MAMEWorld Forums - The Loony Bin - Re: What free basic video trimmer do you recommend?

> > > - No video quality lost.
> > > - Supports mp4 and flash videos.
> >
> > These requirements are mutually incompatible. You can't trim MP4 and flash videos
> > without re-encoding and losing quality.
>
> WRONG !!
>
> There are various tools out there that let you do just that - trim AVI, MP4, FLV, etc
> videos without ANY re-encoding or ANY loss of quality.

First of all, "trim" is an imprecise term that could be interpreted as "crop" or "cut".

There's no way you can crop any vector-quantiser codec video without re-encoding, that won't happen.

You can't do lossless cuts on arbitrary frames in a video containing bidirectionally predicted frames.

Supposed your video uses the following frame type sequence (typical of MPEG2): IPBIPBIPB...
The P frames refer to the previous I frame, and the B frames refer to the previous and next I frame.
The frames will actually be stored/transmitted in the order 12435768...

You can do a lossless cut on an I or P frame, but you can't do a lossless cut on a B frame as it's stored/transmitted after an I frame that comes later in the display sequence, and needs data from it to be decoded. Cutting on the B frame would require re-encoding.

H.264 (common in FLV and MP4 videos) often uses a more complex frame type sequence like IPPBPPIPPBPP...
In this case the first two P frames refer to the I frame before them, the second two P frames refer the the I frame after them, and the B frame refers to the I frames on either side.
Frames are stored transmitted in order 123745678...

With this frame sequence you can only do a lossless cut on frames 1, 2, 3, 7, 8, 9... Cutting on other frames requires re-encoding.

That covers cuts where you're removing the part of the video after the cut point. If you're removing the part of the video before the cut point, there are still a limited set of frames you can cut on without requiring re-encoding, although it's a different set (in the second example it's 1, 5, 6, 7, 11, 12...).

Wishful thinking doesn't change the fact that these formats require re-encoding to allow cuts on arbitrary frames, even if it's only a few frames at the end of the video that would need to be re-encoded.