Trimming a video with Avidemux

[dfisher052]

I'm trying to trim front and back of some videos. The back trims ok but the front is way off. I've tried a 7 second advance but that doesn't always work. It seems to me that there has to be something wrong if it's as much as 7 seconds off. And it's not just 1 but several. With some it works OK but with others it can anywhere from 5 to 8 or 9 seconds off. Any ideas?

[jagabo]

In copy mode you can only trim on keyframes at the start of a segment.

[dfisher052]

What constitutes a segment?

[jagabo]

Anything you keep must start with a keyframe.

The data for most frames in highly compressed video do not contain everything required to reconstruct the frame. The data only contains differences from prior or future frames (for example "this frame is the same as the last one, just make these changes..."). Only keyframes contain all data needed to reconstruct a frame (like a JPEG file). If you remove a keyframe that's referenced by later frames you can no longer reconstruct those later frames.

[Peter Sagan]

You have to use the two "Go to keyframe" buttons. Those will tell you where you can start a cut (for lossless trimming).

If you want to cut from any other point you have to ditch copy-mode and re-encode instead.

[dfisher052]

I've been looking all over but can not find keyframe buttons.

[davexnet]

I believe the up and down arrow keys are key frame navigation

[dfisher052]

Thanks...Found them.

[Hoser Rob]

What types of video? If it's xvid/divx (most .avi files) it's possible to cut on non key frames. It'll do smart rendering to re encode the bits that weren't cut on key frames.

With x264 (most .mp4 or .mkv, actually most video period) or HEVC you can't cut on non key frames without curruption of the output video. It won't do smart rendering qith those formats.

[videobruger]

I've tried a 7 second advance

With h.264/h.265 the keyframes may be 250 frames (10 sec) apart.

[dfisher052]

Thanks for the replys...Hoser Rob, I am cutting MP4's & MKV's and I find that what you said is true. I'm now just cutting on the key frame which is close enough for what I'm doing.