this post was submitted on 19 Dec 2024
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So I take a peek at the Lemmy's source code.
When you see a thumbnail and title of a URL (including YouTube video links) on Lemmy, what happens is that Lemmy server fetched the website HTML file and get the
og:titleandog:image(this convention is called OpenGraph protocol ^1) located inside<head>HTML tag.For example, for this video https://www.youtube.com/watch?v=MbYuEEU5e50, the OpenGraph metadata inside the HTML looks like this:
Try opening the "Create Post" page, then enter URL of a YouTube video. You might notice that a loading spinner will appear. During this, the frontend of Lemmy is actually creating a request to
/api/v3/post/site_metadata?url=[insert your URL]. When the backend side of Lemmy receives the request from front end, it will run this code ^2 to fetch the HTML and parse it to find the OpenGraph metadata. If the metadata is found, the backend will return it to front end, and you willl see recommended title and the "Thumbnail URL" field should be filled.In theory, this should work fine, regardless of any website. I personally tried to view the HTML code of a YouTube video and the HTML file indeed contains OpenGraph metadata.
But it doesn't for YouTube videos in Lemmy somehow. And I'm not sure.
I'm suspecting YouTube thought that Lemmy servers are bots and because of that instead of receiving the HTML file of a YouTube video, instead it received a HTML file of a captcha page. I think I'm gonna try contacting admins of my instance to see whether YouTube URL works or not.
Thats a very helpful response, thank you!
It seems like the only people getting thumbnails on !videos@lemmy.world are from shitjustworks (how fitting lol)
See the other reply: seems that it was caused by Lemmy backend only fetches the first 512kb of the HTML, meanwhile YouTube puts bunch of JavaScript in the beginning of the HTML
Here's the fix that was only merged couple days ago apparently, https://github.com/LemmyNet/lemmy/pull/5266
Crazy, right
Interesting...