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

[Jack]

nick wrote:

> 1. It's is stated that the resolution(height/width) you choose to 
> encode a movie should both be a multiple of 16. Must I use this rule 
> when cropping the movie, or can I also crop the movie as follows:
>
> ... -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?
>
No.  So long as you eventually scale to a multiple of 16, you can crop 
to whatever size you want.

> 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.
>
1.78:1 is the aspect ratio of the video on the DVD, which includes the 
black bars.  The actual film is 2.35:1.  This is because DVDs don't 
support a 2.35:1 aspect ratio.

After cropping, it should be 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?
> Or is Mplayer rescaling the  scale parameters I defined, namely 720:200?
> Or perhaps something else?
>
> 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?

Mplayer always scales to the aspect ratio indicated in the source file, 
if your vo driver supports scaling, unless you specifically specify a 
different aspect ratio on the command line with -aspect.  It's only 
doing that for display purposes, though.  Mencoder won't do that.

>
> 4. How does the autoaspect parameter work?

Check the man pages.

> 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?
>
That depends on what you're playing it back with.  A large number of 
video players expect square pixels in AVI files; anything else will 
display wrong.  If you're targeting, say, Windows Media Player, you'll 
want to scale to 2.35:1. 
If you're going to play it back with mplayer, though, you can safely 
scale to 720x416 (to get to 16), and use autoaspect or aspect=2.35 in 
your lavcopts (and/or -force-avi-aspect 2.35 on the command line) to 
record the aspect ratio in the file, so it will be properly scaled on 
playback.

-jack