HDR Image Accuracy Problem

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. I took
a series of pictures each with 5 exposures, and found out quickly that
was not enough for what I am trying to accomplish. The luminance values
(as displayed by photosphere and the false color I created) looked about
right, with the exception of particularly bright portions of the image,
which were damped by not having enough exposures. The extreme values are
quite important to me.

I decided to start over, and took 11 exposures of a scene: 5
underexposed, 5 overexposed, 1 properly exposed (each a full f-stop
apart) with a Nikon D100. The proper exposure was determined by the
camera using matrix metering. The most underexposed image had a small
portion that seemed to be washed out with white, but I can't take any
more underexposed due to my camera (I don't think this is causing my
problem, anyway). I created an HDR image using photosphere, and chose
'save new response' and 'align images', leaving the rest of the fields
unchecked. Clicked go, and created the hdr image. The image _looks_
good, but I think that the values are likely quite a bit off.

I measured several values in the scene with a luminance meter (recently
calibrated Minolta LS-110) and all of them are 15-20 times less than the
values suggested by photosphere. The values I was seeing with 5
exposures were closer to the actual luminance meter values, but I have a
feeling now, that was a coincidence.

I will be happy to provide the jpegs and/or the hdr that I created if
they will serve to discover my problem.

Any help or insight would be greatly appreciated. Thank you!

Halley Fitzpatrick [ Program Engineer ]
The Pacific Energy Center [ p. 415 972 5238 ]
851 Howard Street [ f. 415 896 1290 ]
San Francisco, CA 94103 [ e. [email protected] ]

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