automated 3pass, how would it improve pic quality?

[deets]

hi,
Ive been using the automated 3 pass mode in megui with x264. Im a little confused about the options, should it give an improved picture over automated 2 pass, and if so, why?

I kinda thought 2 pass was always just for hitting a specified file size so am not quite sure what 3 passes would offer over 2.

the extra time wouldnt be an issue as i have a week and leave the encoding pc on 24 hours, but am i really just wasting that time or should it improve quality in theory?

thanks :)

[Kurth]

Well 3 pass encode give a better quality but just a little.
I did a test with a 2 minutes video and the result
2 Pass PSNR 44.255
3 Pass PSNR 44.280
This is something that your eye will not see any diference.
If you are encoding a movie that will need a lot of time to encode I really think it dont worth using 3 pass encoding for just this little better quality.

[foxyshadis]

3-pass reallocates a little differently, based on any problem areas encountered in 2-pass. Sometimes quality goes up, sometimes it actually goes down. I think the case for 'goes up' tends to be lots of rapid transitions between high and low motion, but I'm not sure. Also shorter clips benefit more from it. But the limitations of the rate-control algorithm tend to weigh more on it than any theoretical limit.

[deets]

interesting. i use low bitrates, 185, so was curios if the 3 passes would help more when the bitrate was spread more thinly.

im mainly doing 30 min tv show DVD rips, so the extra pass is roughly another 30 mins encoding. not too bad and every extra bit of quality could potentially help at such low bitrates.

[Oline 61]

interesting. i use low bitrates, 185, so was curios if the 3 passes would help more when the bitrate was spread more thinly.

im mainly doing 30 min tv show DVD rips, so the extra pass is roughly another 30 mins encoding. not too bad and every extra bit of quality could potentially help at such low bitrates.
Wow, so the final pass is done at realtime speeds?

[foxyshadis]

The best way to find out for your particular show is to encode both ways (which is easy, just save the last two passes), and run ssim test (such as with msu metrics tool) as well as visually scan problem areas in both clips to see if there's any improvement.

[deets]

oh yep, direct comparison is the best way of course, but its also good to know the theory behind the options :)

Oline 61: yeah the psp uses a low res, so the speeds are often on the 20fps region for second/third pass :) first turbo is usually around 70fps

[bond]

my opinion is that 3pass isnt worth it when thinking about the huge encoding time increase, but thats me on my pentium3 866mhz :D

[deets]

haha, oh lordy i dont even wanna think about the frame rates you would get on that beast :P

i will do some testing tonight, have till thursday before i have to catch the plane :D

[DarkZell666]

Well, automated 3-pass has been proven to give significantly better visual results on low-bitrate anime :)

And btw global psnr increase doesn't mean much since the bits are RE-allocated elsewhere (some scenes will have lower psnr, and others a higher one). As foxyshadis said, the fast low/high motion switch scenes are the most affected.

I might be mistaken, please correct if wrong :

I think that the more passes you use, the closer to --crf you get (if a 10-pass encode gave you an average quant of 20, doing a 1-pass encode using --crf 20 would give a very similar result). Except that you can't use --crf xx and achieve a desired filesize out of the box =)

[Sirber]

On anime, 640x480@24FPS, at 318kbps, I don't see a difference between 2 and 3 pass, except it takes 2 more hours :)

[DarkZell666]

Weren't you a 3-pass fr34k a while ago ? :p

[Sirber]

It's cheaper to do 2pass than to buy a 4400+ X2 :D

[Romario]

I never use 3pass, because it's useless in most occasions.

[deets]

if its useless, whats its reason for being there then?

just trying to understand the theory behind it, in what situations would be it useful and what not so.

if you get me :D as a noob, so often i do things (like 2pass) and dont really know why

[nurbs]

I guess its there for the same reason --me esa is there. While it's useless in most cases, if it weren't there people would start clamoring for it.

[Sharktooth]

3passes is there coz sometimes 2passes fail to converge to the specified bitrate...
the legend of "more passes = better quality" is just a legend... not the truth...

[shon3i]

is just a legend... not the truth...
Good legends sometime came a truth. LOL!

[Sharktooth]

well, it may happen in some particular cases but not always ;)

[deets]

3passes is there coz sometimes 2passes fail to converge to the specified bitrate...
the legend of "more passes = better quality" is just a legend... not the truth...

just what i was looking for, thanks :D

[shon3i]

well, it may happen in some particular cases but not alwaysi agree with you

[Romario]

shon3i, on which particular cases do you mean?

[Sirber]

very low bitrate for exemple

[Romario]

Under 250 kbps, I assume?

[Sirber]

depends on the source...

[shon3i]

Under 250 kbps, I assume?
That right but like Sirber says depend on the source.

[Chainmax]

the legend of "more passes = better quality" is just a legend... not the truth...

That's not what pretty much everyone (including you) was saying up to until quite recently. Has something changed or was it always pretty much useless?

[ChronoCross]

I can explain.

I do believe that this myth started with the introduction of nth pass in divx. Divx's whole mindset is the more passes you do the better refrined frametype decision and bitrate distribution will be cause it can gather new data absed on the last pass each time.

However if you look at xvid's two pass, all that is required is a superior algorithm to get better quality. This mentality has also passed to x264 as even in 3 pass the quality increases a miniscule amount overall while certain sections can actually look worse.

That's why it is best to stay with 2pass unless you miss your filesize.

[shon3i]

DivX produce better quality after third pass, using EKG and setting modulation to 0.5 i think.

[ChronoCross]

DivX produce better quality after third pass, using EKG and setting modulation to 0.5 i think.

yup. cause their algo's rely on special adjustments each time. unlike x264 and xvid who don't see much improvement between 2 and 3 pass. they both still beat all their competition.

[Chainmax]

I can explain.

I do believe that this myth started with the introduction of nth pass in divx.

Not in the case of x264 IMO. From its first appearance until quite recently, very knowledgeable people like Sirber, Sharktooth and more (not sure if you too) said that 3 passes were better in x264. This whole "3-pass is overkill" deal has been going on for about a month. I am not complaining or criticising and if I appear like that I apologize as I respect you guys a whole lot. I'm just wondering wether something in the innards of x264 has changed recently or you guys just found out about this.

[ChronoCross]

It's just a new view on things through extended testing. Nothing has so drastically changed that it's not required it's just a differing change of opinion.

[Chainmax]

That makes more sense :).

[Sharktooth]

Not in the case of x264 IMO. From its first appearance until quite recently, very knowledgeable people like Sirber, Sharktooth and more (not sure if you too) said that 3 passes were better in x264. This whole "3-pass is overkill" deal has been going on for about a month. I am not complaining or criticising and if I appear like that I apologize as I respect you guys a whole lot. I'm just wondering wether something in the innards of x264 has changed recently or you guys just found out about this.
I never reccomended 3 passes... unless there were particular cases.

[Chainmax]

Wasn't it included in the highest quality profiles?

[Sharktooth]

It was include in one Anime (max quality) profile coz usually Anime are encoded at low bitrates (and in that case 3 passes "may" be usefull). It was also included in one HQ profile (insane) coz ppl using insane settings do not care about the encoding time... However i changed them coz there was almost no point in using 3 passes. If x264 doesnt hit the desired filesize you can add a 3rd pass by yourself.