trim-bounds.cljs

(defn trimmed-bounds
  [ctx w h]
  (let [data (.-data (.getImageData ctx 0 0 w h))]
    (loop [y 0 minx Infinity miny Infinity maxx -Infinity maxy -Infinity]
      (if (< y h)
        (let [yw (* y w)
              minx' (loop [x 0]
                      (when (< x w)
                        (if (= (aget data (+ (* (+ yw x) 4) 3)) 0)
                          (recur (inc x))
                          x)))
              maxx' (loop [x (- w 1)]
                      (when (> x -1)
                        (if (= (aget data (+ (* (+ yw x) 4) 3)) 0)
                          (recur (dec x))
                          x)))]
          (if (and (nil? minx') (nil? maxx'))
            (recur (inc y) minx miny maxx maxy)
            (recur (inc y) (min minx minx') (min miny y) (max maxx maxx') (max maxy y))))
        [minx miny maxx maxy]))))