gfx: font: add glyph rendering functionality

Helps hexops/mach#877

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

src/gfx/font/main.zig

@@ -1,3 +1,4 @@
+const std = @import("std");
 const mach = @import("../../main.zig");
 const testing = mach.testing;
 const math = mach.math;
@@ -16,12 +17,13 @@ pub const TextRun = TextRunInterface(if (@import("builtin").cpu.arch == .wasm32)
 fn FontInterface(comptime T: type) type {
     assertDecl(T, "initBytes", fn (font_bytes: []const u8) anyerror!T);
     assertDecl(T, "shape", fn (f: *const T, r: *TextRun) anyerror!void);
-    assertDecl(T, "deinit", fn (*const T) void);
+    assertDecl(T, "render", fn (f: *T, allocator: std.mem.Allocator, glyph_index: u32, opt: RenderOptions) anyerror!RenderedGlyph);
+    assertDecl(T, "deinit", fn (*T, allocator: std.mem.Allocator) void);
     return T;
 }
 
 fn TextRunInterface(comptime T: type) type {
-    assertField(T, "font_size_px", u32);
+    assertField(T, "font_size_px", f32);
     assertField(T, "px_density", u8);
     assertDecl(T, "init", fn () anyerror!T);
     assertDecl(T, "addText", fn (s: *const T, []const u8) void);
@@ -47,7 +49,7 @@ fn assertField(comptime T: anytype, comptime name: []const u8, comptime Field: t
 pub const px_per_pt = 4.0 / 3.0;
 
 pub const Glyph = struct {
-    glyph_index: u21,
+    glyph_index: u32,
     cluster: u32,
     advance: Vec2,
     offset: Vec2,
@@ -65,14 +67,31 @@ pub const Glyph = struct {
     // }
 };
 
+pub const RGBA32 = extern struct {
+    r: u8,
+    g: u8,
+    b: u8,
+    a: u8,
+};
+
+pub const RenderOptions = struct {
+    font_size_px: f32,
+};
+
+pub const RenderedGlyph = struct {
+    bitmap: ?[]const RGBA32,
+    width: u32,
+    height: u32,
+};
+
 test {
-    const std = @import("std");
     std.testing.refAllDeclsRecursive(@This());
 
     // Load a font
+    const allocator = std.testing.allocator;
     const font_bytes = @import("font-assets").fira_sans_regular_ttf;
-    const font = try Font.initBytes(font_bytes);
-    defer font.deinit();
+    var font = try Font.initBytes(font_bytes);
+    defer font.deinit(allocator);
 
     // Create a text shaper
     var run = try TextRun.init();
@@ -85,6 +104,10 @@ test {
     run.addText(text);
     try font.shape(&run);
 
+    // Test rendering the first glyph
+    const rendered = try font.render(allocator, 176, .{ .font_size_px = run.font_size_px });
+    _ = rendered;
+
     // TODO: https://github.com/hexops/mach/issues/1048
     // TODO: https://github.com/hexops/mach/issues/1049
     //

src/gfx/font/native/Font.zig

@@ -3,6 +3,9 @@ const ft = @import("mach-freetype");
 const harfbuzz = @import("mach-harfbuzz");
 const TextRun = @import("TextRun.zig");
 const px_per_pt = @import("../main.zig").px_per_pt;
+const RenderedGlyph = @import("../main.zig").RenderedGlyph;
+const RenderOptions = @import("../main.zig").RenderOptions;
+const RGBA32 = @import("../main.zig").RGBA32;
 
 const Font = @This();
 
@@ -11,6 +14,7 @@ var freetype_ready: bool = false;
 var freetype: ft.Library = undefined;
 
 face: ft.Face,
+bitmap: std.ArrayListUnmanaged(RGBA32) = .{},
 
 pub fn initFreetype() !void {
     freetype_ready_mu.lock();
@@ -40,8 +44,9 @@ pub fn shape(f: *const Font, r: *TextRun) anyerror!void {
     const hb_font = harfbuzz.Font.init(hb_face);
     defer hb_font.deinit();
 
-    const font_size_pt = @as(f32, @floatFromInt(r.font_size_px)) / px_per_pt;
-    hb_font.setScale(@as(i32, @intFromFloat(font_size_pt)) * 256, @as(i32, @intFromFloat(font_size_pt)) * 256);
+    const font_size_pt = r.font_size_px / px_per_pt;
+    const font_size_pt_frac: i32 = @intFromFloat(font_size_pt * 256.0);
+    hb_font.setScale(font_size_pt_frac, font_size_pt_frac);
     hb_font.setPTEM(font_size_pt);
 
     // TODO: optionally pass shaping features?
@@ -52,6 +57,53 @@ pub fn shape(f: *const Font, r: *TextRun) anyerror!void {
     r.positions = r.buffer.getGlyphPositions() orelse return error.OutOfMemory;
 }
 
-pub fn deinit(f: *const Font) void {
-    f.face.deinit();
+pub fn render(f: *Font, allocator: std.mem.Allocator, glyph_index: u32, opt: RenderOptions) anyerror!RenderedGlyph {
+    // TODO: DPI configuration
+    const dpi = 72;
+    const font_size_pt = opt.font_size_px / px_per_pt;
+    const font_size_pt_frac: i32 = @intFromFloat(font_size_pt * 64.0);
+    f.face.setCharSize(font_size_pt_frac, font_size_pt_frac, dpi, dpi) catch return error.RenderError;
+    f.face.loadGlyph(glyph_index, .{ .render = true }) catch return error.RenderError;
+
+    const glyph = f.face.glyph();
+    const glyph_bitmap = glyph.bitmap();
+    const buffer = glyph_bitmap.buffer();
+    const width = glyph_bitmap.width();
+    const height = glyph_bitmap.rows();
+    const margin = 1;
+
+    if (buffer == null) return RenderedGlyph{
+        .bitmap = null,
+        .width = width + (margin * 2),
+        .height = height + (margin * 2),
+    };
+
+    // Add 1 pixel padding to texture to avoid bleeding over other textures. This is part of the
+    // render() API contract.
+    f.bitmap.clearRetainingCapacity();
+    const num_pixels = (width + (margin * 2)) * (height + (margin * 2));
+    // TODO: handle OOM here
+    f.bitmap.ensureTotalCapacity(allocator, num_pixels) catch return error.RenderError;
+    f.bitmap.resize(allocator, num_pixels) catch return error.RenderError;
+    for (f.bitmap.items, 0..) |*data, i| {
+        const x = i % (width + (margin * 2));
+        const y = i / (width + (margin * 2));
+        if (x < margin or x > (width + margin) or y < margin or y > (height + margin)) {
+            data.* = RGBA32{ .r = 0, .g = 0, .b = 0, .a = 0 };
+        } else {
+            const alpha = buffer.?[((y - margin) * width + (x - margin)) % buffer.?.len];
+            data.* = RGBA32{ .r = 0, .g = 0, .b = 0, .a = alpha };
+        }
+    }
+
+    return RenderedGlyph{
+        .bitmap = f.bitmap.items,
+        .width = width + (margin * 2),
+        .height = height + (margin * 2),
+    };
+}
+
+pub fn deinit(f: *Font, allocator: std.mem.Allocator) void {
+    f.face.deinit();
+    f.bitmap.deinit(allocator);
 }

src/gfx/font/native/TextRun.zig

@@ -7,14 +7,14 @@ const Glyph = @import("../main.zig").Glyph;
 
 const TextRun = @This();
 
-font_size_px: u32 = 16.0,
+font_size_px: f32 = 16.0,
 px_density: u8 = 1,
 
 // Internal / private fields.
 buffer: harfbuzz.Buffer,
 index: usize = 0,
 infos: []harfbuzz.GlyphInfo = undefined,
-positions: []harfbuzz.GlyphPosition = undefined,
+positions: []harfbuzz.Position = undefined,
 
 pub fn init() anyerror!TextRun {
     return TextRun{
@@ -32,7 +32,7 @@ pub fn next(s: *TextRun) ?Glyph {
     const pos = s.positions[s.index];
     s.index += 1;
     return Glyph{
-        .glyph_index = @intCast(info.codepoint),
+        .glyph_index = info.codepoint,
         // TODO: should we expose this? Is there a browser equivalent? do we need it?
         // .var1 = @intCast(info.var1),
         // .var2 = @intCast(info.var2),