float CanvasRenderingContext2D::getFontBaseline(
const FontMetrics& fontMetrics) const {
// If the font is so tiny that the lroundf operations result in two
// different types of text baselines to return the same baseline, use
// floating point metrics (crbug.com/338908).
// If you changed the heuristic here, for consistency please also change it
// in SimpleFontData::platformInit().
bool useFloatAscentDescent =
fontMetrics.ascent() < 3 || fontMetrics.height() < 2;
switch (state().getTextBaseline()) {
case TopTextBaseline:
return useFloatAscentDescent ? fontMetrics.floatAscent()
: fontMetrics.ascent();
case HangingTextBaseline:
// According to
// http://wiki.apache.org/xmlgraphics-fop/LineLayout/AlignmentHandling
// "FOP (Formatting Objects Processor) puts the hanging baseline at 80% of
// the ascender height"
return useFloatAscentDescent ? (fontMetrics.floatAscent() * 4.0) / 5.0
: (fontMetrics.ascent() * 4) / 5;
case BottomTextBaseline:
case IdeographicTextBaseline:
return useFloatAscentDescent ? -fontMetrics.floatDescent()
: -fontMetrics.descent();
case MiddleTextBaseline:
return useFloatAscentDescent
? -fontMetrics.floatDescent() + fontMetrics.floatHeight() / 2.0
: -fontMetrics.descent() + fontMetrics.height() / 2;
case AlphabeticTextBaseline:
default:
// Do nothing.
break;
}
return 0;
}
static inline float positionOffsetForDecoration(ETextDecoration decoration, const FontMetrics& fontMetrics, float thickness)
{
// FIXME: For SVG Fonts we need to use the attributes defined in the <font-face> if specified.
// Compatible with Batik/Opera.
if (decoration == UNDERLINE)
return fontMetrics.ascent() + thickness * 1.5f;
if (decoration == OVERLINE)
return thickness;
if (decoration == LINE_THROUGH)
return fontMetrics.ascent() * 5.0f / 8.0f;
ASSERT_NOT_REACHED();
return 0.0f;
}
int computeUnderlineOffset(TextUnderlinePosition underlinePosition, const FontMetrics& fontMetrics, const InlineTextBox* inlineTextBox, int textDecorationThickness)
{
// This represents the gap between the baseline and the closest edge of the underline.
int gap = std::max<int>(1, ceilf(textDecorationThickness / 2.0));
// FIXME: The code for visual overflow detection passes in a null inline text box. This means it is now
// broken for the case where auto needs to behave like "under".
// According to the specification TextUnderlinePositionAuto should avoid drawing through glyphs in
// scripts where it would not be appropriate (e.g., ideographs).
// Strictly speaking this can occur whenever the line contains ideographs
// even if it is horizontal, but detecting this has performance implications. For now we only work with
// vertical text, since we already determined the baseline type to be ideographic in that
// case.
TextUnderlinePosition resolvedUnderlinePosition = underlinePosition == TextUnderlinePositionAuto && inlineTextBox && inlineTextBox->root().baselineType() == IdeographicBaseline ? TextUnderlinePositionUnder : TextUnderlinePositionAlphabetic;
switch (resolvedUnderlinePosition) {
case TextUnderlinePositionAlphabetic:
return fontMetrics.ascent() + gap;
case TextUnderlinePositionUnder: {
ASSERT(inlineTextBox);
// Position underline relative to the bottom edge of the lowest element's content box.
const RootInlineBox& rootBox = inlineTextBox->root();
const RenderElement* decorationRenderer = inlineTextBox->parent()->renderer().enclosingRendererWithTextDecoration(TextDecorationUnderline, inlineTextBox->isFirstLine());
float offset;
if (inlineTextBox->renderer().style().isFlippedLinesWritingMode()) {
offset = inlineTextBox->logicalTop();
rootBox.minLogicalTopForTextDecorationLine(offset, decorationRenderer, TextDecorationUnderline);
offset = inlineTextBox->logicalTop() - offset;
} else {
offset = inlineTextBox->logicalBottom();
rootBox.maxLogicalBottomForTextDecorationLine(offset, decorationRenderer, TextDecorationUnderline);
offset -= inlineTextBox->logicalBottom();
}
return inlineTextBox->logicalHeight() + gap + std::max<float>(offset, 0);
}
case TextUnderlinePositionAuto:
ASSERT_NOT_REACHED();
}
ASSERT_NOT_REACHED();
return fontMetrics.ascent() + gap;
}