feat(shader-ast): add trilight lighting model

packages/shader-ast/src/index.ts

@@ -13,3 +13,4 @@ export * from "./std/fog";
 export * from "./std/lambert";
 export * from "./std/raymarch";
 export * from "./std/sdf";
+export * from "./std/trilight";

packages/shader-ast/src/std/trilight.ts

@@ -0,0 +1,42 @@
+import { Sym } from "../api";
+import {
+    add,
+    defn,
+    F32_0,
+    F32_1,
+    mul,
+    neg,
+    ret,
+    sub,
+    sym
+} from "../ast";
+import { abs, dot, max } from "../builtins";
+
+/**
+ * Tom Forsyth's Trilight lighting model.
+ *
+ * https://tomforsyth1000.github.io/papers/trilight/trilight.html
+ *
+ * @param surfNormal vec3
+ * @param lightDir vec3
+ * @param col1 vec3
+ * @param col2 vec3
+ * @param col3 vec3
+ */
+export const trilight = defn(
+    "vec3",
+    "trilight",
+    [["vec3"], ["vec3"], ["vec3"], ["vec3"], ["vec3"]],
+    (n, l, c1, c2, c3) => {
+        let d: Sym<"f32">;
+        return [
+            (d = sym(dot(n, l))),
+            ret(
+                add(
+                    add(mul(c1, max(d, F32_0)), mul(c2, sub(F32_1, abs(d)))),
+                    mul(c3, max(dot(neg(n), l), F32_0))
+                )
+            )
+        ];
+    }
+);