, I found that a macro in ftraster.c:371

#define SCALED(x)  ( ( (x) << ras.scale_shift ) - (ras.precision_half) )

is trying to fit the points into the centre of a pixel. but some certain 
Y value is already at the centre of a pixel. So SCALED moved the Y value 
to the edge of the pixel. I guess this may contribute to the un-evenness 
of a horizontal stroke. So I modified it to:

#define SCALED(x)  ( (x) << ras.scale_shift )

So the whole grid fitting changed, but the output looks much better. 
also the serif at the top appeared. By examining the font file, at 6 
point size at 600dpi, we should have 1 pixel sized serif.

I am pretty much puzzled and need some enlightment. I enclosed the font 
file.


Thanks.

--
Dakai Liu
Software Engineer
R&D
Toshiba-TAP

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