Merge pull request #5052 from rt1301/#5045-filter-doc

Updated the p5.Image filter() method documentation

src/image/p5.Image.js

@@ -664,6 +664,44 @@ p5.Image.prototype.mask = function(p5Image) {
 /**
  * Applies an image filter to a <a href="#/p5.Image">p5.Image</a>
  *
+ * THRESHOLD
+ * Converts the image to black and white pixels depending if they are above or
+ * below the threshold defined by the level parameter. The parameter must be
+ * between 0.0 (black) and 1.0 (white). If no level is specified, 0.5 is used.
+ *
+ * GRAY
+ * Converts any colors in the image to grayscale equivalents. No parameter
+ * is used.
+ *
+ * OPAQUE
+ * Sets the alpha channel to entirely opaque. No parameter is used.
+ *
+ * INVERT
+ * Sets each pixel to its inverse value. No parameter is used.
+ *
+ * POSTERIZE
+ * Limits each channel of the image to the number of colors specified as the
+ * parameter. The parameter can be set to values between 2 and 255, but
+ * results are most noticeable in the lower ranges.
+ *
+ * BLUR
+ * Executes a Gaussian blur with the level parameter specifying the extent
+ * of the blurring. If no parameter is used, the blur is equivalent to
+ * Gaussian blur of radius 1. Larger values increase the blur.
+ *
+ * ERODE
+ * Reduces the light areas. No parameter is used.
+ *
+ * DILATE
+ * Increases the light areas. No parameter is used.
+ *
+ * filter() does not work in WEBGL mode.
+ * A similar effect can be achieved in WEBGL mode using custom
+ * shaders. Adam Ferriss has written
+ * a <a href="https://github.com/aferriss/p5jsShaderExamples"
+ * target='_blank'>selection of shader examples</a> that contains many
+ * of the effects present in the filter examples.
+ *
  * @method filter
  * @param  {Constant} filterType  either THRESHOLD, GRAY, OPAQUE, INVERT,
  *                                POSTERIZE, ERODE, DILATE or BLUR.