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We Built a Video Rendering Engine by Lying to the Browser About What Time It Is

173 points by darshkpatel

by yetihehe

6 subcomments

About 20 years ago there was a similar problem with demoscene creations. It was hard to capture demos in realtime in all their glory. So one guy created a tool[1] that waited for a frame render and presented proper time to demo so that frames would be paced properly. "All popular ways of getting time into the program are wrapped aswell - timeGetTime, QueryPerformanceCounter, you name it. This is necessary so .kkapture can make the program think it runs at a fixed framerate (whatever you specified)."
[1] https://www.farbrausch.de/~fg/kkapture/

by chmod775

0 subcomment

I've done similar shenanigans before. That main loop is probably simplified? It won't work well with anything that uses timing primitives for debouncing (massively slowing such code down, only progressing with each frame). Also a setInterval with, say 5ms may not "look" the same when it's always 1000/fps milliseconds later instead (if you're capturing at 24fps/30fps, that would be a huge difference).
What you should do is put everything that was scheduled on a timeline (every setTimeout, setInterval, requestAnimationFrame), then "play" through it until you arrive at the next frame, rather than calling each setTimeout/setInterval callback only for each frame.
Also their main loop will let async code "escape" their control. You want to make sure the microtask queue is drained before actually capturing anything. If you don't care about performance, you can use something like await new Promise(resolve => setTimeout(resolve, 0)) for this (using the real setTimeout) before you capture your frame. Use the MessageChannel trick if you want to avoid the delay this causes.
For correctness you should also make sure to drain the queue before calling each of the setTimeout/setInterval callbacks.
I'm leaning towards that code being simplified, since they'd probably have noticed the breakage this causes. Or maybe, given that this is their business, their whole solution is vibe-coded and they have no idea why it's sometimes acting strange. Anyone taking bets?

by xnx

0 subcomment

Mentioned at the very end that this is based on https://github.com/Vinlic/WebVideoCreator

by echoangle

4 subcomments

Crazy that this approach seems to be the preferred way to do it. How hard would it be to implement the recording in the browser engine? There you could do it perfectly, right?

by zeta0134

0 subcomment

Ha, my first thought is that I'd likely break this system. My page synchronizes its animation playback rate to an audio worklet, because I need to do both anyway, and some experimentation determined that syncing to audio resulted in smooth frame pacing across most browsers. This means that requestAnimationFrame has the very simple job of presenting the most recently rendered frame. It ignores the system time and, if there isn't a new frame to present yet, does nothing.

by eviks

0 subcomment

Not a single before/after video?

0 subcomment

by NoahZuniga

1 subcomments

This wouldn't work for CSS/svg animations?

by G_o_D

0 subcomment

https://chromewebstore.google.com/detail/scroll-capture/egmh...

0 subcomment

by brcmthrowaway

0 subcomment

Does anyone remember FRAPS?

by tosti

0 subcomment

What a waste of time. Just hook up an hdmi recorder

by amelius

1 subcomments

> The core issue is that browsers are real-time systems. They render frames when they can, skip frames under load, and tie animations to wall-clock time. If your screenshot takes 200ms but your animation expects 16ms frames, you get a stuttery, unwatchable mess.
But by faking the performance of your webpage, maybe you are lying to your potential users too?

by soulofmischief

5 subcomments

This post smells of LLM throughout. Not just the structure (many headings, bullet lists), but the phrasing as well. A few obvious examples:
- no special framework. No library buy-in. Just a URL
- Advance clock. Fire callbacks. Capture. Repeat. Every frame is deterministic, every time.
- We render dozens of frames that nobody will ever see, just to keep Chrome's compositor from going stale.
- The fundamental insight that you could monkey-patch browser time APIs ... is genuinely clever
- Where we diverged
The whole post is like this, but these examples stand out immediately. We haven't quite collectively put a name on this style of writing yet, but anyone who uses these tools daily knows how to spot it immediately.
I'm okay with using LLMs as editors and even drafters, but it's a sign of laziness and carelessness when your entire post feels written by an LLM and the voice isn't your own.
It feels inauthentic and companies like replit should consider the impact on their brand before just letting people write these kind of phoned-in blog posts. Especially after the catastrophe that was the Cloudflare Matrix incident (which they later "edited" and never owned up to).
And the lede is buried at the very end: This is just a vibe-coded modification of https://github.com/Vinlic/WebVideoCreator, and instead of making their changes open source since they're "standing on the shoulders of giants", the modifications are now proprietary.
In the end, being an AI company is no excuse for bad writing.

by marxisttemp

1 subcomments

The prose here reads like it was LLM-generated.
Short sentences. Plenty of newlines. Enumerate everything. Always.

by andrewstuart

4 subcomments

I did this a few years ago. The approach these guys are taking is kinda hacky compared to other better ways - and I've tried most of them.
It works but only in a limited way there's lots of problems and caveats that come up.
I dropped it in the end partly because of all the problems and edge cases, partly because its a solution looking for a problem an AI essentially wipes out any demand for generating video in browsers.
I ended up writing code that modified chromium and grabbed the frames directly from deep in the heartof the rendering system.
It was a big technical challenge and a lot of fun but as I say, fairly pointless.
And there are other solutions that are arguably better - like recording video with OBS / the GPU nvenc engine / with a hardware video capture dongle and there's other ways too that are purely software in Linux that work extremely well.
You can see some of the results I got from my work here:
https://www.youtube.com/watch?v=1Tac2EvogjE
https://www.youtube.com/watch?v=ZwqMdi-oMoo
https://www.youtube.com/watch?v=6GXts_yNl6s
https://www.youtube.com/watch?v=KzFngReJ4ZI
https://www.youtube.com/watch?v=LA6VWZcDANk
In the end if you want to capture browser video - use OBS or ffmpeg with nvenc or something - all the fancy footwork isn’t needed.

by d--b

0 subcomment

This is super smart but doesn't seem very future-proof...

by CloakHQ

0 subcomment

[dead]

by raphaelmolly8

0 subcomment

[dead]