问题
When sharing a link in major websites like Digg and Facebook; it will create thumbnails by capturing main images of the page. How they catch images from a webpage? Does it included loading the whole page (e.g. by cURL) and parsing it (e.g. with preg_match) ? To me, this method is slow and unreliable. Does they have a more practical method?
P.S. I think there should be a practical method for quick crawling the page by skipping some parts (e.g. CSS and JS) to reach src attributes. Any idea?
回答1:
They typcailly look for an image on the page, and scale it down on their servers. Reddit's scraper code shows a good deal of what they do. The Scraper class should give you some good ideas on how to tackle this.
回答2:
JohnD's answer shows that Reddit uses embed.ly as part of their Python solution. Really embed.ly does the hard part of finding the image and they're free under 10,000 requests/mo.
回答3:
They generally use a tool like webkit2png.
回答4:
Some use
<link rel="image_src" href="yourimage.jpg" />
included in the head of the page. See http://www.labnol.org/internet/design/set-thumbnail-images-for-web-pages/6482/
Facebook uses
<meta property="og:image" content="thumbnail_image" />
see: http://developers.facebook.com/docs/share/#basic-tags
来源：https://stackoverflow.com/questions/7462044/how-major-websites-capture-thumbnails-from-a-link