How does "Desired Filesize" affect the codec?

[Bulletproof]

How does Desired Filesize affect the codec? I was used to using Nandub and just supplying my setting and getting the result back. But how exactly is XVID constraining the filesize? I thought I was constraining the bitrate already by just tweaking the curve? Is the curve directly affected by the Desired Filesize? And what if I didn't care about the filesize and just wanted the codec to work freely, would pushing up the desired filesize to 999 work or would that just boost everything unnecessarily?

[Swede]

By 'work freely' do you mean maximize quality? Then you could just use 1 Pass-Quantizer and set it to 2.

[Bulletproof]

I think that might nullify the compression techniques though, I want to apply the compression techniques but not constraint it by a filesize.

[Swede]

I want to apply the compression techniques but not constraint it by a filesize. I'm not sure I follow you here. Do you want it restricted or not? Then how do you want to restict it if not by filesize/bitrate?

[Bulletproof]

I don't want to restrict the filesize. You know how before in the old DivX you use a bitrate calculator to determine how big your file would be? plain old DivX wasn't very "dynamic" and had much less variables to work with so you could get a somewhat close filesize from what you get from the calculator. When you specify options in XVID and such, there are alot of variables, curves and such are modified. Let's say I specify the Desired Filesize to be 200 megs, what is it doing to keep it under 200 megs? What if I wanted to just give XVID the compression parameters and just create the file and not be constraint to just 200 megs. Using a quantizer of 2 eliminates all of the good compression techniques that XVID does. A good example is Nandub, you do not force a filesize with it, you just give it the compression parameters and get a file. If the file is oversized then you change the way the bits are used by compressing the curve more. I dont understand what the desired filesize does in XVID, if you specify a low filesize does it automatically compress the curve for you so it reaches that size?

[Koepi]

Think of the filesize constraint as a bitrate (that's exactly what it does internally).

Since everyone uses a bitrate calc to reach a certain filesize, why not using directly the filesize and leave the rest to the codec?

There won't be something like an average framesize/bitrate directly in xvid.