Implement transparent BG variant.

lib/imager.js

@@ -259,6 +259,20 @@ Imager.prototype = {
       });
     }
 
+    if (variant.transparentBG) {
+      Object.keys(variant.transparentBG).forEach(function (name) {
+        var processFiles = function (cb) {
+          var preset = {
+            name: name,
+            color: variant.transparentBG[name],
+            sep: variant.separator || '_'
+          };
+          self.transparentBG(file, preset, filename, cb);
+        };
+        asyncArr.push(processFiles);
+      });
+    }
+
     async.parallel(asyncArr, function (err, results) {
       var f = _.uniq(results).toString();
 
@@ -442,6 +456,44 @@ Imager.prototype = {
       });
   },
 
+  /**
+   * Convert to transparent background image
+   *
+   * @param {Object} file
+   * @param {Object} preset
+   * @param {String} filename
+   * @param {Function} cb
+   * @return {Imager}
+   * @api public
+   */
+
+  transparentBG: function(file, preset, filename, cb) {
+    var self = this;
+    var ct = file['type'] || file.headers['content-type'];
+    var remoteFile = preset.name + preset.sep + filename;
+    var tempFile = path.join(tempDir, 'imager_' +
+        Math.round(new Date().getTime()) + '_' +
+        Math.floor(Math.random() * 1000) + contentType[ct]);
+
+    var c = preset.color
+
+    var cmd = gm(file['path'])
+        .command('convert')
+        .out('-transparent', c)
+        .write(tempFile, function (err) {
+          if (err) return cb(err);
+          async.each(self.storage, function (storage, cb) {
+            self['pushTo' + storage](tempFile, remoteFile, filename, ct, cb);
+          }, function (err) {
+            fs.unlink(tempFile, function (err) {
+              if (err) console.error(err);
+            });
+            if (err) cb(err);
+            else cb(null, filename);
+          });
+        });
+  },
+
   /**
    * Upload all the variants to Local
    *