Compressibility check/resolution

[eva 01]

There's one thing I don't get: I thought "the higher the resolution, the better the quality". When I do the compressibility check, for a resolution of 640x272, I get a percentage of 47.6 (Bits/(pixels*frames)=0.16); and when I decrease the resolution to 448x192, I get a percentage of 96.4 (Bits/(pixels*frames)=0.323).
So could someone tell me exactly what the compressibility check is about (please tell me something different from what it's said in the guides)
:confused:
Thanx for your help

PS: is it better to have a low resolution with a high percentage or to have a high resolution with a low percentage? (or mid-resolution and mid percentage)

[Got Milk?]

The higher the resolution, the better the quality up to a certain point. After that point, there is too much information for the codec to handle and you end up with a very blocky output.

From the DivX 5 GKnot guide (http://www.doom9.org/gknot-main3.htm):The compressibility check gives you a rough indication of how good a movie can be compressed. The 3 values shown above are related as follows: the 0.338 is the value that the compressibility check returned. If the Bits/(Pixel*Frame) the first value, right above the Load button matches this value you have perfect quality, going for a higher value would be overkill as the movie wouldn't look any better.. The limit of the codec has been reached. The 41.7% value tells you the relationship between the optimal case and what you're going to get at the currently selected resolution. 60% or higher will pretty much guarantee that you'll get pretty good looking results. Going over 80% is a serious overkill and you should rather increase the resolution. Values in the range of 50-60% still look pretty good and 40-50 is still OK, especially when you activate B-frames. The darker a movie gets and the cleaner the source the more you can compress a movie. Using B-frames certain movies will look good even at a 0.12 Bits/(Pixel*Frame value).

[brashquido]

This is my take on things, please correct me if I'm wrong.

Higher resolution = less available storage for each macroblock (block of 16 X 16 pixels which make up the video)

Lower resolution = more available storage for each macroblock

If you put in a target size of 700MB, of which say 650MB is for video that is 2 hours long. That means at the resolution of 640x272@25FPS, there are 680 macroblocks (640/16 = 40, 272/16=17, 40*17=680) and at 25FPS over 2 hours, that means there are a total of 3000 frames. This means each frame has about 221KB of available storage, of which that has to be shared around 680 marcroblocks, giving each macroblock about 334 bytes.

If you lower the resolution to 448x192 (28 X 12 macroblocks = 336 total) means, given that all the other settings remain the same, each macroblock will now have about 676 bytes, which is over double of the higher resolution.

Rule of thumb (unless using high bitrates) is resolution does NOT equal higher quality video. The trick is finding the best trade off between image quality and resolution. I find the resolution of about 480 X 384 (uncropped) to be around the "sweet spot" for encoding to DivX for single CD conversions.