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Metal: compute the underline position from font metrics (F2)

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The underline was drawn at s->ybase + s->underline_position, but
underline_position/underline_thickness are filled in by each backend
(the NS backend computes them in ns_draw_text_decoration) and were left
at 0 here, so the line landed on the baseline and cut through the bottom
of the glyphs.

Add mtl_underline_metrics, mirroring the NS computation: position from
font->underline_position (or descent/2, or the descent line per the
face), thickness from the font (min 1), clamped to stay inside the cell,
and matching a previous underlined run for continuity. Handle single,
double (second line above), and colored underlines; wave falls back to a
straight line at the same position for now.

Verified vs NS: the underline is now a full-width bar just below the
glyph body instead of through it; AE unchanged at ~0.88%.
This commit is contained in:
Andros Fenollosa 2026-06-02 16:37:17 +02:00
parent 2a4fade1fd
commit df385b87fa
1 changed files with 77 additions and 7 deletions
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84
src/mtlterm.m
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@ -1913,6 +1913,57 @@ mtl_draw_glyph_string_box (struct glyph_string *s, MtlFrameData *fd)
[fd fillRect:NSMakeRect (x + w - vth, y, vth, h) color:br]; /* right */
}
/* Compute the underline offset below the baseline and its thickness, mirroring
ns_draw_text_decoration. These depend on font metrics; the glyph string's
underline_position/underline_thickness are otherwise left uninitialized, so
the underline was being drawn at the baseline (offset 0), cutting through the
bottom of the glyphs. Honors the face's descent-line options and uses the
default underline-minimum-offset (1) and x-use-underline-position-properties
(t). */
static void
mtl_underline_metrics (struct glyph_string *s, int *position, int *thickness)
{
/* Match a previous underlined run so a continued underline stays seamless. */
if (s->prev
&& s->prev->face->underline != FACE_UNDERLINE_WAVE
&& s->prev->face->underline >= FACE_UNDERLINE_SINGLE
&& s->prev->underline_thickness > 0
&& (s->prev->face->underline_at_descent_line_p
== s->face->underline_at_descent_line_p)
&& (s->prev->face->underline_pixels_above_descent_line
== s->face->underline_pixels_above_descent_line))
{
*thickness = s->prev->underline_thickness;
*position = s->prev->underline_position;
return;
}
struct font *font = font_for_underline_metrics (s);
int descent = s->y + s->height - s->ybase;
int minimum_offset = 1;
int th = (font && font->underline_thickness > 0) ? font->underline_thickness : 1;
int pos;
if (s->face->underline_at_descent_line_p)
pos = descent - th - s->face->underline_pixels_above_descent_line;
else if (font && font->underline_position >= 0)
pos = font->underline_position;
else if (font)
pos = lround (font->descent / 2.0);
else
pos = minimum_offset;
if (!s->face->underline_pixels_above_descent_line)
pos = max (pos, minimum_offset);
/* Keep the underline inside the cell. */
if (descent <= pos) { pos = descent - 1; th = 1; }
else if (descent < pos + th) th = 1;
*position = pos;
*thickness = th;
}
static void
mtl_draw_glyph_string_impl (struct glyph_string *s)
{
@ -2010,14 +2061,33 @@ mtl_draw_glyph_string_impl (struct glyph_string *s)
pen_x += adv;
}
/* Underline */
if (face && face->underline != FACE_NO_UNDERLINE)
/* Underline. Single/double lines (the common case for buttons and links)
use font-derived position/thickness; wave falls back to a straight line at
the same position for now. */
if (face && face->underline >= FACE_UNDERLINE_SINGLE)
{
int uth = s->underline_thickness > 0 ? s->underline_thickness : 1;
int uybase = s->ybase + s->underline_position;
unsigned long uc = face->underline_defaulted_p
? fg : face->underline_color;
[fd fillRect:NSMakeRect (s->x, uybase, s->width, uth) color:uc];
int position, thickness;
mtl_underline_metrics (s, &position, &thickness);
s->underline_thickness = thickness;
s->underline_position = position;
unsigned long uc = face->underline_defaulted_p ? fg : face->underline_color;
[fd fillRect:NSMakeRect (s->x, s->ybase + position, s->width, thickness)
color:uc];
/* Second line above the first for double underline. */
if (face->underline == FACE_UNDERLINE_DOUBLE_LINE)
{
int p2 = position - thickness - 1;
[fd fillRect:NSMakeRect (s->x, s->ybase + p2, s->width, thickness)
color:uc];
}
}
else if (face && face->underline == FACE_UNDERLINE_WAVE)
{
int position, thickness;
mtl_underline_metrics (s, &position, &thickness);
unsigned long uc = face->underline_defaulted_p ? fg : face->underline_color;
[fd fillRect:NSMakeRect (s->x, s->ybase + position, s->width, thickness)
color:uc];
}
/* Strike-through */