[Radiance-general] HDRI - Camera Response Curve

[Rob Guglielmetti]

Hi Jack, all, Happy Happy, yadda yadda...

Jack de Valpine wrote:

> Hi Greg,
>
> Thanks for the follow-up on this. I think that I have gotten the white 
> balance part correct. I always set it by hand (for example to the sun 
> icon) so the auto white balance feature is effectively off. And I have 
> used aperture priority on the camera however I have used auto 
> bracketing, so maybe I am not getting enough samples? It sound like 
> perhaps the best thing to do is set the aperture manually and vary the 
> exposure time manually as well in order to generate your suggested ~10 
> samples. Although doing everything manually involves a lot of 
> unnecessary touch of the camera.

Yeah, for this calibration sequence, you definitely need more samples 
than a typical auto-bracket will do.  Fixed aperture, varied exposure, 
and shoot from a tripod if you can.  The Canon Rebels are really nice 
because you can adjust the exposure with a little roller wheel; once you 
have the WB and aperture set, you just roll the wheel & click, roll the 
wheel & click, etc.  My Olympus requires a lot more futzing with menus 
to do manual exposure and it's impossible to shoot a decent sequence w/o 
a tripod.

> Once a response curve has been generated is there some way to check or 
> validate it (if that makes any sense)?


I dunno about validating a response curve, but I'd expect you could 
measure the scene with a luminance meter while you're shooting it, and 
then compare those values to some samples from the hdr image.

>
> In the book (the new one;-) you also indicate that the darkest 
> exposure should have no RGB values greater than ~200 and the lightest 
> no values less than ~20. I could certainly figure out a filter routine 
> with pcomb, however there must be a simple way to do this with 
> ImageMagick. The point being to run a quick preprocess check on the 
> the bounding images. Any takers or suggestions for how to do this easily?

I just open the high & low images in Photoshop and look at the 
histogram.  Are you trying to figure out a way to automate this step 
from the command line?