font/freetype: Enable bitmap glyphs for non-color faces (#3837)

This allows for crisp bitmap font rendering once again. Tested with
Terminus (TTF), and at both 1x and 2x DPI the font renders perfectly
(click for 1:1 size):

![Terminus (TTF), size 12, 96
DPI](https://github.com/user-attachments/assets/181ba561-ebe4-49df-aa41-68fc782f92b8)

<img alt="Terminus (TTF), size 12, 192 DPI"
src="https://github.com/user-attachments/assets/d34804fe-4966-42e8-bf9e-bd10570ad443"
width="384" height="50" />

src/font/embedded.zig

@@ -34,3 +34,6 @@ pub const cozette = @embedFile("res/CozetteVector.ttf");
 /// Monaspace has weird ligature behaviors we want to test in our shapers
 /// so we embed it here.
 pub const monaspace_neon = @embedFile("res/MonaspaceNeon-Regular.otf");
+
+/// Terminus TTF is a scalable font with bitmap glyphs at various sizes.
+pub const terminus_ttf = @embedFile("res/TerminusTTF-Regular.ttf");

src/font/face/freetype.zig

@@ -288,7 +288,6 @@ pub const Face = struct {
         self.face.loadGlyph(glyph_id, .{
             .render = true,
             .color = self.face.hasColor(),
-            .no_bitmap = !self.face.hasColor(),
         }) catch return false;
 
         // If the glyph is SVG we assume colorized
@@ -323,14 +322,6 @@ pub const Face = struct {
             // glyph properties before render so we don't render here.
             .render = !self.synthetic.bold,
 
-            // Disable bitmap strikes for now since it causes issues with
-            // our cell metrics and rasterization. In the future, this is
-            // all fixable so we can enable it.
-            //
-            // This must be enabled for color faces though because those are
-            // often colored bitmaps, which we support.
-            .no_bitmap = !self.face.hasColor(),
-
             // use options from config
             .no_hinting = !self.load_flags.hinting,
             .force_autohint = !self.load_flags.@"force-autohint",
@@ -385,7 +376,7 @@ pub const Face = struct {
                 return error.UnsupportedPixelMode;
             };
 
-            log.warn("converting from pixel_mode={} to atlas_format={}", .{
+            log.debug("converting from pixel_mode={} to atlas_format={}", .{
                 bitmap_ft.pixel_mode,
                 atlas.format,
             });
@@ -1005,3 +996,55 @@ test "svg font table" {
 
     try testing.expectEqual(430, table.len);
 }
+
+const terminus_i =
+    \\........
+    \\........
+    \\...#....
+    \\...#....
+    \\........
+    \\..##....
+    \\...#....
+    \\...#....
+    \\...#....
+    \\...#....
+    \\...#....
+    \\..###...
+    \\........
+    \\........
+    \\........
+    \\........
+;
+// Including the newline
+const terminus_i_pitch = 9;
+
+test "bitmap glyph" {
+    const alloc = testing.allocator;
+    const testFont = font.embedded.terminus_ttf;
+
+    var lib = try Library.init();
+    defer lib.deinit();
+
+    var atlas = try font.Atlas.init(alloc, 512, .grayscale);
+    defer atlas.deinit(alloc);
+
+    // Any glyph at 12pt @ 96 DPI is a bitmap
+    var ft_font = try Face.init(lib, testFont, .{ .size = .{ .points = 12 } });
+    defer ft_font.deinit();
+
+    // glyph 77 = 'i'
+    const glyph = try ft_font.renderGlyph(alloc, &atlas, 77, .{});
+
+    // should render crisp
+    try testing.expectEqual(8, glyph.width);
+    try testing.expectEqual(16, glyph.height);
+    for (0..glyph.height) |y| {
+        for (0..glyph.width) |x| {
+            const pixel = terminus_i[y * terminus_i_pitch + x];
+            try testing.expectEqual(
+                @as(u8, if (pixel == '#') 255 else 0),
+                atlas.data[(glyph.atlas_y + y) * atlas.size + (glyph.atlas_x + x)],
+            );
+        }
+    }
+}

src/font/face/freetype_convert.zig

@@ -43,28 +43,16 @@ pub fn monoToGrayscale(alloc: Allocator, bm: Bitmap) Allocator.Error!Bitmap {
     var buf = try alloc.alloc(u8, bm.width * bm.rows);
     errdefer alloc.free(buf);
 
-    // width divided by 8 because each byte has 8 pixels. This is therefore
-    // the number of bytes in each row.
-    const bytes_per_row = bm.width >> 3;
-
-    var source_i: usize = 0;
-    var target_i: usize = 0;
-    var i: usize = bm.rows;
-    while (i > 0) : (i -= 1) {
-        var j: usize = bytes_per_row;
-        while (j > 0) : (j -= 1) {
-            var bit: u4 = 8;
-            while (bit > 0) : (bit -= 1) {
-                const mask = @as(u8, 1) << @as(u3, @intCast(bit - 1));
-                const bitval: u8 = if (bm.buffer[source_i + (j - 1)] & mask > 0) 0xFF else 0;
-                buf[target_i] = bitval;
-                target_i += 1;
+    for (0..bm.rows) |y| {
+        const row_offset = y * @as(usize, @intCast(bm.pitch));
+        for (0..bm.width) |x| {
+            const byte_offset = row_offset + @divTrunc(x, 8);
+            const mask = @as(u8, 1) << @intCast(7 - (x % 8));
+            const bit: u8 = @intFromBool((bm.buffer[byte_offset] & mask) != 0);
+            buf[y * bm.width + x] = bit * 255;
         }
     }
 
-        source_i += @intCast(bm.pitch);
-    }
-
     var copy = bm;
     copy.buffer = buf.ptr;
     copy.pixel_mode = freetype.c.FT_PIXEL_MODE_GRAY;

src/font/res/README.md

@@ -25,6 +25,9 @@ This project uses several fonts which fall under the SIL Open Font License (OFL-
   - [Copyright 2013 Google LLC](https://github.com/googlefonts/noto-emoji/blob/main/LICENSE)
 - Cozette (MIT)
   - [Copyright (c) 2020, Slavfox](https://github.com/slavfox/Cozette/blob/main/LICENSE)
+- Terminus TTF (OFL-1.1)
+  - [Copyright (c) 2010-2020 Dimitar Toshkov Zhekov with Reserved Font Name "Terminus Font"](https://sourceforge.net/projects/terminus-font/)
+  - [Copyright (c) 2011-2023 Tilman Blumenbach with Reserved Font Name "Terminus (TTF)"](https://files.ax86.net/terminus-ttf/)
 
 A full copy of the OFL license can be found at [OFL.txt](./OFL.txt).
 An accompanying FAQ is also available at <https://openfontlicense.org/>.