@woosh/meep-engine/src/core/geom/3d/plane/orient3d_fast.js

Pure JavaScript game engine. Fully featured and production ready.

/**
 * Non-robust
 *
 * Return a positive value if the point pd lies below the
 * plane passing through pa, pb, and pc; "below" is defined so
 * that pa, pb, and pc appear in counterclockwise order when
 * viewed from above the plane.  Returns a negative value if
 * pd lies above the plane.  Returns zero if the points are
 * coplanar.  The result is also a rough approximation of six
 * times the signed volume of the tetrahedron defined by the
 * four points.
 *
 * NOTE: ported from http://www.cs.cmu.edu/afs/cs/project/quake/public/code/predicates.c
 *
 * @param {number[]} points
 * @param {number} a index of point a
 * @param {number} b index of point b
 * @param {number} c index of point c
 * @param {number} d index of point d
 * @returns {number}
 */
export function orient3d_fast(points, a, b, c, d) {
    const a3 = a * 3;
    const d3 = d * 3;
    const b3 = b * 3;
    const c3 = c * 3;

    const d_x = points[d3];
    const d_y = points[d3 + 1];
    const d_z = points[d3 + 2];

    const adx = points[a3] - d_x;
    const ady = points[a3 + 1] - d_y;
    const adz = points[a3 + 2] - d_z;

    const bdx = points[b3] - d_x;
    const bdy = points[b3 + 1] - d_y;
    const bdz = points[b3 + 2] - d_z;

    const cdx = points[c3] - d_x;
    const cdy = points[c3 + 1] - d_y;
    const cdz = points[c3 + 2] - d_z;

    return adx * (bdy * cdz - bdz * cdy)
        + bdx * (cdy * adz - cdz * ady)
        + cdx * (ady * bdz - adz * bdy);
}