Wrong value from hb_ot_layout_get_baseline_with_fallback() w/ MS Gothic font

[lb90]

Hi! It seems that hb_ot_layout_get_baseline_with_fallback() is returning wrong baseline values for the MS Gothic font (C:\Windows\Fonts\msgothic.ttc). We discovered that while investigating an issue with text layout in Pango, see https://discourse.gnome.org/t/when-combining-latin-with-asian-the-latin-glyphs-are-shifted-upwards-on-windows/16010/2.

Consider the following sample:

#include <stdlib.h>
#include <stddef.h>
#include <stdio.h>
#include <assert.h>

#include <hb.h>
#include <hb-ot.h>

#define BASE_DIR \
"C:/Windows/Fonts"

#define FONT_PATH(filename) \
(BASE_DIR "/" filename)

#define NUM_ELEMENTS(arr) \
(sizeof (arr) / sizeof (arr[0]))

const int SCALE = 144 * 1024;
const float PTEM = 108.0f;

static void
print_baseline (const char *path,
                unsigned int index)
{
  hb_blob_t *blob = hb_blob_create_from_file (path);
  hb_face_t *face = hb_face_create(blob, index);
  hb_font_t *font = hb_font_create(face);
  hb_position_t baseline = 0;

  hb_font_set_scale (font, SCALE, SCALE);
  hb_font_set_ptem (font, PTEM);

  hb_ot_layout_get_baseline_with_fallback (font,
                                           HB_OT_LAYOUT_BASELINE_TAG_IDEO_FACE_BOTTOM_OR_LEFT,
                                           HB_DIRECTION_LTR,
                                           HB_TAG ('h','a','n','i'),
                                           0,
                                           &baseline);

  printf ("* %s (index %u)\n", path, index);
  printf ("  baseline BOTTOM_OR_LEFT: %d\n", (int) baseline);

  hb_blob_destroy (blob);
  hb_face_destroy (face);
  hb_font_destroy (font);
}

int main()
{
  struct {
    const char *path;
    unsigned int index;
  } tests[] = {
    {FONT_PATH ("msgothic.ttc"), 0},
    {FONT_PATH ("YuGothM.ttc"), 0},
    {FONT_PATH ("YuGothM.ttc"), 1},
    {FONT_PATH ("BIZ-UDGothicB.ttc"), 0},
    {FONT_PATH ("BIZ-UDGothicB.ttc"), 1},
    {FONT_PATH ("GOTHIC.TTF"), 0}
  };

  for (size_t i = 0; i < NUM_ELEMENTS (tests); i++)
    print_baseline (tests[i].path, tests[i].index);
}

Note: 📝 the code can run on Linux, just mount a Windows partition and change BASE_DIR accordingly UPDATE: nope, you really need Windows: https://bugzilla.redhat.com/show_bug.cgi?id=1377049. Try running stat /mnt/windows/Windows/Fonts/msgothic.ttc and see if you get unsupported reparse tag 0x80000017

We get the following values:

$ ./test-hb-get-baseline.exe
* C:/Windows/Fonts/msgothic.ttc (index 0)
  baseline BOTTOM_OR_LEFT: 14745
* C:/Windows/Fonts/YuGothM.ttc (index 0)
  baseline BOTTOM_OR_LEFT: -10296
* C:/Windows/Fonts/YuGothM.ttc (index 1)
  baseline BOTTOM_OR_LEFT: -10296
* C:/Windows/Fonts/BIZ-UDGothicB.ttc (index 0)
  baseline BOTTOM_OR_LEFT: -2967
* C:/Windows/Fonts/BIZ-UDGothicB.ttc (index 1)
  baseline BOTTOM_OR_LEFT: -2967
* C:/Windows/Fonts/GOTHIC.TTF (index 0)
  baseline BOTTOM_OR_LEFT: -14393

The first value doesn't seem right : 14745.

Interestingly, if we pass 'Hani' instead of 'hani', then the issue doesn't occur anymore:

$ ./test-hb-get-baseline.exe
* C:/Windows/Fonts/msgothic.ttc (index 0)
  baseline BOTTOM_OR_LEFT: -5991
* C:/Windows/Fonts/YuGothM.ttc (index 0)
  baseline BOTTOM_OR_LEFT: -10296
* C:/Windows/Fonts/YuGothM.ttc (index 1)
  baseline BOTTOM_OR_LEFT: -10296
* C:/Windows/Fonts/BIZ-UDGothicB.ttc (index 0)
  baseline BOTTOM_OR_LEFT: -2967
* C:/Windows/Fonts/BIZ-UDGothicB.ttc (index 1)
  baseline BOTTOM_OR_LEFT: -2967
* C:/Windows/Fonts/GOTHIC.TTF (index 0)
  baseline BOTTOM_OR_LEFT: -14393

However Pango relies on hb_ot_tags_from_script_and_language() to get the language and script tags, and that function returns 'hani'.

[behdad]

Humm. This seems to be a bad baseline value in the font. For script hani and baseline tag ideo it has value 0, which is wrong in my understanding. Other fonts I tried had negative values there.

Short of adding a conditional to filter out zero in the code, I don't see a way to fix this.

cc @matthiasclasen

[behdad]

BTW, you can get the same info using hb-info utility.

[behdad]

I'm actually now confused and not sure anymore, how the baseline numbers are to be used across fonts. See this section:
https://learn.microsoft.com/en-us/typography/opentype/spec/base#example-3b-baseline-values-for-four-scripts