Resolution

[Sergejack]

I'm wondering, when a video is resized to a lower resolution in order to improve the compressability, doesn't the loss of information instead handicap the way the codec compute the motion and "stuff" ?

Then, what should be taken in consideration when encoding at low bitrate knowing higer resolution "helps" the codec to "understand" the animation but may (or may not ? O_o) cost more bits.

[jggimi]

I assume you're asking about MPEG-4 compression technologies, because most MPEG-1/2 compression is done with fixed resolutions.

There's a great document that describes very clearly how various MPEG-4 technologies work: Motion Vectors, Quantizers, Matrices ... even though it's the DivX User Guide, it talks to MPEG-4 technologies in general.

The best practice among MPEG-4 encoding members is to use techniques that choose a resolution that gives the best possible quality for any given average bitrate.

This is usually done through a compressibility test. Since each video that you compress will have different compressibility, there is no one-size-fits-all resolution that works for any particular bitrate. A compressibility test uses the formula <value> = bits / (pixels * frames). This single number takes into acount both bitrate and resolution.

The technique is fairly simple: A resolution is selected, and an analysis is done to determine the maximum possible bitrate at that particular resolution. The resulting b/p*f value is used to select a new resolution where the new b/p*f value is 60-80% of the maximum. This is the methodology used by AutoGK and GK for encoding DivX and XviD.

[Sergejack]

I don't understand but a bit of what you'r explaining ^^' (Even if I already read much about motion vector (The whole XVId unofficial FAQ and even before that, the Dirac's algorithme explainations).
I'll try to get info on (Auto)GK.

But just to know : To figure out the compressability will I have to encode over and over again ?

[jggimi]

I don't understand but a bit of what you're explaining...

You've asked about MPEG compression technology. The Guide I pointed you to speaks to the various technologies used in MPEG-4 codecs.

• Unlike my explanations, the Guide's explanations are simple, understandable, and shown with example images to make the concepts even easier to understand.

The guide is written in plain and easy to understand English ... or French, or German, or Japanese.

...To figure out the compressability will I have to encode over and over again ?

No.

• AutoGK does a compression test for you as part of any multipass encoding.
  
• GK has a semi-automatic compression test, which is usually run only once. Subsequent tests in GK are only needed if you change the compressibility of the content. That would happen if you select different codec features/options, or change preprocessing filters/settings.

...I'll try to get info on (Auto)GK...

Start here.