Can specify a pageClipRectFn to dynamically grab page.clipRect() from page data

lib/webshot.js

@@ -189,6 +189,9 @@ function spawnPhantom(site, path, streaming, options, cb) {
   filteredOptions.site = site;
   filteredOptions.path = path;
   filteredOptions.streaming = streaming;
+  if (options.pageClipRectFn) {
+    filteredOptions.pageClipRectFn = options.pageClipRectFn.toString();
+  }
 
   var phantomArgs = [phantomScript, JSON.stringify(filteredOptions)];
 

lib/webshot.phantom.js

@@ -111,6 +111,11 @@ var _takeScreenshot = function(status) {
           throw new Error("Unable to fetch bounds for element " + selector);
         }
       }, options.captureSelector, options.zoomFactor);
+    } else if (options.pageClipRectFn) {
+      // A hack to get the script into the page
+      page.clipRect = page.evaluate(function(fn) {
+        return eval('('+fn+')')();
+      }, options.pageClipRectFn);
     } else {
 
       //Set the rectangle of the page to render

test/options/pageClipRectFn.js

@@ -0,0 +1,34 @@
+var webshot = require('../../lib/webshot')
+  , should = require('should')
+  , fs = require('fs')
+  , im = require('imagemagick')
+  , helper = require('../helper')
+  , pngOutput = helper.pngOutput
+  , fixtures = helper.fixtures;
+
+describe('pageClipRectFn', function() {
+  this.timeout(20000);
+
+  it('screenshots the page given the rect returned from pageClipRectFn', function(done) {
+    var fixture = fixtures[2];
+    // width and height here aren't important, but they're different from fixture.width and fixture.height
+    var options = {
+      pageClipRectFn: function () {
+        return {top: 0, left: 0, width: 500, height: 600};
+      }
+    };
+
+    webshot(fixture.path, pngOutput, options, function(err) {
+      if (err) return done(err);
+
+      im.identify(pngOutput, function(err, features) {
+        if (err) return done(err);
+
+        features.width.should.equal(500);
+        features.height.should.equal(600);
+        done();
+      });
+    });
+  });
+});
+