Merge branch 'add_contrast_filter' of https://github.com/sarthak-saxena/konva into sarthak-saxena-add_contrast_filter

src/filters/Contrast.js

@@ -0,0 +1,70 @@
+(function (Konva) {
+  'use strict';
+  /**
+   * Contrast Filter.
+   * @function
+   * @memberof Konva.Filters
+   * @param {Object} imageData
+   * @example
+   * node.cache();
+   * node.filters([Konva.Filters.Contrast]);
+   * node.contrast(10);
+   */
+
+  Konva.Filters.Contrast = function (imageData) {
+    var adjust = Math.pow((parseInt(this.contrast()) + 100) / 100, 2);
+
+    var data = imageData.data,
+      nPixels = data.length,
+      red = 150,
+      green = 150,
+      blue = 150,
+      i;
+
+    for (i = 0; i < nPixels; i += 4) {
+      red = data[i];
+      green = data[i + 1];
+      blue = data[i + 2];
+
+      //Red channel
+      red /= 255;
+      red -= 0.5;
+      red *= adjust;
+      red += 0.5;
+      red *= 255;
+
+      //Green channel
+      green /= 255;
+      green -= 0.5;
+      green *= adjust;
+      green += 0.5;
+      green *= 255;
+
+      //Blue channel
+      blue /= 255;
+      blue -= 0.5;
+      blue *= adjust;
+      blue += 0.5;
+      blue *= 255;
+
+      red = (red < 0 ? 0 : (red > 255 ? 255 : red));
+      green = (green < 0 ? 0 : (green > 255 ? 255 : green));
+      blue = (blue < 0 ? 0 : (blue > 255 ? 255 : blue));
+
+      data[i] = red;
+      data[i + 1] = green;
+      data[i + 2] = blue;
+    }
+  };
+
+  /**
+   * get/set filter contrast.  The contrast is a number between -100 and 100.
+   * Use with {@link Konva.Filters.Contrast} filter.
+   * @name contrast
+   * @method
+   * @memberof Konva.Node.prototype
+   * @param {Number} contrast value between -100 and 100
+   * @returns {Number}
+   */
+  Konva.Factory.addGetterSetter(Konva.Node, 'contrast', 0, null, Konva.Factory.afterSetFilter);
+})(Konva);