From 5f0daa23b9d9329ba053f9a328d6254191dc9e49 Mon Sep 17 00:00:00 2001
From: Mitchell Hashimoto <m@mitchellh.com>
Date: Sat, 24 Aug 2024 20:51:26 -0700
Subject: [PATCH] font/freetype: synthetic italic

---
 pkg/freetype/face.zig      | 19 +++++++++++++++++++
 src/font/face/freetype.zig | 37 ++++++++++++++++++++++++++++++++++++-
 2 files changed, 55 insertions(+), 1 deletion(-)

diff --git a/pkg/freetype/face.zig b/pkg/freetype/face.zig
index 2724aba40..367d409e5 100644
--- a/pkg/freetype/face.zig
+++ b/pkg/freetype/face.zig
@@ -14,6 +14,11 @@ pub const Face = struct {
         _ = c.FT_Done_Face(self.handle);
     }
 
+    /// Increment the counter of the face.
+    pub fn ref(self: Face) void {
+        _ = c.FT_Reference_Face(self.handle);
+    }
+
     /// A macro that returns true whenever a face object contains some
     /// embedded bitmaps. See the available_sizes field of the FT_FaceRec structure.
     pub fn hasFixedSizes(self: Face) bool {
@@ -170,6 +175,20 @@ pub const Face = struct {
         );
         return intToError(res);
     }
+
+    /// Set the transformation that is applied to glyph images when they are
+    /// loaded into a glyph slot through FT_Load_Glyph.
+    pub fn setTransform(
+        self: Face,
+        matrix: ?*const c.FT_Matrix,
+        delta: ?*const c.FT_Vector,
+    ) void {
+        c.FT_Set_Transform(
+            self.handle,
+            @constCast(@ptrCast(matrix)),
+            @constCast(@ptrCast(delta)),
+        );
+    }
 };
 
 /// An enumeration to specify indices of SFNT tables loaded and parsed by
diff --git a/src/font/face/freetype.zig b/src/font/face/freetype.zig
index a182f1657..d1e375647 100644
--- a/src/font/face/freetype.zig
+++ b/src/font/face/freetype.zig
@@ -37,6 +37,17 @@ pub const Face = struct {
     /// Set quirks.disableDefaultFontFeatures
     quirks_disable_default_font_features: bool = false,
 
+    /// Set to true to apply a synthetic italic to the face.
+    synthetic_italic: bool = false,
+
+    /// The matrix applied to a regular font to create a synthetic italic.
+    const italic_matrix: freetype.c.FT_Matrix = .{
+        .xx = 0x10000,
+        .xy = 0x044ED, // approx. tan(15)
+        .yx = 0,
+        .yy = 0x10000,
+    };
+
     /// Initialize a new font face with the given source in-memory.
     pub fn initFile(lib: Library, path: [:0]const u8, index: i32, opts: font.face.Options) !Face {
         const face = try lib.lib.initFace(path, index);
@@ -119,6 +130,24 @@ pub const Face = struct {
         return "";
     }
 
+    /// Return a new face that is the same as this but has a transformation
+    /// matrix applied to italicize it.
+    pub fn syntheticItalic(self: *const Face, opts: font.face.Options) !Face {
+        // Increase face ref count
+        self.face.ref();
+        errdefer self.face.deinit();
+
+        var f = try initFace(
+            .{ .lib = self.lib },
+            self.face,
+            opts,
+        );
+        errdefer f.deinit();
+        f.synthetic_italic = true;
+
+        return f;
+    }
+
     /// Resize the font in-place. If this succeeds, the caller is responsible
     /// for clearing any glyph caches, font atlas data, etc.
     pub fn setSize(self: *Face, opts: font.face.Options) !void {
@@ -246,6 +275,12 @@ pub const Face = struct {
     ) !Glyph {
         const metrics = opts.grid_metrics orelse self.metrics;
 
+        // If we have synthetic italic, then we apply a transformation matrix.
+        // We have to undo this because synthetic italic works by increasing
+        // the ref count of the base face.
+        if (self.synthetic_italic) self.face.setTransform(&italic_matrix, null);
+        defer if (self.synthetic_italic) self.face.setTransform(null, null);
+
         // If our glyph has color, we want to render the color
         try self.face.loadGlyph(glyph_index, .{
             .render = true,
@@ -265,7 +300,7 @@ pub const Face = struct {
 
         // This bitmap is blank. I've seen it happen in a font, I don't know why.
         // If it is empty, we just return a valid glyph struct that does nothing.
-        if (bitmap_ft.rows == 0) return Glyph{
+        if (bitmap_ft.rows == 0) return .{
             .width = 0,
             .height = 0,
             .offset_x = 0,