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7th February 2008, 05:47 | #1 | Link |
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Bits per pixel value for quality x264?
I'm using bit per pixel koeffitient to calculate target size or bitrate:
Size (kb) = W * H * Num_frames * k/ (8 * 1024) Where W* H * N total number of pixels. k - bits per pixel Bitrate (kbit/s) = W * H * fps * k / 1024 Which gives me clear quality-to-koeffitient dependency, independent from resolution or fps. I.e. for XVid I use k = 0.15-0.4 bit per pixel for h263 quantization, 0.5 for MPEG quantiztion. ____________________ Based on that, what k (bits per pixel) should be for x264? From what I've got, for 1920x1080 used 10000kbit/s, k = 0.16-0.17. Could you express your feeling about good x264 hiqh quality rip. What k are you using? Just want to put it to my Excel. I don't feel like using 1000 kbit/s from profiles for different resolution is a good thing Thank you! |
7th February 2008, 06:02 | #3 | Link | |
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I.e. for tripod based, low details (night shot video) goes to 0.15. Hand-taped, high detailed (faces, leaves, etc), lots of movement went to 0.4. And other values between. I general I put 0.35-0.4 because size is not an issue, so I get around worst cases. But to put something like 0.7 is impractical in case above, right? That's what I'm looking for x264. Hight quality, but not overkill. Assuming I taping nature from hand, crowds, beach with moving waves. Forgot to mention, for x264 I use automated 2-pass, trellis etc. - all settings at max quality-longer compression time Thans Last edited by Vetal; 7th February 2008 at 06:06. |
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7th February 2008, 08:00 | #4 | Link | |
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A good starting point for me has been selecting my maximum bitrate and pixel space, and then minimums for both, and linearly scaling the bitrate. But this probably has some issues maybe due to dimensions/sizes of macroblocks affecting the quality of the encode, etc. What would be a more consistent formula for IQ? Perhaps accounting for "unused" space on the blocks would be in order, anything else? |
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7th February 2008, 08:04 | #5 | Link | |
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7th February 2008, 08:36 | #6 | Link |
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I´ve been wondering the same thing for a while.
After observing several samples of high quality xvid encodes and then transcode them to h264 I´ve reach these values: Resolution, Bitrate 512X384, 800 640X480, 1249 352X480, 687 352X240, 344 384X288, 450 720X480, 1405 I consider that bitrate to be very safe to transcode high quality material and retain it´s transparency. Always using two passes and these settings: --ref 5 --mixed-refs --no-fast-pskip --bframes 16 --b-pyramid --b-rdo --bime --weightb --direct auto --filter -2,-1 --subme 7 --trellis 2 --analyse all --8x8dct --threads auto --thread-input --progress --no-dct-decimate --no-psnr --no-ssim --output "output" "input" ("--me tesa" if you can or "--me umh") ...Or to transcode xvid material I found that CRF 21 is enough, sometimes even CRF 24, for lower quality xvids. |
7th February 2008, 08:59 | #7 | Link | |
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7th February 2008, 09:06 | #8 | Link |
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... and don't work for different resolution with same source (or comparable source). Bitrate / ( H x W x Fps ) ^ 0.75 is a very better HVS value.
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7th February 2008, 09:49 | #9 | Link | |
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Could you explain why the ^0.75 of your formula?? |
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7th February 2008, 09:51 | #10 | Link | |
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It represents the fact that higher framerates do not need linearly higher bitrates (due to the motion being smaller and simpler between frames) and the fact that higher resolution video also doesn't need linearly higher bitrates. |
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7th February 2008, 18:24 | #11 | Link | |
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Is it more x264 specific and XVid should be more or less 0.75? |
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7th February 2008, 18:55 | #12 | Link |
Mr. Sandman
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BPP is useless. as it has been said it doesnt take into consideration the source material compressibility.
also, what you dont understand is your eyes are not an encoder. there are other things other than motion and details that could vary the complexity of a movie. every movie has it's own complexity and compressibility. you cant just throw a number based on incomplete informations and think it will be correct. use a proper compression test instead, and you will have a way more accurate result. if you dont know what im talking about, have a look here
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7th February 2008, 19:36 | #13 | Link |
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Why do you need a "bits per pixel" value anyway ???
* If you are targeting for a certain level of quality and don't care much about filesize, then CRF is the best choice (for the time being) * If you are targeting for a certain filesize (e.g. CD-R or DVD-R), then use the 2-Pass mode with the appropriate target average bitrate.
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7th February 2008, 19:37 | #14 | Link |
The Crazy Idahoan
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There really is only one reason to use 2 pass encoding and that is that you need the film to be under a certain filesize.
Appart from that, if quality is the most important thing then CRF + some AQ is the best way to go to get a good mix of best bitrate for a given quality. |
8th February 2008, 20:12 | #15 | Link |
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Thanks to everyone! I did some tests with CRF and found how wrong am I with BPP. Some video (Baby Einstein, lots of water video, bubbles etc.) went below in BPP, while my DV video got high above my empiric coefficient.
So, my strategy is, to pick some CRF, 16-25 (based on 'x264 explained' link). Get the compressibility estimate with CRF Then do 2-pass if I really inclined and video is important. Though, there is some controversy in forum about CRF vs 2-pass: If small gain in quality of 2-pass worth extra processing time. And is there any gain at all. Please, let me know if there are conceptual mistakes in this approach. Thank you |
8th February 2008, 20:23 | #16 | Link |
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several people have already said that: if you want specific quality, use CRF, and only use 2-pass if you need a specific filesize.
The time where you needed 1000 passes with 10000 compressibility checks is long over! One (1) pass with crf will give you the desired quality, and it is efficient. you'll not get better quality by using 2-pass, only by using more bitrate. |
9th February 2008, 10:26 | #19 | Link |
aka XaS
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That's what happens with 1-pass bitrate-based encoding. 2-pass takes the "useless" bits from low-motion scenes and gives them back to the high-motion scenes. If a 2-pass-encoded video looks ugly, the second pass simply doesn't have enough bits to play with. It's like asking for The Lord Of The Rings in HD to fit into 700MB when in fact it needs over 3GB to start looking good. There, the whole movie is craving for more bits and 2-pass can't do anything about it.
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25th August 2008, 01:15 | #20 | Link | |
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In other words,if i need a certain size which resoltion should i resize to?How can i determine it with megui? |
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