@woosh/meep-engine/src/engine/graphics/sh3/path_tracer/sampling/random_in_hemisphere.js

Pure JavaScript game engine. Fully featured and production ready.

import { v3_dot } from "../../../../../core/geom/vec3/v3_dot.js";

/**
 *
 * @param {function} random
 * @param {number[]} result
 * @param {number} result_offset
 * @param {number[]} normal
 * @param {number} normal_offset
 */
export function random_in_hemisphere(
    random,
    result,
    result_offset,
    normal,
    normal_offset
) {
    const nx = normal[normal_offset + 0];
    const ny = normal[normal_offset + 1];
    const nz = normal[normal_offset + 2];

    const phi = Math.PI * 2 * random();

    const cosTheta = random() * 2 - 1;

    const theta = Math.acos(cosTheta);

    const sinTheta = Math.sin(theta);

    //compute coordinates
    const x = sinTheta * Math.cos(phi);
    const y = sinTheta * Math.sin(phi);
    const z = cosTheta;

    if (v3_dot(x, y, z, nx, ny, nz) < 0) {
        result[result_offset] = -x;
        result[result_offset + 1] = -y;
        result[result_offset + 2] = -z;
    } else {
        //write the result
        result[result_offset] = x;
        result[result_offset + 1] = y;
        result[result_offset + 2] = z;
    }
}