flowchart LR
    vs["vertex shader<br/>writes attr per vertex<br/>(unchanged)"] --> rast["Metal rasterizer"]
    rast -->|"no per-vertex inputs"| x["GetAttributeAtVertex(attr, i)<br/>not expressible"]
    rast --> lin["attr interpolated<br/>without perspective"]
    rast --> bary["no-perspective<br/>barycentrics b"]
    subgraph ps["start of the fragment shader (patch 0019)"]
        direction TB
        d["quad derivatives<br/>dx, dy of attr and b"] --> solve["solve 2x2 system<br/>E1, E2 = attr change per b1, b2"]
        solve --> v["v0 = attr − b1·E1 − b2·E2<br/>v1 = v0 + E1, v2 = v0 + E2"]
    end
    lin --> d
    bary --> d
    v --> use["attr_vertices[i]<br/>error ≤ 6.7e-5 at 250"]