On Mon, 5 Sep 2005, E.Chalaron wrote:
> Acquisition at 1392x 1138 gives me quite a good result on my screen. Once
Computer screen or TV screen? There's a big difference.
> reduced to the PAL standard the all lot is getting darker ... much darker...
That's expected - what looks dim on a computer screen will look fine
on a TV set. TVs and computer monitors have different characteristics.
Using a computer monitor (especially one that isn't calibrated) for
video editing is a good way to produce something that won't look
right on a TV set.
What does the final MPEG-2 burned to a DVD look like on a TV set?
I guess the next question I have is "how are you reducing to the PAL
standard"? Perhaps there's something about how that is being done
that is causing the range to be shifted to the dark side.
As I recall from your earlier comments you're acquiring the data in
video range 4:2:2 (uvuy or 2vuy - same data, just different byte
order for the chroma planes).
Or are you going thru a YUV -> RGB -> YUV conversion step along the
way - that would account for some of the level shift.
Perhaps a look at the script/process you're using would be useful -
maybe there's something in that which is mangling the data somehow.
> An option could be to overexposed slightly at acquisition or gamma correct it.
> Gamma correction is possible with my camera (Allied Vision, Marlin), but it is
> either 0 or 1.... which does not suit all.
Have you tried both settings? I wonder if the 0 means "none"/"disabled"
while 1 enables the gamma of whatever value is wired into the camera
> I can use the gamma correction from yuvcorrect but did not manage to get what
> I wanted by using either both 1/ gamma or shift.
-Y Y_0.95_16_235_24_235
might be what you want. I've found, in my recent conversions of
old/dark tapes, that a gamma adjust of 0.95 works nicely to brighten
the data without overexposing/blowing-out the highlights.
Problem with the current state of the tools is that there's no
immediate visual feedback. You need to run an encoding session,
look at the results, change the script(s), repeat. I use FinalCutPro
to do all my editing and color correction - there you can adjust the
filters and see (on an external TV monitor) the effect.
> Dont tell me to RTFM, already done but I need to start from a lower level like
> "Luma for dummies"..
First lesson is to use a TV monitor and not a computer screen for
video editing (color correction).
What does 'y4mhist' say about your data? Run a few frames thru
'y4mhist' (which understands 4:2:2 so there's no need to convert
the chroma sampling). The Y' table will show the distribution of
luma samples (a waveform monitor would be Nice - but that's probably
outside your budget ;)). If there's a heavy concentration below 32 or
so then the scene is quite dark. Run a histogram before and after
whatever processing is being done to reduce to PAL and see if the
levels are being shifted.
Good Luck!
Cheers,
Steven Schultz
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