[MEncoder-users] encoding speed

[Raimund Berger]

Philippe MONROUX <phi2-monroux at wanadoo.fr> writes:

> De (from) (von) <raimund.berger at gmail.com> :
>
>> >> What do you mean with "big scale"?
>> > example
>> >  bpp=0.08 -> scale=1568:880
>> >  bpp=0.10 -> scale=1408:784
>
>> 
>> I had something different in mind though. It seemed from your wording
>> that you are upscaling the image before encoding, which would be a
>> serious mistake.
>
> You're right. I'm newbie with mencoder. 
>
> It's seems that, with x264, using only crop=... and to wipe out scale=
> ( to get original size) is a <<not so bad choice>>. Isn'it ?

Yes, very much so.

Never feed upscaled video to the encoder. The scaler can not magically
add more detail to the image, it just interpolates the additional
pixels. So the upscaled image does not contain more information than
the original one, but it

* costs the encoder more work (slower)
* eats up your precious bitrate bandwidth for those redundant pixels.

Any upscaling should always be done on playback, not on encoding.

Cropping away black borders on the other hand is highly
recommended. Not only because those borders contain no valuable visual
information, which means that encoding them would just waste bitrate
bandwidth. But also because the hard edges have a negative effect on
compression efficiency. So if you crop, just make sure you completely
eliminate those black border pixels.

After cropping, you can *downscale* the image if you wish. This makes
sense if you want to make a low bit rate encoding, like squeezing a 2
hour action movie into 700 MB. If you don't downscale, the encoded
video might exhibit encoding artifacts (like blockiness and blur) due
to the low bitrate, and those are often perceived as ugly by the
viewer. Downscaling on the other hand of course reduces sharpness and
detail, but in a uniform way. So the end result often looks more
pleasing.

Finally, there's no need for scaling to preserve a specific aspect
ratio. As long as the input file contains all relevant data, like for
example a proper DVD stream does, mencoder will automagically store
the right information in the final AVC video stream, in the form of a
so call pixel aspect ratio. The output aspect ratio will then usually
be just right, generally also when you crop or downscale. So in most
cases there is no need to take extra steps on your side.