added; proto.selectFrame(#). closes #202

lib/args.js

@@ -10,6 +10,13 @@ var argsToArray = require('./utils').argsToArray;
  */
 
 module.exports = function (proto) {
+  // change the specified frame.
+  // See #202.
+  proto.selectFrame = function (frame) {
+    if (typeof frame === 'number')
+      this.sourceFrames = '[' + frame + ']';
+    return this;
+  }
 
   // define the sub-command to use, http://www.graphicsmagick.org/utilities.html
   proto.command = proto.subCommand = function subCommand (name){

test/selectFrame.js

@@ -0,0 +1,20 @@
+var assert = require('assert');
+var fs = require('fs')
+
+module.exports = function (_, dir, finish, gm) {
+  var m = gm(dir + '/original.gif[0]')
+
+  if (!gm.integration)
+    return finish();
+
+  m.identify('%#', function (err, hash1) {
+    if (err) return finish(err);
+
+    m.selectFrame(2).identify('%#', function (err, hash2) {
+      if (err) return finish(err);
+
+      assert.ok(hash1.toString().trim() !== hash2.toString().trim())
+      finish();
+    })
+  })
+}