Fix 'text-underline-position: under' to use em height ascent/descent

third_party/WebKit/Source/platform/fonts/SimpleFontData.cpp

 /*
  * Copyright (C) 2005, 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 21 matching lines @@
 #include <unicode/unorm.h>
 #include <unicode/utf16.h>
 #include <memory>
 #include "SkPath.h"
 #include "SkTypeface.h"
 #include "SkTypes.h"
 #include "platform/fonts/FontDescription.h"
 #include "platform/fonts/VDMXParser.h"
 #include "platform/fonts/skia/SkiaTextMetrics.h"
 #include "platform/geometry/FloatRect.h"
+#include "platform/wtf/ByteOrder.h"
 #include "platform/wtf/MathExtras.h"
 #include "platform/wtf/PtrUtil.h"
 #include "platform/wtf/allocator/Partitions.h"
 #include "platform/wtf/text/CharacterNames.h"
 #include "platform/wtf/text/Unicode.h"
 
 namespace blink {
 
 const float kSmallCapsFontSizeMultiplier = 0.7f;
 const float kEmphasisMarkFontSizeMultiplier = 0.5f;
@@ skipping 320 matching lines @@
 PassRefPtr<SimpleFontData> SimpleFontData::CreateScaledFontData(
     const FontDescription& font_description,
     float scale_factor) const {
   const float scaled_size =
       lroundf(font_description.ComputedSize() * scale_factor);
   return SimpleFontData::Create(
       FontPlatformData(platform_data_, scaled_size),
       IsCustomFont() ? CustomFontData::Create() : nullptr);
 }
 
+// Internal leadings can be distributed to ascent and descent.
+// -------------------------------------------
+//           | - Internal Leading (in ascent)
+//           |--------------------------------
+//  Ascent - |              |
+//           |              |
+//           |              | - Em height
+// ----------|--------------|
+//           |              |
+// Descent - |--------------------------------
+//           | - Internal Leading (in descent)
+// -------------------------------------------
+LayoutUnit SimpleFontData::EmHeightAscent(FontBaseline baseline_type) const {
+  if (baseline_type == kAlphabeticBaseline) {
+    if (!em_height_ascent_)
+      ComputeEmHeightMetrics();
+    return em_height_ascent_;
+  }
+  LayoutUnit em_height = LayoutUnit::FromFloatRound(PlatformData().size());
+  return em_height - em_height / 2;
+}
+
+LayoutUnit SimpleFontData::EmHeightDescent(FontBaseline baseline_type) const {
+  if (baseline_type == kAlphabeticBaseline) {
+    if (!em_height_descent_)
+      ComputeEmHeightMetrics();
+    return em_height_descent_;
+  }
+  LayoutUnit em_height = LayoutUnit::FromFloatRound(PlatformData().size());
+  return em_height / 2;
+}
+
+static std::pair<int16_t, int16_t> TypoAscenderAndDescender(
+    SkTypeface* typeface) {
+  // TODO(kojii): This should move to Skia once finalized. We can then move

[bungeman-chromium, 2017/05/03 13:56:37]

What is the reason for not using SkPaint::getFontMetrics and using the fAscent /
fDescent from that? Those are almost always these values anyway, and when
they're not they're some other value the font specified for this purpose. Isn't
that information already available? It appears you're already 'normalizing'
these anyway, so is there a reason that only and exactly the 'typo' versions are
valid?

+  // EmHeightAscender/Descender to FontMetrics.
+  int16_t buffer[2];
+  size_t size = typeface->getTableData(SkSetFourByteTag('O', 'S', '/', '2'), 68,

[drott, 2017/05/08 08:04:00]

What's 68 here? Could you make a constant for this?

[kojii, 2017/05/09 04:37:16]

It's the offset. Skia has access to os2table so they don't need it but we need
to hard code until this is moved to Skia. Should have commented, thank you
pointing out.

+                                       sizeof(buffer), buffer);
+  if (size == sizeof(buffer)) {
+    return std::make_pair(static_cast<int16_t>(ntohs(buffer[0])),
+                          -static_cast<int16_t>(ntohs(buffer[1])));
+  }
+  return std::make_pair(0, 0);
+}
+
+void SimpleFontData::ComputeEmHeightMetrics() const {
+  // Compute em height metrics from OS/2 sTypoAscender and sTypoDescender.
+  SkTypeface* typeface = platform_data_.Typeface();
+  int16_t typo_ascender, typo_descender;
+  std::tie(typo_ascender, typo_descender) = TypoAscenderAndDescender(typeface);
+  if (typo_ascender > 0 &&
+      NormalizeEmHeightMetrics(typo_ascender, typo_ascender + typo_descender)) {
+    return;
+  }
+
+  // As the last resort, compute em height metrics from our ascent/descent.
+  const FontMetrics& font_metrics = GetFontMetrics();
+  if (NormalizeEmHeightMetrics(font_metrics.FloatAscent(),
+                               font_metrics.FloatHeight())) {
+    return;
+  }
+  NOTREACHED();
+}
+
+bool SimpleFontData::NormalizeEmHeightMetrics(float ascent,
+                                              float height) const {
+  if (height <= 0 || ascent < 0 || ascent > height)
+    return false;
+  // While the OpenType specification recommends the sum of sTypoAscender and
+  // sTypoDescender to equal 1em, most fonts do not follow. Most Latin fonts
+  // set to smaller than 1em, and many tall scripts set to larger than 1em.
+  // https://www.microsoft.com/typography/otspec/recom.htm#OS2
+  // To ensure the sum of ascent and descent is the "em height", normalize by
+  // keeping the ratio of sTypoAscender:sTypoDescender.
+  // This matches to how Gecko computes "em height":

[drott, 2017/05/08 08:04:00]

Would be great if you had a Mozilla DXR link to where it happens in Gecko.

[kojii, 2017/05/09 04:37:16]

Yeah, not great at reading Mozilla code yet...

+  // https://github.com/whatwg/html/issues/2470#issuecomment-291425136
+  float em_height = PlatformData().size();
+  em_height_ascent_ = LayoutUnit::FromFloatRound(ascent * em_height / height);
+  em_height_descent_ =
+      LayoutUnit::FromFloatRound(em_height) - em_height_ascent_;
+  return true;
+}
+
 FloatRect SimpleFontData::PlatformBoundsForGlyph(Glyph glyph) const {
   if (!platform_data_.size())
     return FloatRect();
 
   static_assert(sizeof(glyph) == 2, "Glyph id should not be truncated.");
 
   SkRect bounds;
   SkiaTextMetrics(&paint_).GetSkiaBoundsForGlyph(glyph, &bounds);
   return FloatRect(bounds);
 }
 
 float SimpleFontData::PlatformWidthForGlyph(Glyph glyph) const {
   if (!platform_data_.size())
     return 0;
 
   static_assert(sizeof(glyph) == 2, "Glyph id should not be truncated.");
 
   return SkiaTextMetrics(&paint_).GetSkiaWidthForGlyph(glyph);
 }
 
 }  // namespace blink