feat(shader-ast-stdlib): add additive() HOF

packages/shader-ast-stdlib/src/index.ts

@@ -7,6 +7,7 @@ export * from "./fog/linear";
 export * from "./lighting/lambert";
 export * from "./lighting/trilight";
 
+export * from "./math/additive";
 export * from "./math/clamp";
 export * from "./math/cross2";
 export * from "./math/dist-chebyshev";

packages/shader-ast-stdlib/src/math/additive.ts

@@ -0,0 +1,68 @@
+import {
+    add,
+    assign,
+    defn,
+    float,
+    FLOAT0,
+    FLOAT05,
+    FloatSym,
+    forLoop,
+    Func1,
+    inc,
+    lt,
+    mul,
+    Prim,
+    ret,
+    sym
+} from "@thi.ng/shader-ast";
+
+/**
+ * Higher order function. Takes an AST type ID, a single-arg scalar
+ * function `fn`, number of octaves (default: 4) and an optional
+ * function name. Returns a new function which computes the summed value
+ * of `fn` over the given number octaves and accepts 3 args:
+ *
+ * - position (float)
+ * - octave shift (float)
+ * - octave decay (usually 0.5)
+ *
+ * For each octave `i` [0..oct), the function is (in principle)
+ * evaluated as:
+ *
+ * n += decay / exp2(i) * fn(pos * exp2(i) + i * shift)
+ *
+ * @param fn
+ * @param oct
+ * @param name
+ */
+export const additive = <T extends Prim>(
+    type: T,
+    fn: Func1<T, "float">,
+    oct = 4,
+    name = "additive"
+) =>
+    defn("float", name, [[type], [type], ["float"]], (pos, shift, decay) => {
+        let n: FloatSym;
+        let amp: FloatSym;
+        return [
+            (n = sym(FLOAT0)),
+            (amp = sym(FLOAT05)),
+            forLoop(
+                sym("float", float(0)),
+                (i) => lt(i, float(oct)),
+                inc,
+                (i) => [
+                    assign(
+                        n,
+                        add(
+                            n,
+                            mul(amp, fn(<any>add(<any>pos, mul(i, <any>shift))))
+                        )
+                    ),
+                    assign(amp, mul(amp, decay)),
+                    assign(pos, <any>mul(<any>pos, 2))
+                ]
+            ),
+            ret(n)
+        ];
+    });