tdf#55469 Consistent line spacing across platforms

We current use platform APIs to calculate line spacing, however
different platforms behave differently:

* FreeType and Core Text will prefer hhea table over OS/2, and OS/2 Typo
  metrics over Win ones.
* GDI’s TEXTMETRIC only uses OS/2 Win metrics, while NEWTEXTMETRIC seems
  to use Typo one, but we use only the old TEXTMETRIC.

So we get inconsistent line spacing and we have no control which of
three competing sets of line spacing metrics we end up using.

The current conventional wisdom is that:
* hhea metrics should be used, since hhea is a mandatory font table and
  should always be present.
* But if OS/2 is present, it should be used since it is mandatory in
  Windows.
  OS/2 has Typo and Win metrics, but the later was meant to control
  text clipping not line spacing and can be ridiculously large.
  Unfortunately many Windows application incorrectly use the Win metrics
  (thanks to GDI’s TEXTMETRIC) and old fonts might be designed with this
  in mind, so OpenType introduced a flag for fonts to indicate that they
  really want to use Typo metrics. So for best backward compatibility:
  * Use Win metrics if available.
  * Unless USE_TYPO_METRICS flag is set, in which case use Typo metrics.

This patch does this by reading the hhea and OS/2 tables directly and
implementing the algorithm above.

Quick comparison with Microsoft Office 2016 shows similar line spacing
as the new line spacing here, so I guess we are improving compatibility
as well.

Change-Id: I4541e67e3e14508e3529e73083056a09de02e637
Reviewed-on: https://gerrit.libreoffice.org/31053
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Khaled Hosny <khaledhosny@eglug.org>

1 files changed, 72 insertions, 0 deletions

diff --git a/vcl/source/font/fontmetric.cxx b/vcl/source/font/fontmetric.cxx
index 9cde929ce649..3d7bf9a0b375 100644
--- a/vcl/source/font/fontmetric.cxx
+++ b/vcl/source/font/fontmetric.cxx
@@ -24,6 +24,7 @@
 #include "impfontmetric.hxx"
 #include "impfontmetricdata.hxx"
 #include "PhysicalFontFace.hxx"
+#include "sft.hxx"
 
 #include <vector>
 #include <set>
@@ -214,6 +215,7 @@ bool ImplFontMetric::operator==( const ImplFontMetric& r ) const
 
 ImplFontMetricData::ImplFontMetricData( const FontSelectPattern& rFontSelData )
     : FontAttributes( rFontSelData )
+    , mnHeight ( rFontSelData.mnHeight )
     , mnWidth ( rFontSelData.mnWidth )
     , mnOrientation( (short)(rFontSelData.mnOrientation) )
     , mnAscent( 0 )
@@ -424,4 +426,74 @@ void ImplFontMetricData::ImplInitAboveTextLineSize()
     mnAboveWUnderlineOffset = nCeiling + (nIntLeading + 1) / 2;
 }
 
+/*
+ * Calculate line spacing:
+ *
+ * - hhea metrics should be used, since hhea is a mandatory font table and
+ *   should always be present.
+ * - But if OS/2 is present, it should be used since it is mandatory in
+ *   Windows.
+ *   OS/2 has Typo and Win metrics, but the later was meant to control
+ *   text clipping not line spacing and can be ridiculously large.
+ *   Unfortunately many Windows application incorrectly use the Win metrics
+ *   (thanks to GDI’s TEXTMETRIC) and old fonts might be designed with this
+ *   in mind, so OpenType introduced a flag for fonts to indicate that they
+ *   really want to use Typo metrics. So for best backward compatibility:
+ *   - Use Win metrics if available.
+ *   - Unless USE_TYPO_METRICS flag is set, in which case use Typo metrics.
+*/
+void ImplFontMetricData::ImplCalcLineSpacing(const std::vector<uint8_t>& rHheaData,
+        const std::vector<uint8_t>& rOS2Data, int nUPEM)
+{
+    mnAscent = mnDescent = mnExtLeading = mnIntLeading = 0;
+
+    double fScale = static_cast<double>(mnHeight) / nUPEM;
+
+    vcl::TTGlobalFontInfo rInfo;
+    memset(&rInfo, 0, sizeof(vcl::TTGlobalFontInfo));
+    GetTTFontMterics(rHheaData, rOS2Data, &rInfo);
+
+    // Try hhea table first.
+    if (rInfo.ascender || rInfo.descender)
+    {
+        mnAscent     =  rInfo.ascender  * fScale;
+        mnDescent    = -rInfo.descender * fScale;
+        mnExtLeading =  rInfo.linegap   * fScale;
+    }
+
+    // But if OS/2 is present, prefer it.
+    if (rInfo.winAscent || rInfo.winDescent || rInfo.typoAscender || rInfo.typoDescender)
+    {
+        if (mnAscent == 0 && mnDescent == 0)
+        {
+            mnAscent     = rInfo.winAscent  * fScale;
+            mnDescent    = rInfo.winDescent * fScale;
+            mnExtLeading = 0;
+        }
+
+        const uint16_t kUseTypoMetricsMask = 1 << 7;
+        if (rInfo.fsSelection & kUseTypoMetricsMask)
+        {
+            mnAscent     =  rInfo.typoAscender  * fScale;
+            mnDescent    = -rInfo.typoDescender * fScale;
+            mnExtLeading =  rInfo.typoLineGap   * fScale;
+        }
+    }
+
+    if (mnAscent || mnDescent)
+        mnIntLeading = mnAscent + mnDescent - mnHeight;
+
+    SAL_INFO("vcl.gdi.fontmetric",
+                  "fsSelection: "   << rInfo.fsSelection
+             << ", typoAscender: "  << rInfo.typoAscender
+             << ", typoDescender: " << rInfo.typoDescender
+             << ", typoLineGap: "   << rInfo.typoLineGap
+             << ", winAscent: "     << rInfo.winAscent
+             << ", winDescent: "    << rInfo.winDescent
+             << ", ascender: "      << rInfo.ascender
+             << ", descender: "     << rInfo.descender
+             << ", linegap: "       << rInfo.linegap
+             );
+}
+
 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */