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QFontEngine: Read minimum left and right glyph bearings from 'hhea' table

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This table has values precomputed based on every single glyph in
the font, not just the subset we use as a fallback, which should
improve both performance and correctness.

The fallback codepath of computing the minimum values based on a
subset of the characters in the font is left in, as we still need
that for bitmap fonts, and some font tables that have invalid
values.

Change-Id: I71aac1e09c9f9de80446b023ba15a9e2afe7d226
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>
This commit is contained in:
Tor Arne Vestbø 2015-08-27 18:27:57 +02:00
parent 5f3529be32
commit 9c71f55ef9
1 changed files with 55 additions and 19 deletions
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74
src/gui/text/qfontengine.cpp
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@ -581,34 +581,70 @@ qreal QFontEngine::minLeftBearing() const
return m_minLeftBearing;
}
#define q16Dot16ToFloat(i) ((i) / 65536.0)
#define kMinLeftSideBearingOffset 12
#define kMinRightSideBearingOffset 14
qreal QFontEngine::minRightBearing() const
{
if (m_minRightBearing == kBearingNotInitialized) {
// To balance performance and correctness we only look at a subset of the
// possible glyphs in the font, based on which characters are more likely
// to have a left or right bearing.
static const ushort characterSubset[] = {
'(', 'C', 'F', 'K', 'V', 'X', 'Y', ']', '_', 'f', 'r', '|',
127, 205, 645, 884, 922, 1070, 12386
};
// Try the 'hhea' font table first, which covers the entire font
QByteArray hheaTable = getSfntTable(MAKE_TAG('h', 'h', 'e', 'a'));
if (hheaTable.size() >= int(kMinRightSideBearingOffset + sizeof(qint16))) {
const uchar *tableData = reinterpret_cast<const uchar *>(hheaTable.constData());
Q_ASSERT(q16Dot16ToFloat(qFromBigEndian<quint32>(tableData)) == 1.0);
// The font may have minimum bearings larger than 0, so we have to start at the max
m_minLeftBearing = m_minRightBearing = std::numeric_limits<qreal>::max();
qint16 minLeftSideBearing = qFromBigEndian<qint16>(tableData + kMinLeftSideBearingOffset);
qint16 minRightSideBearing = qFromBigEndian<qint16>(tableData + kMinRightSideBearingOffset);
for (uint i = 0; i < (sizeof(characterSubset) / sizeof(ushort)); ++i) {
const glyph_t glyph = glyphIndex(characterSubset[i]);
if (!glyph)
continue;
// The table data is expressed as FUnits, meaning we have to take the number
// of units per em into account. Since pixelSize already has taken DPI into
// account we can use that directly instead of the point size.
int unitsPerEm = emSquareSize().toInt();
qreal funitToPixelFactor = fontDef.pixelSize / unitsPerEm;
glyph_metrics_t glyphMetrics = const_cast<QFontEngine *>(this)->boundingBox(glyph);
// Some fonts on OS X (such as Gurmukhi Sangam MN, Khmer MN, Lao Sangam MN, etc.), have
// invalid values for their NBSPACE left bearing, causing the 'hhea' minimum bearings to
// be way off. We detect this by assuming that the minimum bearsings are within a certain
// range of the em square size.
static const int largestValidBearing = 4 * unitsPerEm;
// Glyphs with no contours shouldn't contribute to bearings
if (!glyphMetrics.width || !glyphMetrics.height)
continue;
if (qAbs(minLeftSideBearing) < largestValidBearing)
m_minLeftBearing = minLeftSideBearing * funitToPixelFactor;
if (qAbs(minRightSideBearing) < largestValidBearing)
m_minRightBearing = minRightSideBearing * funitToPixelFactor;
}
m_minLeftBearing = qMin(m_minLeftBearing, glyphMetrics.leftBearing().toReal());
m_minRightBearing = qMin(m_minRightBearing, glyphMetrics.rightBearing().toReal());
// Fallback in case of missing 'hhea' table (bitmap fonts e.g.) or broken 'hhea' values
if (m_minLeftBearing == kBearingNotInitialized || m_minRightBearing == kBearingNotInitialized) {
// To balance performance and correctness we only look at a subset of the
// possible glyphs in the font, based on which characters are more likely
// to have a left or right bearing.
static const ushort characterSubset[] = {
'(', 'C', 'F', 'K', 'V', 'X', 'Y', ']', '_', 'f', 'r', '|',
127, 205, 645, 884, 922, 1070, 12386
};
// The font may have minimum bearings larger than 0, so we have to start at the max
m_minLeftBearing = m_minRightBearing = std::numeric_limits<qreal>::max();
for (uint i = 0; i < (sizeof(characterSubset) / sizeof(ushort)); ++i) {
const glyph_t glyph = glyphIndex(characterSubset[i]);
if (!glyph)
continue;
glyph_metrics_t glyphMetrics = const_cast<QFontEngine *>(this)->boundingBox(glyph);
// Glyphs with no contours shouldn't contribute to bearings
if (!glyphMetrics.width || !glyphMetrics.height)
continue;
m_minLeftBearing = qMin(m_minLeftBearing, glyphMetrics.leftBearing().toReal());
m_minRightBearing = qMin(m_minRightBearing, glyphMetrics.rightBearing().toReal());
}
}
if (m_minLeftBearing == kBearingNotInitialized || m_minRightBearing == kBearingNotInitialized)