bit rate and ref frames

[chazz spacey]

i have a video:


and i want to compress it again at the same quality(at least 99.999%) with x264.
my question is: do i need that many ref frames? can i get same quality if i set bit rate for example to 10000 kbps(2 pass) and ref frames to 1 and leave level at auto?
encoding + compression time at 12 ref frames at 5000 kbps with 7 minutes video is 3 minutes more than at 10000 kbps at 3 ref frames. can we say that bit rate and ref frames inversely proportional?
wait a minute... this video is actually encoded with x264. i think i don't need to re-encode it as i just cut some part of the video, no any editing. can i disable encoding and just use compression in x264 codec? i know avidemux does saving without any encoding ad compression. but it has some problems which is another story.

edit: actually i need to encode as i need to add hardcode subtitles.

[smrpix]

i think i don't need to re-encode it as i just cut some part of the video, no any editing. can i disable encoding and just use compression in x264 codec?

If you cut on I-frames you don't need to re-encode.

(As a matter of definition -- cutting, deleting or joining are all called "editing" regardless of the sophistication of those edits.)

edit: actually i need to reencode as i need to add hardcode subtitles.

edit: yes you do.

[chazz spacey]

i have a video:

[Attachment 28811 - Click to enlarge]

and i want to compress it again at the same quality(at least 99.999%) with x264.
my question is: do i need that many ref frames? can i get same quality if i set bit rate for example to 10000 kbps(2 pass) and ref frames to 1 and leave level at auto?
encoding + compression time at 12 ref frames at 5000 kbps with 7 minutes video is 3 minutes more than at 10000 kbps at 3 ref frames. can we say that bit rate and ref frames inversely proportional?

answer?

[Cornucopia]

If you do stuff to the file such that it needs re-encoding, and if that re-encoding is using a lossy codec, then the option for getting "99.999%" quality is almost 0%. Even with a high(er) bitrate. But 95-97%, that's possible.

IIWY, I'd stop messing with the details of the settings, as it is clear you do not fully understand their meaning/ramifications.

Reference Frames:

Description: Selects the maximum number of reference frames that can be used. Referenced frames are frames that refer to other frames (eg. if both frames are similar). Having a high referenced frame will improve quality but slow up encoding. For typical content, a reference frame of 3 to 5 is recommended. For content with a lot of repetition (eg. animation), a reference frame of 8 to 10 can be used. If a large number of reference frames is selected, then the "Mixed References" options should also be selected to allow x264 greater control (see "Analysis Options" section).
Command Line: --ref n
(where 'n' is the number of reference frames)

You need as many ref frames as your content warrants for that intended quality or bitrate level. Depending upon who encoded your "source" and whether they knew what they were doing, and what your source is, it could be correct already, or not.

If you truly want a file as close in quality as your source, stop using 1pass or 2pass VBR and use CRF with a low value (<16). Set your target profile and tuning (if necessary) and the rate factor and then live with what filesize you're given.

Scott

[chazz spacey]

i set crf to 22 and when i check the bitrate it is 5374 kbps. and i use this bitrate in 2 pass.
and here are the results:
crf


2pass




so, if i set the bitrate of crf compressed video for 2 pass will it give me the same quality?

[Cornucopia]

CRF uses 1 pass. It doesn't need a 2nd pass. You get consistent quality, at the expense of unknowable (possibly larger) filesize. If, on the other hand, you choose 2pass VBR (NOT the same thing as CRF), you get consistent filesize, at the expense of unknowable (possibly worse) quality. To use VBR EFFICIENTLY, you should do 2pass vs. 1pass, because VBR wants to "look ahead". With 1pass, it can only look ahead 1 frame or 1 GOP (maybe 2). With 2pass, it can look ahead to anywhere/everywhere in the file (because it has already done it in the first pass). CRF doesn't look ahead, which is why its levels might be a little more "wild" and/or higher. It is possible to constrain both so that they put out equivalent filesizes (rarely identical) and near-equivalent quality at that size. But why do that? - use the tool that best suits your needs.

Scott

[chazz spacey]

i meant 2pass VBR not 2 pass crf

[chazz spacey]

so, do they give the same quality when they are at same avarage bitrate?(crf and 2 pass vbr). if not then why?

[Cornucopia]

At the same AVERAGE bitrate, they are the same AVERAGE quality. There will be some times throughout the file when one is better quality than the other, and vice versa.

Both (at equal AVG bitrate) are giving you similar, but not identical "guesses" of image.

Scott