Avg quant at the top of analyse.log is NOT correct

[jonny]

The correct way to obtain the average quant in analyse.log is to sum the quant in analyse.log and divide by the number of frames.

Avg quant at the top of analyse.log is NOT correct.
You can find why here:
http://forums.divx.com/viewtopic.php?topic=34345&forum=6

From Sparky139 (DARC):

"Average quantizer" is basically a quantizer that will be used for encoding frames with average complexity in the clip. An example: your clip has 100 frames with complexity 1000, 100 frames with complexity 2000 and 100 frames with complexity 3000. Average complexity is 2000. Frames with this complexity will receive the quantizer equal to "average quantizer". Quantizer for frames with complexity 1000 will be lower, and for frames with complexity 3000 will be higher.
This explanation does not take into account B frames, rate error correction and a few other things, but it shows the general idea.

This is correct definition of the "Average quantizer" you usually find in the top of analyse.log.

What about to make this Sticky?

[jonny]

In addiction to this, there is a relationship between final average quant and compressibility test value.

It's something like this:

Red, green & blue are 3 different clip.
With every clip i've slowly incremented the bitrate and every time i've calculated:
1-comp. test value
2-final average quant encoding in 2-pass mode

http://jonny.leffe.dnsalias.com/g2.gif
(average quants in the graphic are calculated summing the quant in analyse.log and dividing by the number of frames)


Let's see what happen if you use the average quant reported at the top of analyse.log:
http://jonny.leffe.dnsalias.com/g1.gif


You can see that the first graphic is a lot better than the second.