[mythtv-users] Commercial cutting & DVD burning

[David]

On Mon, 7 Mar 2005, Praveen Shenoy wrote:

>
>> Thanks to everybody for the replies. David, thx for the steps. A few 
>> question though. I installed avidemux as you suggested and opned one 
>> of my 30 minute clips. Maybe I missed something or didn't do it 
>> right. Here is what I came across....
>> I also read some docs about cutting. The only way I could do was jump 
>> to the next black frame or the keyframe and then having marked the 
>> point a mark point b and cut it. But clicking on find next black 
>> frame, it kind of was going thru the video at the same pace as it 
>> would play, maybe a little faster but definitely did not finish in 5 
>> mins. So, I am thinking I am missing something. Are you using the 
>> cutlist when you start avidemux? Can you give me some more info on 
>> cutting the clips once I open it in avidemux? 
>
This mail must have fallen foul of a filter - anyway...

The exact cutting process is important!!

Removing a section (advert)
* Scroll through to where you want the cut to start
* Now hit the A button to mark the selection-start. This can be on any 
frame type (at least I've never had a problem)
* Click through to the end of the advert (I click to the right of the 
slider in the scroll bar to get a rough place - much quicker than using 
the keyframe advance)
* Now, click to the next (or previous) keyframe.
* Now use the B button to mark the selection-end
* Hit Delete

Trimming a file (removing leading credits)
* Go to the first frame
* Now hit the A button to mark the selection-start
* Scroll to the frame you want as the first frame
* Now, click to the next (or previous) keyframe
* Now use the B button to mark the selection-end
* Hit Delete

Trimming a file (removing trailing credits)
* Scroll to the frame you want as the last frame
* Now, click to the next (or previous) keyframe
* Now, advance 1 frame (press '6') --- This is important on older 
versions of avidemux.
* Now hit the A button
* Advance to the last frame and hit the B button
* Hit Delete

Notes:
* I never use the find black frame - too slow.
* dragging the slider in the scrollbar is actually really good!!

The reason for the frame advance is that the last frame must be a 
keyframe (or avidemux complains).
If you put your 'cursor' on a keyframe, and mark it as the A-point, 
avidemux deletes from-and-including the A-point.


The reason for all this is to do with the way mpeg holds video.
You know how cine film is a series of still images played one after the 
other.
Well mpeg does have still images (a keyframe or I-frame) and then it 
stores the changes between that still image and the next frame. These 
frames are called P-frames or B-frames and they don't contain images, 
just info about the bits of the image that have changed. This is how 
mpeg does compression.

So imagine you're going through the video and you show a picture. Then 
you add a bit of red here and take away some green there... and you get 
the next picture. Do this 12 times or so. Eventually, just in case a 
glitch crept in, we have another keyframe.

OK, now do it again...  show a keyframe and then make some changes, then 
more changes.
Now, imagine we've cut a section so skip ahead in the video to a random 
B- or P-frame.
This frame contains changes - but they're for a completely different 
image. So if you apply these changes you end up with a pixellated 
multicolour screen for a few frames (until a new I-frame appears).


HTH - I'll post it on the wiki and include some stuff about avidemux 
video settings too...

David