Question about First and Second passes

[xosuitehearts]

Hi there everyone,

I've been sort of curious about what the point of doing 2 passes is for when Im converting a vob down to an avi?

A lot of this process is time consuming and I'm somewhat curious about how I can speed this up and if only doing 1 pass would be the right idea.

[zmaster]

In the coding 2pass codec is better distributes the bit rate, say so. But you certainly can use a single pass encoding.

[BigDid]

Hi and welcome to the forum.

In addition 2 passes can target a given size therefore needed for CD or DVD storage. If not needed, 1 pass will do the job; same size for 1 or 2passes being (nearly) same quality.

Did

[yetanotherid]

I've done single pass encodes and compared them with a dual pass encode of the same final quality and it makes no real difference to the end result.

AutoGK has standalone player compatibility settings in it's hidden options (for DivX type DVD players etc). If you enable it, one of the things it does is to limit the maximum bitrate if necessary so the player can cope, however that limitation only works in 2 pass mode.

Aside from that, 1 pass gives you a set quality while the file size will be a suprise, 2 pass lets you pick the file size and AutoGK adjusts the over-all quality to fit.
One of the things which does frustrate me about AutoGK is the fact that you have to pick the file size and then AutoGK runs it's compression test (in 2 pass mode) and offers a quality prediction. If you want to use 2 pass encoding but you're after a certain quality as well, then sometimes it takes a few guesses at the file size. It'd be nice if AutoGK ran the compression test and then let you adjust the file size to achieve a desired 2 pass quality. AutoGK is still my preferred DVD to AVI conversion program though because of the way it intelligently resizes and crops video and the fact it makes it hard for the user to mess things up.

[yetanotherid]

By the way, is there a reason you're converting to AVI? It's just that these days.... unless you're converting for playback on a particular device... x.264 rather than XviD is probably a better option.

[Marshy]

I've done single pass encodes and compared them with a dual pass encode of the same final quality and it makes no real difference to the end result.



And this based on what; hard fact of every high or low motion scene or just your subjective visual opinion? Doesn't 2pass allocate bitrate better in scenes whch need it or those that not. Your comment is vaque, back it up with proof and show us.
:)

[yetanotherid]

And this based on what; hard fact of every high or low motion scene or just your subjective visual opinion? Doesn't 2pass allocate bitrate better in scenes whch need it or those that not. Your comment is vaque, back it up with proof and show us.

No hard facts. Just my subjective visual opinion. Is there another measurement of quality I should be using?

I've converted movies each way, stopped them on various identical frames and looked for differences... even after zooming in. I've watched them at normal speed side by side on identical monitors, and any differences I could see were only when pausing on identical frames and zooming in, but if there were differences they were so tiny I really had to look to see them, and I still couldn't say one method consistently gave better results than the other. They were just sometimes very, very slightly different. When watching the movies playing side by side if there really was a quality difference I doubt I could spot it even with bionic eyes.

I couldn't say whether 2 pass allocates bitrate better than single pass. I suspect it might sometimes allocate it more efficiently but that possibly doesn't equate to a difference in perceived quality. There must be some difference as I've encoded video to a certain quality (say AutoGK's 75%) then used the resulting file size for a 2 pass encode. I've done that quite a few times and the resulting quality of the 2 pass encode is never exactly 75%. It's only a little different, say 74% or 76% percent at the most, but that's not a quality difference anyone's ever going to be able to see and as I said maybe it's more a difference in encoding efficiency rather than perceived quality itself.
Thinking about it, it may even be partly due to the fact that a single pass encode just stops when it's done, whereas when I pick the same file size for the 2 pass encode I can only do so to the nearest MB.

As I said in an earlier post 2 pass encodes with the standalone player compatibility option enabled keeps the maximum bitrate in check whereas single pass doesn't, but even then I've compared encodes done both ways and even during high bitrate moments I can't see a difference.

I guess the fact that I can encode to a certain quality, then do a 2 pass encode using the first one's file size, and when more often than not AutoGK reports a first pass quality as being no more than around 1% different to the single pass encode, it helps me confidently advise someone there's no real quality difference between the two methods.

Hopefully that's taken some of the vagueness out of my reply for you. If you know of a way to measure perceived quality that's better than subjectively perceiving it, please let me know. I'd be keen to try it.

[Marshy]

Yes I see I made you work hard for that reply dude and it is not as vague now. I'd imagine you are keen to try anything in the nicest possible sense as your enthusiasm shows. Nice one!

[yetanotherid]

Yes I see I made you work hard for that reply dude and it is not as vague now.

Not really. It was so easy I got bored typing it. ;)
I've been using AutoGK for a long time.

I'd imagine you are keen to try anything in the nicest possible sense as your enthusiasm shows. Nice one!

I'm not sure what you're imagining there, but thanks.

[Marshy]

I'm not sure what you're imagining there, but thanks.

Imagining hell no, meaning is you would take on new challenges.

[Ghitulescu]

No hard facts. Just my subjective visual opinion. Is there another measurement of quality I should be using?

Hopefully that's taken some of the vagueness out of my reply for you. If you know of a way to measure perceived quality that's better than subjectively perceiving it, please let me know. I'd be keen to try it.
So you're not offering hard evidence the very same way you pretend from the others?

[yetanotherid]

So you're not offering hard evidence the very same way you pretend from the others?

Aside from the fact that if I do a single pass encode at a particular quality, then use it's file size for a 2 pass encode and it usually comes out at the same quality (+or- 1% or so)?
That's not enough?

Did you miss where I asked if there is a way to measure perceived quality.... or did you post simply to be facetious?

[azmoth]

I'm interested too, yetanotherid, where is your hard evidence you are not offering and would criticize others if they posted the same exact comment???

[Groucho2004]

Ghitulescu, yetanotherid, azmoth:

Get a room already!

[azmoth]

I would like to see some evidence(if possible) that 2 pass is more beneficial than 1 pass in terms of quality and the topic is interesting.:cool:

[manono]

Cut it out. While yetanotherid is sometimes the instigator, it looks to me that you guys are ganging up on him here. And for no good reason, that I can tell. It's generally known that the only reason to run two passes is if you need a certain size (like a 700MB CD), ignoring the AutoGK standalone compatibility settings for the moment. I, for one, will always run single VBR passes (directly in VDub(Mod) and not AutoGK) when file size isn't a consideration. I know I'll get the quality I want and that quality will be even throughout the AVI. For that reason alone (even quality throughout) I consider single-pass encoding actually superior to 2-pass encoding.

Ghitulescu and azmoth, I'm talking to you.

[yetanotherid]

I'm interested too, yetanotherid, where is your hard evidence you are not offering and would criticize others if they posted the same exact comment???

There's no need to emphasise words when asking such a straightforward question, it achieves nothing but to indicate you're emphasis is an attempt to compensate for a lack of validity.

Firstly, I wouldn't criticise anyone for posting such a detailed and logical explanation as why they hold an opinion on a matter which, to the best of my knowledge, can't be proved.
Should I disagree with them I'd probably say so, I might criticise their rationale, and I may even upset them by offering my own explanations as to why I disagree, but I certainly wouldn't criticise their attempt to provide an explanation.

Secondly, your current demand for proof, given your track record of refusing to answer questions asked of you, dismissing other's explanations for no better reason than "I'm not going to change my mind", when combined with your own unwillingness to supply proof supporting your opinion when it's practical to do so, is evidence of your own hypocrisy, not mine, and given I've stated several times I don't know if there is a way to offer definitive proof of what I'm saying, to then ask me why I've not done so would seem to either be a willingness to ask a silly question, or an inability to realise why it's silly.

I have though stated at least twice I've performed numerous single pass encodes at a particular quality, then used the resulting file size for a 2 pass encode and the quality invariably matches.... give or take a percent, and that I consider this to be evidence which supports my claim. While I'd understand if you disagreed as to whether this proves anything, and stated why you disagreed, because you instead asked why I've not attempted to offer any proof, I can only assume your question is in respect to a definition of the word proof with which I'm for some reason unfamiliar.

If you don't think my explanation is correct then by all means say so, but if you're going to do so, please for once have an equally logical argument on offer to support your own opinion and reasons for stating I'm incorrect. If you find the topic interesting, try contributing to it in a manner which errr.... contributes.

[azmoth]

Tanty? Gee, even simple questions stirs deep angst when original question is open to anyone to answer and nothing more than that. Note to myself: Be kind and considerate and respectful to all. Period.. :D

Anyway thanks to user for making the effort to answer though and one may find this surprising, but one actually agrees with with him/her on some points regarding quality, though there was not much there substancewise in the overall monologue as it is a hard one to pin down, I grant one that. :D

And there is also; if the source is good the output will be good, good in good out so to speak, if bitrate is reasonably high. What does, errr, one think?

[manono]

Thread closed. Go challenge each other in some forum I don't moderate. Any more in this one and I'll issue strikes first and ask questions later. You two guys should be ashamed of yourselves. This is a hobby. This is a forum for the free exchange of information. It's not for testosterone exhibitions.