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Chrome RenderText分析(2)

2013-12-05 11:40 by Clingingboy, ... 阅读, ... 评论, 收藏, 编辑

 

Chrome RenderText分析(1)

继续分析以下步骤

image

 

一.TextRun结构

struct TextRun {
  TextRun();
  ~TextRun();

  ui::Range range;
  Font font;
  // A gfx::Font::FontStyle flag to specify bold and italic styles.
  // Supersedes |font.GetFontStyle()|. Stored separately to avoid calling
  // |font.DeriveFont()|, which is expensive on Windows.
  int font_style;

  // TODO(msw): Disambiguate color/style from TextRuns for proper glyph shaping.
  //            See an example: http://www.catch22.net/tuts/uniscribe-mysteries
  SkColor foreground;
  bool strike;
  bool diagonal_strike;
  bool underline;

  int width;
  // The cumulative widths of preceding runs.
  int preceding_run_widths;

  SCRIPT_ANALYSIS script_analysis;

  scoped_ptr<WORD[]> glyphs;
  scoped_ptr<WORD[]> logical_clusters;
  scoped_ptr<SCRIPT_VISATTR[]> visible_attributes;
  int glyph_count;

  scoped_ptr<int[]> advance_widths;
  scoped_ptr<GOFFSET[]> offsets;
  ABC abc_widths;
  SCRIPT_CACHE script_cache;

 private:
  DISALLOW_COPY_AND_ASSIGN(TextRun);
};

TextRun可以理解为一个输出结果

  1. ScriptItemize输出script_analysis
  2. ScriptShape输出glyphs,logical_clusters,visible_attributes,glyph_count
  3. ScriptPlace输出advance_widths,offsets,abc_widths(真正想要的宽度结果)

二.ScriptShape

  1. 根据文字的长度初始化相关的缓冲区
  2. 选择文字字体
  3. 调用ScriptShape来填充TextRun
  4. 如果调用ScriptShape失败的话则使用SCRIPT_FONTPROPERTIES的默认值来填充
void RenderTextWin::LayoutTextRun(internal::TextRun* run) {
  const size_t run_length = run->range.length();
  const wchar_t* run_text = &(GetLayoutText()[run->range.start()]);
  Font original_font = run->font;
  LinkedFontsIterator fonts(original_font);
  bool tried_cached_font = false;
  bool tried_fallback = false;
  // Keep track of the font that is able to display the greatest number of
  // characters for which ScriptShape() returned S_OK. This font will be used
  // in the case where no font is able to display the entire run.
  int best_partial_font_missing_char_count = INT_MAX;
  Font best_partial_font = original_font;
  bool using_best_partial_font = false;
  Font current_font;

  run->logical_clusters.reset(new WORD[run_length]);
  while (fonts.NextFont(&current_font)) {
    HRESULT hr = ShapeTextRunWithFont(run, current_font);

    bool glyphs_missing = false;
    if (hr == USP_E_SCRIPT_NOT_IN_FONT) {
      glyphs_missing = true;
    } else if (hr == S_OK) {
      // If |hr| is S_OK, there could still be missing glyphs in the output.
      // http://msdn.microsoft.com/en-us/library/windows/desktop/dd368564.aspx
      const int missing_count = CountCharsWithMissingGlyphs(run);
      // Track the font that produced the least missing glyphs.
      if (missing_count < best_partial_font_missing_char_count) {
        best_partial_font_missing_char_count = missing_count;
        best_partial_font = run->font;
      }
      glyphs_missing = (missing_count != 0);
    } else {
      NOTREACHED() << hr;
    }

    // Use the font if it had glyphs for all characters.
    if (!glyphs_missing) {
      // Save the successful fallback font that was chosen.
      if (tried_fallback)
        successful_substitute_fonts_[original_font.GetFontName()] = run->font;
      return;
    }

    // First, try the cached font from previous runs, if any.
    if (!tried_cached_font) {
      tried_cached_font = true;

      std::map<std::string, Font>::const_iterator it =
          successful_substitute_fonts_.find(original_font.GetFontName());
      if (it != successful_substitute_fonts_.end()) {
        fonts.SetNextFont(it->second);
        continue;
      }
    }

    // If there are missing glyphs, first try finding a fallback font using a
    // meta file, if it hasn't yet been attempted for this run.
    // TODO(msw|asvitkine): Support RenderText's font_list()?
    if (!tried_fallback) {
      tried_fallback = true;

      Font fallback_font;
      if (ChooseFallbackFont(cached_hdc_, run->font, run_text, run_length,
                             &fallback_font)) {
        fonts.SetNextFont(fallback_font);
        continue;
      }
    }
  }

  // If a font was able to partially display the run, use that now.
  if (best_partial_font_missing_char_count < static_cast<int>(run_length)) {
    // Re-shape the run only if |best_partial_font| differs from the last font.
    if (best_partial_font.GetNativeFont() != run->font.GetNativeFont())
      ShapeTextRunWithFont(run, best_partial_font);
    return;
  }

  // If no font was able to partially display the run, replace all glyphs
  // with |wgDefault| from the original font to ensure to they don't hold
  // garbage values.
  // First, clear the cache and select the original font on the HDC.
  ScriptFreeCache(&run->script_cache);
  run->font = original_font;
  SelectObject(cached_hdc_, run->font.GetNativeFont());

  // Now, get the font's properties.
  SCRIPT_FONTPROPERTIES properties;
  memset(&properties, 0, sizeof(properties));
  properties.cBytes = sizeof(properties);
  HRESULT hr = ScriptGetFontProperties(cached_hdc_, &run->script_cache,
                                       &properties);
  if (hr == S_OK) {
    // Finally, initialize |glyph_count|, |glyphs| and |visible_attributes| on
    // the run (since they may not have been set yet).
    run->glyph_count = run_length;
    memset(run->visible_attributes.get(), 0,
           run->glyph_count * sizeof(SCRIPT_VISATTR));
    for (int i = 0; i < run->glyph_count; ++i) {
      run->glyphs[i] = IsWhitespace(run_text[i]) ? properties.wgBlank :
                                                   properties.wgDefault;
    }
  }

  // TODO(msw): Don't use SCRIPT_UNDEFINED. Apparently Uniscribe can
  //            crash on certain surrogate pairs with SCRIPT_UNDEFINED.
  //            See https://bugzilla.mozilla.org/show_bug.cgi?id=341500
  //            And http://maxradi.us/documents/uniscribe/
  run->script_analysis.eScript = SCRIPT_UNDEFINED;
}

HRESULT RenderTextWin::ShapeTextRunWithFont(internal::TextRun* run,
                                            const Font& font) {
  // Update the run's font only if necessary. If the two fonts wrap the same
  // PlatformFontWin object, their native fonts will have the same value.
  if (run->font.GetNativeFont() != font.GetNativeFont()) {
    const int font_size = run->font.GetFontSize();
    const int font_height = run->font.GetHeight();
    run->font = font;
    DeriveFontIfNecessary(font_size, font_height, run->font_style, &run->font);
    ScriptFreeCache(&run->script_cache);
  }

  // Select the font desired for glyph generation.
  SelectObject(cached_hdc_, run->font.GetNativeFont());

  HRESULT hr = E_OUTOFMEMORY;
  const size_t run_length = run->range.length();
  const wchar_t* run_text = &(GetLayoutText()[run->range.start()]);
  // Max glyph guess: http://msdn.microsoft.com/en-us/library/dd368564.aspx
  size_t max_glyphs = static_cast<size_t>(1.5 * run_length + 16);
  while (hr == E_OUTOFMEMORY && max_glyphs < kMaxGlyphs) {
    run->glyph_count = 0;
    run->glyphs.reset(new WORD[max_glyphs]);
    run->visible_attributes.reset(new SCRIPT_VISATTR[max_glyphs]);
    hr = ScriptShape(cached_hdc_,
                     &run->script_cache,
                     run_text,
                     run_length,
                     max_glyphs,
                     &run->script_analysis,
                     run->glyphs.get(),
                     run->logical_clusters.get(),
                     run->visible_attributes.get(),
                     &run->glyph_count);
    max_glyphs *= 2;
  }
  return hr;
}

三.ScriptPlace

run->abc_widths是计算的结果

void RenderTextWin::LayoutVisualText() {
  DCHECK(!runs_.empty());

  if (!cached_hdc_)
    cached_hdc_ = CreateCompatibleDC(NULL);

  HRESULT hr = E_FAIL;
  string_size_.set_height(0);
  for (size_t i = 0; i < runs_.size(); ++i) {
    internal::TextRun* run = runs_[i];
    LayoutTextRun(run);

    string_size_.set_height(std::max(string_size_.height(),
                                     run->font.GetHeight()));
    common_baseline_ = std::max(common_baseline_, run->font.GetBaseline());

    if (run->glyph_count > 0) {
      run->advance_widths.reset(new int[run->glyph_count]);
      run->offsets.reset(new GOFFSET[run->glyph_count]);
      hr = ScriptPlace(cached_hdc_,
                       &run->script_cache,
                       run->glyphs.get(),
                       run->glyph_count,
                       run->visible_attributes.get(),
                       &(run->script_analysis),
                       run->advance_widths.get(),
                       run->offsets.get(),
                       &(run->abc_widths));
      DCHECK(SUCCEEDED(hr));
    }
  }

  // Build the array of bidirectional embedding levels.
  scoped_ptr<BYTE[]> levels(new BYTE[runs_.size()]);
  for (size_t i = 0; i < runs_.size(); ++i)
    levels[i] = runs_[i]->script_analysis.s.uBidiLevel;

  // Get the maps between visual and logical run indices.
  visual_to_logical_.reset(new int[runs_.size()]);
  logical_to_visual_.reset(new int[runs_.size()]);
  hr = ScriptLayout(runs_.size(),
                    levels.get(),
                    visual_to_logical_.get(),
                    logical_to_visual_.get());
  DCHECK(SUCCEEDED(hr));

  // Precalculate run width information.
  size_t preceding_run_widths = 0;
  for (size_t i = 0; i < runs_.size(); ++i) {
    internal::TextRun* run = runs_[visual_to_logical_[i]];
    run->preceding_run_widths = preceding_run_widths;
    const ABC& abc = run->abc_widths;
    run->width = abc.abcA + abc.abcB + abc.abcC;
    preceding_run_widths += run->width;
  }
  string_size_.set_width(preceding_run_widths);
}