feat(shader-ast-stdlib): add normal2()

packages/shader-ast-stdlib/src/tex/normal.ts

@@ -0,0 +1,50 @@
+import type { Sampler2DSym, Vec2Sym, Vec2Term } from "@thi.ng/shader-ast";
+import { F, S2D, V2, V3 } from "@thi.ng/shader-ast/api/types";
+import { defn, ret } from "@thi.ng/shader-ast/ast/function";
+import { vec2, vec3 } from "@thi.ng/shader-ast/ast/lit";
+import { add, sub } from "@thi.ng/shader-ast/ast/ops";
+import { sym } from "@thi.ng/shader-ast/ast/sym";
+import { $x, $y } from "@thi.ng/shader-ast/ast/swizzle";
+import { normalize } from "@thi.ng/shader-ast/builtin/math";
+import { texture } from "@thi.ng/shader-ast/builtin/texture";
+
+const sample1 = (tex: Sampler2DSym, uv: Vec2Sym, unit: Vec2Term) =>
+	sub($x(texture(tex, sub(uv, unit))), $x(texture(tex, add(uv, unit))));
+
+/**
+ * Computes the surface normal from a grayscale texture at given `uv` position
+ * and using given `step` to define the half-distances between the 4 neighbor
+ * lookups (north/south/east/west). The `z` value is used as the normal's Z
+ * component (prior to normalization). `eps` is being added to all components to
+ * avoid zero-vectors.
+ *
+ * @remarks
+ * The function only uses the X (red) component of the texture.
+ *
+ * @param tex
+ * @param uv
+ * @param step
+ * @param z
+ * @param eps
+ */
+export const normal2 = defn(
+	V3,
+	null,
+	[S2D, V2, V2, F, F],
+	(tex, uv, step, z, eps) => {
+		let sx: Vec2Sym;
+		let sy: Vec2Sym;
+		return [
+			(sx = sym(vec2($x(step), 0))),
+			(sy = sym(vec2(0, $y(step)))),
+			ret(
+				normalize(
+					add(
+						vec3(sample1(tex, uv, sx), sample1(tex, uv, sy), z),
+						eps
+					)
+				)
+			),
+		];
+	}
+);