Source: resample.js

/**
 * @module resample
 */

/**
 * Resample the input arrays using nearest neighbor value selection.
 * @param {TypedArray[]} valueArrays The input arrays to resample
 * @param {number} inWidth The width of the input rasters
 * @param {number} inHeight The height of the input rasters
 * @param {number} outWidth The desired width of the output rasters
 * @param {number} outHeight The desired height of the output rasters
 * @returns {TypedArray[]} The resampled rasters
 */
export function resampleNearest(valueArrays, inWidth, inHeight, outWidth, outHeight) {
  const relX = inWidth / outWidth;
  const relY = inHeight / outHeight;
  return valueArrays.map((array) => {
    const newArray = new (Object.getPrototypeOf(array).constructor)(outWidth * outHeight);
    for (let y = 0; y < outHeight; ++y) {
      const cy = Math.min(Math.round(relY * y), inHeight - 1);
      for (let x = 0; x < outWidth; ++x) {
        const cx = Math.min(Math.round(relX * x), inWidth - 1);
        newArray[(y * outWidth) + x] = array[(cy * inWidth) + cx];
      }
    }
    return newArray;
  });
}

// simple linear interpolation, code from:
// https://en.wikipedia.org/wiki/Linear_interpolation#Programming_language_support
function lerp(v0, v1, t) {
  return ((1 - t) * v0) + (t * v1);
}

/**
 * Resample the input arrays using bilinear interpolation.
 * @param {TypedArray[]} valueArrays The input arrays to resample
 * @param {number} inWidth The width of the input rasters
 * @param {number} inHeight The height of the input rasters
 * @param {number} outWidth The desired width of the output rasters
 * @param {number} outHeight The desired height of the output rasters
 * @returns {TypedArray[]} The resampled rasters
 */
export function resampleBilinear(valueArrays, inWidth, inHeight, outWidth, outHeight) {
  const relX = inWidth / outWidth;
  const relY = inHeight / outHeight;

  return valueArrays.map((array) => {
    const newArray = new (Object.getPrototypeOf(array).constructor)(outWidth * outHeight);
    for (let y = 0; y < outHeight; ++y) {
      const rawY = relY * y;

      const yl = Math.floor(rawY);
      const yh = Math.min(Math.ceil(rawY), (inHeight - 1));

      for (let x = 0; x < outWidth; ++x) {
        const rawX = relX * x;
        const tx = rawX % 1;

        const xl = Math.floor(rawX);
        const xh = Math.min(Math.ceil(rawX), (inWidth - 1));

        const ll = array[(yl * inWidth) + xl];
        const hl = array[(yl * inWidth) + xh];
        const lh = array[(yh * inWidth) + xl];
        const hh = array[(yh * inWidth) + xh];

        const value = lerp(
          lerp(ll, hl, tx),
          lerp(lh, hh, tx),
          rawY % 1,
        );

        newArray[(y * outWidth) + x] = value;
      }
    }
    return newArray;
  });
}

/**
 * Resample the input arrays using nearest neighbor value selection.
 * @param {TypedArray[]} valueArrays The input arrays to resample
 * @param {number} inWidth The width of the input rasters
 * @param {number} inHeight The height of the input rasters
 * @param {number} outWidth The desired width of the output rasters
 * @param {number} outHeight The desired height of the output rasters
 * @param {string} [method = 'nearest'] The desired resampling method
 * @returns {TypedArray[]} The resampled rasters
 */
export function resample(valueArrays, inWidth, inHeight, outWidth, outHeight, method = 'nearest') {
  switch (method.toLowerCase()) {
    case 'nearest':
      return resampleNearest(valueArrays, inWidth, inHeight, outWidth, outHeight);
    case 'bilinear':
    case 'linear':
      return resampleBilinear(valueArrays, inWidth, inHeight, outWidth, outHeight);
    default:
      throw new Error(`Unsupported resampling method: '${method}'`);
  }
}