[POC] Naïve right-to-left labeling

src/mbgl/text/glyph_set.cpp

@@ -71,6 +71,85 @@ void align(Shaping &shaping, const float justify, const float horizontalAlign,
  }
 }
 
+// Returns true if the glyph is a strong right-to-left glyph.
+bool isRTL(uint32_t glyph) {
+ // Use the major RTL Unicode code blocks as a rough approximation.
+ return ((glyph >= 0x600 && glyph <= 0x6ff && (glyph < 0x660 || glyph > 0x669) && glyph != 0x66b && glyph != 0x66c && (glyph < 0x6f0 || glyph > 0x6f9)) /* Arabic */
+ || (glyph >= 0x750 && glyph <= 0x77f) /* Arabic Supplement */
+ || (glyph >= 0x8a0 && glyph <= 0x8ff) /* Arabic Extended-A */
+ || (glyph >= 0x590 && glyph <= 0x5ff) /* Hebrew */
+ || (glyph >= 0x700 && glyph <= 0x74f) /* Syriac */
+ || (glyph >= 0x780 && glyph <= 0x7bf) /* Thaana */);
+}
+
+// Reverse the string to right-to-left orientation if necessary.
+void reverse(std::vector<PositionedGlyph> &positionedGlyphs, const std::map<uint32_t, SDFGlyph> &sdfs) {
+ size_t numRTLChars = 0;
+ for (auto& positionedGlyph : positionedGlyphs) {
+ auto glyph = positionedGlyph.glyph;
+ if (isRTL(glyph)) {
+ numRTLChars++;
+ }
+ }
+ // Treat the entire string as right-to-left if most characters are strong right-to-left characters.
+ bool isMostlyRTL = numRTLChars >= static_cast<float>(positionedGlyphs.size()) / 2.0f;
+ if (isMostlyRTL) {
+ // Reverse the entire string.
+ for (auto& glyph : positionedGlyphs) {
+ glyph.x *= -1;
+ auto it = sdfs.find(glyph.glyph);
+ if (it != sdfs.end()) {
+ glyph.x -= it->second.metrics.advance;
+ }
+ }
+ // For bilingual labels, unreverse short spans of LTR text in predominantly RTL strings.
+ for (auto start = positionedGlyphs.begin(), end = start; end != positionedGlyphs.end(); ++end) {
+ if (start != end && (isRTL(end->glyph) || end->y != (end - 1)->y)) {
+ // End of LTR run or line wrap.
+ auto left = (end - 1)->x;
+ auto right = start->x;
+ auto leftSDF = sdfs.find((end - 1)->glyph);
+ if (leftSDF != sdfs.end()) {
+ right -= leftSDF->second.metrics.advance;
+ }
+ for (; start != end; ++start) {
+ start->x = left + right - start->x;
+ auto sdf = sdfs.find(start->glyph);
+ if (sdf != sdfs.end()) {
+ start->x += sdf->second.metrics.advance;
+ }
+ }
+ ++start;
+ } else if (isRTL(end->glyph)) {
+ ++start;
+ }
+ }
+ } else {
+ // For bilingual labels, reverse short spans of RTL text in predominantly LTR strings.
+ for (auto start = positionedGlyphs.begin(), end = start; end != positionedGlyphs.end(); ++end) {
+ if (start != end && (!isRTL(end->glyph) || end->y != (end - 1)->y)) {
+ // End of RTL run or line wrap.
+ auto left = start->x;
+ auto right = (end - 1)->x;
+ auto rightSDF = sdfs.find((end - 1)->glyph);
+ if (rightSDF != sdfs.end()) {
+ right += rightSDF->second.metrics.advance;
+ }
+ for (; start != end; ++start) {
+ start->x = left + right - start->x;
+ auto sdf = sdfs.find(start->glyph);
+ if (sdf != sdfs.end()) {
+ start->x -= sdf->second.metrics.advance;
+ }
+ }
+ ++start;
+ } else if (!isRTL(end->glyph)) {
+ ++start;
+ }
+ }
+ }
+}
+
 void justifyLine(std::vector<PositionedGlyph> &positionedGlyphs, const std::map<uint32_t, SDFGlyph> &sdfs, uint32_t start,
  uint32_t end, float justify) {
  PositionedGlyph &glyph = positionedGlyphs[end];
@@ -159,6 +238,7 @@ void GlyphSet::lineWrap(Shaping &shaping, const float lineHeight, const float ma
 
  justifyLine(positionedGlyphs, sdfs, lineStartIndex, uint32_t(positionedGlyphs.size()) - 1, justify);
  align(shaping, justify, horizontalAlign, verticalAlign, maxLineLength, lineHeight, line, translate);
+ reverse(positionedGlyphs, sdfs);
 
  // Calculate the bounding box
  shaping.top += -verticalAlign * height;