Different sizes using the same file and compression (Indeo/MSP) !!!

[ppravier]

While I wanted to obtain an AVI file the closest possible to a given size by adjusting the quality slider I noticed a very strange thing : I did twice the same compression using the same quality settings, the same file, everything unchanged : The two AVI files had slightly different sizes (1-2 % different).
This is really incredible, because all the parameters where absolutelly the same. I checked carefully everything.
I did several times the compression of my AVI file (starting everytime from the same source file), at different quality pourcentages. For the same poucentage value I obtained evey time a different size !

[Grigory]

This may happen if the compression uses some random component.
Or, is it possible that the slider position is different, but digits that show its position are rounded and are the same only on screen?

Grigory

[ppravier]

Thank you for your answer.

About the random elements in the compression scheme, I know nothing about it. If this is true, this is a complete explanation of the different sizes. Is there any ressources on the net explaining how this Indeo codec works ? (I found nothing).

About the slider I don't think that what you say is possible. In fact it looks like it is 100 pixels long, and for that reason the numerical values may be enough to match all the slider's positions, and for each position there is a different number.

What is the most disappointing for me is that I can obtain a bigger file with a quality of 45% than a file obtained with a quality of 46%.

[Grigory]

I am sorry, I don't know any details about Indeo5 codec, exept that it uses wavelet algorithm.

I can, however, imagine that it is possible to use some random element inside codec. For example, motion detection part may be based on some randomized search method, which can increase the speed in comparison with regular search.

Considering the quality slider, I suppose that for 45 and 46 values, the algorithm changes slightly to provide statistically slightly lower file size for lower settings, however, for one video file the result may be reversed.
This is not very unusual.
Quality slider controls the size of (actually) variable bitrate movie. If you need exact file size, you may have to use fixed bitrate compression.

Grigory