@woosh/meep-engine/src/engine/graphics/impostors/octahedral/grid/OctahedralUvEncoder.js

Pure JavaScript game engine. Fully featured and production ready.

import { vec3 } from "gl-matrix";
import { v3_dot } from "../../../../../core/geom/vec3/v3_dot.js";
import { sign_not_zero } from "../../../../../core/math/sign_not_zero.js";
import { UvEncoder } from "./UvEncoder.js";

export class OctahedralUvEncoder extends UvEncoder {
    uv_to_unit(output, input) {
        let px = input[0] * 2 - 1;
        let pz = input[1] * 2 - 1;

        const abs_x = Math.abs(px);
        const abs_z = Math.abs(pz);
        const py = 1 - abs_x - abs_z;

        if (py < 0) {
            px = sign_not_zero(px) * (1 - abs_z);
            pz = sign_not_zero(pz) * (1 - abs_x);
        }

        output[0] = px;
        output[1] = py;
        output[2] = pz;

        vec3.normalize(output, output);
    }

    unit_to_uv(output, input) {

        const input_x = input[0];
        const input_y = input[1];
        const input_z = input[2];

        const octant_x = sign_not_zero(input_x);
        const octant_y = sign_not_zero(input_y);
        const octant_z = sign_not_zero(input_z);

        // |x| + |y| + |z| = 1
        const sum = v3_dot(input_x, input_y, input_z, octant_x, octant_y, octant_z);

        let octahedron_x = input_x / sum;
        const octahedron_y = input_y / sum;
        let octahedron_z = input_z / sum;

        if (octahedron_y < 0) {
            const absolute_x = Math.abs(octahedron_x);
            const absolute_z = Math.abs(octahedron_z);

            octahedron_x = octant_x * (1 - absolute_z);
            octahedron_z = octant_z * (1 - absolute_x);
        }

        output[0] = octahedron_x;
        output[1] = octahedron_z;
    }
}