Implement rule 17

* Convert the kerning into a proper right margin for mathnormal, mathscr and
  mathcal (fonts with \fontdimen2 == 0 as per rule 17 from TeXBook).

* Simulate the fact that characters in mathnormal fonts have a 0-width left
  bearing.

* Implement subscript positioning in the case of rule 17 using negative italic
  correction (kerning_).

src/mathed/InsetMath.h

@@ -230,7 +230,7 @@ public:
 	/// math stuff usually isn't allowed in text mode
 	virtual bool allowedIn(mode_type mode) const { return mode == MATH_MODE; }
 
-	/// superscript kerning
+	/// Italic correction as described in InsetMathScript.h
 	virtual int kerning(BufferView const *) const { return 0; }
 	///
 	bool isInToc() const { return true; }

src/mathed/InsetMathChar.cpp

@@ -29,6 +29,10 @@
 #include "support/lstrings.h"
 #include "support/textutils.h"
 
+#include <algorithm>
+
+using namespace std;
+
 
 namespace lyx {
 
@@ -105,20 +109,38 @@ Inset * InsetMathChar::clone() const
 
 void InsetMathChar::metrics(MetricsInfo & mi, Dimension & dim) const
 {
-	bool const mathfont = isMathFont(mi.base.fontname);
-	if (mathfont && subst_) {
+	string const & f = mi.base.fontname;
+	if (isMathFont(f) && subst_) {
 		// If the char has a substitute, draw the replacement symbol
 		// instead, but only in math mode.
 		mathedSymbolDim(mi.base, dim, subst_);
 		kerning_ = mathed_char_kerning(mi.base.font, *subst_->draw.rbegin());
 		return;
-	} else if (!slanted(char_) && mi.base.fontname == "mathnormal") {
+	} else if (!slanted(char_) && f == "mathnormal") {
 		Changer dummy = mi.base.font.changeShape(UP_SHAPE);
 		dim = theFontMetrics(mi.base.font).dimension(char_);
+		kerning_ = 0;
 	} else {
 		frontend::FontMetrics const & fm = theFontMetrics(mi.base.font);
 		dim = fm.dimension(char_);
-		kerning_ = fm.rbearing(char_) - dim.wid;
+		kerning_ = mathed_char_kerning(mi.base.font, char_);
+		// cmmi has almost no left bearing: simulate this
+		if (f == "mathnormal") {
+			dim.wid += max(-fm.lbearing(char_), 0);
+		}
+		// Rule 17 from Appendix G
+		// These are the fonts with fontdimen(2)==0.
+		// To properly handle the case fontdimen(2)!=0 (that is for all other
+		// math fonts), where the italic correction must also be converted into
+		// kerning but only at the end of strings of characters with the same
+		// font, one would need a more elaborate implementation in MathRow. For
+		// now the case fontdimen(2)==0 is the most important.
+		if (f == "mathnormal" || f == "mathscr" || f == "mathcal") {
+			dim.wid += kerning_;
+			// We use a negative value to tell InsetMathScript to move the
+			// subscript leftwards instead of the superscript rightwards
+			kerning_ = -kerning_;
+		}
 	}
 }
 
@@ -138,6 +160,10 @@ void InsetMathChar::draw(PainterInfo & pi, int x, int y) const
 			return;
 		}
 	}
+	// cmmi has almost no left bearing: simulate this
+	if (pi.base.fontname == "mathnormal") {
+		x += max(-theFontMetrics(pi.base.font).lbearing(char_), 0);
+	}
 	pi.draw(x, y, char_);
 }
 

src/mathed/InsetMathScript.cpp

@@ -226,7 +226,8 @@ int InsetMathScript::dx0(BufferView const & bv) const
 {
 	LASSERT(hasDown(), return 0);
 	Dimension const dim = dimension(bv);
-	return hasLimits() ? (dim.wid - down().dimension(bv).width()) / 2 : nwid(bv);
+	return hasLimits() ? (dim.wid - down().dimension(bv).width()) / 2
+		: nwid(bv) + min(nker(&bv), 0);
 }
 
 
@@ -234,7 +235,8 @@ int InsetMathScript::dx1(BufferView const & bv) const
 {
 	LASSERT(hasUp(), return 0);
 	Dimension const dim = dimension(bv);
-	return hasLimits() ? (dim.wid - up().dimension(bv).width()) / 2 : nwid(bv) + nker(&bv);
+	return hasLimits() ? (dim.wid - up().dimension(bv).width()) / 2
+		: nwid(bv) + max(nker(&bv), 0);
 }
 
 
@@ -265,10 +267,8 @@ int InsetMathScript::ndes(BufferView const & bv) const
 
 int InsetMathScript::nker(BufferView const * bv) const
 {
-	if (!nuc().empty()) {
-		int kerning = nuc().kerning(bv);
-		return kerning > 0 ? kerning : 0;
-	}
+	if (!nuc().empty())
+		return nuc().kerning(bv);
 	return 0;
 }
 
@@ -320,9 +320,9 @@ void InsetMathScript::metrics(MetricsInfo & mi, Dimension & dim) const
 			dim.wid = max(dim.wid, dimdown.width());
 	} else {
 		if (hasUp())
-			dim.wid = max(dim.wid, nker(mi.base.bv) + dimup.width());
+			dim.wid = max(dim.wid, max(nker(mi.base.bv), 0) + dimup.width());
 		if (hasDown())
-			dim.wid = max(dim.wid, dimdown.width());
+			dim.wid = max(dim.wid, min(nker(mi.base.bv), 0) + dimdown.width());
 		dim.wid += nwid(bv);
 	}
 	int na = nasc(bv);

src/mathed/InsetMathScript.h

@@ -135,7 +135,10 @@ private:
 	int nasc(BufferView const &) const;
 	/// returns descent of nucleus if any
 	int ndes(BufferView const &) const;
-	/// returns superscript kerning of nucleus if any
+	/// Italic correction: amount of displacement between subscript and
+	/// superscript in math mode as per Appendix G, rule 18f.  A positive value
+	/// shifts the superscript to the right, and a negative value shifts the
+	/// subscript to the left.
 	int nker(BufferView const * bv) const;
 	/// where do we have to draw the scripts?
 	bool hasLimits() const;

src/mathed/InsetMathSideset.h

@@ -104,7 +104,7 @@ private:
 	int nasc(BufferView const &) const;
 	/// returns descent of nucleus if any
 	int ndes(BufferView const &) const;
-	/// returns subscript and superscript kerning of nucleus if any
+	/// Italic correction as described in InsetMathScript.h
 	int nker(BufferView const * bv) const;
 	/// Whether there are two left scripts or one single cell
 	bool scriptl_; 

src/mathed/InsetMathSpecialChar.cpp

@@ -61,7 +61,7 @@ void InsetMathSpecialChar::metrics(MetricsInfo & mi, Dimension & dim) const
 	} else {
 		frontend::FontMetrics const & fm = theFontMetrics(mi.base.font);
 		dim = fm.dimension(char_);
-		kerning_ = fm.rbearing(char_) - dim.wid;
+		kerning_ = mathed_char_kerning(mi.base.font, char_);
 	}
 }
 

src/mathed/MathData.h

@@ -166,7 +166,7 @@ public:
 	int slevel() const { return slevel_; }
 	/// additional super/subscript shift
 	int sshift() const { return sshift_; }
-	/// superscript kerning
+	/// Italic correction as described in InsetMathScript.h
 	int kerning(BufferView const *) const { return kerning_; }
 	///
 	void swap(MathData & ar) { base_type::swap(ar); }

src/mathed/MathSupport.cpp

@@ -556,7 +556,7 @@ int mathed_char_width(FontInfo const & font, char_type c)
 int mathed_char_kerning(FontInfo const & font, char_type c)
 {
 	frontend::FontMetrics const & fm = theFontMetrics(font);
-	return fm.rbearing(c) - fm.width(c);
+	return max(0, fm.rbearing(c) - fm.width(c));
 }