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LibWeb: Use more precise font metrics when doing inline layout

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We now position inline-level boxes based on ascent and descent metrics
from the font in use. This makes our basic text layouts look a lot more
like those produced by other browsers. :^)

I've tried to match the terminology used by the CSS Inline Layout spec.

This will regress Acid2 a little bit, and probably various other sites,
but on the whole it's the direction we should be heading, so let's go.
This commit is contained in:
Andreas Kling 2022-03-29 22:33:44 +02:00
parent fae8fde913
commit 6a4247bee9
4 changed files with 59 additions and 74 deletions
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77
Userland/Libraries/LibWeb/Layout/LineBuilder.cpp
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@ -146,32 +146,37 @@ void LineBuilder::update_last_line()
break;
}
auto fragment_baseline = [&](auto const& fragment) -> float {
float fragment_baseline = 0;
if (fragment.layout_node().is_text_node()) {
fragment_baseline = fragment.layout_node().font().baseline();
} else {
auto const& box = verify_cast<Layout::Box>(fragment.layout_node());
fragment_baseline = box_baseline(m_formatting_state, box);
}
return fragment_baseline;
};
auto line_box_baseline = [&] {
float line_box_baseline = 0;
for (auto& fragment : line_box.fragments()) {
auto baseline = fragment_baseline(fragment);
if (fragment.height() < m_context.containing_block().line_height())
baseline += (m_context.containing_block().line_height() - fragment.height()) / 2;
// NOTE: For fragments with a <length> vertical-align, shift the fragment baseline down by the length.
auto const& font = fragment.layout_node().font();
auto const line_height = fragment.layout_node().line_height();
auto const font_metrics = font.pixel_metrics();
auto const typographic_height = font_metrics.ascent + font_metrics.descent;
auto const leading = line_height - typographic_height;
auto const half_leading = leading / 2;
// The CSS specification calls this AD (A+D, Ascent + Descent).
float fragment_baseline = 0;
if (fragment.layout_node().is_text_node()) {
fragment_baseline = font_metrics.ascent;
} else {
auto const& box = verify_cast<Layout::Box>(fragment.layout_node());
fragment_baseline = box_baseline(m_formatting_state, box);
}
fragment_baseline += half_leading;
// Remember the baseline used for this fragment. This will be used when painting the fragment.
fragment.set_baseline(fragment_baseline);
// NOTE: For fragments with a <length> vertical-align, shift the line box baseline down by the length.
// This ensures that we make enough vertical space on the line for any manually-aligned fragments.
if (auto length_percentage = fragment.layout_node().computed_values().vertical_align().template get_pointer<CSS::LengthPercentage>(); length_percentage && length_percentage->is_length())
baseline += length_percentage->length().to_px(fragment.layout_node());
fragment_baseline += length_percentage->length().to_px(fragment.layout_node());
// Store the baseline on the fragment. This is used when painting.
fragment.set_baseline(baseline);
line_box_baseline = max(line_box_baseline, baseline);
line_box_baseline = max(line_box_baseline, fragment_baseline);
}
return line_box_baseline;
}();
@ -192,7 +197,7 @@ void LineBuilder::update_last_line()
auto y_value_for_alignment = [&](CSS::VerticalAlign vertical_align) {
switch (vertical_align) {
case CSS::VerticalAlign::Baseline:
return m_current_y + line_box_baseline - fragment_baseline(fragment) + fragment.border_box_top();
return m_current_y + line_box_baseline - fragment.baseline() + fragment.border_box_top();
case CSS::VerticalAlign::Top:
return m_current_y + fragment.border_box_top();
case CSS::VerticalAlign::Middle:
@ -202,7 +207,7 @@ void LineBuilder::update_last_line()
case CSS::VerticalAlign::TextBottom:
case CSS::VerticalAlign::TextTop:
// FIXME: These are all 'baseline'
return m_current_y + line_box_baseline - fragment_baseline(fragment) + fragment.border_box_top();
return m_current_y + line_box_baseline - fragment.baseline() + fragment.border_box_top();
}
VERIFY_NOT_REACHED();
};
@ -217,14 +222,34 @@ void LineBuilder::update_last_line()
}
}
fragment.set_offset({ new_fragment_x, new_fragment_y });
fragment.set_offset({ new_fragment_x, floorf(new_fragment_y) });
uppermost_box_top = min(uppermost_box_top, fragment.top_of_inline_level_box(m_formatting_state));
lowermost_box_bottom = max(lowermost_box_bottom, fragment.bottom_of_inline_level_box(m_formatting_state));
float top_of_inline_box = 0;
float bottom_of_inline_box = 0;
{
// FIXME: Support inline-table elements.
if (fragment.layout_node().is_replaced_box() || fragment.layout_node().is_inline_block()) {
auto const& fragment_box_state = m_formatting_state.get(static_cast<Box const&>(fragment.layout_node()));
top_of_inline_box = fragment.offset().y() - fragment_box_state.margin_box_top();
bottom_of_inline_box = fragment.offset().y() + fragment_box_state.content_height + fragment_box_state.margin_box_bottom();
} else {
auto font_metrics = fragment.layout_node().font().pixel_metrics();
auto typographic_height = font_metrics.ascent + font_metrics.descent;
auto leading = fragment.layout_node().line_height() - typographic_height;
auto half_leading = leading / 2;
top_of_inline_box = fragment.offset().y() + fragment.baseline() - font_metrics.ascent - half_leading;
bottom_of_inline_box = fragment.offset().y() + fragment.baseline() + font_metrics.descent + half_leading;
}
if (auto length_percentage = fragment.layout_node().computed_values().vertical_align().template get_pointer<CSS::LengthPercentage>(); length_percentage && length_percentage->is_length())
bottom_of_inline_box += length_percentage->length().to_px(fragment.layout_node());
}
uppermost_box_top = min(uppermost_box_top, top_of_inline_box);
lowermost_box_bottom = max(lowermost_box_bottom, bottom_of_inline_box);
}
// 3. The line box height is the distance between the uppermost box top and the lowermost box bottom.
line_box.m_height = max(m_context.containing_block().line_height(), lowermost_box_bottom - uppermost_box_top);
line_box.m_height = lowermost_box_bottom - uppermost_box_top;
line_box.m_bottom = m_current_y + line_box.m_height;
line_box.m_baseline = line_box_baseline;