Resolutions and Scaling (mencoder)

[kharan5876]

Maybe this is more of a general encoding question but I use linux so I figured I could maybe get more specialized help here.

If you scale down the resolution from the original dvd 740x480 is this a way to decrease the filesize and/or increase quality?

If I keep the same bitrate but scale the output resolution size, will the resulting filesize be smaller or will it be the same because of the savings being used by the encoder.

If so, how would one go about finding the "magic numbers" for optimal gains.

[nm]

If you scale down the resolution from the original dvd 740x480 is this a way to decrease the filesize and/or increase quality?
Possibly, depending on how high bitrate and which encoder you are using. For example x264 can encode DVD resolution video (720x480 or 720x576 for PAL) at ~1000 kbps with quite acceptable quality (these things are of course subjective), and the quality will decrease if you scale the resolution down a lot. If you would like to encode at lower bitrates such as 300-500 kbps with x264, you'd probably get better quality by resizing to 640x360 or even lower resolutions.

If I keep the same bitrate but scale the output resolution size, will the resulting filesize be smaller or will it be the same because of the savings being used by the encoder.
The file size would stay the same because it is defined by the bitrate (as long as the encoder's rate control works properly). Bitrate tells the encoder how many bits it is allowed to use per time unit in average. Since you don't change the video duration or timing by changing the resolution, you end up with the same size.

If so, how would one go about finding the "magic numbers" for optimal gains.
By encoding many, many times with different resolutions and bitrates. :) Or by deciding on a resolution and encoding in CQ (or CRF for x264) mode instead of trying to guess a suitable bitrate for each source.

Considering the storage space and efficient encoders that are available nowadays, people usually want to keep most of the original quality and just use a high enough bitrate so that downscaling is not necessary.