Orgnaization & cleanup

draw/shaders/source/backdrop_downsample.frag

@@ -2,9 +2,9 @@
 
 // Backdrop downsample fragment shader.
 // Reads source_texture (full-resolution snapshot of pre-bracket framebuffer contents) and
-// writes a downsampled copy at factor 1, 2, 4, 8, or 16. The output is the working texture
-// (sized at full swapchain resolution); larger factors only fill a sub-rect of it via the
-// CPU-set viewport. See backdrop.odin for the factor selection table (Flutter-style).
+// writes a downsampled copy at factor 1, 2, or 4. The output is the working texture (sized
+// at full swapchain resolution); larger factors only fill a sub-rect of it via the CPU-set
+// viewport. See backdrop.odin for the factor selection table (Flutter-style).
 //
 // Shader paths by factor:
 //
@@ -15,15 +15,12 @@
 //   factor=2: each output covers a 2×2 source block. Single bilinear tap at the shared
 //             corner reads all 4 source pixels with 0.25 weight.
 //
-//   factor>=4: each output covers a (factor)×(factor) source block. We use 4 bilinear taps,
-//              each at the shared corner of a (factor/2)×(factor/2) sub-block. Each tap reads
-//              4 source pixels uniformly; combined, the 4 taps sample 16 source pixels arranged
-//              uniformly across the block. This is an approximation of a true (factor)² box
-//              filter — exact at factor=4 (16 pixels = full coverage), undersampled at factor=8
-//              (16 pixels of 64) and factor=16 (16 of 256). Flutter uses a richer 13-tap COD-
-//              style downsample shader at high factors; we accept the simpler 4-tap pattern
-//              for now since the high-factor cases come with large kernels that mask any
-//              residual aliasing.
+//   factor=4: each output covers a 4×4 source block. We use 4 bilinear taps, each at the
+//             shared corner of a 2×2 sub-block. Each tap reads 4 source pixels uniformly;
+//             combined, the 4 taps sample 16 source pixels arranged uniformly across the
+//             block (full coverage at factor=4). The factor>=4 path is structured so the
+//             same shader code would extend to factor=8 (16 pixels of 64) or factor=16 (16
+//             of 256) if the CPU-side cap is ever raised, though the current cap is 4.
 //
 // The viewport+scissor are set by the CPU to limit output to the layer's work region in
 // working-texture coords (work_region_phys / factor), clamped to the texture bounds.