DirectWrite to detect GDI only fonts.

src/ports/SkScalerContext_win_dw.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkTypes.h"
 #undef GetGlyphIndices
 
 #include "SkDWrite.h"
 #include "SkDWriteGeometrySink.h"
 #include "SkEndian.h"
 #include "SkGlyph.h"
 #include "SkHRESULT.h"
 #include "SkMaskGamma.h"
 #include "SkMatrix22.h"
 #include "SkOTTable_EBLC.h"
 #include "SkOTTable_EBSC.h"
 #include "SkOTTable_gasp.h"
+#include "SkOTTable_maxp.h"
 #include "SkPath.h"
 #include "SkScalerContext.h"
 #include "SkScalerContext_win_dw.h"
 #include "SkTScopedComPtr.h"
 #include "SkTypeface_win_dw.h"
 
 #include <dwrite.h>
 
 static bool isLCD(const SkScalerContext::Rec& rec) {
     return SkMask::kLCD16_Format == rec.fMaskFormat ||
            SkMask::kLCD32_Format == rec.fMaskFormat;
 }
 
+static bool is_hinted_without_gasp(DWriteFontTypeface* typeface) {
+    AutoTDWriteTable<SkOTTableMaximumProfile> maxp(typeface->fDWriteFontFace.get());
+    if (!maxp.fExists) {
+        return false;
+    }
+    if (maxp.fSize < sizeof(SkOTTableMaximumProfile::Version::TT)) {
+        return false;
+    }
+    if (maxp->version.version != SkOTTableMaximumProfile::Version::TT::VERSION) {
+        return false;
+    }
+
+    if (0 == maxp->version.tt.maxSizeOfInstructions) {
+        // No hints.
+        return false;
+    }
+
+    AutoTDWriteTable<SkOTTableGridAndScanProcedure> gasp(typeface->fDWriteFontFace.get());
+    return !gasp.fExists;
+}
+
 /** A PPEMRange is inclusive, [min, max]. */
 struct PPEMRange {
     int min;
     int max;
 };
 
 /** If the rendering mode for the specified 'size' is gridfit, then place
  *  the gridfit range into 'range'. Otherwise, leave 'range' alone.
  */
 static void expand_range_if_gridfit_only(DWriteFontTypeface* typeface, int size, PPEMRange* range) {
@@ skipping 203 matching lines @@
 
     // If rotated but the horizontal text could have used a bitmap,
     // render high quality rotated glyphs but measure using bitmap metrics.
     } else if (treatLikeBitmap) {
         fTextSizeRender = gdiTextSize;
         fRenderingMode = DWRITE_RENDERING_MODE_CLEARTYPE_NATURAL_SYMMETRIC;
         fTextureType = DWRITE_TEXTURE_CLEARTYPE_3x1;
         fTextSizeMeasure = gdiTextSize;
         fMeasuringMode = DWRITE_MEASURING_MODE_GDI_CLASSIC;
 
+    // Fonts that have hints but no gasp table get non-symmetric rendering.
+    // Usually such fonts have low quality hints which were never tested
+    // with anything but GDI ClearType classic. Such fonts often rely on
+    // drop out control in the y direction in order to be legible.
+    } else if (is_hinted_without_gasp(typeface)) {
+        fTextSizeRender = gdiTextSize;
+        fRenderingMode = DWRITE_RENDERING_MODE_CLEARTYPE_NATURAL;
+        fTextureType = DWRITE_TEXTURE_CLEARTYPE_3x1;
+        fTextSizeMeasure = realTextSize;
+        fMeasuringMode = DWRITE_MEASURING_MODE_NATURAL;
+
     // The normal case is to use natural symmetric rendering and linear metrics.
     } else {
         fTextSizeRender = realTextSize;
         fRenderingMode = DWRITE_RENDERING_MODE_CLEARTYPE_NATURAL_SYMMETRIC;
         fTextureType = DWRITE_TEXTURE_CLEARTYPE_3x1;
         fTextSizeMeasure = realTextSize;
         fMeasuringMode = DWRITE_MEASURING_MODE_NATURAL;
     }
 
     if (this->isSubpixel()) {
@@ skipping 80 matching lines @@
     DWRITE_FONT_METRICS dwfm;
     fTypeface->fDWriteFontFace->GetMetrics(&dwfm);
     SkScalar advanceX = SkScalarMulDiv(fTextSizeMeasure,
                                        SkIntToScalar(gm.advanceWidth),
                                        SkIntToScalar(dwfm.designUnitsPerEm));
 
     SkVector vecs[1] = { { advanceX, 0 } };
     if (DWRITE_MEASURING_MODE_GDI_CLASSIC == fMeasuringMode ||
         DWRITE_MEASURING_MODE_GDI_NATURAL == fMeasuringMode)
     {
+        // DirectWrite produced 'compatible' metrics, but while close,
+        // the end result is not always an integer as it would be with GDI.
+        vecs[0].fX = SkScalarRoundToScalar(advanceX);
         fG_inv.mapVectors(vecs, SK_ARRAY_COUNT(vecs));
     } else {
         fSkXform.mapVectors(vecs, SK_ARRAY_COUNT(vecs));
     }
 
     glyph->fAdvanceX = SkScalarToFixed(vecs[0].fX);
     glyph->fAdvanceY = SkScalarToFixed(vecs[0].fY);
 }
 
 void SkScalerContext_DW::generateMetrics(SkGlyph* glyph) {
@@ skipping 305 matching lines @@
                                        NULL, //advances
                                        NULL, //offsets
                                        1, //num glyphs
                                        FALSE, //sideways
                                        FALSE, //rtl
                                        geometryToPath.get()),
          "Could not create glyph outline.");
 
     path->transform(fSkXform);
 }