this post was submitted on 06 Dec 2024
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Technology
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YouTube automatically generate videos in lower resolution of the one uploaded.
So when you watch a 4k video and switch to 1080, you are no longer watching the original video but a re-encoded one by YouTube itself which could have more artifacts since it's resized and compressed.
I dunno the exact specs (like bit rate, etc.), someone will probably add them in another reply.
I believe YouTube always re-encodes the video, so the video will contain (extra) compression artefacts even if you’re watching at the original resolution. However, I also believe YouTube’s exact compression parameters aren’t public, so I don’t believe anyone outside of YouTube itself knows for sure which videos are compressed in which ways.
What I do know is that different content also compresses in different ways, simply because the video can be easier/harder to compress. IIRC, shows like last week tonight (mostly static camera looking at a host) are way easier to compress than higher paced content, which (depending on previously mentioned unknown parameters) could have a large impact on the amount of artefacts. This makes it more difficult to compare different video’s uploaded at their different resolutions.
Just to be clear it is probably a good thing that YouTube re-encodes all videos. Videos are a highly complex format and decoders are prone to security vulnerabilities. By transcoding everything (in a controlled sandbox) YouTube takes most of this risk on and makes it highly unlikely that the resulting video that they serve to the general public is able to exploit any bugs in decoders.
Plus YouTube serves videos in a variety of formats and resolutions (and now different bitrates within a resolution). So even if they did try to preserve the original encoding where possible you wouldn't get it most of the time because there is a better match for your device.
I agree that, theoretically speaking, YouTube might be protecting some end users from this type of attack. However, the main reason YouTube re-encodes video is to reduce (their) bandwidth usage. I think it’s very kind towards YouTube to view this as a free service to the general public, when it’s mostly a cost-cutting measure.