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Take surrogate pairs into account for caretPositions #306

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Take surrogate pairs into account for caretPositions #306

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49 changes: 40 additions & 9 deletions packages/troika-three-text/src/Typesetter.js
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Original file line number Diff line number Diff line change
Expand Up @@ -105,6 +105,10 @@ export function createTypesetter(resolveFonts, bidi) {
// In the future we may consider a full Unicode line breaking algorithm impl: https://www.unicode.org/reports/tr14
const BREAK_AFTER_CHARS = new RegExp(`${lineBreakingWhiteSpace}|[\\-\\u007C\\u00AD\\u2010\\u2012-\\u2014\\u2027\\u2056\\u2E17\\u2E40]`)

// glyphs that start with a code point within the surrogate range should be treated as surrogate pairs
const HIGH_SURROGATE_START = 0xd800;
const HIGH_SURROGATE_END = 0xdbff;

/**
* Load and parse all the necessary fonts to render a given string of text, then group
* them into consecutive runs of characters sharing a font.
Expand Down Expand Up @@ -517,13 +521,17 @@ export function createTypesetter(resolveFonts, bidi) {
caretPositions[charIndex * 4 + 2] = line.baseline + fontData.caretBottom + anchorYOffset //common bottom y
caretPositions[charIndex * 4 + 3] = line.baseline + fontData.caretTop + anchorYOffset //common top y

// If we skipped any chars from the previous glyph (due to ligature subs), fill in caret
// positions for those missing char indices; currently this uses a best-guess by dividing
// If we skipped any chars from the previous glyph (due to ligature subs/surrogates), fill in caret
// positions for those missing char indices; currently ligatures uses a best-guess by dividing
// the ligature's width evenly. In the future we may try to use the font's LigatureCaretList
// table to get better interior caret positions.
const ligCount = charIndex - prevCharIndex
if (ligCount > 1) {
fillLigatureCaretPositions(caretPositions, prevCharIndex, ligCount)
const charCount = charIndex - prevCharIndex
if (charCount > 1) {
if (isSurrogate(text.slice(prevCharIndex, charIndex))) {
fillSurrogateCaretPositions(caretPositions, prevCharIndex, charCount)
} else {
fillLigatureCaretPositions(caretPositions, prevCharIndex, charCount)
}
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I feel like this may not be a strict if/else situation ... you could potentially have ligatures formed from codepoints that are surrogates. Maybe a better approach would be to modify fillLigatureCaretPositions to check for surrogates stepwise within its loop?

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You're correct, took a deeper look into things and I didn't realize the tons of other characters that are considered surrogate pairs that aren't emojis and could very likely be part of ligature substitutions. Thanks for calling this out

}
prevCharIndex = charIndex
}
Expand Down Expand Up @@ -597,11 +605,15 @@ export function createTypesetter(resolveFonts, bidi) {
}
})

// Fill in remaining caret positions in case the final character was a ligature
// Fill in remaining caret positions in case the final character was a ligature/surrogate
if (caretPositions) {
const ligCount = text.length - prevCharIndex;
if (ligCount > 1) {
fillLigatureCaretPositions(caretPositions, prevCharIndex, ligCount)
const charCount = text.length - prevCharIndex
if (charCount > 1) {
if (isSurrogate(text.slice(prevCharIndex, charIndex))) {
fillSurrogateCaretPositions(caretPositions, prevCharIndex, charCount)
} else {
fillLigatureCaretPositions(caretPositions, prevCharIndex, charCount)
}
}
}
}
Expand Down Expand Up @@ -677,6 +689,25 @@ export function createTypesetter(resolveFonts, bidi) {
}
}

function fillSurrogateCaretPositions(caretPositions, charStartIndex, charCount) {
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This could be simplified to just a single caretPositions.copyWithin() call. (Edit: if this function stays; see other comment.)

const charStartX = caretPositions[charStartIndex * 4]
const charEndX = caretPositions[charStartIndex * 4 + 1]
const charBottom = caretPositions[charStartIndex * 4 + 2]
const charTop = caretPositions[charStartIndex * 4 + 3]
for (let i = 0; i < charCount; i++) {
const startIndex = (charStartIndex + i) * 4
caretPositions[startIndex] = charStartX;
caretPositions[startIndex + 1] = charEndX;
caretPositions[startIndex + 2] = charBottom
caretPositions[startIndex + 3] = charTop
}
}

function isSurrogate(text) {
const firstCodeUnit = text.charCodeAt(0);
return firstCodeUnit >= HIGH_SURROGATE_START && firstCodeUnit <= HIGH_SURROGATE_END;
}

function now() {
return (self.performance || Date).now()
}
Expand Down