[XviD-devel] Slow encoding - pondering the cause

[Christoph Lampert]

On Mon, 24 May 2004, J. Grant wrote:
> I wonder if the different resolution 592 x 448 makes a difference at the 
> same bitrate?
> 
> Is there an optimal resolution for encoding a video at bitrate 1000 ? 
> For instance I could encode at 640x480 or at 1024x768, the former would 
> have more quality in the fewer pixels and the latter would have less 
> quality in more pixels.  Which is more suitable?  Perhaps there is a 
> comparison graph?

The higher the resolution, the more detailed the image can be. 
With fewer pixels, there are fewer details. 

But if you have higher resolution at same bitrate, the compression ratio 
becomes higher, and if it's too high. you might end with a blocky or 
blurry image. 

So, there always has to be a compromise, and which is the best bitrate at 
given resolution or which is the best resolution at given bitrate you 
cannot really tell in advance. It depends on very many factors. 

As a rule of thumb, I usually calculate the "bits-per-pixel", that is you 
calculate how many pixels are shown per second, e.g. for 640x480 at 25fps
it's 640*480*25 = 7680000 pixels per second. That you divide by the 
bitrate. E.g. with 1000kbps, it's 1000000 bits per second, so you end up 
with  .13 bits per pixel which is a reasonable value. 
Below 0.1 is often too low, above 0.2 is often not needed. 
Many settings influence this, and the most important is the video 
material that you try to encode. 
E.g., if you have B-frames actice, a somewhat lower number is acceptable 
than without, and action material with explosions and fast motion etc. 
should get more bits than e.g. a sitcom. 

Please check the forums e.g. at doom9.net for the general encoding stuff, 
since this list is intended for development.  

gruel