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Explain vertical font metrics and refactor UpdateLineMetrics. (flutter/engine#11795)

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This commit is contained in:
Gary Qian 2019-10-02 18:08:48 -04:00 committed by GitHub
parent b904f5c1fd
commit c2b238781e
2 changed files with 89 additions and 33 deletions
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116
engine/src/flutter/third_party/txt/src/txt/paragraph_txt.cc vendored
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@ -1108,39 +1108,10 @@ void ParagraphTxt::Layout(double width) {
double max_ascent = strut_.ascent + strut_.half_leading;
double max_descent = strut_.descent + strut_.half_leading;
double max_unscaled_ascent = 0;
auto update_line_metrics = [&](const SkFontMetrics& metrics,
const TextStyle& style,
PlaceholderRun* placeholder_run) {
if (!strut_.force_strut) {
double ascent;
double descent;
if (style.has_height_override) {
// Scale the ascent and descent such that the sum of ascent and
// descent is `fontsize * style.height * style.font_size`.
double metrics_height = -metrics.fAscent + metrics.fDescent;
ascent = (-metrics.fAscent / metrics_height) * style.height *
style.font_size;
descent = (metrics.fDescent / metrics_height) * style.height *
style.font_size;
} else {
// Use the font-provided ascent, descent, and leading directly.
ascent = (-metrics.fAscent + metrics.fLeading / 2);
descent = (metrics.fDescent + metrics.fLeading / 2);
}
ComputePlaceholder(placeholder_run, ascent, descent);
max_ascent = std::max(ascent, max_ascent);
max_descent = std::max(descent, max_descent);
}
max_unscaled_ascent = std::max(placeholder_run == nullptr
? -metrics.fAscent
: placeholder_run->baseline_offset,
max_unscaled_ascent);
};
for (const PaintRecord& paint_record : paint_records) {
update_line_metrics(paint_record.metrics(), paint_record.style(),
paint_record.GetPlaceholderRun());
UpdateLineMetrics(paint_record.metrics(), paint_record.style(),
max_ascent, max_descent, max_unscaled_ascent,
paint_record.GetPlaceholderRun());
}
// If no fonts were actually rendered, then compute a baseline based on the
@ -1151,7 +1122,8 @@ void ParagraphTxt::Layout(double width) {
font.setTypeface(GetDefaultSkiaTypeface(style));
font.setSize(style.font_size);
font.getMetrics(&metrics);
update_line_metrics(metrics, style, nullptr);
UpdateLineMetrics(metrics, style, max_ascent, max_descent,
max_unscaled_ascent, nullptr);
}
// Calculate the baselines. This is only done on the first line.
@ -1202,6 +1174,84 @@ void ParagraphTxt::Layout(double width) {
longest_line_ = max_right_ - min_left_;
}
void ParagraphTxt::UpdateLineMetrics(const SkFontMetrics& metrics,
const TextStyle& style,
double& max_ascent,
double& max_descent,
double& max_unscaled_ascent,
PlaceholderRun* placeholder_run) {
if (!strut_.force_strut) {
double ascent;
double descent;
if (style.has_height_override) {
// Scale the ascent and descent such that the sum of ascent and
// descent is `fontsize * style.height * style.font_size`.
//
// The raw metrics do not add up to fontSize. The state of font
// metrics is a mess:
//
// Each font has 4 sets of vertical metrics:
//
// * hhea: hheaAscender, hheaDescender, hheaLineGap.
// Used by Apple.
// * OS/2 typo: typoAscender, typoDescender, typoLineGap.
// Used sometimes by Windows for layout.
// * OS/2 win: winAscent, winDescent.
// Also used by Windows, generally will be cut if extends past
// these metrics.
// * EM Square: ascent, descent
// Not actively used, but this defines the 'scale' of the
// units used.
//
// `Use Typo Metrics` is a boolean that, when enabled, prefers
// typo metrics over win metrics. Default is off. Enabled by most
// modern fonts.
//
// In addition to these different sets of metrics, there are also
// multiple strategies for using these metrics:
//
// * Adobe: Set hhea values to typo equivalents.
// * Microsoft: Set hhea values to win equivalents.
// * Web: Use hhea values for text, regardless of `Use Typo Metrics`
// The hheaLineGap is distributed half across the top and half
// across the bottom of the line.
// Exceptions:
// Windows: All browsers respect `Use Typo Metrics`
// Firefox respects `Use Typo Metrics`.
//
// This pertains to this code in that it is ambiguous which set of
// metrics we are actually using via SkFontMetrics. This in turn
// means that if we use the raw metrics, we will see differences
// between platforms as well as unpredictable line heights.
//
// A more thorough explanation is available at
// https://glyphsapp.com/tutorials/vertical-metrics
//
// Doing this ascent/descent normalization to the EM Square allows
// a sane, consistent, and reasonable line height to be specified,
// though it breaks with what is done by any of the platforms above.
double metrics_height = -metrics.fAscent + metrics.fDescent;
ascent =
(-metrics.fAscent / metrics_height) * style.height * style.font_size;
descent =
(metrics.fDescent / metrics_height) * style.height * style.font_size;
} else {
// Use the font-provided ascent, descent, and leading directly.
ascent = (-metrics.fAscent + metrics.fLeading / 2);
descent = (metrics.fDescent + metrics.fLeading / 2);
}
ComputePlaceholder(placeholder_run, ascent, descent);
max_ascent = std::max(ascent, max_ascent);
max_descent = std::max(descent, max_descent);
}
max_unscaled_ascent =
std::max(placeholder_run == nullptr ? -metrics.fAscent
: placeholder_run->baseline_offset,
max_unscaled_ascent);
};
double ParagraphTxt::GetLineXOffset(double line_total_advance,
size_t line_number,
bool justify_line) {

6
engine/src/flutter/third_party/txt/src/txt/paragraph_txt.h vendored
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@ -359,6 +359,12 @@ class ParagraphTxt : public Paragraph {
bool IsStrutValid() const;
void UpdateLineMetrics(const SkFontMetrics& metrics,
const TextStyle& style,
double& max_ascent,
double& max_descent,
double& max_unscaled_ascent,
PlaceholderRun* placeholder_run);
// Calculate the starting X offset of a line based on the line's width and
// alignment.
double GetLineXOffset(double line_total_advance,