Merge PR #1310 into master

lib/EpdFont/EpdFont.cpp

@@ -17,9 +17,9 @@ void EpdFont::getTextBounds(const char* string, const int startX, const int star
 
   int32_t cursorXFP = fp4::fromPixel(startX);  // 12.4 fixed-point accumulator
   int lastBaseX = startX;
-  int lastBaseAdvanceFP = 0;  // 12.4 fixed-point
+  int lastBaseLeft = 0;
+  int lastBaseWidth = 0;
   int lastBaseTop = 0;
-  constexpr int MIN_COMBINING_GAP_PX = 1;
   uint32_t cp;
   uint32_t prevCp = 0;
   while ((cp = utf8NextCodepoint(reinterpret_cast<const uint8_t**>(&string)))) {
@@ -35,20 +35,16 @@ void EpdFont::getTextBounds(const char* string, const int startX, const int star
       continue;
     }
 
-    int raiseBy = 0;
-    if (isCombining) {
-      const int currentGap = glyph->top - glyph->height - lastBaseTop;
-      if (currentGap < MIN_COMBINING_GAP_PX) {
-        raiseBy = MIN_COMBINING_GAP_PX - currentGap;
-      }
-    }
+    const int raiseBy = isCombining ? combiningMark::raiseAboveBase(glyph->top, glyph->height, lastBaseTop) : 0;
 
     if (!isCombining && prevCp != 0) {
       cursorXFP += getKerning(prevCp, cp);  // 4.4 fixed-point kern
     }
 
     const int cursorXPixels = fp4::toPixel(cursorXFP);  // snap 12.4 fixed-point to nearest pixel
-    const int glyphBaseX = isCombining ? (lastBaseX + fp4::toPixel(lastBaseAdvanceFP / 2)) : cursorXPixels;
+    const int glyphBaseX =
+        isCombining ? combiningMark::centerOver(lastBaseX, lastBaseLeft, lastBaseWidth, glyph->left, glyph->width)
+                    : cursorXPixels;
     const int glyphBaseY = startY - raiseBy;
 
     *minX = std::min(*minX, glyphBaseX + glyph->left);
@@ -58,7 +54,8 @@ void EpdFont::getTextBounds(const char* string, const int startX, const int star
 
     if (!isCombining) {
       lastBaseX = cursorXPixels;
-      lastBaseAdvanceFP = glyph->advanceX;  // 12.4 fixed-point
+      lastBaseLeft = glyph->left;
+      lastBaseWidth = glyph->width;
       lastBaseTop = glyph->top;
       cursorXFP += glyph->advanceX;  // 12.4 fixed-point advance
       prevCp = cp;

lib/EpdFont/EpdFontData.h

@@ -28,6 +28,37 @@ constexpr int toPixel(int32_t fp) { return static_cast<int>((fp + HALF) >> FRAC_
 constexpr float toFloat(int32_t fp) { return fp / static_cast<float>(1 << FRAC_BITS); }
 }  // namespace fp4
 
+/// Helpers for positioning Unicode combining marks (U+0300 ff.) over a
+/// preceding base glyph without GPOS anchor tables.
+namespace combiningMark {
+
+constexpr int MIN_GAP_PX = 1;
+
+/// Compute the cursor-X at which to render a combining mark so its bitmap
+/// is visually centered over the base glyph's bitmap.
+constexpr int centerOver(int baseCursorPos, int baseLeft, int baseWidth, int markLeft, int markWidth) {
+  return baseCursorPos + baseLeft + baseWidth / 2 - markWidth / 2 - markLeft;
+}
+
+/// Rotated-90CW variant of centerOver.  In the rotated coordinate system
+/// renderCharImpl uses (cursorY - left) instead of (cursorX + left), so
+/// every left/width term inverts sign.
+constexpr int centerOverRotated90CW(int baseCursorPos, int baseLeft, int baseWidth, int markLeft, int markWidth) {
+  return baseCursorPos - baseLeft - baseWidth / 2 + markWidth / 2 + markLeft;
+}
+
+/// For combining marks that sit entirely above the baseline, compute how many
+/// pixels to raise the mark so there is at least MIN_GAP_PX between its bottom
+/// edge and the top of the base glyph.  Returns 0 for marks that extend to or
+/// below the baseline (e.g. cedilla, dot-below, ogonek).
+constexpr int raiseAboveBase(int markTop, int markHeight, int baseTop) {
+  if (markTop - markHeight <= 0) return 0;
+  const int gap = markTop - markHeight - baseTop;
+  return (gap < MIN_GAP_PX) ? (MIN_GAP_PX - gap) : 0;
+}
+
+}  // namespace combiningMark
+
 /// Fixed-point conventions used by EpdGlyph and EpdFontData:
 ///   advanceX:   12.4 unsigned fixed-point in uint16_t  (use fp4::toPixel)
 ///   kernMatrix:  4.4 signed fixed-point in int8_t      (use fp4::toPixel)