Multipass Encoding with Filters

[dimitrik]

Hi, I'm hoping someone can answer this for me.

I'm encoding an uncompressed avi with DivX 5.05 using multipass encoding (in VirtualDub) and I'm also applying a filter.

My question is: should I be applying the filter on every pass, or just the 1st one? Or the last one? It occurs to me that by applying it every time I'm probably slowing it needlessly and maybe I should only apply it to the last pass, but I'm not sure.

And a 2nd question on multipass encoding - is there a real benefit in doing more than 2 passes - I can see almost no difference in filesize and I actually think the 2nd pass looks better than the 3rd

Any advice would be great.


____________________________
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[khp]

Re: Multipass Encoding with Filters

Originally posted by dimitrik

My question is: should I be applying the filter on every pass, or just the 1st one? Or the last one? It occurs to me that by applying it every time I'm probably slowing it needlessly and maybe I should only apply it to the last pass, but I'm not sure.

When doing multipass encoding, all passes should be preformed on the same source, when you change the filter settings, you are effectivly changeing the source, that the divx encoder sees. So changeing the filters will produce less than optimal results.
If you do change the filters between passes, you should use the filters you intend to use, durring the last pass since that's the pass where the encoding happens, all the previous passes are only used to improve the bitrate distribution,, and are therefore less important. But again, optimally all passes should use the same filters.

If you are using very cpu intensive filters, and you have lot's of spare harddisk space, it might be more efficient to apply the filters to the source, and encode it using lossless compression, like huffyuv. And then reencode the filtered huffyuv compressed file, using 2-pass divx encoding, without any filters.

Originally posted by dimitrik

And a 2nd question on multipass encoding - is there a real benefit in doing more than 2 passes - I can see almost no difference in filesize and I actually think the 2nd pass looks better than the 3rd

Using more than one pass for divx encoding, serves only one purpose. To improve the bitrate distribution.

Going from 1 to 2 passes often produces a significant improvement. Because the bitrate distribution can be improved a lot, using the knowledge gained from the first pass. Doing more than 2 passes can only slightly improve the bitrate distribution, because it's already very close to the optimal using only 2 passes.

[dimitrik]

That makes everything clear, thanks!