Replace leading by more useful x-height. Use OS/2 font metrics when available.

Use otmMacAscent/otmMacDescent rather than otmAscent/otmDescent.
Actually both should be available as they have different purposes...

(bzr r14430.1.5)

5 files changed, 66 insertions, 21 deletions

diff --git a/src/libnrtype/FontInstance.cpp b/src/libnrtype/FontInstance.cpp
index 6b2e030d1..5853d5217 100644
--- a/src/libnrtype/FontInstance.cpp
+++ b/src/libnrtype/FontInstance.cpp
@@ -21,6 +21,7 @@
 #include FT_BBOX_H
 #include FT_TRUETYPE_TAGS_H
 #include FT_TRUETYPE_TABLES_H
+#include FT_GLYPH_H
 #include <pango/pangoft2.h>
 #include <2geom/pathvector.h>
 #include <2geom/path-sink.h>
@@ -507,7 +508,7 @@ void font_instance::LoadGlyph(int glyph_id)
     }
 }
 
-bool font_instance::FontMetrics(double &ascent,double &descent,double &leading)
+bool font_instance::FontMetrics(double &ascent,double &descent,double &xheight)
 {
     if ( pFont == NULL ) {
         return false;
@@ -516,34 +517,77 @@ bool font_instance::FontMetrics(double &ascent,double &descent,double &leading)
     if ( theFace == NULL ) {
         return false;
     }
+
+    // CSS2 recommends using the OS/2 values sTypoAscender and sTypoDescender
+    // for the ascender and descender values:
+    //   http://www.w3.org/TR/CSS2/visudet.html#sTypoAscender
+    // The typographic ascender and descender are taken from the
+    // otmMacAscent and otmMacDescent values:
+    //   http://microsoft.public.win32.programmer.gdi.narkive.com/LV6k4BDh/msdn-documentation-outlinetextmetrics-clarification
+    // The otmAscent and otmDescent values are the maxiumum ascent and maxiumum
+    // descent of all the glyphs in a font.
+
 #ifdef USE_PANGO_WIN32
     OUTLINETEXTMETRIC otm;
     if ( !GetOutlineTextMetrics(parent->hScreenDC,sizeof(otm),&otm) ) {
         return false;
     }
+
     double scale=1.0/parent->fontSize;
-    ascent=fabs(otm.otmAscent*scale);
-    descent=fabs(otm.otmDescent*scale);
-    leading=fabs(otm.otmLineGap*scale);
+    ascent=fabs(otm.otmMacAscent*scale);
+    descent=fabs(otm.otmMacDescent*scale);
+    xheight=fabs(otm.otmXHeight*scale);
+    // May not be necessary... but if OS/2 table is missing or not version 2 or higher,
+    // xheight might be set to 0.
+    if( xheight = 0.0 ) xheight = ascent/2.0;
     //otmSubscriptSize, otmSubscriptOffset, otmSuperscriptSize, otmSuperscriptOffset, 
 #else
     if ( theFace->units_per_EM == 0 ) {
         return false; // bitmap font
     }
-    ascent=fabs(((double)theFace->ascender)/((double)theFace->units_per_EM));
-    descent=fabs(((double)theFace->descender)/((double)theFace->units_per_EM));
-    leading=fabs(((double)theFace->height)/((double)theFace->units_per_EM));
-    leading-=ascent+descent;
+    
+    TT_OS2*  os2 = (TT_OS2*)FT_Get_Sfnt_Table( theFace, FT_SFNT_OS2 );       
+    if( os2 ) {
+        ascent =fabs(((double)os2->sTypoAscender)/((double)theFace->units_per_EM));
+        descent=fabs(((double)os2->sTypoDescender)/((double)theFace->units_per_EM));
+    } else {
+        ascent =fabs(((double)theFace->ascender)/((double)theFace->units_per_EM));
+        descent=fabs(((double)theFace->descender)/((double)theFace->units_per_EM));
+    }
+
+    // We must find x-height ourselves! First try OS/2 table.
+    if( os2 && os2->version >= 0x0002 && os2->version != 0xffffu ) {
+        // Only os/2 version 2 and above have sxHeight, 0xffff marks "old Mac fonts" without table
+        xheight=fabs(((double)os2->sxHeight)/((double)theFace->units_per_EM));
+    } else {
+        // Measure 'x' height in font. Recommended option by XSL standard if no sxHeight.
+        FT_UInt index = FT_Get_Char_Index( theFace, 'x' );
+        if( index != 0 ) {
+            FT_Load_Glyph( theFace, index, FT_LOAD_NO_SCALE );
+            xheight = (fabs)(((double)theFace->glyph->metrics.height/(double)theFace->units_per_EM));
+        } else {
+            // No 'x' in font!
+            xheight = ascent/2.0;
+        }
+    }
 #endif
 
-    // CSS dictates em size is ascent + descent
+    // CSS dictates em size is ascent + descent.... but this doesn't seem to be used in practice.
+    // The em size is what the font reports...
     double em = ascent + descent;
     if( em <= 0 ) {
-        return false; // Pathological
+       return false; // Pathological
     }
     ascent /= em;
     descent /= em;
-    leading /= em;
+    xheight /= em;
+
+    // gchar* font_name = pango_font_description_to_string( descr );
+    // std::cout <<  "Font: " << (font_name ? font_name : ("Null")) << std::endl;
+    // g_free( font_name );
+    // std::cout << "  ascent: " << ascent << "  descent: " << descent
+    //           << "  x-height: " << xheight << "\n" << std::endl;
+
     return true;
 }
 
diff --git a/src/libnrtype/Layout-TNG-Compute.cpp b/src/libnrtype/Layout-TNG-Compute.cpp
index 8f7f6bca0..6b5697b10 100644
--- a/src/libnrtype/Layout-TNG-Compute.cpp
+++ b/src/libnrtype/Layout-TNG-Compute.cpp
@@ -1121,7 +1121,7 @@ void Layout::Calculator::_computeFontLineHeight(font_instance *font, double font
         *line_height_multiplier = 1.0;
     }
     else {
-	font->FontMetrics(font_metrics->ascent, font_metrics->descent, font_metrics->leading);
+	font->FontMetrics(font_metrics->ascent, font_metrics->descent, font_metrics->xheight);
     }
     *font_metrics *= font_size;
 
@@ -1819,13 +1819,13 @@ void Layout::_calculateCursorShapeForEmpty()
     FontMetrics line_height;
     if (font) {
         const_cast<font_instance*>(font)->FontSlope(caret_slope_run, caret_slope_rise);
-        font->FontMetrics(line_height.ascent, line_height.descent, line_height.leading);
+        font->FontMetrics(line_height.ascent, line_height.descent, line_height.xheight);
         line_height *= font_size;
         font->Unref();
     } else {
         line_height.ascent = font_size * 0.85;      // random guesses
         line_height.descent = font_size * 0.15;
-        line_height.leading = 0.0;
+        line_height.xheight = 0.0;
     }
     double caret_slope = atan2(caret_slope_run, caret_slope_rise);
     _empty_cursor_shape.height = font_size / cos(caret_slope);
diff --git a/src/libnrtype/Layout-TNG.h b/src/libnrtype/Layout-TNG.h
index 6875fa14a..c2895e05f 100644
--- a/src/libnrtype/Layout-TNG.h
+++ b/src/libnrtype/Layout-TNG.h
@@ -610,7 +610,7 @@ public:
 
     /**
      * Keep track of font metrics. Two use cases:
-     * 1. Keep track of ascent and descent of an individual font.
+     * 1. Keep track of ascent, descent, and x-height of an individual font.
      * 2. Keep track of effective ascent and descent that includes half-leading.
      *
      * Note: Leading refers to the "external" leading which is added (subtracted) due to
@@ -626,16 +626,16 @@ public:
 
         double ascent;
         double descent;
-        double leading; // Not used... to be removed (requires change to font->FontMetrics()
+        double xheight;
 
         // CSS 2.1 dictates that font-size is based on em-size which is defined as ascent + descent
         inline double emSize() const {return ascent + descent;}
         // Alternatively name function for use 2.
         inline double lineSize() const { return ascent + descent; }
-        inline void setZero() {ascent = descent = 0.0;}
+        inline void setZero() {ascent = descent = xheight = 0.0;}
 
         // For scaling for 'font-size'.
-        inline FontMetrics& operator*=(double x) {ascent *= x; descent *= x; return *this;}
+        inline FontMetrics& operator*=(double x) {ascent *= x; descent *= x; xheight *= x; return *this;}
 
         /// Save the larger values of ascent and descent between this and other. Needed for laying
         /// out a line with mixed font-sizes, fonts, or line spacings.
@@ -646,6 +646,7 @@ public:
 
         inline double getAscent() const {return ascent; }
         inline double getDescent() const {return descent; }
+        inline double getXheight() const {return xheight; }
     };
 
     /// see _enum_converter()
diff --git a/src/libnrtype/TextWrapper.cpp b/src/libnrtype/TextWrapper.cpp
index 380e9ba3f..124f3f7b4 100644
--- a/src/libnrtype/TextWrapper.cpp
+++ b/src/libnrtype/TextWrapper.cpp
@@ -862,7 +862,7 @@ void text_wrapper::MeasureBoxes(void)
     for (int i = 0; i < nbBox; i++) {
         boxes[i].ascent = 0;
         boxes[i].descent = 0;
-        boxes[i].leading = 0;
+        boxes[i].xheight = 0;
         boxes[i].width = 0;
 
         PangoFont *curPF = glyph_text[boxes[i].g_st].font;
@@ -870,7 +870,7 @@ void text_wrapper::MeasureBoxes(void)
             PangoFontDescription *pfd = pango_font_describe(curPF);
             font_instance *curF = f_src->Face(pfd);
             if ( curF ) {
-                curF->FontMetrics(boxes[i].ascent, boxes[i].descent, boxes[i].leading);
+                curF->FontMetrics(boxes[i].ascent, boxes[i].descent, boxes[i].xheight);
                 curF->Unref();
             }
             pango_font_description_free(pfd);
diff --git a/src/libnrtype/one-box.h b/src/libnrtype/one-box.h
index c868cf23f..c3b36a3ce 100644
--- a/src/libnrtype/one-box.h
+++ b/src/libnrtype/one-box.h
@@ -9,7 +9,7 @@
 // this time for sp-typeset
 struct one_box {
     int g_st, g_en; ///< First and last glyph of this word.
-    double ascent, descent, leading;
+    double ascent, descent, xheight;
     double width;
     bool word_start, word_end;
 };