code: function (inData, outData, width, height, options) {
          /* options:
           *  - dx: horizontal offset (in pixels) of the effect.
           *  - dy: vertical offset (in pixels) of the effect.
           *  - radius: the radius of the effect in pixels.
           *  - zoom: the amount of bulge (0.0-1.0).
           */
          options = defaultOptions(options, {dx: 0, dy: 0, radius: 0, zoom: 0});
          var m1 = options.radius;
          var m2 = clamp(options.zoom, 0, 1);
          return polar(inData, outData, options.dx, options.dy, width, height, function (d, a) {
              return [d * smoothstep(0, m2, d / m1), a];
          });
      }
docs: Distortion filters