Coordinates

Sub-cell offsets aren’t shared code across backends

retroglyph-window’s Presenter trait specifies the sub-cell offset (Tile::dx/dy) and spill contract once, so the CPU rasterizer (retroglyph-software) and the GPU ones (retroglyph-gl, retroglyph-wgpu) produce the same pixels without mirrored per-backend comments that reference each other and drift when only one is touched. What the contract does not specify is a shared implementation: the GPU backends shift a quad’s vertex position in their vertex shader, while retroglyph-software shifts origin_x/origin_y in a CPU blit. These are irreducibly different mechanics that must nonetheless agree on the same four points (offsets are unscaled font pixels, backgrounds stay unshifted, spill is uniform in all four directions, and a two-pass background-then-glyph draw is what makes that uniformity possible); see the Presenter trait’s own doc comment for those four points.