Bug 1815679 - Fix errors in mapping font-style:oblique values to the 'slnt' axis in variable fonts. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D169335

gfx/thebes/gfxFont.cpp

@@ -870,11 +870,15 @@ void gfxShapedText::AdjustAdvancesForSyntheticBold(float aSynBoldOffset,
 }
 
 float gfxFont::AngleForSyntheticOblique() const {
-  // If the style doesn't call for italic/oblique, or if the face already
-  // provides it, no synthetic style should be added.
-  if (mStyle.style == FontSlantStyle::NORMAL || !mStyle.allowSyntheticStyle ||
-      !mFontEntry->IsUpright() || mFontEntry->HasSlantVariation()) {
-    return 0.0f;
+  // First check conditions that mean no synthetic slant should be used:
+  if (mStyle.style == FontSlantStyle::NORMAL) {
+    return 0.0f;  // Requested style is 'normal'.
+  }
+  if (!mStyle.allowSyntheticStyle) {
+    return 0.0f;  // Synthetic obliquing is disabled.
+  }
+  if (!mFontEntry->MayUseSyntheticSlant()) {
+    return 0.0f;  // The resource supports "real" slant, so don't synthesize.
   }
 
   // If style calls for italic, and face doesn't support it, use default
@@ -885,7 +889,7 @@ float gfxFont::AngleForSyntheticOblique() const {
                : FontSlantStyle::DEFAULT_OBLIQUE_DEGREES;
   }
 
-  // Default or custom oblique angle
+  // OK, we're going to use synthetic oblique: return the requested angle.
   return mStyle.style.ObliqueAngle();
 }
 

gfx/thebes/gfxFontEntry.cpp

@@ -1349,18 +1349,14 @@ void gfxFontEntry::GetVariationsForStyle(nsTArray<gfxFontVariation>& aResult,
     // The 'ital' axis is normally a binary toggle; intermediate values
     // can only be set using font-variation-settings.
     aResult.AppendElement(gfxFontVariation{HB_TAG('i', 't', 'a', 'l'), 1.0f});
-  } else if (HasSlantVariation()) {
+  } else if (aStyle.style != StyleFontStyle::NORMAL && HasSlantVariation()) {
     // Figure out what slant angle we should try to match from the
     // requested style.
-    float angle = aStyle.style.IsNormal() ? 0.0f
-                  : aStyle.style.IsItalic()
-                      ? FontSlantStyle::DEFAULT_OBLIQUE_DEGREES
-                      : aStyle.style.ObliqueAngle();
+    float angle = aStyle.style.SlantAngle();
     // Clamp to the available range, unless the face is a user font
     // with no explicit descriptor.
     if (!(IsUserFont() && (mRangeFlags & RangeFlags::eAutoSlantStyle))) {
-      angle =
-          SlantStyle().Clamp(FontSlantStyle::FromFloat(angle)).ObliqueAngle();
+      angle = SlantStyle().Clamp(FontSlantStyle::FromFloat(angle)).SlantAngle();
     }
     // OpenType and CSS measure angles in opposite directions, so we have to
     // invert the sign of the CSS oblique value when setting OpenType 'slnt'.

gfx/thebes/gfxFontEntry.h

@@ -181,8 +181,9 @@ class gfxFontEntry {
   bool IsItalic() const { return SlantStyle().Min().IsItalic(); }
   bool IsOblique() const { return SlantStyle().Min().IsOblique(); }
   bool IsUpright() const { return SlantStyle().Min().IsNormal(); }
-  inline bool SupportsItalic();
-  inline bool SupportsBold();  // defined below, because of RangeFlags use
+  inline bool SupportsItalic();  // defined below, because of RangeFlags use
+  inline bool SupportsBold();
+  inline bool MayUseSyntheticSlant();
   bool IgnoreGDEF() const { return mIgnoreGDEF; }
   bool IgnoreGSUB() const { return mIgnoreGSUB; }
 
@@ -834,6 +835,22 @@ inline bool gfxFontEntry::SupportsBold() {
           HasBoldVariableWeight());
 }
 
+inline bool gfxFontEntry::MayUseSyntheticSlant() {
+  if (!IsUpright()) {
+    return false;  // The resource is already non-upright.
+  }
+  if (HasSlantVariation()) {
+    if (mRangeFlags & RangeFlags::eAutoSlantStyle) {
+      return false;
+    }
+    if (!SlantStyle().IsSingle()) {
+      return false;  // The resource has a 'slnt' axis, and has not been
+                     // clamped to just its upright setting.
+    }
+  }
+  return true;
+}
+
 // used when iterating over all fonts looking for a match for a given character
 struct GlobalFontMatch {
   GlobalFontMatch(uint32_t aCharacter, uint32_t aNextCh,

gfx/thebes/gfxFontUtils.h

@@ -1262,7 +1262,7 @@ static inline double StyleDistance(const mozilla::SlantStyleRange& aRange,
     return kReverse;
   }
 
-  const double kDefaultAngle = mozilla::FontSlantStyle::OBLIQUE.ObliqueAngle();
+  const double kDefaultAngle = mozilla::FontSlantStyle::DEFAULT_OBLIQUE_DEGREES;
 
   if (aTargetStyle.IsItalic()) {
     if (minStyle.IsOblique()) {

layout/style/ServoStyleConstsInlines.h

@@ -1078,7 +1078,14 @@ inline bool StyleFontStyle::IsOblique() const {
   return !IsItalic() && !IsNormal();
 }
 
-inline float StyleFontStyle::ObliqueAngle() const { return ToFloat(); }
+inline float StyleFontStyle::ObliqueAngle() const {
+  MOZ_ASSERT(IsOblique());
+  return ToFloat();
+}
+
+inline float StyleFontStyle::SlantAngle() const {
+  return IsNormal() ? 0 : IsItalic() ? DEFAULT_OBLIQUE_DEGREES : ObliqueAngle();
+}
 
 using FontStretch = StyleFontStretch;
 using FontSlantStyle = StyleFontStyle;

servo/ports/geckolib/cbindgen.toml

@@ -906,7 +906,9 @@ renaming_overrides_prefixing = true
 
  inline bool IsItalic() const;
  inline bool IsOblique() const;
- inline float ObliqueAngle() const;
+ inline float ObliqueAngle() const;  // Only for use when IsOblique() is true
+ inline float SlantAngle() const;  // Returns angle for any font-style, including
+                                   // normal/italic as well as explicit oblique
 """
 
 "FontStretch" = """