bug#54646: 29.0.50; set-fontset-font and font clipping issues

[Visuwesh]

[Friday April 01, 2022] Eli Zaretskii wrote:

>> From: Visuwesh <visuweshm@gmail.com>
>> Cc: Robert Pluim <rpluim@gmail.com>,  54646@debbugs.gnu.org
>> Date: Fri, 01 Apr 2022 20:28:06 +0530
>> 
>> >   hb_font_t *hb_font
>> >     = font->driver->begin_hb_font
>> >     ? font->driver->begin_hb_font (font, &position_unit)
>> >     : NULL;
>> >
>> > The value of position_unit then affects the values returned in the
>> > Lisp glyph object used to display the grapheme cluster:
>> >
>> >       xoff = lround (pos[i].x_offset * position_unit);
>> >       yoff = - lround (pos[i].y_offset * position_unit);
>> >       wadjust = lround (pos[i].x_advance * position_unit);
>> >       if (xoff || yoff || wadjust != metrics.width)
>> >    LGLYPH_SET_ADJUSTMENT (lglyph, CALLN (Fvector,
>> >                                          make_fixnum (xoff),
>> >                                          make_fixnum (yoff),
>> >                                          make_fixnum (wadjust)));
>> >
>> > I'd be interested in what happens there in the "good" vs the "bad"
>> > cases.
>> >
>> > If we pass the same information to HarfBuzz, and it returns different
>> > results, then it's probably a problem in HarfBuzz.
>> 
>> I get the same value for position_unit just after begin_hb_font call and
>> just after setting the value of wadjust, in the bad and the good case:
>> 0.015625.  In case I was not clear, here's a patch that shows where I
>> added the printf calls
>> 
>> diff --git a/src/hbfont.c b/src/hbfont.c
>> index 2721a66120..887e0c0e86 100644
>> --- a/src/hbfont.c
>> +++ b/src/hbfont.c
>> @@ -490,6 +490,7 @@ hbfont_shape (Lisp_Object lgstring, Lisp_Object 
>> direction)
>>      : NULL;
>>    if (!hb_font)
>>      return make_fixnum (0);
>> +  printf("position_unit begin_hb_font: %f\n", position_unit);
>>  
>>    hb_bool_t success = hb_shape_full (hb_font, hb_buffer, NULL, 0, NULL);
>>    if (font->driver->end_hb_font)
>> @@ -593,6 +594,7 @@ hbfont_shape (Lisp_Object lgstring, Lisp_Object 
>> direction)
>>        xoff = lround (pos[i].x_offset * position_unit);
>>        yoff = - lround (pos[i].y_offset * position_unit);
>>        wadjust = lround (pos[i].x_advance * position_unit);
>> +      printf("position_unit after lround: %f\n", position_unit);
>>        if (xoff || yoff || wadjust != metrics.width)
>>      LGLYPH_SET_ADJUSTMENT (lglyph, CALLN (Fvector,
>>                                            make_fixnum (xoff),
>> 
>> So I see "position_unit begin_hb_font: 0.0015625" and "position_unit
>> after lround: 0.0015625" in the good and the bad case.
>
> So we pass the same data to HarfBuzz and get back different results in
> xoff, yoff, and wadjust?
>
> IOW, the results of shaping are different in the two cases, although
> the inputs are identical?  Can you print the other values involved in
> the data that gets put into lglyph, and see whether any of it is
> different between the two cases?
>
> the lglyph data is shown in this excerpt from the code:
>
>       LGLYPH_SET_CHAR (lglyph, chars[char_idx]);
>       LGLYPH_SET_CODE (lglyph, info[i].codepoint);
>
>       unsigned code = info[i].codepoint;
>       font->driver->text_extents (font, &code, 1, &metrics);
>       LGLYPH_SET_WIDTH (lglyph, metrics.width);
>       LGLYPH_SET_LBEARING (lglyph, metrics.lbearing);
>       LGLYPH_SET_RBEARING (lglyph, metrics.rbearing);
>       LGLYPH_SET_ASCENT (lglyph, metrics.ascent);
>       LGLYPH_SET_DESCENT (lglyph, metrics.descent);
>
>       xoff = lround (pos[i].x_offset * position_unit);
>       yoff = - lround (pos[i].y_offset * position_unit);
>       wadjust = lround (pos[i].x_advance * position_unit);
>       if (xoff || yoff || wadjust != metrics.width)
>       LGLYPH_SET_ADJUSTMENT (lglyph, CALLN (Fvector,
>                                             make_fixnum (xoff),
>                                             make_fixnum (yoff),
>                                             make_fixnum (wadjust)));
>
> WHat is different between the two cases in this data?  Does the call
> to font->driver->text_extents produce different data in 'metrics',
> perhaps?  Do the values in pos[i] structure differ?  Something else?

TBH, I'm not even sure if I am comparing the data for the same set of
characters but AFAICT, the values don't seem to differ.  Is there a way
to print the concerned character so I can make better comparisons?

I don't think it is of any help but I attached two text files: bad-case
and good-case.  bad-case has all the data for the clipped text, and
good-case for the non-clipped text (for the same font size, at least I
hope so...).

bad-case
Description: Bad case

good-case
Description: Good case