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freetype: route FC_MATRIX through cairo for color glyphs

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Color glyphs (COLR/CBDT/SVG) go through cairo on a second FT_Face,
self->face_for_cairo, opened in ensure_cairo_resources. That face
never receives the FT_Set_Transform installed on self->face in
face_from_descriptor (#9990): cairo owns FT_Set_Transform on its own
face and derives it from the font matrix on every render
(_cairo_ft_unscaled_font_set_scale in cairo-ft-font.c). The only
channel that reaches color-glyph rasterization is the cairo font
matrix, not the FT face transform, so FC_MATRIX is silently dropped on
that path. Stock fontconfig rules do not apply FC_MATRIX to color
fonts, so this is a hand-built config edge case.

Factor the two cairo_set_font_size() call sites in set_cairo_font_size
and fit_cairo_glyph into apply_cairo_font_size(), which calls
cairo_set_font_matrix() with size * FC_MATRIX when has_matrix is set,
and falls through to cairo_set_font_size() otherwise. The non-matrix
path is bit-identical to before. For pure shears (xx=1, yy=1)
cairo_scaled_font_glyph_extents reads the post-shear bbox so the
shrink loop bounds the destination correctly; the user-visible effect
is that a sheared color glyph reports a wider bbox and shrinks more
aggressively to fit a cell than its upright sibling. Acceptable for
the hand-built edge case this PR scopes to.

FT_Matrix stores xx,xy,yx,yy in row-major order;
cairo_matrix_init takes xx,yx,xy,yy. Same matrix, transposed argument
order - pinned with a comment because it is easy to flip.

Refs: #9990
This commit is contained in:
Strykar
2026-05-13 19:36:36 +05:30
parent a28abf1573
commit ee937bdd1b
1 changed files with 24 additions and 2 deletions
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26
kitty/freetype.c
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@@ -871,11 +871,33 @@ pt_to_px(double pt, double dpi) {
return ((long)round((pt * (dpi / 72.0))));
}
static void
apply_cairo_font_size(Face *self, unsigned sz_px) {
// The cairo path uses self->face_for_cairo (a second FT_Face opened in
// ensure_cairo_resources), which never receives the FT_Set_Transform set
// on self->face in face_from_descriptor. cairo owns FT_Set_Transform on
// its face and derives it from the font matrix on every render
// (_cairo_ft_unscaled_font_set_scale in cairo-ft-font.c), so the only
// channel that reaches glyph rasterization is the cairo font matrix
// itself. Encode FC_MATRIX there.
// FT_Matrix is xx,xy,yx,yy (row-major); cairo_matrix_init takes
// xx,yx,xy,yy. Same matrix, transposed argument order.
if (!self->has_matrix) { cairo_set_font_size(self->cairo.cr, sz_px); return; }
double s = (double)sz_px;
double xx = self->matrix.xx / 65536.0;
double xy = self->matrix.xy / 65536.0;
double yx = self->matrix.yx / 65536.0;
double yy = self->matrix.yy / 65536.0;
cairo_matrix_t m;
cairo_matrix_init(&m, xx * s, yx * s, xy * s, yy * s, 0, 0);
cairo_set_font_matrix(self->cairo.cr, &m);
}
static void
set_cairo_font_size(Face *self, double size_in_pts) {
unsigned sz_px = pt_to_px(size_in_pts, (self->xdpi + self->ydpi) / 2.0);
if (self->cairo.size_in_px == sz_px) return;
cairo_set_font_size(self->cairo.cr, sz_px);
apply_cairo_font_size(self, sz_px);
self->cairo.size_in_px = sz_px;
}
@@ -885,7 +907,7 @@ fit_cairo_glyph(Face *self, cairo_glyph_t *g, cairo_text_extents_t *bb, cairo_sc
double ratio = MIN(width / bb->width, height / bb->height);
unsigned sz = (unsigned)(ratio * self->cairo.size_in_px);
if (sz >= self->cairo.size_in_px) sz = self->cairo.size_in_px - 2;
cairo_set_font_size(self->cairo.cr, sz);
apply_cairo_font_size(self, sz);
sf = cairo_get_scaled_font(self->cairo.cr);
cairo_scaled_font_glyph_extents(sf, g, 1, bb);
self->cairo.size_in_px = sz;