fix identify for calculated format

fix #859

Author: SvyatoslavPozhydaev

lib/getters.js

@@ -12,6 +12,12 @@ module.exports = function (gm) {
 
   const IDENTIFYING = 1;
   const IDENTIFIED = 2;
+  
+  /**
+   * calculated formats https://imagemagick.org/script/escape.php
+   * @type {RegExp}
+   */
+  const CALCULATED_FORMATS = /%k|%@|%#/
 
   /**
    * Map getter functions to output names.
@@ -268,8 +274,11 @@ module.exports = function (gm) {
   function makeArgs (self, val) {
     var args = [
         'identify'
-      , '-ping'
     ];
+    
+    if (!isFormatCalculated(val.format)) {
+      args.push('-ping')
+    }
 
     if (val.format) {
       args.push('-format', val.format);
@@ -282,6 +291,14 @@ module.exports = function (gm) {
     args = args.concat(self.src());
     return args;
   }
+  
+  /**
+   * @param {string|undefined} format
+   * @returns {boolean}
+   */
+  function isFormatCalculated(format) {
+    return !format || !!CALCULATED_FORMATS.test(format);
+  }
 
   /**
    * Map exif orientation codes to orientation names.