Bug 1721612 - (followup) Rename AdjustAdvancesForSyntheticBold to App…

…lyTrackingToClusters, and clamp the adjusted advance to avoid becoming negative. r=gfx-reviewers,lsalzman

This is not necessary to fix the observed bug, but as a precaution against excessive
negative tracking, let's clamp the adjusted advance so that it can't become negative
(which seems unlikely to end well).

Also rename the method, given that it is not only used for synthetic bold adjustments
any longer.

(No change in behavior, except in the edge-case of a font that has such small
advances and extreme tracking that it tries to go backwards...)

Depends on D193288

Differential Revision: https://phabricator.services.mozilla.com/D193289

gfx/thebes/gfxFont.cpp

@@ -840,18 +840,18 @@ bool gfxShapedText::FilterIfIgnorable(uint32_t aIndex, uint32_t aCh) {
   return false;
 }
 
-void gfxShapedText::AdjustAdvancesForSyntheticBold(float aSynBoldOffset,
-                                                   uint32_t aOffset,
-                                                   uint32_t aLength) {
-  int32_t synAppUnitOffset = aSynBoldOffset * mAppUnitsPerDevUnit;
+void gfxShapedText::ApplyTrackingToClusters(float aTrackingAdjustment,
+                                            uint32_t aOffset,
+                                            uint32_t aLength) {
+  int32_t appUnitAdjustment = aTrackingAdjustment * mAppUnitsPerDevUnit;
   CompressedGlyph* charGlyphs = GetCharacterGlyphs();
   for (uint32_t i = aOffset; i < aOffset + aLength; ++i) {
     CompressedGlyph* glyphData = charGlyphs + i;
     if (glyphData->IsSimpleGlyph()) {
       // simple glyphs ==> just add the advance
       int32_t advance = glyphData->GetSimpleAdvance();
       if (advance > 0) {
-        advance += synAppUnitOffset;
+        advance = std::max(0, advance + appUnitAdjustment);
         if (CompressedGlyph::IsSimpleAdvance(advance)) {
           glyphData->SetSimpleGlyph(advance, glyphData->GetSimpleGlyph());
         } else {
@@ -872,14 +872,10 @@ void gfxShapedText::AdjustAdvancesForSyntheticBold(float aSynBoldOffset,
         if (!details) {
           continue;
         }
-        if (IsRightToLeft()) {
-          if (details[0].mAdvance > 0) {
-            details[0].mAdvance += synAppUnitOffset;
-          }
-        } else {
-          if (details[detailedLength - 1].mAdvance > 0) {
-            details[detailedLength - 1].mAdvance += synAppUnitOffset;
-          }
+        auto& advance = IsRightToLeft() ? details[0].mAdvance
+                                        : details[detailedLength - 1].mAdvance;
+        if (advance > 0) {
+          advance = std::max(0, advance + appUnitAdjustment);
         }
       }
     }
@@ -3341,7 +3337,7 @@ bool gfxFont::ShapeText(DrawTarget* aDrawTarget, const char16_t* aText,
       // synthetic bold: we want to apply between clusters, not to
       // non-spacing glyphs within a cluster. So we can reuse that
       // helper here.
-      aShapedText->AdjustAdvancesForSyntheticBold(tracking, aOffset, aLength);
+      aShapedText->ApplyTrackingToClusters(tracking, aOffset, aLength);
     }
     return true;
   }
@@ -3357,8 +3353,8 @@ void gfxFont::PostShapingFixup(DrawTarget* aDrawTarget, const char16_t* aText,
     const Metrics& metrics = GetMetrics(aVertical ? nsFontMetrics::eVertical
                                                   : nsFontMetrics::eHorizontal);
     if (metrics.maxAdvance > metrics.aveCharWidth) {
-      aShapedText->AdjustAdvancesForSyntheticBold(GetSyntheticBoldOffset(),
-                                                  aOffset, aLength);
+      aShapedText->ApplyTrackingToClusters(GetSyntheticBoldOffset(), aOffset,
+                                           aLength);
     }
   }
 }

gfx/thebes/gfxFont.h

@@ -1036,8 +1036,8 @@ class gfxShapedText {
     return mDetailedGlyphs->Get(aCharIndex);
   }
 
-  void AdjustAdvancesForSyntheticBold(float aSynBoldOffset, uint32_t aOffset,
-                                      uint32_t aLength);
+  void ApplyTrackingToClusters(float aTrackingAdjustment, uint32_t aOffset,
+                               uint32_t aLength);
 
   // Mark clusters in the CompressedGlyph records, starting at aOffset,
   // based on the Unicode properties of the text in aString.

gfx/thebes/gfxMacFont.cpp

@@ -178,7 +178,7 @@ bool gfxMacFont::ShapeText(DrawTarget* aDrawTarget, const char16_t* aText,
         // synthetic bold: we want to apply between clusters, not to
         // non-spacing glyphs within a cluster. So we can reuse that
         // helper here.
-        aShapedText->AdjustAdvancesForSyntheticBold(tracking, aOffset, aLength);
+        aShapedText->ApplyTrackingToClusters(tracking, aOffset, aLength);
       }
       return true;
     }