drop half of the frames without reencoding

[mathmax]

Hello

I have a .mts file 1920 *1080 that has been up converted to 59.94fps. Each frames are doubled. In order to reduce the size before uploading, I would like to drop half of the frames without reencoding. Is it possible with some tools (and maybe the help of avisynth)?

[jagabo]

It's not possible unless it uses an all intra-frame format. All high compression codecs use inter-frame encoding. Most compressed frames only contain information about the changes from frame to frame. If you drop that information for one frame all the frames after it will be corrupt, until the next keyframe.

For example, with a five frame sequence:

frame 1) A
frame 2) previous frame + B = A+B
frame 3) previous frame + C = A+B+C
frame 4) previous frame + D = A+B+C+D
frame 5) previous frame + E = A+B+C+D+E

If you drop frames 2 and 4:

frame 1) A
frame 3) previous frame + C = A+C
frame 5) previous frame + E = A+C+E

As you can see, if you drop frames the final product is wrong.

[mathmax]

ah yes.. sure. I didn't thought about this problem. When I demux an select a part of the video.. I should then only be able to cut after a group of pictures and not to a precise frame, right?

[Cornucopia]

Correct, unless you re-encode (either the affected GOP only, or the whole thing)...

Scott

[jagabo]

Any section you keep must start with a keyframe. They can end anywhere (if you end on a B frame the last few frames may be corrupt). Some editors only allow ending in keyframes too.