[MEncoder-users] mEncoder newbie

[A. Schmid]

Rob Langley wrote:
> Hi
> 
> 
> In the example below the resulting 5 second mp4 shows noise
> (horizontal coloured lines) for the first few seconds then a very
> pixilated few seconds of the actual footage.  I thought if I used
> –ovc copy it does just that copies each frame

Yes, and this is exactly why you get a distorted image!
All MPEG-ish video streams have I-Frames and, between them, P- (and 
possibly B-) Frames. I-Frames are much like JPEG still-images, P-Frames 
are just _differences_ from the preceding I-Frame (this is also where 
motion compensation takes place), dramatically saving filesize.

Now guess what happens, if you take a random frame from the video 
stream? You'll probably a P-Frame (there is only one I-Frame in 10 
seconds or something), meaning you cannot display the image (because you 
dont have the preceding I-Frame).
This is what happens with your "-ss 1 -ovc copy", and therefore you get 
garbage - up to the next I-Frame in the stream.

If you want the correct image, then you need to either re-encode this 
first few seconds, or you need to start the copied section at an 
I-Frame. Both is not easy with mencoder - better use some video editing 
software like avidemux.
Or you just re-encode the whole cut-out section, which may result in 
some quality loss.

> I also note when encoding the message ‘Skipping Frame’ many many
> times is this normal?

That depends. A few skips can be neccessary to keep audio and video in 
sync, always depending on output and input framerate. The same is true 
for "duplicate frames".

Regards,
Alex