>This is my first post in this list so hi !
Howdy!
>Hope you could help in this as i am a begginner in the video world.
>What i am trying to achieve is to take a video stream, extract couple of
>frames, modify them (add another picture on top) and put them back in to
>the video stream.
>
>This is what i do :
>
>(lav2yuv -f 1 myMovie.mov | y4mtoppm | <composite command>| ppmtoy4m -F
>25:1 | strip.sh;... ;lav2yuv -f 2 myMovie.mov | y4mtoppm | <composite
>command>| ppmtoy4m -F 25:1 | strip.sh;) | ffmpeg -f yuv4mpeg pipe -i -
>-sameq -b 8500 res.mov
>
>Note: the strip.sh remove Y4M unwanted headers
(Does 'strip.sh' need to be applied to all stream chunks, or just all
except for the first? I don't know what ffmpeg is expecting....)
>Everythings work not too bad except i can notice a luminance difference
>between frame i had extracted and the rest of the video stream. It is
>like a white transparent shadow (it s more visible from Quicktime Player
>though ...)
...
>So i concluded y4mtoppm or ppmtoy4m is the culprit (or both) . I know
>the subsampling and colorspace operations is lossy so i guess the shadow
>comes from this loss. ...
Hmm... I have a suggestion for something which could help diagnosing
the problem. Could you extract one (problematic) frame from your
original source, and then pass it through "y4mtoppm | ppmtoy4m" to
get a modified frame, and then mail both the original and modified
to me (as the plain, raw, short YUV4MPEG2 streams)?
What I plan to do (and you could do this, too) is to run each through
the new "flatten" mode of y4mtopnm ("y4mtopnm -f"), and then open
the resulting PGM's in gimp and see what actually happened to each
channel.
Even though it is lossy, I wouldn't have expected a noticeable difference
in one "y4mtoppm | ppmtoy4m" cycle, so it could be a bug. (Or, it could
be that your original stream/frame has a lot of pixels with Y'<16, which
are getting clipped -- which would brighten them.)
>Alternatively i though i could use YUV data instead of PPM (so i could
>work directly with yuv without subsampling) , i tried to extract one yuv
>single frame but imageMagick only displays crap. ...:
You could try using "lavpipe" with "matteblend.flt"; I'm not sure if anyone
has used these in years, though. This would do all the work with YUV4MPEG2
streams (if simple alpha-compositing is all you need).
-matt m.
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