Why Constant quality over Average bit rate?

[Obioban]

It seems like everyone who is in the know prefers constant quality to average bit rate. Why is that?

Logically, to my mind, average quality makes more sense to me-- some scenes are more intense than others and need more data to be properly encoded. Average allows those scenes to have more and the other scenes to have less, as required.

... but that doesn't seem to be what people choose, including the preset makers.

What's the thought process behind that?

[Deleted User 11865]

2-pass Average Bitrate should give you consistent quality across a given source (as you said, scenes that are easy to compress will get less bits, while more difficult scenes will get more). But Constant Quality does the same in that regard.

However, quality will vary from source to source. Clean sources with very little motion (The Social Network) will look much better than grainy sources (Saving Private Ryan) given the same average bitrate.

Whereas Constant Quality will give you consistent quality across multiple encodes, too.

[Deleted User 13735]

Constant Quality renders almost twice as fast.

[Obioban]

If you don't care at all about encode times, for a given bit rate, should the output be better with 2 pass avg than with constant quality?

I'm not super concerned about consistency from one movie file to the next... so much as getting the most quality I can out of my files.

Currently I'm just using "High"

[rollin_eng]

If you don't care at all about encode times, for a given bit rate, should the output be better with 2 pass avg than with constant quality?

It depends.

If you ran a cq encode and the resulting encode had a bitrate of 2000 then you ran a 2 pass encode at 2000 you should get the same result.

But if you ran a cq encode and the resulting encode had a bitrate of 3000 then you ran a 2 pass encode at 2000 your 2 pass would be worse.

This is why cq is better, the encoder is better at picking the bit rate than you.

[Obioban]

It depends.

If you ran a cq encode and the resulting encode had a bitrate of 2000 then you ran a 2 pass encode at 2000 you should get the same result.

But if you ran a cq encode and the resulting encode had a bitrate of 3000 then you ran a 2 pass encode at 2000 your 2 pass would be worse.

This is why cq is better, the encoder is better at picking the bit rate than you.

That's where I'm confused.

If you ran a aq encode at 2000 shouldn't it use more bitrate in the visually intensive scenes (where it's needed) and less bit rate on the less intensive scenes? Resulting in a higher average visual quality than something that was 2000 the entire time (cq)?

It seems like average quality should result in a more efficient use of the same resources.

[Deleted User 11865]

If you don't care at all about encode times, for a given bit rate, should the output be better with 2 pass avg than with constant quality?

If a CQ of RF 20 gives you an average bitrate of, say, 1888 Kbps for one source, and that you re-encode the same source source using the same settings and 1888 Kbps 2-pass ABR, both encodes should look almost the same (so neither will be better).

[Deleted User 11865]

2000 the entire time (cq)?

Constant Quality does not mean constant bit rate. At all.

[Obioban]

2000 the entire time (cq)?

Constant Quality does not mean constant bit rate. At all.

AAAHHHHHHH.

Lol, that completely changes everything. Thanks!

[rollin_eng]

Constant Quality does not mean constant bit rate. At all.

^^This.

Think of cq as saying 'I want my encode to be 95% of the original and use whatever bitrate necessary to do that'. This means you can have massive bit rate swings if a movie needs it.

[jamiemlaw]

Am I right in thinking that you get constant bit rate, which is as it sounds; then you get constant rate factor, which is variable bit rate (better); and then you get constant quality, which is variable rate factor (best)?

[Deleted User 11865]

Am I right in thinking that you get constant bit rate, which is as it sounds; then you get constant rate factor, which is variable bit rate (better); and then you get constant quality, which is variable rate factor (best)?

No, Constant Quality means CRF (Constant Rate Factor) when using x264, and CQP (Constant Quantizer) when using other encoders.
But CRF is better than CQP.

[jamiemlaw]

Constant quantizer: that's the term I was looking for! Got a few things muddled there.

Okay, so CRF better than CQP better than CBR?

And CRF implies VQP, and CQP implies VBR?

[Deleted User 11865]

Constant quantizer: that's the term I was looking for! Got a few things muddled there.

Okay, so CRF better than CQP better than CBR?

And CRF implies VQP, and CQP implies VBR?

Yes.