Implement basic kerning when rendering text.

Only old-style `kern` tables are supported--modern GPOS-based kerning is not supported.

src/describescreen.cpp

@@ -19,6 +19,17 @@ void TextWindow::ScreenEditTtfText(int link, uint32_t v) {
     SS.TW.edit.request = hr;
 }
 
+void TextWindow::ScreenToggleTtfKerning(int link, uint32_t v) {
+    hRequest hr = { v };
+    Request *r = SK.GetRequest(hr);
+
+    SS.UndoRemember();
+    r->extraPoints = !r->extraPoints;
+
+    SS.MarkGroupDirty(r->group);
+    SS.ScheduleShowTW();
+}
+
 void TextWindow::ScreenSetTtfFont(int link, uint32_t v) {
     int i = (int)v;
     if(i < 0) return;
@@ -205,8 +216,11 @@ void TextWindow::DescribeSelection() {
                 Printf(false, "%FtTRUETYPE FONT TEXT%E");
                 Printf(true, "  font = '%Fi%s%E'", e->font.c_str());
                 if(e->h.isFromRequest()) {
-                    Printf(false, "  text = '%Fi%s%E' %Fl%Ll%f%D[change]%E",
+                    Printf(true, "  text = '%Fi%s%E' %Fl%Ll%f%D[change]%E",
                         e->str.c_str(), &ScreenEditTtfText, e->h.request().v);
+                    Printf(true, "  %Fd%f%D%Ll%s  apply kerning",
+                           &ScreenToggleTtfKerning, e->h.request().v,
+                           e->extraPoints ? CHECK_TRUE : CHECK_FALSE);
                     Printf(true, "  select new font");
                     SS.fonts.LoadAll();
                     // Not using range-for here because we use i inside the output.

src/drawentity.cpp

@@ -475,7 +475,8 @@ void Entity::GenerateBezierCurves(SBezierList *sbl) const {
             Vector v = topLeft.Minus(botLeft);
             Vector u = (v.Cross(n)).WithMagnitude(v.Magnitude());
 
-            SS.fonts.PlotString(font, str, sbl, botLeft, u, v);
+            // `extraPoints` is storing kerning boolean
+            SS.fonts.PlotString(font, str, sbl, extraPoints, botLeft, u, v);
             break;
         }
 

src/request.cpp

@@ -90,7 +90,8 @@ void Request::Generate(IdList<Entity,hEntity> *entity,
     // Request-specific generation.
     switch(type) {
         case Type::TTF_TEXT: {
-            double actualAspectRatio = SS.fonts.AspectRatio(font, str);
+            // `extraPoints` is storing kerning boolean
+            double actualAspectRatio = SS.fonts.AspectRatio(font, str, extraPoints);
             if(EXACT(actualAspectRatio != 0.0)) {
                 // We could load the font, so use the actual value.
                 aspectRatio = actualAspectRatio;

src/ttf.cpp

@@ -108,11 +108,11 @@ TtfFont *TtfFontList::LoadFont(const std::string &font)
 }
 
 void TtfFontList::PlotString(const std::string &font, const std::string &str,
-                             SBezierList *sbl, Vector origin, Vector u, Vector v)
+                             SBezierList *sbl, bool kerning, Vector origin, Vector u, Vector v)
 {
     TtfFont *tf = LoadFont(font);
     if(!str.empty() && tf != NULL) {
-        tf->PlotString(str, sbl, origin, u, v);
+        tf->PlotString(str, sbl, kerning, origin, u, v);
     } else {
         // No text or no font; so draw a big X for an error marker.
         SBezier sb;
@@ -123,11 +123,11 @@ void TtfFontList::PlotString(const std::string &font, const std::string &str,
     }
 }
 
-double TtfFontList::AspectRatio(const std::string &font, const std::string &str)
+double TtfFontList::AspectRatio(const std::string &font, const std::string &str, bool kerning)
 {
     TtfFont *tf = LoadFont(font);
     if(tf != NULL) {
-        return tf->AspectRatio(str);
+        return tf->AspectRatio(str, kerning);
     }
 
     return 0.0;
@@ -331,7 +331,7 @@ static int CubicTo(const FT_Vector *c1, const FT_Vector *c2, const FT_Vector *p,
 }
 
 void TtfFont::PlotString(const std::string &str,
-                         SBezierList *sbl, Vector origin, Vector u, Vector v)
+                         SBezierList *sbl, bool kerning, Vector origin, Vector u, Vector v)
 {
     ssassert(fontFace != NULL, "Expected font face to be loaded");
 
@@ -344,6 +344,7 @@ void TtfFont::PlotString(const std::string &str,
     outlineFuncs.delta    = 0;
 
     FT_Pos dx = 0;
+    uint32_t prevGid = 0;
     for(char32_t cid : ReadUTF8(str)) {
         uint32_t gid = FT_Get_Char_Index(fontFace, cid);
         if (gid == 0) {
@@ -382,6 +383,13 @@ void TtfFont::PlotString(const std::string &str,
          */
         FT_BBox cbox;
         FT_Outline_Get_CBox(&fontFace->glyph->outline, &cbox);
+
+        // Apply Kerning, if any:
+        FT_Vector kernVector;
+        if(kerning && FT_Get_Kerning(fontFace, prevGid, gid, FT_KERNING_DEFAULT, &kernVector) == 0) {
+            dx += kernVector.x;
+        }
+
         FT_Pos bx = dx - cbox.xMin;
         // Yes, this is what FreeType calls left-side bearing.
         // Then interchangeably uses that with "left-side bearing". Sigh.
@@ -402,14 +410,16 @@ void TtfFont::PlotString(const std::string &str,
         // And we're done, so advance our position by the requested advance
         // width, plus the user-requested extra advance.
         dx += fontFace->glyph->advance.x;
+        prevGid = gid;
     }
 }
 
-double TtfFont::AspectRatio(const std::string &str) {
+double TtfFont::AspectRatio(const std::string &str, bool kerning) {
     ssassert(fontFace != NULL, "Expected font face to be loaded");
 
     // We always request a unit size character, so the aspect ratio is the same as advance length.
     double dx = 0;
+    uint32_t prevGid = 0;
     for(char32_t chr : ReadUTF8(str)) {
         uint32_t gid = FT_Get_Char_Index(fontFace, chr);
         if (gid == 0) {
@@ -424,7 +434,14 @@ double TtfFont::AspectRatio(const std::string &str) {
             break;
         }
 
+        // Apply Kerning, if any:
+        FT_Vector kernVector;
+        if(kerning && FT_Get_Kerning(fontFace, prevGid, gid, FT_KERNING_DEFAULT, &kernVector) == 0) {
+            dx += (double)kernVector.x / capHeight;
+        }
+
         dx += (double)fontFace->glyph->advance.x / capHeight;
+        prevGid = gid;
     }
 
     return dx;

src/ttf.h

@@ -24,8 +24,8 @@ public:
     bool LoadFromResource(FT_LibraryRec_ *fontLibrary, bool keepOpen = false);
 
     void PlotString(const std::string &str,
-                    SBezierList *sbl, Vector origin, Vector u, Vector v);
-    double AspectRatio(const std::string &str);
+                    SBezierList *sbl, bool kerning, Vector origin, Vector u, Vector v);
+    double AspectRatio(const std::string &str, bool kerning);
 
     bool ExtractTTFData(bool keepOpen);
 };
@@ -43,8 +43,8 @@ public:
     TtfFont *LoadFont(const std::string &font);
 
     void PlotString(const std::string &font, const std::string &str,
-                    SBezierList *sbl, Vector origin, Vector u, Vector v);
-    double AspectRatio(const std::string &font, const std::string &str);
+                    SBezierList *sbl, bool kerning, Vector origin, Vector u, Vector v);
+    double AspectRatio(const std::string &font, const std::string &str, bool kerning);
 };
 
 #endif

src/ui.h

@@ -402,6 +402,7 @@ public:
     // All of these are callbacks from the GUI code; first from when
     // we're describing an entity
     static void ScreenEditTtfText(int link, uint32_t v);
+    static void ScreenToggleTtfKerning(int link, uint32_t v);
     static void ScreenSetTtfFont(int link, uint32_t v);
     static void ScreenUnselectAll(int link, uint32_t v);
 

test/request/ttf_text/test.cpp

@@ -6,6 +6,12 @@ TEST_CASE(normal_roundtrip) {
     CHECK_SAVE("normal.slvs");
 }
 
+TEST_CASE(kerning_roundtrip) {
+    CHECK_LOAD("kerning.slvs");
+    CHECK_RENDER("kerning.png");
+    CHECK_SAVE("kerning.slvs");
+}
+
 TEST_CASE(normal_migrate_from_v20) {
     CHECK_LOAD("normal_v20.slvs");
     CHECK_SAVE("normal.slvs");