clipRect
clipRect {object}
This property defines the rectangular area of the web page to be rasterized when page.render is invoked. If no clipping rectangle is set, page.render will process the entire web page.
Examples
var webPage = require('webpage');
var page = webPage.create();
page.clipRect = {
top: 14,
left: 3,
width: 400,
height: 300
};