From 8eff74ef7680f40bbcc03d634ad1ad11417c29f3 Mon Sep 17 00:00:00 2001
From: Mitchell Hashimoto <m@mitchellh.com>
Date: Mon, 1 Jun 2026 19:44:10 -0700
Subject: [PATCH] font: add glyf rasterizer

---
 src/font/glyf_rasterize.zig | 832 ++++++++++++++++++++++++++++++++++++
 src/font/main.zig           |   1 +
 src/font/opentype/glyf.zig  |   6 +-
 3 files changed, 836 insertions(+), 3 deletions(-)
 create mode 100644 src/font/glyf_rasterize.zig

diff --git a/src/font/glyf_rasterize.zig b/src/font/glyf_rasterize.zig
new file mode 100644
index 000000000..f1b1eb3e7
--- /dev/null
+++ b/src/font/glyf_rasterize.zig
@@ -0,0 +1,832 @@
+//! Rasterization for OpenType glyf outlines.
+//!
+//! This module intentionally lives in `font` rather than `font/opentype`
+//! because I wanted to keep `font/opentype` dependency free on the font
+//! package.
+
+const std = @import("std");
+const assert = std.debug.assert;
+const Allocator = std.mem.Allocator;
+const z2d = @import("z2d");
+
+const face = @import("face.zig");
+const glyf = @import("opentype/glyf.zig");
+
+/// Metrics describing the authored glyf coordinate space, since
+/// a glyf table doesn't contain this on its own.
+pub const DesignMetrics = struct {
+    /// Units-per-em for outline/design coordinates.
+    units_per_em: u32,
+
+    /// Authored advance width in design units.
+    advance_width: u32,
+
+    /// Authored line height in design units.
+    line_height: u32,
+};
+
+/// An owned, tightly packed alpha8 bitmap.
+pub const Bitmap = struct {
+    width: u32,
+    height: u32,
+    data: []u8,
+
+    // An empty 0x0 bitmap.
+    pub const empty: Bitmap = .{ .width = 0, .height = 0, .data = "" };
+
+    pub fn initEmpty(alloc: Allocator, width: u32, height: u32) Allocator.Error!Bitmap {
+        const data = try alloc.alloc(u8, @as(usize, width) * @as(usize, height));
+        @memset(data, 0);
+        return .{ .width = width, .height = height, .data = data };
+    }
+
+    pub fn deinit(self: *Bitmap, alloc: Allocator) void {
+        alloc.free(self.data);
+        self.* = undefined;
+    }
+};
+
+pub const Error = Allocator.Error || z2d.Path.Error || z2d.painter.FillError;
+
+/// Rasterize a decoded glyf outline to a full-cell alpha bitmap.
+///
+/// The returned bitmap is always `grid_metrics.cell_width * cell_width` by
+/// `grid_metrics.cell_height`. `opts.constraint` is applied using the same
+/// `face.RenderOptions.Constraint` machinery used by the platform font
+/// backends.
+///
+/// The caller owns the returned bitmap.
+pub fn rasterize(
+    alloc: Allocator,
+    outline: glyf.Glyf.Outline,
+    design: DesignMetrics,
+    opts: face.RenderOptions,
+) Error!Bitmap {
+    assert(design.units_per_em > 0);
+    assert(design.advance_width > 0);
+    assert(design.line_height > 0);
+
+    // Calculate our final width/height.
+    const width: u32 = std.math.mul(
+        u32,
+        opts.grid_metrics.cell_width,
+        opts.cell_width orelse 1,
+    ) catch std.math.maxInt(u32);
+    const height = opts.grid_metrics.cell_height;
+    assert(width > 0 and height > 0);
+
+    // If we have no contours or points then we have no drawable shape, but the
+    // caller still asked for a cell-sized bitmap. Return that full bitmap with
+    // zero coverage so downstream atlas/upload code doesn't need a separate
+    // size contract for empty glyphs.
+    if (outline.contours.len == 0 or outline.points.len == 0) return Bitmap.initEmpty(alloc, width, height);
+
+    // Glyf entries have a header bounding box, but this rasterizer operates on
+    // the decoded Outline only. Recompute bounds from the decoded coordinate
+    // data so placement and scaling follow the geometry we actually draw. If a
+    // source glyf header disagrees with its points, the point data is the safer
+    // source of truth for rasterization; the header belongs in decode-time
+    // validation/metadata, not this font-level drawing API.
+    const bounds: Bounds = bounds: {
+        var bounds: Bounds = .{
+            .x_min = @floatFromInt(outline.points[0].x),
+            .y_min = @floatFromInt(outline.points[0].y),
+            .x_max = @floatFromInt(outline.points[0].x),
+            .y_max = @floatFromInt(outline.points[0].y),
+        };
+        for (outline.points[1..]) |p| {
+            const x: f64 = @floatFromInt(p.x);
+            const y: f64 = @floatFromInt(p.y);
+            bounds.x_min = @min(bounds.x_min, x);
+            bounds.y_min = @min(bounds.y_min, y);
+            bounds.x_max = @max(bounds.x_max, x);
+            bounds.y_max = @max(bounds.y_max, y);
+        }
+        break :bounds bounds;
+    };
+
+    // Degenerate point bounds can't produce filled area and would make the
+    // point-to-bitmap transform divide by zero, so return a full transparent
+    // bitmap just like an empty outline.
+    if (bounds.width() == 0 or bounds.height() == 0) return Bitmap.initEmpty(alloc, width, height);
+
+    // Build the surface we'll draw on. This is a simple alpha8 drawing.
+    var sfc: z2d.Surface = try .init(
+        .image_surface_alpha8,
+        alloc,
+        @intCast(width),
+        @intCast(height),
+    );
+    defer sfc.deinit(alloc);
+
+    var path: z2d.Path = .empty;
+    defer path.deinit(alloc);
+
+    const placement: Placement = .init(bounds, design, opts);
+    for (0..outline.contours.len) |i| try appendContourPath(
+        alloc,
+        &path,
+        outline.contour(i),
+        bounds,
+        placement,
+    );
+
+    try z2d.painter.fill(
+        alloc,
+        &sfc,
+        &.{ .opaque_pattern = .{
+            .pixel = .{ .alpha8 = .{ .a = 255 } },
+        } },
+        path.nodes.items,
+        .{},
+    );
+
+    return .{
+        .width = width,
+        .height = height,
+        .data = try alloc.dupe(u8, std.mem.sliceAsBytes(sfc.image_surface_alpha8.buf)),
+    };
+}
+
+const Bounds = struct {
+    x_min: f64,
+    y_min: f64,
+    x_max: f64,
+    y_max: f64,
+
+    fn width(self: Bounds) f64 {
+        return self.x_max - self.x_min;
+    }
+
+    fn height(self: Bounds) f64 {
+        return self.y_max - self.y_min;
+    }
+};
+
+/// Cell-relative pixel rectangle where the decoded outline bounds should be
+/// rasterized within the output bitmap.
+///
+/// This is deliberately the placement of the outline's computed point bounds,
+/// not the full declared advance/line-height box. `advance_width` and
+/// `line_height` describe the design-space layout box the outline was drawn
+/// within: they include intentional bearings and whitespace around the visible
+/// points. We use that declared box when applying `RenderOptions.Constraint` so
+/// sizing and alignment preserve those bearings consistently with other font
+/// backends; once that is resolved, we rasterize only the actual outline bounds
+/// into this rectangle.
+///
+/// ```text
+/// output bitmap / terminal cell
+/// ╭────────────────────────────────────────────────────────────────────────╮ top
+/// │                                                                        │
+/// │   declared advance/line-height box                                     │
+/// │   (outer layout box used for constraints)                              │
+/// │   ╭────────────────────────────────────────────────────────────────╮   │
+/// │   │                                                                │   │
+/// │◀──────── x ────────▶╭────────── width ──────────╮                  │   │
+/// │   │                 │ Placement                 │ ▲ height         │   │
+/// │   │                 │ outline point bounds      │ │                │   │
+/// │   │                 │ pixels to draw            │ │                │   │
+/// │   │                 │                           │ │                │   │
+/// │   │                 ╰───────────────────────────╯ ▼                │   │
+/// │   ╰─────────────────▲──────────────────────────────────────────────╯   │
+/// │                     │ y                                                │
+/// ╰────────────────────────────────────────────────────────────────────────╯ bottom
+///              x is measured from the bitmap left to the Placement left.
+///              y is measured from the bitmap bottom to the Placement bottom.
+///              bitmap_height is the full top-to-bottom bitmap height.
+/// ```
+///
+/// Constraints are applied to the outer box so the whitespace remains part of
+/// alignment decisions. `Placement` is the inner rectangle after that outer box
+/// has been constrained.
+const Placement = struct {
+    /// Left edge of the rasterized outline bounds in bitmap pixels, measured
+    /// from the bitmap's left edge.
+    x: f64,
+
+    /// Bottom edge of the rasterized outline bounds in bitmap pixels, measured
+    /// from the bitmap's bottom edge. This matches the cell-relative y axis
+    /// used by font.face.GlyphSize and is converted to z2d's y-down axis when
+    /// points are transformed.
+    y: f64,
+
+    /// Width of the rasterized outline bounds in bitmap pixels after applying
+    /// font.face.RenderOptions.Constraint.
+    width: f64,
+
+    /// Height of the rasterized outline bounds in bitmap pixels after applying
+    /// font.face.RenderOptions.Constraint.
+    height: f64,
+
+    /// Full bitmap height in pixels, used to convert cell-relative y-up-ish
+    /// placement into the y-down coordinate system used by z2d surfaces.
+    bitmap_height: f64,
+
+    /// Calculate where the decoded point bounds should land in the output
+    /// bitmap.
+    ///
+    /// The glyf protocol supplies declared metrics (`units_per_em`,
+    /// `advance_width`, and `line_height`) in design units, while Ghostty's
+    /// font constraint code works in cell-relative pixels. We first map the em
+    /// square to one cell height, matching the linked glyph rasterizer's
+    /// baseline model where design-space `y=0` is the bottom/baseline of the em
+    /// and `y=units_per_em` is its top. Then we describe the actual outline
+    /// bounds as a relative sub-rectangle of the declared advance/line-height
+    /// box. That declared box includes any intentional side bearings or
+    /// vertical whitespace around the outline; constraints should apply to that
+    /// layout box rather than to the tight point bounds alone. This returns the
+    /// final pixel rectangle for only the outline bounds that we will rasterize.
+    fn init(
+        bounds: Bounds,
+        design: DesignMetrics,
+        opts: face.RenderOptions,
+    ) Placement {
+        // Start with protocol-like design units mapped so that the em square
+        // occupies one cell. This makes units_per_em the scale reference and
+        // preserves the linked rasterizer's y=0 baseline/bottom behavior.
+        // Callers can then use RenderOptions.Constraint to fit/cover/stretch/
+        // align the declared advance/line-height box using existing font logic.
+        const scale = @as(f64, @floatFromInt(opts.grid_metrics.cell_height)) /
+            @as(f64, @floatFromInt(design.units_per_em));
+
+        // Convert the decoded point bounds into the same pixel coordinate space
+        // expected by RenderOptions.Constraint. This rectangle is the visible
+        // outline bounds, not the full advance/line-height layout box.
+        const glyph: face.GlyphSize = .{
+            .width = bounds.width() * scale,
+            .height = bounds.height() * scale,
+            .x = bounds.x_min * scale,
+            .y = bounds.y_min * scale,
+        };
+
+        // Convert the declared layout box to pixels. This is the box that
+        // carries intentional bearings/whitespace and should be constrained.
+        const group_width = @as(f64, @floatFromInt(design.advance_width)) * scale;
+        const group_height = @as(f64, @floatFromInt(design.line_height)) * scale;
+
+        // Apply the same fit/cover/stretch/alignment/padding rules used by
+        // normal font rendering. The result is still the outline bounds, but
+        // placed as if its containing advance/line-height box was constrained.
+        const constraint: face.RenderOptions.Constraint = constraint: {
+            var constraint = opts.constraint;
+            if (group_width > 0 and group_height > 0) {
+                // Tell Constraint that `glyph` is a sub-rectangle of the
+                // declared layout box. Constraint will size/align the outer box
+                // and then return the corresponding transformed inner box.
+                constraint.relative_width = glyph.width / group_width;
+                constraint.relative_height = glyph.height / group_height;
+                constraint.relative_x = glyph.x / group_width;
+                constraint.relative_y = glyph.y / group_height;
+            }
+            break :constraint constraint;
+        };
+        const constrained = constraint.constrain(
+            glyph,
+            opts.grid_metrics,
+            opts.constraint_width,
+        );
+
+        // Store the final outline placement plus the full bitmap height needed
+        // later to flip from cell-relative y to z2d's y-down surface space.
+        return .{
+            .x = constrained.x,
+            .y = constrained.y,
+            .width = constrained.width,
+            .height = constrained.height,
+            .bitmap_height = @floatFromInt(opts.grid_metrics.cell_height),
+        };
+    }
+};
+
+const Point = struct {
+    x: f64,
+    y: f64,
+};
+
+/// Append one contour to a z2d path.
+///
+/// Glyf contours are quadratic outlines with explicit on-curve points and
+/// off-curve control points. Consecutive off-curve points imply an on-curve
+/// point halfway between them, and a contour may begin with an off-curve point.
+/// This normalizes those cases while walking the closed contour and emits z2d
+/// line/cubic-curve operations in bitmap coordinates.
+fn appendContourPath(
+    alloc: Allocator,
+    path: *z2d.Path,
+    contour: []const glyf.Glyf.Outline.Point,
+    bounds: Bounds,
+    placement: Placement,
+) Error!void {
+    if (contour.len == 0) return;
+
+    const first = contour[0];
+    const last = contour[contour.len - 1];
+
+    var current: Point = undefined;
+    var i: usize = 0;
+
+    // Choose the starting on-curve point for this closed contour. If the first
+    // point is off-curve then the contour logically starts either at the final
+    // on-curve point, or at the implied midpoint between the final and first
+    // off-curve points.
+    if (first.on_curve) {
+        i = 1;
+        current = transformPoint(
+            first,
+            bounds,
+            placement,
+        );
+    } else if (last.on_curve) {
+        current = transformPoint(
+            last,
+            bounds,
+            placement,
+        );
+    } else {
+        current = midpoint(
+            transformPoint(last, bounds, placement),
+            transformPoint(first, bounds, placement),
+        );
+    }
+
+    // Move to the beginning
+    try path.moveTo(alloc, current.x, current.y);
+
+    // Go through the points and connect em!
+    while (i < contour.len) {
+        const p = contour[i];
+
+        // On-curve points connect to the current point with a straight line.
+        if (p.on_curve) {
+            current = transformPoint(p, bounds, placement);
+            try path.lineTo(alloc, current.x, current.y);
+            i += 1;
+            continue;
+        }
+
+        // Off-curve points are quadratic control points. The following point is
+        // either the curve endpoint or, if it is also off-curve, contributes an
+        // implied on-curve endpoint halfway between the two controls.
+        const control = transformPoint(p, bounds, placement);
+        const next = contour[(i + 1) % contour.len];
+        const end = if (next.on_curve) transformPoint(
+            next,
+            bounds,
+            placement,
+        ) else midpoint(
+            control,
+            transformPoint(next, bounds, placement),
+        );
+
+        // z2d paths only expose cubic curves, so convert the TrueType
+        // quadratic segment to an equivalent cubic segment before appending it.
+        const c1 = Point{
+            .x = current.x + ((2.0 / 3.0) * (control.x - current.x)),
+            .y = current.y + ((2.0 / 3.0) * (control.y - current.y)),
+        };
+        const c2 = Point{
+            .x = end.x + ((2.0 / 3.0) * (control.x - end.x)),
+            .y = end.y + ((2.0 / 3.0) * (control.y - end.y)),
+        };
+        try path.curveTo(
+            alloc,
+            c1.x,
+            c1.y,
+            c2.x,
+            c2.y,
+            end.x,
+            end.y,
+        );
+
+        current = end;
+
+        // If we consumed an explicit on-curve endpoint then skip it; otherwise
+        // the next off-curve point still needs to be used as the control point
+        // for the following quadratic segment.
+        i += if (next.on_curve) 2 else 1;
+    }
+
+    try path.close(alloc);
+}
+
+/// Convert a decoded glyf point from design-space coordinates to z2d bitmap
+/// coordinates.
+///
+/// `bounds` describes the decoded outline's point/control bounds in glyf
+/// design units. `placement` describes where those bounds should land in the
+/// output bitmap after constraints are applied. Glyf coordinates are y-up; z2d
+/// surfaces are y-down, so this also flips the y axis using
+/// `placement.bitmap_height`.
+fn transformPoint(
+    p: glyf.Glyf.Outline.Point,
+    bounds: Bounds,
+    placement: Placement,
+) Point {
+    const scale_x = placement.width / bounds.width();
+    const scale_y = placement.height / bounds.height();
+    const x_design: f64 = @floatFromInt(p.x);
+    const y_design: f64 = @floatFromInt(p.y);
+    return .{
+        .x = placement.x + ((x_design - bounds.x_min) * scale_x),
+        .y = placement.bitmap_height - placement.y -
+            ((y_design - bounds.y_min) * scale_y),
+    };
+}
+
+/// Return the implied on-curve point between two off-curve TrueType control
+/// points.
+fn midpoint(a: Point, b: Point) Point {
+    return .{
+        .x = (a.x + b.x) / 2.0,
+        .y = (a.y + b.y) / 2.0,
+    };
+}
+
+fn testMetrics(width: u32, height: u32) @import("Metrics.zig") {
+    return .{
+        .cell_width = width,
+        .cell_height = height,
+        .cell_baseline = 0,
+        .underline_position = height,
+        .underline_thickness = 1,
+        .strikethrough_position = height / 2,
+        .strikethrough_thickness = 1,
+        .overline_position = 0,
+        .overline_thickness = 1,
+        .box_thickness = 1,
+        .cursor_thickness = 1,
+        .cursor_height = height,
+        .icon_height = @floatFromInt(height),
+        .icon_height_single = @floatFromInt(height),
+        .face_width = @floatFromInt(width),
+        .face_height = @floatFromInt(height),
+        .face_y = 0,
+    };
+}
+
+test "glyf_rasterize: empty outline returns empty bitmap" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var bm = try rasterize(alloc, .{ .points = &.{}, .contours = &.{} }, .{
+        .units_per_em = 1000,
+        .advance_width = 1000,
+        .line_height = 1000,
+    }, .{
+        .grid_metrics = testMetrics(20, 20),
+    });
+    defer bm.deinit(alloc);
+
+    try testing.expectEqual(@as(u32, 20), bm.width);
+    try testing.expectEqual(@as(u32, 20), bm.height);
+    try testing.expectEqual(@as(usize, 20 * 20), bm.data.len);
+    for (bm.data) |v| try testing.expectEqual(@as(u8, 0), v);
+}
+
+test "glyf_rasterize: square fills bitmap center" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const outline: glyf.Glyf.Outline = .{
+        .points = &.{
+            .{ .x = 0, .y = 0, .on_curve = true },
+            .{ .x = 1000, .y = 0, .on_curve = true },
+            .{ .x = 1000, .y = 1000, .on_curve = true },
+            .{ .x = 0, .y = 1000, .on_curve = true },
+        },
+        .contours = &.{3},
+    };
+
+    var bm = try rasterize(alloc, outline, .{
+        .units_per_em = 1000,
+        .advance_width = 1000,
+        .line_height = 1000,
+    }, .{
+        .grid_metrics = testMetrics(20, 20),
+    });
+    defer bm.deinit(alloc);
+
+    try testing.expect(bm.data[10 * bm.width + 10] > 200);
+}
+
+test "glyf_rasterize: quadratic contour renders" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const outline: glyf.Glyf.Outline = .{
+        .points = &.{
+            .{ .x = 0, .y = 0, .on_curve = true },
+            .{ .x = 500, .y = 1000, .on_curve = false },
+            .{ .x = 1000, .y = 0, .on_curve = true },
+        },
+        .contours = &.{2},
+    };
+
+    var bm = try rasterize(alloc, outline, .{
+        .units_per_em = 1000,
+        .advance_width = 1000,
+        .line_height = 1000,
+    }, .{
+        .grid_metrics = testMetrics(20, 20),
+    });
+    defer bm.deinit(alloc);
+
+    var nonzero = false;
+    for (bm.data) |v| nonzero = nonzero or v != 0;
+    try testing.expect(nonzero);
+}
+
+test "glyf_rasterize: consecutive off-curve points render" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const outline: glyf.Glyf.Outline = .{
+        .points = &.{
+            .{ .x = 0, .y = 0, .on_curve = true },
+            .{ .x = 250, .y = 1000, .on_curve = false },
+            .{ .x = 750, .y = 1000, .on_curve = false },
+            .{ .x = 1000, .y = 0, .on_curve = true },
+        },
+        .contours = &.{3},
+    };
+
+    var bm = try rasterize(alloc, outline, .{
+        .units_per_em = 1000,
+        .advance_width = 1000,
+        .line_height = 1000,
+    }, .{
+        .grid_metrics = testMetrics(20, 20),
+    });
+    defer bm.deinit(alloc);
+
+    var nonzero = false;
+    for (bm.data) |v| nonzero = nonzero or v != 0;
+    try testing.expect(nonzero);
+}
+
+test "glyf_rasterize: units per em controls baseline scale" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const outline: glyf.Glyf.Outline = .{
+        .points = &.{
+            .{ .x = 0, .y = 0, .on_curve = true },
+            .{ .x = 1000, .y = 0, .on_curve = true },
+            .{ .x = 1000, .y = 1000, .on_curve = true },
+            .{ .x = 0, .y = 1000, .on_curve = true },
+        },
+        .contours = &.{3},
+    };
+
+    var bm = try rasterize(alloc, outline, .{
+        .units_per_em = 2000,
+        .advance_width = 1000,
+        .line_height = 1000,
+    }, .{
+        .grid_metrics = testMetrics(20, 20),
+    });
+    defer bm.deinit(alloc);
+
+    // With a 2000-unit em in a 20px cell, this 1000-unit square occupies the
+    // bottom half of the cell. This matches the linked rasterizer's y=0
+    // baseline/bottom behavior and proves units_per_em is the scale reference.
+    try testing.expect(bm.data[15 * bm.width + 5] > 200);
+    try testing.expectEqual(@as(u8, 0), bm.data[5 * bm.width + 5]);
+}
+
+test "glyf_rasterize: degenerate outline returns full empty bitmap" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const outline: glyf.Glyf.Outline = .{
+        .points = &.{
+            .{ .x = 0, .y = 0, .on_curve = true },
+            .{ .x = 1000, .y = 0, .on_curve = true },
+            .{ .x = 500, .y = 0, .on_curve = true },
+        },
+        .contours = &.{2},
+    };
+
+    var bm = try rasterize(alloc, outline, .{
+        .units_per_em = 1000,
+        .advance_width = 1000,
+        .line_height = 1000,
+    }, .{
+        .grid_metrics = testMetrics(20, 20),
+    });
+    defer bm.deinit(alloc);
+
+    try testing.expectEqual(@as(u32, 20), bm.width);
+    try testing.expectEqual(@as(u32, 20), bm.height);
+    try testing.expectEqual(@as(usize, 20 * 20), bm.data.len);
+    for (bm.data) |v| try testing.expectEqual(@as(u8, 0), v);
+}
+
+test "glyf_rasterize: contour can start off curve with final on curve point" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const outline: glyf.Glyf.Outline = .{
+        .points = &.{
+            .{ .x = 500, .y = 1000, .on_curve = false },
+            .{ .x = 1000, .y = 0, .on_curve = true },
+            .{ .x = 0, .y = 0, .on_curve = true },
+        },
+        .contours = &.{2},
+    };
+
+    var bm = try rasterize(alloc, outline, .{
+        .units_per_em = 1000,
+        .advance_width = 1000,
+        .line_height = 1000,
+    }, .{
+        .grid_metrics = testMetrics(20, 20),
+    });
+    defer bm.deinit(alloc);
+
+    var nonzero = false;
+    for (bm.data) |v| nonzero = nonzero or v != 0;
+    try testing.expect(nonzero);
+}
+
+test "glyf_rasterize: contour can start with implied midpoint" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const outline: glyf.Glyf.Outline = .{
+        .points = &.{
+            .{ .x = 250, .y = 1000, .on_curve = false },
+            .{ .x = 1000, .y = 0, .on_curve = true },
+            .{ .x = 750, .y = 1000, .on_curve = false },
+            .{ .x = 0, .y = 0, .on_curve = true },
+        },
+        .contours = &.{3},
+    };
+
+    var bm = try rasterize(alloc, outline, .{
+        .units_per_em = 1000,
+        .advance_width = 1000,
+        .line_height = 1000,
+    }, .{
+        .grid_metrics = testMetrics(20, 20),
+    });
+    defer bm.deinit(alloc);
+
+    var nonzero = false;
+    for (bm.data) |v| nonzero = nonzero or v != 0;
+    try testing.expect(nonzero);
+}
+
+test "glyf_rasterize: multiple contours render independently" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const outline: glyf.Glyf.Outline = .{
+        .points = &.{
+            .{ .x = 0, .y = 0, .on_curve = true },
+            .{ .x = 400, .y = 0, .on_curve = true },
+            .{ .x = 400, .y = 400, .on_curve = true },
+            .{ .x = 0, .y = 400, .on_curve = true },
+            .{ .x = 600, .y = 600, .on_curve = true },
+            .{ .x = 1000, .y = 600, .on_curve = true },
+            .{ .x = 1000, .y = 1000, .on_curve = true },
+            .{ .x = 600, .y = 1000, .on_curve = true },
+        },
+        .contours = &.{ 3, 7 },
+    };
+
+    var bm = try rasterize(alloc, outline, .{
+        .units_per_em = 1000,
+        .advance_width = 1000,
+        .line_height = 1000,
+    }, .{
+        .grid_metrics = testMetrics(20, 20),
+    });
+    defer bm.deinit(alloc);
+
+    try testing.expect(bm.data[16 * bm.width + 4] > 200);
+    try testing.expect(bm.data[4 * bm.width + 16] > 200);
+    try testing.expectEqual(@as(u8, 0), bm.data[10 * bm.width + 10]);
+}
+
+test "glyf_rasterize: non-zero bearings preserve declared whitespace" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const outline: glyf.Glyf.Outline = .{
+        .points = &.{
+            .{ .x = 250, .y = 0, .on_curve = true },
+            .{ .x = 750, .y = 0, .on_curve = true },
+            .{ .x = 750, .y = 1000, .on_curve = true },
+            .{ .x = 250, .y = 1000, .on_curve = true },
+        },
+        .contours = &.{3},
+    };
+
+    var bm = try rasterize(alloc, outline, .{
+        .units_per_em = 1000,
+        .advance_width = 1000,
+        .line_height = 1000,
+    }, .{
+        .grid_metrics = testMetrics(20, 20),
+    });
+    defer bm.deinit(alloc);
+
+    try testing.expectEqual(@as(u8, 0), bm.data[10 * bm.width + 2]);
+    try testing.expect(bm.data[10 * bm.width + 10] > 200);
+    try testing.expectEqual(@as(u8, 0), bm.data[10 * bm.width + 17]);
+}
+
+test "glyf_rasterize: negative y coordinates descend below baseline" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const outline: glyf.Glyf.Outline = .{
+        .points = &.{
+            .{ .x = 0, .y = -250, .on_curve = true },
+            .{ .x = 1000, .y = -250, .on_curve = true },
+            .{ .x = 1000, .y = 750, .on_curve = true },
+            .{ .x = 0, .y = 750, .on_curve = true },
+        },
+        .contours = &.{3},
+    };
+
+    var bm = try rasterize(alloc, outline, .{
+        .units_per_em = 1000,
+        .advance_width = 1000,
+        .line_height = 1000,
+    }, .{
+        .grid_metrics = testMetrics(20, 20),
+    });
+    defer bm.deinit(alloc);
+
+    try testing.expectEqual(@as(u8, 0), bm.data[2 * bm.width + 10]);
+    try testing.expect(bm.data[10 * bm.width + 10] > 200);
+    try testing.expect(bm.data[18 * bm.width + 10] > 200);
+}
+
+test "glyf_rasterize: two-cell bitmap and constraint render within width" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const outline: glyf.Glyf.Outline = .{
+        .points = &.{
+            .{ .x = 0, .y = 0, .on_curve = true },
+            .{ .x = 1000, .y = 0, .on_curve = true },
+            .{ .x = 1000, .y = 1000, .on_curve = true },
+            .{ .x = 0, .y = 1000, .on_curve = true },
+        },
+        .contours = &.{3},
+    };
+
+    var bm = try rasterize(alloc, outline, .{
+        .units_per_em = 1000,
+        .advance_width = 1000,
+        .line_height = 1000,
+    }, .{
+        .grid_metrics = testMetrics(20, 20),
+        .cell_width = 2,
+        .constraint_width = 2,
+        .constraint = .{
+            .size = .cover,
+            .align_horizontal = .center,
+            .align_vertical = .center,
+        },
+    });
+    defer bm.deinit(alloc);
+
+    try testing.expectEqual(@as(u32, 40), bm.width);
+    try testing.expectEqual(@as(u32, 20), bm.height);
+    try testing.expectEqual(@as(u8, 0), bm.data[10 * bm.width + 2]);
+    try testing.expect(bm.data[10 * bm.width + 20] > 200);
+    try testing.expectEqual(@as(u8, 0), bm.data[10 * bm.width + 37]);
+}
+
+test "glyf_rasterize: line height does not change unconstrained em scale" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const outline: glyf.Glyf.Outline = .{
+        .points = &.{
+            .{ .x = 0, .y = 0, .on_curve = true },
+            .{ .x = 1000, .y = 0, .on_curve = true },
+            .{ .x = 1000, .y = 1000, .on_curve = true },
+            .{ .x = 0, .y = 1000, .on_curve = true },
+        },
+        .contours = &.{3},
+    };
+
+    var bm = try rasterize(alloc, outline, .{
+        .units_per_em = 1000,
+        .advance_width = 1000,
+        .line_height = 2000,
+    }, .{
+        .grid_metrics = testMetrics(20, 20),
+    });
+    defer bm.deinit(alloc);
+
+    try testing.expect(bm.data[10 * bm.width + 10] > 200);
+    try testing.expect(bm.data[2 * bm.width + 10] > 200);
+    try testing.expect(bm.data[17 * bm.width + 10] > 200);
+}
diff --git a/src/font/main.zig b/src/font/main.zig
index a8522afe1..0c218bbda 100644
--- a/src/font/main.zig
+++ b/src/font/main.zig
@@ -15,6 +15,7 @@ pub const Collection = @import("Collection.zig");
 pub const DeferredFace = @import("DeferredFace.zig");
 pub const Face = face.Face;
 pub const Glyph = @import("Glyph.zig");
+pub const glyf_rasterize = @import("glyf_rasterize.zig");
 pub const Metrics = @import("Metrics.zig");
 pub const opentype = @import("opentype.zig");
 pub const shape = @import("shape.zig");
diff --git a/src/font/opentype/glyf.zig b/src/font/opentype/glyf.zig
index fb9621221..6f94264c8 100644
--- a/src/font/opentype/glyf.zig
+++ b/src/font/opentype/glyf.zig
@@ -25,10 +25,10 @@ pub const Glyf = struct {
     pub const Outline = struct {
         /// List of contour end points. Calculate the full list of
         /// points using points[prev...this+1]
-        contours: []sfnt.uint16,
+        contours: []const sfnt.uint16,
 
         /// The backing storage of all points in the entry.
-        points: []Point,
+        points: []const Point,
 
         /// A single decoded point in a simple glyph contour.
         pub const Point = struct {
@@ -41,7 +41,7 @@ pub const Glyf = struct {
         ///
         /// The returned slice references `points` and is invalidated when
         /// this outline is deinitialized.
-        pub fn contour(self: Outline, index: usize) []Point {
+        pub fn contour(self: Outline, index: usize) []const Point {
             const start = if (index == 0)
                 0
             else