Created attachment 6773 [details] [review]
patch to use specified bitmap strike for glyph availability check

When environmental variable is set as FC_BITMAP_SIZE="12,14,0,16,18",
fc-cache lookups 12pixel strike for first. If the font provide it,
fc-cache checks the glyph availability by glyph data by 12pixel strike.
If the font has no 12pixel strike, fc-cache lookups 14pixel in next.
If the font has no 12/14pixel strikes, fc-cache lookups vector data
in next. If the font has no 12/14pixel strike or vector data, fc-cache
lookups 16pixel strike in next...

Keith, anything to pick from this bug?

we'd need to have a font that doesn't currently work; every bitmap-only TTF I've created has a glyf table, making this patch unnecessary.

Created attachment 12576 [details]
TTF-packed font that I and Qianqian Fang discussed

Behdad, thank you for reminding this to Keith.

This is the bitmap-only TrueType font that I
and Qianqian Fang discussed (and I wrote the
patch for). The file is too large to fit within 1MB,
I divide it into 2 parts.

Created attachment 12577 [details]
TTF-packed font that I and Qianqian Fang discussed

here is part 2.

fontconfig-2.4.91 generates following line into cache:

"wqy-bsong.ttf" 0 "WenQuanYi Bitmap Song,文泉驿点阵宋体
:familylang=en,zh-sg:style=medium:stylelang=en:
fullname=WenQuanYi Bitmap Song-medium:fullnamelang=en:slant=0:weight=100:width=100
:pixelsize=12,13,15,16:spacing=100:foundry=unknown
:antialias=False:index=0:outline=False:scalable=False
:charset=:lang=:fontversion=45875
:capability=otlayout\\:DFLT:fontformat=TrueType:decorative=False"

As you can find, the charset is empty.
That's what I and Qianqian Fang were trying to fix.

Created attachment 16336 [details] [review]
Look for glyphs in a bitmap when the glyf table is missing

This patch checks fonts which have no glyphs to see if they are fixed-size true type fonts. If so, it then selects a single size (nearest 16 pixels) and uses that to measure the font coverage.

Please test and report back if it doesn't work for you.

I haven't heard any response to the proposed patch (which is in master), so I'm closing this bug.

At present I don't have a time to execute comprehensive checking,
but I confirmed that wqy-bsong.ttf (I attached ever) is recognized
as non-empty TTF. Anyway the situation was improved. Thanks.

There are two points I have to consider.

* the patch tries to check bitmap table when FcCharSetCount is zero.
  If a TTF has only a few outline glyphs (e.g. Roman characters) and
  many bitmap glyphs (e.g. CJK ideographs), how fontconfig should
  handle it?

* the patch check the pixel size which is the nearest size to 16pixel.
  I guess 10-14 pixels are popular among CJK bitmap only fonts.
  I wish if the size is configurable in ./configure options.
  

If the font has any outline characters, it will need a glyf table to map them. Until we find a counter example, we'll assume that any sane font will have a complete glyf table if it has any glyf entries.

Also, it only looks at a single bitmap instance; assuming (once again, until we find a counter example) that a font will have all of the same bitmaps at each size, and not subset them at some sizes.

I strongly prefer not providing configuration until we find a place where it is needed, and even then, it would be best to figure out a mechanism which doesn't require that the user "know" about the fonts they are installing.

OK, when I find a TTF described as above (the coverage of glyf
table is smaller than that of bitmap tables), I will file another
entry in bugzilla. Thus I have no objection to close this bug now.

*** Bug 81375 has been marked as a duplicate of this bug. ***