decimating doesn't decrease size of file??

[mb1]

Hi everybody,
I'm using Virtual Dub. To decrease size of files I usually decimate them by 2, and that usually decreases their size about 40-50%. But lately I got movies which after decimation have almost the same size as before, they decrease e.g., from 713 Mb to 708 Mb. And Virtual Dub shows that the smaller ones are really decimated, having only half of the frames. Somebody knows what's going on?

Best regards
Maciej

[setarip]

"But lately I got movies which after decimation have almost the same size as before"

From your statement, I gather that you're downloading (rather than creating) videos. If so, the difference could be the result of fewer keyframes or a different codec...

[mb1]

Yes, I am downloading. I don't understand your statement: you mean that I decimate only the keyframes?
I think this is not the case, Virtual Dub shows the number of ALL frames, and this number says me I have only half of them, compared to the original movie. And I don't understand why the size of the file isn't also decreased, as should happen, according to the Virtual Dub help.
I also use the same codec as in the original movie.

[Echo147]

Decimating any number of frames isn't guaranteed to reduce the filesize drastically, because DivX bitrate is specified *per second*, not per number of frames processed.

In theory, decimating your frames by 2 should allow you to half the bitrate in DivX's compression properties, and achieve the same picture quality.

The motion can't be looking too good though...