Rigorously treat font scale factors.

The old values were chosen without a good understanding of font
metrics.

src/drawconstraint.cpp

@@ -83,7 +83,7 @@ void Constraint::DoLabel(Vector ref, Vector *labelPos, Vector gr, Vector gu) {
 
     std::string s = Label();
     double swidth  = ssglStrWidth(s, th),
-           sheight = ssglStrHeight(th);
+           sheight = ssglStrCapHeight(th);
 
     // By default, the reference is from the center; but the style could
     // specify otherwise if one is present, and it could also specify a
@@ -157,7 +157,7 @@ int Constraint::DoLineTrimmedAgainstBox(Vector ref, Vector a, Vector b) {
     double pixels = 1.0 / SS.GW.scale;
     std::string s = Label();
     double swidth  = ssglStrWidth(s, th) + 4*pixels,
-           sheight = ssglStrHeight(th) + 8*pixels;
+           sheight = ssglStrCapHeight(th) + 8*pixels;
 
     struct {
         Vector n;
@@ -371,7 +371,7 @@ void Constraint::DoArcForAngle(Vector a0, Vector da, Vector b0, Vector db,
         *ref = (*ref).ScaledBy(0.5).Plus(disp.offset);
         gu = gu.WithMagnitude(1);
         Vector trans =
-            (*ref).Plus(gu.ScaledBy(-1.5*ssglStrHeight(Style::DefaultTextHeight())));
+            (*ref).Plus(gu.ScaledBy(-1.5*ssglStrCapHeight(Style::DefaultTextHeight())));
         ssglWriteTextRefCenter("angle between skew lines", Style::DefaultTextHeight(),
             trans, gr, gu, LineCallback, this);
     }

src/drawentity.cpp

@@ -679,7 +679,7 @@ void Entity::DrawOrGetDistance(void) {
                 ssglWriteText(str, th, mm2, u, v, NULL, NULL);
             } else {
                 Vector pos = mm2.Plus(u.ScaledBy(ssglStrWidth(str, th)/2)).Plus(
-                                      v.ScaledBy(ssglStrHeight(th)/2));
+                                      v.ScaledBy(ssglStrCapHeight(th)/2));
                 Point2d pp = SS.GW.ProjectPoint(pos);
                 dogd.dmin = min(dogd.dmin, pp.DistanceTo(dogd.mp) - 10);
                 // If a line lies in a plane, then select the line, not

src/glhelper.cpp

@@ -36,7 +36,21 @@ static const VectorGlyph &GetVectorGlyph(char32_t chr) {
     return GetVectorGlyph(0xfffd); // replacement character
 }
 
+// The internal font metrics are as follows:
+//   * Cap height (measured on "A"):     87
+//   * Ascender   (measured on "h"):     87
+//   * Descender  (measured on "p"):    -30
+//   * Font size  (ascender+descender): 126
+// Internally and in the UI, the vector font is sized using cap height.
 #define FONT_SCALE(h) ((h)/87.0)
+double ssglStrCapHeight(double h)
+{
+    return /*cap height*/87.0 * FONT_SCALE(h) / SS.GW.scale;
+}
+double ssglStrFontSize(double h)
+{
+    return /*font size*/126.0 * FONT_SCALE(h) / SS.GW.scale;
+}
 double ssglStrWidth(const std::string &str, double h)
 {
     int width = 0;
@@ -51,11 +65,6 @@ double ssglStrWidth(const std::string &str, double h)
     }
     return width * FONT_SCALE(h) / SS.GW.scale;
 }
-double ssglStrHeight(double h)
-{
-    // The characters have height ~90, as they appear in the table.
-    return 90.0 * FONT_SCALE(h) / SS.GW.scale;
-}
 void ssglWriteTextRefCenter(const std::string &str, double h, Vector t, Vector u, Vector v,
                             ssglLineFn *fn, void *fndata)
 {
@@ -63,7 +72,7 @@ void ssglWriteTextRefCenter(const std::string &str, double h, Vector t, Vector u
     v = v.WithMagnitude(1);
 
     double scale = FONT_SCALE(h)/SS.GW.scale;
-    double fh = ssglStrHeight(h);
+    double fh = ssglStrCapHeight(h);
     double fw = ssglStrWidth(str, h);
 
     t = t.Plus(u.ScaledBy(-fw/2));

src/solvespace.h

@@ -355,8 +355,9 @@ void ssglWriteText(const std::string &str, double h, Vector t, Vector u, Vector
     ssglLineFn *fn, void *fndata);
 void ssglWriteTextRefCenter(const std::string &str, double h, Vector t, Vector u, Vector v,
     ssglLineFn *fn, void *fndata);
+double ssglStrCapHeight(double h);
+double ssglStrFontSize(double h);
 double ssglStrWidth(const std::string &str, double h);
-double ssglStrHeight(double h);
 void ssglLockColorTo(RgbaColor rgb);
 void ssglStippledLine(Vector a, Vector b, double width,
                       int stippleType, double stippleScale, bool maybeFat);