[MEncoder-users] Why use TOOLS/calcbpp.pl and scale ?

[Martin Collins]

On Thu, 2 Mar 2006 13:43:50 +0200
dusty0 at freemail.gr wrote:

> When I do a PAL dvd-rip (using xvid usually) why shouldn't
> I use the aspect ratio found in the DVD and use one suggested
> by calcbpp.pl ? Almost all the times I have checked up to now,
> the good aspect ratio values from calcbpp are smaller than the one
> that's inside the DVD.. Won't that lead to a quality loss ?

IIRC calcbpp.pl assumes you will be scaling to square pixels and that
you have a specific bitrate you want to hit. In this case you may get
a better looking encode by down-scaling as well.

I prefer not to scale to square pixels but to set the aspect-ratio
for runtime scaling. If you care more about quality than size set
the max quantiser rather than a bitrate.

> Furthermore I must also scale the movie to the value suggested
> by calcbpp for the aspect ratio that I choose.
> I think that scaling leads to quality loss.

Scaling down can reduce noise and increase apparent sharpness, albeit
with a loss of actual detail. DVDs are usually pretty clean and sharp
to begin with though, so you only notice the loss of detail.

The point is to minimise quality loss for a given file size by
scaling to a picture size that can be accurately encoded at the
bitrate you specify. A smaller artifact-free picture looks better
than a larger crappy one.

> It seems that I am a bit confused here. :(
> I thought that for a good dvd-rip I should,
> 
> 1)crop the black borders

Yup, making sure both dimensions are divisible by 16.

> 2)use the aspect ratio that inside the dvd.

Cropping the borders will change the AR. You should calculate the
right new AR.

> 3)don't scale because that leads to quality loss.

I prefer not to scale if I can afford enough bits. But if bits are
scarce a modest down-scale can gain you more than it loses.

> 4)use a denoiser filter (optional)

It usually saves bits but costs time.

> For my dvd-rips I copy the sound found in the dvd

So do I if it is 5.1, otherwise I use vorbis.

To summarise: you've got the right idea, don't bother with calcbpp
unless your videos must be viewable with players that don't understand
aspect-ratio and must be below a certain filesize.

Martin