How to change MKV from 50 fps to 25 fps skipping every second frame?

[andreyyshore]

So, I have an HD MKV file with 50fps. I want to change it to 25 fps and I want every second frame to be skipped (to keep it in sync with the audio). Is there a way to do this without losing video quality?
EDIT: mkvmerge doesn't skip frames.

[adom]

I'd like to hear a solution too (but using avisynth)

As I said in other threat I'v been playing with avisynth for couple of days and found how to make it:

changeFPS(25) - will delete every second frame

convertFPS(25,zone=224,vbi=49) - will make some blending while converting frame rate ("vbi" value I found on the net for pal movies) - but to apply this command I have to convert to YUY2 - and here I'm lost - what parameters should I use and do I have to convert back to YV12 at the end? - output movie is xvid in avi with res. 800x448

[jagabo]

AviSynth simple decimation:

SelectEvery(2)

The video will have to be reencoded in any case.

but to apply this command I have to convert to YUY2... do I have to convert back to YV12 at the end?

That depends on your encoder -- whether it accepts YUY2 as an input. If not, just ConvertToYV12() again.

[adom]

Thanks jagabo - it is clear now.

[jagabo]

With SelectEvery() you can also specify which of the pair to keep (if you need to for some reason):

SelectEvery(2,0) keeps the first of each pair
SelectEvery(2,1) keeps the second of each pair

SelectEvery(2,0,0) keeps the first of each pair and also duplicates it, retaining the same frame rate
Ie, if your original frames are 0,1,2,3,4,5,6,7.... after SelectEvery(2,0,0) you have 0,0,2,2,4,4,6,6...

[Gavino]

And just to round off this group of Select functions (or is it this select group of functions?), there is also:
SelectEven() - equivalent to SelectEvery(2, 0)
SelectOdd() - equivalent to SelectEvery(2, 1)

[andreyyshore]

I don't get it. I don't understand how to use avisynth.

[jagabo]

New to AviSynth - start here

http://avisynth.org/mediawiki/Main_Page#New_to_AviSynth_-_start_here

[andreyyshore]

OK, so I tried using the AviSynth script with VirtualDub on a smaller file (only 3 minutes/94 MB instead of much more). The video format is H264. Thanks to the K-Lite Codec Pack that was already installed, I could use H264 with AviSynth. I chose the "Direct stream copy" option in VirtualDub (I've never used it before) and the resulted AVI was 6 GB in size! Is it uncompressed? If yes, why is it uncompressed if the video track is supposed to be copied directly?

[Baldrick]

OK, so I tried using the AviSynth script with VirtualDub on a smaller file (only 3 minutes/94 MB instead of much more). The video format is H264. Thanks to the K-Lite Codec Pack that was already installed, I could use H264 with AviSynth. I chose the "Direct stream copy" option in VirtualDub (I've never used it before) and the resulted AVI was 6 GB in size! Is it uncompressed? If yes, why is it uncompressed if the video track is supposed to be copied directly?

You must choose a compression, video->compression. The "avisynth" stream is uncompressed.

[jagabo]

Direct Stream Copy mode (the compressed video is copied from the input file to the output file without being decompressed and recompressed, and without any filtering) only works with AVI files opened directly in VirtualDub. AviSynth's output to VirtualDub will always be uncompressed video frames. If you use Direct Stream Copy mode with an AVS script you will get uncompressed video frames in your output. If you want compressed video you need to use Video -> Compression... to select a codec and set up the compression parameters.

[andreyyshore]

Aha. Thanks.