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Adding primitive support for glyph clusters and n-to-m mapping. #4438

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Mili (Yi) Zhang (milizhang) wants to merge 2 commits into from
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Adding primitive support for glyph clusters and n-to-m mapping. #4438

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7aeab71
Adding primitive support for glyph clusters and n-to-m mapping.
Feb 2, 2020
dd09ff2
Address lint and formatting errors.
Feb 2, 2020
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50 changes: 39 additions & 11 deletions src/renderer/dx/CustomTextLayout.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -398,36 +398,62 @@ CustomTextLayout::CustomTextLayout(gsl::not_null<IDWriteFactory1*> const factory
// Walk through advances and space out characters that are too small to consume their box.
for (auto i = run.glyphStart; i < (run.glyphStart + run.glyphCount); i++)
{
// Advance is how wide in pixels the glyph is
auto& advance = _glyphAdvances.at(i);

// Offsets is how far to move the origin (in pixels) from where it is
auto& offset = _glyphOffsets.at(i);
const auto iClusterBegin = i;

// Get how many columns we expected the glyph to have and multiply into pixels.
UINT16 columns = 0;
{
// Because of typographic features such as ligatures, it is well possible for a glyph to represent
// multiple code points. Previous calls to IDWriteTextAnalyzer::GetGlyphs stores the mapping
// Because of typographic features such as ligatures, it is well possible for a cluster of glyph to
// represent multiple code points. Previous calls to IDWriteTextAnalyzer::GetGlyphs stores the mapping
// information between code points and glyphs in _glyphClusters.
// To properly allocate the columns for such glyphs, we need to find all characters that this glyph
// is representing and add column counts for all the characters together.
// To properly allocate the columns for such glyph clusters, we need to find all characters that this
// cluster is representing and add column counts for all the characters together.
// Inside a glyph cluster, we should try our best to respect the advances and offsets to keep glyphs properly
// positioned. This means the glyph offset adjustment should be applied to all glyphs in cluster, and the
// column-based advance adjustment should be applied on the last glyph of the cluster.

// Find the range for current glyph run in _glyphClusters.
const auto runStartIterator = _glyphClusters.begin() + run.textStart;
const auto runEndIterator = _glyphClusters.begin() + run.textStart + run.textLength;

// Find the range of characters that the current glyph is representing.
const auto firstIterator = std::find(runStartIterator, runEndIterator, i - run.glyphStart);

@reli-msft Renzhi Li (reli-msft) Feb 4, 2020

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These finds may impact performance.
Could we iterate through characters instead?

@milizhang Mili (Yi) Zhang (milizhang) Feb 4, 2020

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I think considering that we are only searching within a single run, I personally think the impact should be pretty close to ignorable.

const auto lastIterator = std::find(runStartIterator, runEndIterator, i - run.glyphStart + 1);
const auto lastIterator = std::find_if(firstIterator, runEndIterator, [iGlyph = (i - run.glyphStart)](auto j) -> bool { return j > iGlyph; });

// Add all allocated column counts together.
for (auto j = firstIterator; j < lastIterator; j++)
{
const auto charIndex = std::distance(_glyphClusters.begin(), j);
columns += _textClusterColumns.at(charIndex);
}

// See if we need to find the end of glyph cluster
if (i + 1 < (run.glyphStart + run.glyphCount) &&
(runEndIterator == lastIterator || *lastIterator > (i - run.glyphStart + 1)))
{
if (runEndIterator == lastIterator)
{
// We are not yet at the end of the glyph run, but nothing in the cluster array maps to the next glyph.
// This means everything until end of run belongs to the same glyph cluster.
i = run.glyphStart + run.glyphCount - 1;
}
else
{
// We found the starting index of next glyph cluster.
// Decrement by one will get us the end of current cluster.
i = run.glyphStart + *lastIterator - 1;
}
}
}

// At this point, we have:
// - iClusterBegin: index of the first glyph in the glyph cluster
// - i: index of the last glyph in the glyph cluster
// - columns: number of terminal columns this cluster should occupy

// Advance is how wide in pixels the glyph is
auto& advance = _glyphAdvances.at(i);

const auto advanceExpected = static_cast<float>(columns * _width);

// If what we expect is bigger than what we have... pad it out.
Expand All @@ -438,7 +464,9 @@ CustomTextLayout::CustomTextLayout(gsl::not_null<IDWriteFactory1*> const factory

// Move the X offset (pixels to the right from the left edge) by half the excess space
// so half of it will be left of the glyph and the other half on the right.
offset.advanceOffset += diff / 2;
// Here we need to move every glyph in the cluster.
for (auto j = iClusterBegin; j <= i; j++)
_glyphOffsets.at(j).advanceOffset += diff / 2;

// Set the advance to the perfect width we want.
advance = advanceExpected;
Expand Down