[Radiance-general] HDR Image Accuracy Problem

[Rob Guglielmetti]

Fitzpatrick, Halley D wrote:

>
> Hello! This is my first post/question...
>
> I have been using Radiance on and off for about a year and a half, and 
> now I am delving into high dynamic range images. I am trying to create 
> accurate and precise false color luminance maps from HDR images.  <snip>
>
Hi Halley,

Don't know if you remember me, but I met you at AEC a couple months ago 
when I interviewed here...

Anyhoo, have you read the photosphere intro?  I know Photosphere is an 
OSX application, but you should download it (from www.anyhere.com) 
anyway just for the "getting started" guide. It is an excellent 
reference for, well, getting started with HDR photography.  The first 
thing you need to do is shoot a good scene and use that for capturing 
your camera response curve.  Come to think of it, what *are* you using 
to create the HDRs?  I highly recommend Greg's Photosphere, or if you 
don't have a Mac handy, the linux-based hdrgen.  Once you have the 
camera response saved, you can try to get some more hdr sequences and 
test the accuracy again.  Bear in mind that really high dynamic range 
scenes are difficult if not impossible to capture in their entirety.  In 
particular, direct sun is the real challenge.  Also make sure that auto 
white balance is switched off, and that you are using aperture priority 
to control the exposures.

The way to test if you are capturing the entire dynamic range is that 
your darkest image should have no white pixels, and your brightest image 
should have no black pixels.  Also bear in mind that if you are 
comparing luminances at areas of very high contrast, you will also have 
trouble getting accurate samples from your luminance meter, and thus 
will have trouble getting numbers to match up.  If you follow the 
getting started guide, shoot a good scene for the camera response curve, 
and take sufficient images to capture the dynamic range of your scenes, 
you *can* get accurate HDR images. I've got some crap around here 
somewhere that shows what happens when you don't take enough shots of 
the scene, and I also have my calibration image sequence, I just have to 
find them.  They may be useful to you, so if you want them I'll send 
them to you (when I find them). 

Have fun.

- Rob Guglielmetti
www.rumblestrip.org