Colour curves noisy?

DJPadre wrote on 6/14/2007, 8:22 PM
is it jstu me or do the curves seem to add excessive noise to teh image?

I was messin around and was trying to work out what was causing some noise on my video and it turned out to be the curves filter..

I then replacesd then with the levels filter, and although its not the same, i can come rather close to the same results with levels as opposed to using curves..

thing is, i can boost my video about 2 stops using levels and not notice too much noise, but doing the same with curves.. forget it..

any thoughts?

Comments

Chienworks wrote on 6/14/2007, 8:57 PM
Let me guess that you're talking about noise in the darker areas. Brightening a video with color covers will increase the contrast in the shadow areas. There is always noise in those areas anyway and the increased contrast will make it much more noticeable.

Levels will brighten without necessarily preserving the black point, so the contrast of the dark areas won't be increased as much.
DJPadre wrote on 6/15/2007, 9:02 AM
nah mate, noise in ALL areas..
if exposure is perfect, then i dont gt noise, but if its slightly off (up or down) then its really noticable..

What ive needed todo was correct teh expsosure with levels furst, then run the curves after levels along the chain

anyone else have any ideas (apart from contrast issues) as to why curves are so dirty?
GlennChan wrote on 6/15/2007, 3:08 PM
If the slope of the curve is high, then that will exaggerate/bring out noise more. Slope = how steep the curve it.

With levels, the "curve" is more like a line with the same slope throughout. So unlike color curves, you don't have the situation where one part of the curve is a lot steeper than the rest.

(Color curves can ultimately do the same thing levels can, but the interface is really klunky if you want to do that exactly.)
farss wrote on 6/15/2007, 4:54 PM
To put it another way slope = gain. Gain amplifies eveything including the noise.

Bob.
MH_Stevens wrote on 6/15/2007, 6:23 PM
I think in "Curves" it is the absolute increase in the y-axis that is the gain for any luminance level. The slope of the "curves" line equals the differential luminance gain divided by the luminance value at that point. Example to prove: You make a hump in the center of the "curves" unmodified line such that you have a gain of say 5db at luminance 150, 12db at 200 and back to 5 at 230. You have added positive gain over the whole range of luminance 150 to 230, yet the first half of the hump curve has positive slope whilst the second half has negative slope, but both areas represent gain. The slope only indicates the rate at which you are changing gain. Example, at the top of the hump the gain on each side is the same and the slope of the curve is zero, a horizontal line tangential to the top of the hump.

Having bored you with some math lets consider how to relate this to noise in the blacks. You need raise the gain in this region slowly, a shallow curve and when you get to about 100 start moving up to a 45 deg slope and late then soften back of to the highlights. You now have the famous "S" curve. Play with the bottom of the this curve with the clip playing at as big as a resolution as you have power for so you can monitor the noise. I promise you that when you are skilled in "curves" it is way more useful than levels.
DJPadre wrote on 6/15/2007, 8:09 PM
hmm..

ok, so were messing with gain as we boost the curve up, however, all im really doing with the curves is trying to crush down some black and boost the whites to just under peak.. controlling it with levels if need be (prior to curves).. prety much an increase in contract but without the midtones being overly affected

thing with crushing balcks is that i lose alot of detail.. and im tryin to move away from that "look"

hmm...

as for monitoring the noise, ive noticed on HDV its atrocious.. on DV its passable, on DV encoded to HuffyYUV its clean as a whistle, which makes me wonder ..

maybe 64bit vegas wont be such a bad thing.. pity Vista is gonna hold it back for a while..

dual boot system comin up methinks...
farss wrote on 6/15/2007, 11:48 PM
ive noticed on HDV its atrocious.. on DV its passable, on DV encoded to HuffyYUV its clean as a whistle, which makes me wonder ..

Are you talking about the same piece of footage?

I've found it much easier to push the vision from 1/2" cameras around than that from the smaller ones.