RF and File Size

[CraigHB]

Hello, first post here.

Been using Handbrake a while (great software BTW) and I'm having a bit of an issue with something. What I'd like to do it target a file size using Constant Quality. Right now the only way I can see to do that is with trial and error. I don't want to use Average Bitrate because from what I understand image quality can vary and I don't want that.

Is there a way to calculate the correct Constant Quality RF number for a target file size without resorting to trial and error?

[rollin_eng]

No, Constant Quality means just that. Bitrate will vary to maintain a certain quality.

[mduell]

The size will vary from movie to movie depending on the length and complexity of encoding it. Grainy high motion scenes like Saving Private Ryan take a lot more bits to get the same quality than clean slow moving scenes like The Social Network.

Encode a variety of movies to see what size you typically get on average and adjust from there. Rule of thumb is changing RF by 6 points will double or halve the size.

[CraigHB]

Thanks, I understand that, but file size varies with RF and I'd like to select an RF that arrives at a file size within some range.

[Deleted User 11865]

For the same output bitrate, 2-pass average bitrate and constant RF are the same (only a computer could tell the two apart). The difference is, you cannot predict file size when using CRF, but you cannot predict quality when using 2-pass ABR.

[CraigHB]

I can't see where to edit the posts here. Wanted to add that I'd like to be able to find the resulting file size of a video without having to actually encode it. Thought maybe there might be a tool to do that or something. So I guess I'm stuck with encoding a movie at various RF values to see how they come out (trial and error).

[rollin_eng]

If you want a fixed size use 2 pass ABR.

[CraigHB]

For the same output bitrate, 2-pass average bitrate and constant RF are the same (only a computer could tell the two apart). The difference is, you cannot predict file size when using CRF, but you cannot predict quality when using 2-pass ABR.

So that brings up a point I'm not clear on with Average Bitrate. From what I understand bitrate varies depending on what's happening from frame to frames. Does Average Bitrate allow those fluctuations or does it throttle quality to maintain a constant bitrate at any small interval?

Thing is if the quality is not all over the place with Average Bitrate, then I could just use that. But everything I read says not to use it unless you have an actual bitrate restriction, which I don't.

[rollin_eng]

If you don’t have bitrate restrictions why are you trying to restrict the bitrate?

[BradleyS]

RF naturally varies greatly with the source material's complexity, so you'll be stuck with trial and error for every source. If you really care about size, 2-pass average bit rate will still allow quality to vary while maintaining an overall average bit rate, hence its name and purpose.

[Deleted User 11865]

For the same output bitrate, 2-pass average bitrate and constant RF are the same (only a computer could tell the two apart). The difference is, you cannot predict file size when using CRF, but you cannot predict quality when using 2-pass ABR.

So that brings up a point I'm not clear on with Average Bitrate. From what I understand bitrate varies depending on what's happening from frame to frames. Does Average Bitrate allow those fluctuations or does it throttle quality to maintain a constant bitrate at any small interval?

You're thinking of Constant Bitrate. 1-pass Average Bitrate also has some restrictions by not knowing video oroperties ahead of time, but 2-pass ABR and Constant RF are equivalent (except each serve a different purpose).

[CraigHB]

Okay, thanks for the replies people.

It sounds like 2 pass Average Bit rate is what I'm looking for. If quality holds fairly constant from frame to frame I'll be good with that. I just didn't want to have quality go way down for higher bitrate sections of the video compared to lower bitrate sections.

[Deleted User 13735]

Time (Sec) x Bitrate (Mbps) x .125 = File Size (MB)
Bitrate means Average video bitrate plus the sum of audio bitrates.

Unless you need a constrained bandwidth for streaming and broadcast servers, there is no need to know the exact bitrate. Storage size is no longer an issue.

[mduell]

It sounds like 2 pass Average Bit rate is what I'm looking for. If quality holds fairly constant from frame to frame I'll be good with that. I just didn't want to have quality go way down for higher bitrate sections of the video compared to lower bitrate sections.

Frame to frame is consistent with 2 pass (well, subject to the x264 psychovisual model), but movie to movie variation will still occur.

[CraigHB]

Yeah I expect that movie to movie quality will vary. Thing is I want to store a bunch of movies on my laptop computer's SSD and I don't have a huge amount of space on that drive. I want movie files to be more on the smaller side without getting too much visual loss. I end up doing trial and error with Constant Quality which becomes time consuming. I've tried a few now with 2 pass Average Bitrate and it seems to be doing what I want. I think that will work for me.

[rollin_eng]

If you post your logs we might be able to suggest some settings.