From f75f3d40e434a3f5faee36b88307ab62e6090cc1 Mon Sep 17 00:00:00 2001
From: Yiheng Wu <36156959+jingkaimori@users.noreply.github.com>
Date: Wed, 18 Jun 2025 21:37:50 +0800
Subject: [PATCH] Calculate font unit per pixel dynamically (#1629)

This PR turns font unit per pixel (a "magic number") into actually
computed value. Previously the "font unit per pixel" is hardcoded as
float `4.0`, because `units_per_EM` field of unscii is 64, and default
font size is 16. If another font is loaded, layout will be corrupted by
invalid scaling of `glyphPositions`. Note that only transformation from
font unit to pixel is affected, no visual change is made, and width of
glyph(character) is not changed.
---
 src/graphics.zig | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/graphics.zig b/src/graphics.zig
index 37fa046a..88663fad 100644
--- a/src/graphics.zig
+++ b/src/graphics.zig
@@ -843,10 +843,10 @@ pub const TextBuffer = struct { // MARK: TextBuffer
 		// Merge it all together:
 		self.glyphs = allocator.alloc(GlyphData, glyphInfos.len);
 		for(self.glyphs, 0..) |*glyph, i| {
-			glyph.x_advance = @as(f32, @floatFromInt(glyphPositions[i].x_advance))/4.0;
-			glyph.y_advance = @as(f32, @floatFromInt(glyphPositions[i].y_advance))/4.0;
-			glyph.x_offset = @as(f32, @floatFromInt(glyphPositions[i].x_offset))/4.0;
-			glyph.y_offset = @as(f32, @floatFromInt(glyphPositions[i].y_offset))/4.0;
+			glyph.x_advance = @as(f32, @floatFromInt(glyphPositions[i].x_advance))/TextRendering.fontUnitsPerPixel;
+			glyph.y_advance = @as(f32, @floatFromInt(glyphPositions[i].y_advance))/TextRendering.fontUnitsPerPixel;
+			glyph.x_offset = @as(f32, @floatFromInt(glyphPositions[i].x_offset))/TextRendering.fontUnitsPerPixel;
+			glyph.y_offset = @as(f32, @floatFromInt(glyphPositions[i].y_offset))/TextRendering.fontUnitsPerPixel;
 			glyph.character = @intCast(parser.parsedText.items[textIndexGuess[i]]);
 			glyph.index = glyphInfos[i].codepoint;
 			glyph.cluster = glyphInfos[i].cluster;
@@ -1149,6 +1149,7 @@ const TextRendering = struct { // MARK: TextRendering
 	var textureWidth: i32 = 1024;
 	const textureHeight: i32 = 16;
 	var textureOffset: i32 = 0;
+	var fontUnitsPerPixel: f32 = undefined;
 
 	fn ftError(errorCode: hbft.FT_Error) !void {
 		if(errorCode == 0) return;
@@ -1176,6 +1177,7 @@ const TextRendering = struct { // MARK: TextRendering
 		try ftError(hbft.FT_Set_Pixel_Sizes(freetypeFace, 0, textureHeight));
 		harfbuzzFace = hbft.hb_ft_face_create_referenced(freetypeFace);
 		harfbuzzFont = hbft.hb_font_create(harfbuzzFace);
+		fontUnitsPerPixel = @as(f32, @floatFromInt(freetypeFace.*.units_per_EM))/@as(f32, @floatFromInt(textureHeight));
 
 		glyphMapping = .init(main.globalAllocator);
 		glyphData = .init(main.globalAllocator);