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Re: Custom Curves



>> Four control points are not enough in certain cases.

>Agreed.  I think that must have been a decision made in the interest of
>simplifying operation of custom-curves in the days before a graphical user
>interface.  With DUI things could be different.

What would be really cool is a variable gamma-knee type of adjustment. You
would then be able to pick where on your custom gamma curve you want this to
happen, the amount of "bend" to add, and the direction.. All by only using
two soft-knobs. Just being able to bend over (down) the whites could open up
a lot of neat options.

> The range of the curve should exceed the present 0 to 100 limit.  This
>way you could get a strong white compression to emulate a print, for
>example.

Since this is done on the primary boards in 16 bits, it shouldn't be a problem.

>To complete the Primaries board mechanism I described above... there are two
>sets of lookup tables, yes, you guessed it:  inside and outside window.  So,
>yes, you can have a normal gamma curve "outside" and a custom-curve
>"inside".

Actually there are four lookup tables. Two sets of inside and outside
tables, one set is in the data path, and the second set is being updated
with the next scene of values.. switched or mixed (soft edge) at the
vertical rate.

>I'm not sure how the firmware maps the 0 to 100 limits into what the lookup
>table sees.  This would be a daVinci question.

The Y Matrix board is where the black (16) and white (235) levels are
clipped, the Y channel has only one DSP and therefore when you adjust the
black clip or white clip levels you see the DSP being updated (green/purple
flashes, etc..) on the video output.

>Hmmm... I have to think about this... but I think that you could avoid
>using-up your power window if your key input was available.  You'd just feed
>like a full field of 50% luminance (or whatever) and key to that.  Force the
>cursor "inside" and setup your custom curve.

You could also run a paint program on the SGI and take the digital output
into the daVinci key channel input.. This way you can "draw" on the screen
what you want to correct (I believe varying the peak white level of the
drawing should vary your key level) You could even take the video from the
daVinci into the SGI and draw on it and then feed it back into the key
channel!!! Of course you will need to change the area you do not want to
effect to black.


Rob Chambers, AScT.
Senior Engineer,
Vancouver Television.


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