moji-3.js

let current_data = [...];//An h * w array from current step of radar image
let next_data = [...];//An h * w array from next step of radar image

let new_data = [];
for (let i = 0; i < h; i++) {
    for (let j = 0; j < w; j++) {
        new_data[i * w + j] = parseInt((current_data[i * w + j] + next_data[i * w + j]) / 2);
    }
}
let polygons = contours()
    .size([w, h])
    .thresholds(smoothValues)
    (new_data);