Does the transcoding process cause drop in graphic quality

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maplestoryfall
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Does the transcoding process cause drop in graphic quality

Post by maplestoryfall » Thu Jan 04, 2018 9:51 pm

Will the transcoding cause drop in graphic quality? For example if i have got a .mkv video thats shared with the server, when i play it on ps3 would it get lower quality

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Nadahar
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Re: Does the transcoding process cause drop in graphic quality

Post by Nadahar » Thu Jan 04, 2018 11:55 pm

In most cases, and your example, yes. If the video stream has to be converted into another codec, there will be quality loss. If the video stream is only packed into another container/file format (remuxed) there won't be any loss of quality.

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SubJunk
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Re: Does the transcoding process cause drop in graphic quality

Post by SubJunk » Fri Jan 05, 2018 10:33 am

There are two answers depending on what you mean; Nadahar is right that there is always a loss in quality, but usually it would be impossible to notice.

Every video that gets released on the internet is transcoded already, often several times (a common situation is that the TV station receives it in one format which has already been transcoded during the editing process, then they broadcast it in something like MPEG-2, then the person doing the release captures and records it in MPEG-2 or H.264, then does a longer conversion using settings to get the desired filesize/quality/compatibility balance) so by the time you are seeing it there have already been potentially 5 or more transcodes done to the video and we never notice a loss in quality, and it's the same when we do an extra one on top.

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Nadahar
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Re: Does the transcoding process cause drop in graphic quality

Post by Nadahar » Fri Jan 05, 2018 10:45 am

@SubJunk I have to disagree a bit here. I've been experimenting with reencoding some x264 files into x265, and I ended up getting files that were around the same size in x265 as the originals to not see the degradation from just one conversion (it was single pass though). I think transcoding impacts quality quite a lot if your source material is sharp - the fact that a lot of material is already blurry so you can't tell the difference is another matter.

The encoding done by production companies and broadcasters are often done with very expensive specialized equipment that can handle real-time encoding that we can only dream of. Rippers often use higher quality settings and two-pass encoding because they don't have to do it real time. UMS has to both do it in real time and do it using often quite low quality and generic equipment, which means that the quality reduction is likely to be higher at this step than at any of the previous.

Regardless, if transcoding a media makes it playable on a device it wouldn't otherwise play on, there's not much choice. I wouldn't want to transcode something could potentially be played untranscoded though, because of the quality loss and other disadvantages like controlling audio and subtitle tracks, seeking etc.

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SubJunk
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Re: Does the transcoding process cause drop in graphic quality

Post by SubJunk » Fri Jan 05, 2018 10:55 am

The settings we use in UMS for transcoding are pretty high quality - with H.264 output we use the same quality settings as pro encoding groups do, and the same tool (x264). x264 and x265 seem to do objectively well against expensive solutions.
Two-pass settings aren't often used by rippers anymore with H.264 releases since x264 released the CRF setting. A lot of times you can see the settings they used with MediaInfo.
Because we are streaming over the network instead of caring about filesize, we are able to transcode at high quality in realtime, since the network only has to load 1 second of video per second of playback, which is pretty achievable even at high bitrates.

x265 still isn't up to the polished state of x264 though and there are lots of things that affect it, even down to whether the pixels are divisible by 16. x265 is coming along really well though and maybe this year we will see them implement the kind of advanced compression and analysis features that x264 has. It took x264 many years to get to its current amazing state

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