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

[David Herrmann]

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.

Could you tell me the font-name so I can reproduce this?

I changed your commit-message to mention why this fix is correct and
then applied the patch. I hope you don't mind.
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.

Thanks a lot for reporting this!
David

>         pthread_mutex_lock(&face->glyph_lock);
>         ret = shl_hashtable_insert(face->glyphs, (void*)(long)id, glyph);
> --
> 1.8.1.5
>
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