Clarification concerning 2 pass x264 encodes...

[Lathe]

I know that there are a LOT of threads discussing 2 pass vs. CRF encoding and I've read many of them. So, I THINK that I have a basic idea as to how they are different and in what circumstances you would use one or the other. However, there are a couple of angles that I don't quite understand.

1. When using 2 pass encoding, the first pass is for the purpose of analyzing the stream so as to determine where more bits are needed in the more complex scenes and where less are needed in the simpler scenes, right? Therefore, if a person states that they did a 2 pass encode @ 14k for example, does that mean then NOT a CBR of 14k, right, because then what would be the purpose of the 2 passes, correct? So, would that mean that the resulting encode would then have an AVERAGE bit rate of 14k (in other words the bit rate that you have chosen for the 2 pass encode) and that the busier scenes would in actuality have MORE than 14k bit rate if needed and some would have LESS than 14k if not needed?

2. Using 2 pass encoding then, does that mean (generally) that the first pass of analysis then in actuality determines the best CRF to use on the 2nd pass in order to meet whatever bit rate or file output size you have chosen. Thus meaning that the 2nd pass of a 2 pass encode uses the SAME basic engine that a 1 pass CRF encode would use. Is that correct...?

For example, say I used BDRB to compress a Blu-ray to fit a BD25, and I selected a 2 pass mode. Does the first pass then determine what CRF to use on the 2nd pass in order to hit the right average bit rate in order to fit the BD25? And, in so doing, the actual bits used scene to scene will vary based upon the complexity, and yet do so within the parameters that you have set? (in this case the output size) So, would the process (basically) be the same as if you happened to KNOW the exact CRF to choose beforehand in order to fit the BD25?

I just want to make sure I am understanding this correctly.

Thanks!

[jagabo]

1) Yes.
2) Yes.

And Yes. There are very small differences between the two but they're negligible. If you run CRF encode and it delivers X average bitrate, then encode using 2-pass VBR requesting X average bitrate, the two resulting files will look nearly identical, and bitrate distribution will be similar.

[Lathe]

1) Yes.
2) Yes.

And Yes. There are very small differences between the two but they're negligible. If you run CRF encode and it delivers X average bitrate, then encode using 2-pass VBR requesting X average bitrate, the two resulting files will look nearly identical, and bitrate distribution will be similar.

Awesome, thanks jagaboo! It's nice to have someone understand and answer so succinctly!

[Cornucopia]

Answer 1 could refer to max bitrate, but it isn't likely.

Answer 2 is that a simpler way of looking at the two is that they are equal, but in practice, 2pass vbr is often CONSTRAINED by a set max bitrate, avg bitrate, min bitrate, and sometimes more esoteric things like vbv buffer, whereas cfr usually isn't constrained. Difference shows up in the wildness of the variations. Overall average bitrate should still be the VERY similar though.

Scott

[Lathe]

Answer 1 could refer to max bitrate, but it isn't likely.

Answer 2 is that a simpler way of looking at the two is that they are equal, but in practice, 2pass vbr is often CONSTRAINED by a set max bitrate, avg bitrate, min bitrate, and sometimes more esoteric things like vbv buffer, whereas cfr usually isn't constrained. Difference shows up in the wildness of the variations. Overall average bitrate should still be the VERY similar though.

Scott

Ah, I kind of thought that perhaps CRF had a bit more breathing room. Thanks mate!

[_Al_]

2pass vbr is often CONSTRAINED by a set max bitrate, avg bitrate, min bitrate, and sometimes more esoteric things like vbv buffer, whereas cfr usually isn't constrained

Same settings , including buffers should be taken as granted, because anything can happen, like as you say CRF's bitrate can go higher for a scene as oppose 2pass VBR.

But I answer rather because folks even might want encode 1pass CRF with both buffers restrained a bit (or just gently), not knowing it. Many use 2pass to squeeze video somewhere without needing EXACT size, say 2GB, they just might need something about 2GB or even more precisely 2GB average per movie, per video to save space (on phone card, cloud, whatever). AND using CRF and limiting buffers you can boost low bitrate scenes that are THE scenes that give almost exclusively trouble, not nicely lit scenes. You lower CRF one or max two numbers lower than you'd do it normally. Perhaps just one is enough.

There might be like 80% (20% burns on disc or some other reason ?) of folks out there encoding 2pass, not even needing using 2pass, they just think they have to encode like that. As soon as you come up with max bitrate that most likely is enough for you and your videos, you can set CRF much lower and restrain buffers. Not severely, just about, gently. Do couple of videos without any restrains, check bitrates in bitrateviewer for a considerable amount of videos and then you are ready using 1pass only. For couple of scenes that will be restrained you get boost to tackle possible banding that is more visible. So I know you have to orient yourself whats going on first.

Or you find someone who gives you values for command line to get valuable number, most likely average per movie at the end. I never heard of this here, as oppose nonsense questions, like what is the best setting ...

Or just CRF without buffer restrains gives one an average volume per movie that might be even exact number that person doing 2pass all the time accepts and waste a time all the time to run everything twice the time. And in both cases (CRF and buffer restrain or not) it is far more scientific method that 2pass to average.

[Lathe]

2pass vbr is often CONSTRAINED by a set max bitrate, avg bitrate, min bitrate, and sometimes more esoteric things like vbv buffer, whereas cfr usually isn't constrained

Same settings , including buffers should be taken as granted, because anything can happen, like as you say CRF's bitrate can go higher for a scene as oppose 2pass VBR.

But I answer rather because folks even might want encode 1pass CRF with both buffers restrained a bit (or just gently), not knowing it. Many use 2pass to squeeze video somewhere without needing EXACT size, say 2GB, they just might need something about 2GB or even more precisely 2GB average per movie, per video to save space (on phone card, cloud, whatever). AND using CRF and limiting buffers you can boost low bitrate scenes that are THE scenes that give almost exclusively trouble, not nicely lit scenes. You lower CRF one or max two numbers lower than you'd do it normally. Perhaps just one is enough.

There might be like 80% (20% burns on disc or some other reason ?) of folks out there encoding 2pass, not even needing using 2pass, they just think they have to encode like that. As soon as you come up with max bitrate that most likely is enough for you and your videos, you can set CRF much lower and restrain buffers. Not severely, just about, gently. Do couple of videos without any restrains, check bitrates in bitrateviewer for a considerable amount of videos and then you are ready using 1pass only. For couple of scenes that will be restrained you get boost to tackle possible banding that is more visible. So I know you have to orient yourself whats going on first.

Or you find someone who gives you values for command line to get valuable number, most likely average per movie at the end. I never heard of this here, as oppose nonsense questions, like what is the best setting ...

Or just CRF without buffer restrains gives one an average volume per movie that might be even exact number that person doing 2pass all the time accepts and waste a time all the time to run everything twice the time. And in both cases (CRF and buffer restrain or not) it is far more scientific method that 2pass to average.

Yes, very good! Thank you kindly for taking the time to explain this in such an excellent and easy to understand way!