thi-ng/umbrella

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packages/distance-transform/src/transform.ts

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import type { IGrid2D, NumericArray } from "@thi.ng/api";
import type { DTMetric } from "./api.js";
import { EUCLEDIAN } from "./metric.js";

/**
 * Takes an {@link IGrid2D} and transforms it into a distance field using the
 * provided distance metric (default: {@link EUCLEDIAN}). Any non-zero values in
 * the input grid are used as seed locations for the distance field. Returns a
 * plain `Float32Array` of distance values. If `normalize` is > 0 (default: 1),
 * the result values will be normalized to the [0,normalize] interval.
 *
 * @remarks
 * The returned array will always be tightly packed, regardless of strides given
 * in input grid.
 *
 * Based on: "A general algorithm for computing Distance Transforms in linear
 * time", A. Meijster, J.B.T.M. Roerdink and W.H. Hesselink,
 *
 * Reference: http://www.cs.rug.nl/~roe/publications/dt.pdf
 *
 * @example
 * ```ts
 * // this example uses functionality from these packages:
 * // - thi.ng/pixel
 * // - thi.ng/random
 *
 * // create image with 100 random pixels set
 * const img = intBuffer(256, 256, GRAY8);
 * for(let i = 0; i < 100; i++) {
 *   img.setAt(SYSTEM.int() % img.width, SYSTEM.int() % img.height, 255);
 * }
 *
 * // compute distance field (aka voronoi)
 * const dt = distanceTransform(img, EUCLEDIAN);
 * // wrap as float pixel buffer
 * const dtImg = floatBuffer(img.width, img.height, FLOAT_GRAY, dt);
 * // ...and display
 * dtImg.blitCanvas(canvas2d(img.width, img.height, document.body).canvas);
 * ```
 *
 * @param grid - 
 * @param metric - 
 * @param normalize - 
 */
export const distanceTransform = (
    {
        data: spix,
        size: [width, height],
        stride: [sx, sy],
        offset,
    }: IGrid2D<NumericArray, number>,
    { dist, sep, post }: DTMetric = EUCLEDIAN,
    normalize = 1
) => {
    post = post || ((x) => x);
    const dest = new Float32Array(width * height);
    const g = new Uint32Array(width * height);
    const s = new Uint32Array(width);
    const t = new Uint32Array(width);
    const MAX = 0xffffffff;

    for (let x = 0; x < width; x++) {
        g[x] = spix[offset + x * sx] === 0 ? MAX : 0;
        for (let y = 1; y < height; y++) {
            const i = y * width + x;
            g[i] =
                spix[offset + y * sy + x * sx] === 0
                    ? Math.min(MAX, 1 + g[i - width])
                    : 0;
        }
        for (let y = height - 1; y-- > 0; ) {
            const i = x + y * width;
            const q = g[i + width];
            q < g[i] && (g[i] = Math.min(MAX, 1 + q));
        }
    }

    let maxD = -Infinity;
    for (let y = 0; y < height; y++) {
        const I = y * width;
        let q = 0;
        s[0] = t[0] = 0;
        for (let u = 1; u < width; u++) {
            while (
                q >= 0 &&
                dist(t[q], s[q], g[I + s[q]]) > dist(t[q], u, g[I + u])
            )
                q--;
            if (q < 0) {
                q = 0;
                s[0] = u;
            } else {
                const w = 1 + sep(s[q], u, g[I + s[q]], g[I + u]);
                if (w < width) {
                    q++;
                    s[q] = u;
                    t[q] = w;
                }
            }
        }
        for (let u = width; u-- > 0; ) {
            let d = (dest[u + I] = post(dist(u, s[q], g[I + s[q]])));
            d > maxD && (maxD = d);
            u === t[q] && q--;
        }
    }

    if (normalize > 0 && maxD > 1e-6) {
        maxD = normalize / maxD;
        for (let i = dest.length; i-- > 0; ) dest[i] *= maxD;
    }

    return dest;
};