[MPlayer-users] Question about crop, scale, aspect ratio, multiple of 16

[Ulrich Schweitzer]

On Thursday 16 December 2004 20:48, nick wrote:

> ... -vf crop=720:428:0:74
>
> These crop values are suggested by cropdetect and cut away the bars
> prefectly.  Is it a problem that 428 is not a multiple of 16?

As others have already told you: No problem as long as you rescale to a 
multiple of 16.

> 2. I have a hard time understanding what scaling actually is/does. I
> understand that the croped resolution should be scaled to get a bpp
> value larger that 0.20. Now, when I run the following command, just as
> a test:

Going for a specific bpp value is not really a good idea. Somebody on this 
list determined that you get a constant quality at constant bits per 
sqareroot of pixels a while ago (search the list archive if you are 
interested). But even that is no good criterion in my opinion as the 
video material you are encoding has a huge impact. If there is a lot of 
movement or noise you will need a lot more bits.
Bottom line: Encode the movie and look at it. If you like the quality, 
fine. If not, reencode with a higher bitrate. Over time you will get a 
feeling for what bitrate you need for what kind of movie.

> The first thing I don't understand is why the Movie-Aspect says 1.78:1.
> When I look at the back cover of the dvd, is says Ratio 2.35:1.

All DVDs are either 4:3 or 16:9. If the movie on the DVD is 2.35:1, then 
the DVD is 16:9 plus there are black bars in the picture to make the 
movie 2.35:1

> The next part "prescaling to correct movie aspect". What is Mplayer
> doing here? Scaling the croped resolution with the 1.78:1 aspect ratio?
> Scaling the croped resolution with the 2.35:1 aspect ratio?

Exactly.

> 3. If the "multiple of 16" rule aplies to scaling, how can I get an
> resolution that has those specifications. Since Mplayer automatically
> adjusts the scaled resolutions accoring to the aspect ratio?

Mencoder doesn't do that and that's what counts.

> 4. How does the autoaspect parameter work?
> Should I when scaling try to keep as close to 2.35:1 as possible, or
> can I just use autoaspect in the chain to fix the aspect ratio?

It depends on if you want to watch the encoded movie only with mplayer or 
also with other players. If you never use other players, keep as many 
pixels as possible and store the correct aspect ratio in the avi with 
-force-aspect-ration 2.35 during encoding. Mplayer will display the avi 
with the correct aspect ratio but all other players won't.

If you want to keep your avis compatible with other players, scale to as 
close to 2.35:1 as possible while obeying the multiple of 16 rule and 
still use -force-aspect-ration 2.35. That way mplayer will display the 
avi at the exact aspect ratio while all other players are slightly off. 
The error is so small that you won't even notice it if you look for it.

HTH
Ulrich
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