;; add a brush that blends by lightening the color
(add-linear-brush
 :transparent2
 {:blend {:from "ONE" :to "ONE"}
 :vertex-shader {(g/gl-position) default-vertex-position
 v-value a-value}
 :fragment-shader
 {(g/gl-frag-color) (g/vec4 u-color 1)}})

;; save where the stroke started (needed for sin wave)
(set! stroke-started
      (fn [point]
          (swap! app-state assoc :start-point point)))

;; make a sin wave
(set! constrain-stroke
      (fn [point dir]
          (let [p (geom/+ point (v/vec2 (nth dir 0) 0))]
            (v/vec2 (nth p 0)
                    (+ (nth (:start-point @app-state) 1)
                       (* (.sin js/Math (/ (nth p 0) 50)) 30))
                    ))))

;; lock to the x-axis
(set! constrain-stroke
      (fn [point dir]
          (geom/+ point (v/vec2 (nth dir 0) 0))))

(repl-swap! :current-brush :transparent2)
(repl-swap! :background-color "#b0ccbb")