It works!

src/assets/Textures.zig

@@ -170,6 +170,11 @@ fn loadTexture(engine: *Engine, filename: []const u8, usage: Texture.Usage, temp
     var img = try stbi.Image.loadFromMemory(file_buf, usage.samplesPerTexel());
     defer img.deinit();
     std.debug.assert(img.num_components == usage.samplesPerTexel());
+    if (usage == .normal) {
+        for (img.data) |*sample| {
+            sample.* = sample.* +% 128;
+        }
+    }
 
     var texture = try Texture.init(engine, .{
         .width = img.width,