metal: handle HiDPI scaling

src/renderer/Metal.zig

@@ -1,4 +1,11 @@
 //! Renderer implementation for Metal.
+//!
+//! Open questions:
+//!
+//!   - This requires a "px_scale" uniform to account for pixel scaling
+//!     issues with Retina. I'm not 100% sure why this is necessary and why
+//!     this doesn't happen with OpenGL.
+//!
 pub const Metal = @This();
 
 const std = @import("std");
@@ -64,7 +71,16 @@ const GPUCell = extern struct {
 };
 
 const GPUUniforms = extern struct {
+    /// The projection matrix for turning world coordinates to normalized.
+    /// This is calculated based on the size of the screen.
     projection_matrix: math.Mat,
+
+    /// A scale factor to apply to all pixels given as input (including
+    /// in this uniform i.e. cell_size). This is due to HiDPI screens (Retina)
+    /// mismatch with the window.
+    px_scale: [2]f32,
+
+    /// Size of a single cell in pixels, unscaled.
     cell_size: [2]f32,
 };
 
@@ -292,11 +308,8 @@ pub fn render(
                 @floatCast(f32, bounds.size.height),
                 0,
             ),
-
-            .cell_size = .{
-                self.cell_size.width * scaleX,
-                self.cell_size.height * scaleY,
-            },
+            .px_scale = .{ scaleX, scaleY },
+            .cell_size = .{ self.cell_size.width, self.cell_size.height },
         };
     }
 

src/shaders/cell.metal

@@ -8,6 +8,7 @@ enum Mode : uint8_t {
 
 struct Uniforms {
   float4x4 projection_matrix;
+  float2 px_scale;
   float2 cell_size;
 };
 
@@ -45,8 +46,11 @@ vertex VertexOut uber_vertex(
   VertexIn input [[ stage_in ]],
   constant Uniforms &uniforms [[ buffer(1) ]]
 ) {
+  // TODO: scale with cell width
+  float2 cell_size = uniforms.cell_size * uniforms.px_scale;
+
   // Convert the grid x,y into world space x, y by accounting for cell size
-  float2 cell_pos = uniforms.cell_size * input.grid_pos;
+  float2 cell_pos = cell_size * input.grid_pos;
 
   // Turn the cell position into a vertex point depending on the
   // vertex ID. Since we use instanced drawing, we have 4 vertices
@@ -62,9 +66,6 @@ vertex VertexOut uber_vertex(
   position.x = (vid == 0 || vid == 1) ? 1.0f : 0.0f;
   position.y = (vid == 0 || vid == 3) ? 0.0f : 1.0f;
 
-  // TODO: scale
-  float2 cell_size = uniforms.cell_size;
-
   VertexOut out;
   out.mode = input.mode;
   out.color = float4(input.color) / 255.0f;
@@ -73,14 +74,14 @@ vertex VertexOut uber_vertex(
     // Calculate the final position of our cell in world space.
     // We have to add our cell size since our vertices are offset
     // one cell up and to the left. (Do the math to verify yourself)
-    cell_pos = cell_pos + uniforms.cell_size * position;
+    cell_pos = cell_pos + cell_size * position;
 
     out.position = uniforms.projection_matrix * float4(cell_pos.x, cell_pos.y, 0.0f, 1.0f);
     break;
 
   case MODE_FG:
-    float2 glyph_size = float2(input.glyph_size);
-    float2 glyph_offset = float2(input.glyph_offset);
+    float2 glyph_size = float2(input.glyph_size) * uniforms.px_scale;
+    float2 glyph_offset = float2(input.glyph_offset) * uniforms.px_scale;
 
     // TODO: downsampling
 

src/shaders/cell.v.glsl

@@ -147,7 +147,7 @@ void main() {
         glyph_offset_calc.y = cell_size_scaled.y - glyph_offset_calc.y;
 
         // Calculate the final position of the cell.
-        cell_pos = cell_pos + glyph_size_downsampled * position + glyph_offset_calc;
+        cell_pos = cell_pos + (glyph_size_downsampled * position) + glyph_offset_calc;
         gl_Position = projection * vec4(cell_pos, cell_z, 1.0);
 
         // We need to convert our texture position and size to normalized