.hss: shader graphs

An .hss file is a shader written as a graph of nodes. It covers looks that a standard material can't, such as an unlit glow, a rim effect, or moving vertices.

The format may change between releases.

Why a graph

Objects in UNAVI come from other people. A bad shader can hang the GPU, so the app never runs shader code it was handed.

A graph is a list of nodes, where each node can only use nodes listed before it. That rules out loops, and the app compiles the graph into a shader itself. wired:shading caps a graph at 128 nodes, 16 public inputs, and 4 texture samples.

Structure

A graph has up to two networks.

• Surface sets the color of each pixel. It is either Lit, shaded like a normal material, or Unlit, drawn as-is.
• Displacement is optional, and moves vertices.

A graph also has up to 16 public inputs. Each prim using the graph can override them, so one graph can look different on every prim.

Example

unavi-physgun outlines the object it has grabbed with this graph.

(
    public_inputs: [
        Color((1.0, 0.62, 0.16, 1.0)), // 0: tint, overridden per player
        Float(1.0),                    // 1: unused by the hull; see below
        Float(2.5),                    // 2: intensity
    ],
    surface: (
        nodes: [
// lines 18-23 omitted
            Fresnel(power: Input(1)),
            Saturate(x: Node(0)),
            Mul(a: Node(1), b: Input(2)),
            Mul(a: Input(0), b: Node(2)),
// lines 28-31 omitted
            Extract(v: Node(3), channel: 0),
            Extract(v: Node(3), channel: 1),
            Extract(v: Node(3), channel: 2),
            Combine4(x: Node(4), y: Node(5), z: Node(6), w: Const(Float(0.0))),
        ],
        output: Unlit((color: Node(7))),
        blend: Add,
        cull: Front,
// lines 40-42 omitted
        cast_shadows: false,
    ),
)

beam.hss in the same crate also uses a displacement network, to bend its tractor beam.

Use in .hsda

A prim's shader attribute names the .hss file, and can override public inputs by index.

shader: (
    path: "./glow.hss",
    overrides: {},
),