[MEncoder-users] Flashvideo for YouTube using mencoder

[Ben]

On Fri, Feb 22, 2008 at 3:03 PM, Rich Felker <dalias at aerifal.cx> wrote:
> On Fri, Feb 22, 2008 at 08:10:27AM +0100, Ben wrote:
>  > On Fri, Feb 22, 2008 at 6:58 AM, RC <cooleyr at gmail.com> wrote:
>  >
>  > >  > keyint=148:
>  > >
>  > >  The larger the keyint, the better the quality (potentially).
>  >
>  > Hmm.. isn't the keyframe the one frame saved in maximum quality?
>
>  No, it's one frame saved independently of previous frames to
>  facilitate seeking. If keyint is too large it reduces the number of
>  possible seek destinations but always improves quality.

That's actually contradictory to reality. If I reduce the number of
keyframes, the bitrate of the video lowers, meaning quality
requirement is lower. I don't think it improves quality, ever.

>From wikipedia:

Video compression

In video compression, a key frame, also known as an Intra Frame, is a
frame in which a complete image is stored in the data stream. In lossy
video compression, only changes that occur from one frame to the next
are stored in the data stream, in order to greatly reduce the amount
of information that must be stored. This technique capitalizes on the
fact that most video sources (such as a typical movie) have only small
changes in the image from one frame to the next. Whenever a drastic
change to the image occurs, such as when switching from one camera
shot to another, or at a scene change, a key frame must be created.
The entire image for the frame must be output because the visual
difference between the two frames is so great that the new image
cannot be reproduced incrementally from the previous frame.

Because lossy video compression only stores incremental changes
between frames (except for key frames), it is not possible to fast
forward or rewind to any arbitrary spot in the video stream. That is
because the data for a given frame only represents how that frame was
different from the preceding frame. For that reason it is beneficial
to include key frames at arbitrary intervals while encoding video. For
example, a key frame may be output once for each 10 seconds of video,
even though the video image does not change enough visually to warrant
the automatic creation of the key frame. That would allow seeking
within the video stream at a minimum of 10 second intervals. The down
side is that the resulting video stream will be larger in size because
many key frames were added when they were not necessary for the visual
representation of the frame.


I'm sorry, this might not apply to FLV encoding by mencoder, but
either way, it verifies my view on keyframe-usage.

JB