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LibGfx+LibWeb: Support per-glyph font fallbacks in canvas text painting

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Instead of using the first font from the FontCascadeList for all glyphs
in a text, we perform a text shaping process that finds a suitable font
for each glyph and returns a list of glyph runs, where each glyph run
represents consecutive glyphs using the same font.
This commit is contained in:
Aliaksandr Kalenik 2025-04-19 21:36:26 +02:00 • committed by Andreas Kling
commit f6b0851c38
Notes: github-actions[bot] 2025-04-21 07:52:18 +00:00
3 changed files with 49 additions and 4 deletions
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40
Libraries/LibGfx/TextLayout.cpp
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@ -1,6 +1,7 @@
/*
* Copyright (c) 2018-2020, Andreas Kling <andreas@ladybird.org>
* Copyright (c) 2021, sin-ack <sin-ack@protonmail.com>
* Copyright (c) 2024-2025, Aliaksandr Kalenik <kalenik.aliaksandr@gmail.com>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
@ -12,6 +13,45 @@
namespace Gfx {
Vector<NonnullRefPtr<GlyphRun>> shape_text(FloatPoint baseline_start, Utf8View string, FontCascadeList const& font_cascade_list)
{
if (string.length() == 0)
return {};
Vector<NonnullRefPtr<GlyphRun>> runs;
auto it = string.begin();
auto substring_begin_offset = string.iterator_offset(it);
Font const* last_font = &font_cascade_list.font_for_code_point(*it);
FloatPoint last_position = baseline_start;
auto add_run = [&runs, &last_position](Utf8View string, Font const& font) {
auto run = shape_text(last_position, 0, string, font, GlyphRun::TextType::Common, {});
last_position.translate_by(run->width(), 0);
runs.append(*run);
};
while (it != string.end()) {
auto code_point = *it;
auto const* font = &font_cascade_list.font_for_code_point(code_point);
if (font != last_font) {
auto substring = string.substring_view(substring_begin_offset, string.iterator_offset(it) - substring_begin_offset);
add_run(substring, *last_font);
last_font = font;
substring_begin_offset = string.iterator_offset(it);
}
++it;
}
auto end_offset = string.iterator_offset(it);
if (substring_begin_offset < end_offset) {
auto substring = string.substring_view(substring_begin_offset, end_offset - substring_begin_offset);
add_run(substring, *last_font);
}
return runs;
}
RefPtr<GlyphRun> shape_text(FloatPoint baseline_start, float letter_spacing, Utf8View string, Gfx::Font const& font, GlyphRun::TextType text_type, ShapeFeatures const& features)
{
static hb_buffer_t* buffer = hb_buffer_create();