[MPlayer-users] how to "step backforward" ?

[peng shao]

On Mon, May 25, 2009 at 5:11 AM, Oliver Seitz <info at vtnd.de> wrote:
>> Hi dear all. I know there is a function to step forward by one single
>> frame, by pressing
>> . in mplayer. But I did not find the reverse function to step
>> backforward. Is there
>> this function in mplayer? Thank you.
>
> No, it's not. It is not that easy. I'm not a developer, but my thoughts
> about it are as follows:
>
> Video files consist of intra-frames and inter-frames. Intra-frames contain
> all the neccessary data for one single frame and can be decoded easily.
> Inter-frames only contain the differences of the new frame in respect of
> the frame that was just displayed last. So, if you're stepping back to an
> intra-frame, you could do all the calculation backwards which looks like a
> lot of code to be possibly written. Or you could search backwards to the
> last intra-frame and do all the calculations as normal until you reach the
> frame you desire. That looks like a lot of system load for a little
> effect.
>
> Then video files are usually designed to be read from start to end. There
> are means implemented to help the player program always easily find the
> start of the next frame in the stream. There is no help available within
> the file to assist the player program finding the *previous* frame. That
> might be another problem.
>
> Those are some of the difficulties I suspect to arise.
>
> But, that question is rising frequently these days. Perhaps some of the
> developers will have a look at it...
>
> Greets,
> Kiste
>
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>
Dear Oliver:

Thank you very much for your explanation. That is very reasonable.
And, for recent such questions I think
I know part of the reason.... There are some video players on Windows
claiming they have such function
and it seems some users are pretty satisfied with it... Though I doubt
they can really "go back one frame"....
maybe they just go back for a very short time...

Thank you!