Expose libvips' median filter operation (#1161)

lib/operation.js

@@ -156,6 +156,26 @@ function sharpen (sigma, flat, jagged) {
   return this;
 }
 
+/**
+ * Apply median filter using vips_rank( in, out, m, m, m * m / 2 );
+ * when used witout parameters the defaul window is 3x3
+ * @param {Number} [size] square mask size: size x size
+ * @returns {Sharp}
+ * @throws {Error} Invalid parameters
+ */
+function median (size) {
+  if (!is.defined(size)) {
+    // No arguments: default to 3x3
+    this.options.medianSize = 3;
+  } else if (is.integer(size) && is.inRange(size, 1, 1000)) {
+    // Numeric argument: specific sigma
+    this.options.medianSize = size;
+  } else {
+    throw new Error('Invalid median size ' + size);
+  }
+  return this;
+}
+
 /**
  * Blur the image.
  * When used without parameters, performs a fast, mild blur of the output image.
@@ -444,6 +464,7 @@ module.exports = function (Sharp) {
     flip,
     flop,
     sharpen,
+    median,
     blur,
     extend,
     flatten,