PDA

View Full Version : How to change the gamma curve in a PS way?



Thomas M.
04-15-2005, 05:09 AM
I'm getting a bit pi**ed off here as I'm trying to find a way to change the output curve of LW in a efficent and precise way. I need to match the .hdr output directly in LW to the one from my Nikon D70. By doing a series of exposures from over- to underexposed (grey card) I've been able to draw a camera response curve. Doing this in 1/3 f-stops generated quite a precise curve.

Now in LW I did something similar by changing the light intensity in the same way and rendered a series of "exposures". Measuring the brightness of each render enabled me to draw a LW "response curve".

Unfortunately (and as expected) these two curves don't match. LW is much harder. But not neccessarily in the shadow area, but more in the middle and bright regions. Trying to use FP gamma doesn't help, as it "bends" the curve in the region that doesn't need to be softened. The shadows get much to bright. I'd like the shadows to stay like they are. So I do need a tool like the PS curves function, so that I can change the appearance of the curve in a precise and efficent way.

I already played around with all the filters and also VDR. There's also a strange thing going on as the brightness gets to bright. Suddenly the corrected output isn't white anymore, but black. Kind of a solarization. I don't where it comes from and how you can avoid it, but it makes a lot of the filters useless if the superbright areas aren't white any longer, but black. So I pulled my fingers back from VDR, which is in the way you need to use it quite user un-friendly.

So anybody an idea how to change the output, so it does match my "master" curve? I do need to keep the .hdr information. Is it really a problem to have something like a curve filter for hdr in LW? As far as I do know HDR shop it isn't capable of this stuff, too.

If there's no choice to do this in LW I'd do it in an external program, but which one?

Well, I'd be glad to receive some input.

Cheers
Thomas