[kmscon-devel] [PATCH] font: pango: fix a rendering problem of certain CJK fonts

[David Herrmann]

Hi

On Sun, Mar 10, 2013 at 1:59 PM, Chang Liu <cl91tp at gmail.com> wrote:
> Hi David
>
> 2013/3/9 David Herrmann <dh.herrmann at gmail.com>
>>
>> Hi
>>
>> On Sat, Mar 9, 2013 at 8:41 AM, Chang Liu <cl91tp at gmail.com> wrote:
>> > This patch fixes a rendering problem of certain CJK fonts which causes
>> > the rendered CJK characters to appear downward-shifted.
>> >
>> > For some CJK fonts, pango_layout_get_pixel_extent() returns different
>> > logical_rect for CJK glyphs and Latin glyphs. Therefore, using -rec.y
>> > as the y position of the baseline will render the CJK glyphs downward-
>> > shifted. face.baseline will be a better choice.
>> >
>> > ---
>> >  src/font_pango.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/src/font_pango.c b/src/font_pango.c
>> > index c2954c6..0964123 100644
>> > --- a/src/font_pango.c
>> > +++ b/src/font_pango.c
>> > @@ -199,7 +199,7 @@ static int get_glyph(struct face *face, struct
>> > kmscon_glyph **out,
>> >         bitmap.pixel_mode = FT_PIXEL_MODE_GRAY;
>> >         bitmap.buffer = glyph->buf.data;
>> >
>> > -       pango_ft2_render_layout_line(&bitmap, line, -rec.x, -rec.y);
>> > +       pango_ft2_render_layout_line(&bitmap, line, -rec.x,
>> > face->baseline);
>>
>> Ok, lets summarize how this is supposed to work (as I myself don't
>> remember it correctly):
>> We calculate the width/height of a cell by drawing the basic ASCII
>> characters in a cache and measuring them. Every other glyph is then
>> supposed to fit into multiples of these cells (multiples only in
>> horizontal direction).
>> "face->baseline" is set according to the baseline of this measurement
>> (that is, the ASCII characters).
>>
>> So what this basically does is calculating a bounding box for our
>> glyphs and computing the logical origin. Every glyph that we draw
>> should be drawn at the logical origin (which is face->baseline) and
>> clipped according to this box.
>> So we do have to use face->baseline here, indeed. Using -rec.y aligns
>> to the bottom edge instead, which is obviously wrong.
>
> Yes, my thoughts exactly.
>
>> Could you tell me the font-name so I can reproduce this?
>
> For Latin languages I am using  Lucida Console and for Chinese I am using
> Microsoft YaHei. Both of them are default fonts in MS Win 8 which I
> illegally ^_^
> downloaded from the Internet. Not sure if you could do this in Europe but I
> guess
> you might need a valid license of Win 8 before you could legally use these
> fonts.
> However these fonts are available in Win 7 too and I don't think they
> underwent
> substantial changes since then. Since most laptop came with a pre-installed
> Win 7
> I think you can legally copy these fonts from a Win 7 installation and use
> them
> for testing. They are located in C:\Windows\Fonts and named lucida.ttf and
> msyh{,bd}.ttc.
>
> There are also a bunch of CJK fonts in MS Win 7 which I tested. Look for
> .ttf or .ttc
> files which are larger than 1 MB, they are most certainly CJK fonts. They
> are rendered
> correctly after the patch is applied.

Thanks a lot. I will try to get some of these fonts and test it.

>>
>>
>> I changed your commit-message to mention why this fix is correct and
>> then applied the patch. I hope you don't mind.
>
> No, not at all.
>>
>> Commit: e874518153e509b74aa8e3e52b02642718eb5575
>
>
>> There's also ./docs/unicode-test.txt. Could you check whether there
>> are symbols in it that show this wrong behavior? If not, could you
>> tell me what symbols you used in your screenshots so I can add them.
>>
> Yes. The 4th line (empty line included), the line with a `Unicode' in it, is
> Chinese
> for `What is Unicode?'. All the Chinese characters in that line show this
> behavior.

Ouh, nice to hear that these lines don't contain any insulting text. I
copied it from some websites and had no idea what it meant ;)

Thanks a lot for the help!
David