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Old 23rd February 2020, 03:20   #1  |  Link
F3low
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Why do I lose more quality when I encode 4K compared to 1080p?.

Hello,

I usually download and encode 1080p videos to HEVC 10-bit CRF 23 preset slow and same resolution, I found that those settings give a good balance between file size and quality. The thing is I noticed that when I download a 4k video and encode it to 1080p using those same settings there's a noticeable loss of quality but smaller file size compared to if I download the 1080p version of the video and encode it to HEVC, therefore I have to lower the CRF to 18 to retain more details.

Is there an explanation for that?.

Last edited by F3low; 23rd February 2020 at 03:36.
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Old 23rd February 2020, 11:04   #2  |  Link
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Same CRF for different sources or versions of the same movie does not mean same visual quality or same compression. You can only compare the quality at the same file size, means you will have to adjust the CRF for each source accordingly, or use 2-pass encode to hit the same file size for comparison.
Another reason could be that the 4k version is just an upscaled version of the 1080p 'original', or that your downscaling algo smoothes (blurs) the picture slightly.

Last edited by Sharc; 23rd February 2020 at 11:07.
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Old 23rd February 2020, 13:21   #3  |  Link
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Sharc;1902022, or that your downscaling algo smoothes (blurs) the picture slightly
Yeah, this is what I thought of...
Some of the up/downscalers soften the image to a noticable degree.
Tell us how's your workflow.
And some settings in x265 might also blur your downscale.
As long as we dont know about your workflow, it's only guessing...
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Old 23rd February 2020, 17:06   #4  |  Link
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Yeah, this is what I thought of...
Some of the up/downscalers soften the image to a noticable degree.
Tell us how's your workflow.
And some settings in x265 might also blur your downscale.
As long as we dont know about your workflow, it's only guessing...
I don't think that the 4k videos I download are upscaled 1080p because they have more details if you zoom in.. I'm not sure what you mean by workflow... as I said I encode my videos to 1080p HEVC 10-bit CRF 23 preset slow ( I add this line min-cu-size=8 to force the resolution because sometimes I have to upscale SD videos to 1080p to merge them with the native FHD vids) and everything else is default.

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Old 23rd February 2020, 18:33   #5  |  Link
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CRF 23 is already very low in regards to detail retention. As you add more detail, more of it will be lost because of the constraints of the codec.
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Old 23rd February 2020, 19:21   #6  |  Link
F3low
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Originally Posted by RanmaCanada View Post
CRF 23 is already very low in regards to detail retention. As you add more detail, more of it will be lost because of the constraints of the codec.

I'm no expert but according to my tests with 1080p/720p CRF 23 is the sweet spot when it comes to quality/size balance.. sure you can go lower to retain more details but you will also get a bigger size.. the primary point of encoding to HEVC is better compression as far as I know.
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Old 23rd February 2020, 19:29   #7  |  Link
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CRF 23? You get what you ask for!
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Old 23rd February 2020, 19:55   #8  |  Link
Sharc
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As I wrote before you cannot expect the same quality with the same CRF for different sources. The OP's UHD and HD are different sources even though it might be the same movie.
CRF does not mean a certain absolute quality. It means Constant Rate Factor, nothing else. It aims at a constant viewing quality throughout the video, but not same quality between different videos from different sources.

Last edited by Sharc; 23rd February 2020 at 20:09.
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Old 23rd February 2020, 20:10   #9  |  Link
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As I wrote before you cannot expect the same quality with the same CRF for different sources. The OP's UHD and HD are different sources even though it might be the same movie.
CRF does not mean a certain absolute quality. It means Constant Rate Factor, nothing else. It aims at a constant viewing quality throughout the video, but not same quality between different videos from different sources.
I understand, thanks for the explanation.
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Old 23rd February 2020, 20:14   #10  |  Link
F3low
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CRF 23? You get what you ask for!
Well, it's subjective... I rarely use a lower CRF (like this situation) otherwise I might as well not encode at all since the file size won't go down by much and might even go up in some cases! (I'm not kidding I remember a few months ago I had 2 videos about 1.5GB each using CRF 23 the quality went down but the file size went up I just had to accept it since I had to merge them with other videos).

Last edited by F3low; 23rd February 2020 at 20:16.
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Old 23rd February 2020, 22:29   #11  |  Link
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......I'm not kidding I remember a few months ago I had 2 videos about 1.5GB each using CRF 23 the quality went down but the file size went up .
This is well possible eg when the source has lot of encoding artefacts or noise which boost the bitrate to reproduce the artifacts and the noise details for any CRF.
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Old 23rd February 2020, 22:30   #12  |  Link
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Storage cost is more or less a non-issue nowadays. When I last checked, the price/TB is about 20 bucks.

When it comes to quality, I'd rather downscale a 1080p movie to 720p than compromise on bitrate (I usually encode at CRF 16~18). Most movies don't have resolution worth full HD anyway except some Pixar animation flicks.

I can't speak for 4K since I don't have a 4K display.
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Old 23rd February 2020, 22:59   #13  |  Link
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Originally Posted by Groucho2004 View Post
Storage cost is more or less a non-issue nowadays. When I last checked, the price/TB is about 20 bucks.

When it comes to quality, I'd rather downscale a 1080p movie to 720p than compromise on bitrate (I usually encode at CRF 16~18). Most movies don't have resolution worth full HD anyway except some Pixar animation flicks.

I can't speak for 4K since I don't have a 4K display.
I believe you're talking about HDDs, I don't use them even for backups because they're just too slow and inconvenient.. I just bought a 500GB SSD and a 128GB SD card for backups and they're already full! I'm waiting for another SD card right now, storage gets filled very quickly.

I currently have about 7 SD videos, 2 FHD and 3 HD that I have to edit
and merge an I'm debating whether to upscale/downscale everything to FHD or HD.

Last edited by F3low; 23rd February 2020 at 23:01.
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Old 23rd February 2020, 23:00   #14  |  Link
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This is well possible eg when the source has lot of encoding artefacts or noise which boost the bitrate to reproduce the artifacts and the noise details for any CRF.
That's good to know what would you recommend me to do with files like those? Would setting a target bitrate or file size be a good idea?.
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Old 23rd February 2020, 23:12   #15  |  Link
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I believe you're talking about HDDs, I don't use them even for backups because they're just too slow and inconvenient.. I just bought a 500GB SSD and a 128GB SD card for backups and they're already full! I'm waiting for another SD card right now, storage gets filled very quickly.
Yes, I was referring to HDDs preferably mounted into a NAS. I have a NAS with 10 TB that holds all movies, etc. and an identical NAS serving as a backup unit. I find it very convenient since I have everything in one place and I can stream to any device I want (TV, tablet, phone).

Even though traffic "only" goes over gigabit LAN, it's certainly not too slow for its purpose.
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Last edited by Groucho2004; 23rd February 2020 at 23:16.
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Old 24th February 2020, 00:22   #16  |  Link
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That's good to know what would you recommend me to do with files like those? Would setting a target bitrate or file size be a good idea?.
No. Setting a target bitrate would reduce the file size but degrade the quality even more.
If the source is noisy you could denoise it with a denoising filter before encoding. If you are however too ambitious with denoising you will also loose picture details. You have to find the appropriate balance. Selecting a slower encoder preset may also help.
You may also consider to reencode the audio for a lower bitrate or change the audio format. Audio is sometimes just bloated.

Last edited by Sharc; 24th February 2020 at 00:32.
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Old 24th February 2020, 02:20   #17  |  Link
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Originally Posted by F3low View Post
I'm no expert but according to my tests with 1080p/720p CRF 23 is the sweet spot when it comes to quality/size balance.. sure you can go lower to retain more details but you will also get a bigger size.. the primary point of encoding to HEVC is better compression as far as I know.
And you're asking the "experts"*. I'm not the only one who has said CRF 23 is bad. X265 tops out at CRF 27, so you're already at the far end of the stick. For 4k encodes, I personally do CRF 14-16, and 1080P pending on source is 17-20, all at slower or very slow.

*I am not claiming to be one, but I have been encoding with x265 since its inception. Though I may not know the complete ins and outs as some other users, I do have years of experience that tell me the CRF you are using is detrimental to quality.
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Old 24th February 2020, 03:17   #18  |  Link
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If the point is to compress already compressed content to a smaller size a large quality loss is expected. H.264 and H.265 have bad generational efficiency losses when used at standard content delivery bit rates.

Recompressing already highly compressed H.264/H.265 is a very different use case than recompressing a bluray, for bluray or UHD 14-20 is what I use as well. I don't re-compress highly compressed video but if I did CRF 23 does sound reasonable. The quality will go down but using much lower the entire operation would be pointless, it would end up both larger and worse quality.
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Old 24th February 2020, 14:01   #19  |  Link
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If the point is to compress already compressed content to a smaller size a large quality loss is expected. H.264 and H.265 have bad generational efficiency losses when used at standard content delivery bit rates.

Recompressing already highly compressed H.264/H.265 is a very different use case than recompressing a bluray, for bluray or UHD 14-20 is what I use as well. I don't re-compress highly compressed video but if I did CRF 23 does sound reasonable. The quality will go down but using much lower the entire operation would be pointless, it would end up both larger and worse quality.

That's what I was trying to say. I don't encode blu-ray, I try to find the highest quality content of what I'm looking for on the internet then I encode it to reduce size and store it.
I do care about quality if I weren't I wouldn't even have posted this thread... When I notice that the quality went down noticeably I lower the CRF to 18. Usually when using CRF 23 you'd have to stop the videos and take screenshots or open both video in a media player then compare to notice the difference and in my case I find the small loss totally worth it for the size I'm getting.
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Old 24th February 2020, 14:45   #20  |  Link
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I don't encode blu-ray, I try to find the highest quality content of what I'm looking for on the internet then I encode it to reduce size and store it.
Hm, you might want to check out forum rule 6. Discussing illegally obtained content is frowned upon here.
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