LibGfx: Determine slope attributes for TrueTypeFonts from tables

First checks the OS/2 table for an oblique selection flag before
falling back on italic flags.

Userland/Libraries/LibGfx/BitmapFont.h

@@ -37,7 +37,7 @@ public:
     u16 weight() const override { return m_weight; }
     void set_weight(u16 weight) { m_weight = weight; }
 
-    u8 slope() const { return m_slope; }
+    virtual u8 slope() const override { return m_slope; }
     void set_slope(u8 slope) { m_slope = slope; }
 
     Glyph glyph(u32 code_point) const override;

Userland/Libraries/LibGfx/Font.h

@@ -101,6 +101,7 @@ public:
     FontMetrics metrics(u32 code_point) const;
 
     virtual u8 presentation_size() const = 0;
+    virtual u8 slope() const = 0;
 
     virtual u16 weight() const = 0;
     virtual Glyph glyph(u32 code_point) const = 0;

Userland/Libraries/LibGfx/TrueTypeFont/Font.cpp

@@ -474,6 +474,24 @@ u16 Font::weight() const
     return 400;
 }
 
+u8 Font::slope() const
+{
+    // https://docs.microsoft.com/en-us/typography/opentype/spec/os2
+    constexpr u16 italic_selection_bit { 1 };
+    constexpr u16 oblique_selection_bit { 512 };
+    // https://docs.microsoft.com/en-us/typography/opentype/spec/head
+    constexpr u16 italic_style_bit { 2 };
+
+    if (m_os2.selection() & oblique_selection_bit)
+        return 2;
+    if (m_os2.selection() & italic_selection_bit)
+        return 1;
+    if (m_head.style() & italic_style_bit)
+        return 1;
+
+    return 0;
+}
+
 bool Font::is_fixed_width() const
 {
     // FIXME: Read this information from the font file itself.
@@ -549,6 +567,11 @@ u16 OS2::weight_class() const
     return be_u16(m_slice.offset_pointer((u32)Offsets::WeightClass));
 }
 
+u16 OS2::selection() const
+{
+    return be_u16(m_slice.offset_pointer((u32)Offsets::Selection));
+}
+
 i16 OS2::typographic_ascender() const
 {
     return be_i16(m_slice.offset_pointer((u32)Offsets::TypographicAscender));

Userland/Libraries/LibGfx/TrueTypeFont/Font.h

@@ -57,6 +57,7 @@ public:
     String family() const;
     String variant() const;
     u16 weight() const;
+    u8 slope() const;
     bool is_fixed_width() const;
 
 private:
@@ -122,6 +123,7 @@ public:
     // Gfx::Font implementation
     virtual NonnullRefPtr<Font> clone() const override { return *this; } // FIXME: clone() should not need to be implemented
     virtual u8 presentation_size() const override { return m_point_height; }
+    virtual u8 slope() const override { return m_font->slope(); }
     virtual u16 weight() const override { return m_font->weight(); }
     virtual Gfx::Glyph glyph(u32 code_point) const override;
     virtual bool contains_glyph(u32 code_point) const override { return m_font->glyph_id_for_code_point(code_point) > 0; }

Userland/Libraries/LibGfx/TrueTypeFont/Tables.h

@@ -133,12 +133,14 @@ class OS2 {
 public:
     enum class Offsets {
         WeightClass = 4,
+        Selection = 62,
         TypographicAscender = 68,
         TypographicDescender = 70,
         TypographicLineGap = 72,
     };
 
     u16 weight_class() const;
+    u16 selection() const;
     i16 typographic_ascender() const;
     i16 typographic_descender() const;
     i16 typographic_line_gap() const;