need help from tmpeg users

[symbiote]

I have deciced to make a very graphical "bit rate guesser" for use with tmpeg and divx 5. (yes, another brc...for no apparent reason)

One thing I'm finding difficult though is that the ratio of file size to bitrate doesn't remain constant when comparing different bitrates. I was wondering if anyone has ever "charted/plotted" in some kind of graph how things actually correlate and how to make these kinds of compensations when estimating...(?)

alternatively, if any tmpeg users would like to help the venture along with some mathematical expertese, that would be even better.

Being crap at maths, I'm unsure how to structure the equations to make accurate compensations.

A VERY ROUGH beta is at http://plang.lawngnome.org/tbrg.dcr is anybody is interested. but at the moment I haven't got sufficient data to amke accurate estimates. so the guesses will be horribly wrong at best. it illustrates the style of the interface i intend to use though.....i'll only bother making it look nicer if i can get the functionality hapening.

Thanks for reading

please email me if you can help - plang@lawngnome.org

[khp]

Re: need help from tmpeg users

Originally posted by symbiote

One thing I'm finding difficult though is that the ratio of file size to bitrate doesn't remain constant when comparing different bitrates.

This can be caused by several factors.
1. Structural overhead. For divx avi files this is approximatly 60 bytes (480 bits) per frame, I don't recall the overhead for mpeg-1/2.
2. Did you include the audio in your calculations ?
3. At sufficiently high bitrate, the video quality will become saturated. Which means that a futher bitrate increase won't result in higher filesize. When this limit is reached depends on the compressability of the movie.

[symbiote]

Damn, that's going to complicate the hell out of things.

Thanks for the info...

I first found this problem when i compared the video only file sizes of the same divx at three different bitrates. The filesieze does indeed increase less beyond a certain bitrate (from what i can tell plotting it in a chart), I was kinda hoping though that this point would remain constant between different video input types. But given that it doesn't, shit.

How do the other BRCs get around these kinds of issues? flask and tmpeg give me completely different file sizes (and quality) from the same input using the same codec at the same bitrate. This is why i wanted to make a tmpeg specific calc...
I'm really going to need help from tmpeg veterans it seems, i havn't encoded a great deal as yet.

I'm encoding everything at 1-pass, because I've had an array of seemingly strupid and unexplained errors when trying 2-pass. I also can't seem to locate anywhere to easily read up on the details of how the two modes compare and how to use them effectively.

anyway-

thanks again.

[khp]

Originally posted by symbiote
Damn, that's going to complicate the hell out of things.

No Kidding

Originally posted by symbiote
How do the other BRCs get around these kinds of issues?

They don't. The only way you can do it, is by preforming a compressability test on the source file. This is usually done by compressing a part of the source at maximum quality. Divx5 is usually very accurate in 2-pass mode as long as your target filesize is between 50 and 95% of the maximum. I don't have enough experience with Mpeg-1/2 to make any meaningfull estimation on mpeg-1/2 filesize prediction, and of course it depends on the encoding engine.

Originally posted by symbiote
flask and tmpeg give me completely different file sizes (and quality) from the same input using the same codec at the same bitrate.

Flask and tmpeg does not use the same encoding engine for mpeg encoding.

Originally posted by symbiote
I'm encoding everything at 1-pass, because I've had an array of seemingly strupid and unexplained errors when trying 2-pass.

Then the problem may well be that tmpeg can't accuratly hit the desired filesize in one pass. Have you checked out Doom9 tmpeg guide ?

http://www.doom9.org/mpg/tmpg-main.htm

Also see the guides at http://www.vcdhelp.com/

[symbiote]

Perhaps the best i can do then would be a tool to calculate and reccomend a new bitrate if it's been done once but is too small or too large. atleast i wouldn't be encoding things 5 times to get nice cd fits.

Thanks again for your help. Muchly Appreciated.