[cairo] xlib backend mis-draws fonts with partial bitmap strike

Keith Packard keithp at keithp.com
Tue Oct 30 00:05:29 PDT 2007

Sazanami Mincho is a Japanese font which offers several bitmap strikes,
however coverage of those strikes is not complete. If the first glyph
has a bitmap version, cairo assumes that all of the glyphs will and
creates the font in A1 format. However, glyphs which do not have bitmap
strikes will get mis-rendered as they are first drawn to an A8 image and
then poorly copied to A1. However, if the first glyph does not have a
bitmap strike, it will be drawn anti-aliased, and all of the remaining
glyphs will end up getting copied to an A8 surface.

It seems like we have three options:

 1) Identify bitmap strike sizes and draw all glyphs as A1 surfaces.
 2) Draw bitmap glyphs as bitmaps and other glyphs as A8 surfaces.
 3) Identify whether a bitmap strike is complete and if not, create
    all glyphs as A8 glyphs in case a non-bitmap glyph is requested.

The first would require that we discover bitmap strikes in the FreeType
backend and turn off the antialias flag. I think this is fairly simple
to do.

The second would require splitting the font into two objects; XRender
glyphsets can only be in a single format. This seems like slightly more
work, but certainly not impossible.

The third seems crazy; eliminating all benefit of bitmap strikes for at
least this font.

keith.packard at intel.com
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