Page.render() returns false

Question

I'm using phantomjs for taking screenshots of several webpages. To get a screenshot image i have used the following code.

var page = require('webpage').create(); 
page.viewportSize = { width: 1200,height: 800};
page.open('http://<randomwebpageAddress>', function(status) {
  console.log(status);
  console.log(page.render('screenshot.png'));
  phantom.exit();
});

but the result for some page is

status = "success" but page.render returns false

my question is:

when does page.render() return false?

Answer 1

From the docs, the return value of render() is not important.

Answer 2

Although the documentation says that there is no return type for render(), it always returns a boolean denoting the success of the rendering operation.

The function will always return false if the content is empty, which should never happen.

Since the render() function is capable of rendering into multiple formats such as PNG, JPG, GIF, PDF, the return value may have slightly different meanings.

PDF

false can only be returned, if the format options where set incorrectly.

GIF (pre v2)

It only returns false when the destination file is not writable.

JPG, PNG (and GIF)

The return value comes from QImage::save() and denotes a successful rendering and file write.