understanding compressibility

[plissken]

After I run a compressibility test, and the test returns a figure of (for instance) 0.317, what am I looking at? I figured the smaller this number, the more compressible the DVD video is. The bigger the number, the less it can be compressed. Or have I got it the wrong way?

[hakko504]

You're looking at the wrong number!
The number you quote is a bit/pixel value and indeed it means that the less it is the more compressible the video is, but the important thing is the % value shown next to it. If you look carefully you'll find two bpp values, one based on the settings you have chosen for the video, and one from the compression test. The compression test represents a sample of the video at maximum quality. Now look at the percentage (defined as [calculated value]/[compression test result]).
If it is >100% then you can't reach the bitrate specified at the current resolution, and you must either lower the final size of the video (in MB) or increase the resolution.
If it is between 80% and 100% then you will get a very good video, but some versions of DivX have problems reaching the desired filesize, and you should again consider the previous recommendations
If it is between 60% and 80% then you have reached optimum.
If it is between 40% and 60% then you should consider increasing the video bitrate, by adding a CD or lowering the audio bitrate, or decrease the video resolution slightly. Feel free to encode a clip and look at it though, if you intend to watch it on TV it may very well be sufficient to use these settings.
If it is below 40% then you sholuld increase the bitrate or decrease resolution. This will not look good.

[plissken]

Thanks for that. I know I was looking at the bit/pixel ratio and I knew it was of more concern because it's a figure for comparison whereas the percentage was a scale I could alter. I always adjust the percentage to 60% by altering the filesize and resolution.

So, the smaller the bit/pixel ratio returned by the compressibility test, then the more compressible. Good. I wanted to be sure of that.

Another thing. Does the compressibility test result take into account the filters you use? Your current resolution and filesize? Your current DivX codec settings? If it does, by altering the resolution, you'd be invalidating the compressibility result. Or would you have to run another compressibility test after changing your settings?

[jggimi]

Yes, filter choices can make a difference in compressibility. Examples: B-frames, resizing method, temporal smoothers, and other noise reducers.

Changes in resolution, alone, produce less change than changing filtration.

I often run many different filter previews, and multiple compressibility tests, prior to a complete encoding. I will do it every time I change the filtration. If the only change is to resolution, I sometimes skip the additional compression test, if the resolution isn't changing by large factors.