[HDR-photo] Re: RAW to EXR

[Geraldine Joffre]

On Mon, 30 Apr 2007 18:55:14 -0700, Justin Johnson wrote
> Is there a way to convert RAW files to HDR or EXR without applying  
> any adjustments during the conversion?  It seems that all the  
> aplications that do this raw conversion require that a white balance
>  be used.
> [...]
> Scene-referred by definition is the actual linear float values of
> the  scene.  If this is the case, and HDR images generated by these  
> applications are scene-referred, then how is it possible that they
>  allow for a white balance adjustment to be applied?
>
> It seems to me that applying any sort of color temp adjustment would
>  change the information and could no longer be called  "scene-referred".

This is a rather good question, and I have been thinking about exactly the
same for some time now, though without getting to a satisfactory answer yet.
Anyway, I am glad to be able to discuss this and will give you the results of
my thoughts on it so far.

I believe that you are correct to say that white balance adjustments should
not be applied to HDR images, as otherwise they theoretically can no longer be
called  "scene-referred" anymore. However, it has been the norm since the
first uses of HDR images in computer graphics to create HDR images that are
color balanced. Maybe, the reason for that was because the first scientific
publications on HDR images date back from a time when digital cameras were
still rare and use of RAW files uncommon -- at that time, source photographs
for assembling HDR images were mostly scans from films, i.e. already white
balanced images.

I don't know of any application that currently allows to convert RAW files to
HDR images without applying white balance adjustments, but this is something
we are considering implementing in Photomatix. The rationale being to delay
any tone and color processing on the HDR image to the Tone Mapping stage. This
way the scene-referred nature of the HDR image will be respected, and the user
will have full control over color adjustments in the tone mapping process, the
same way a RAW converter allows both tone and color adjustments over raw data.

One issue though, is that the colors of an HDR image file that is not white
balanced will look wrong when opened in applications that supports HDR image
formats, as these applications expect the data to have been already white
balanced. To avoid that, it may be better to save pre-calculated color values,
i.e. with the  "As Shot" white balance applied, and at the same time provide
in the metadata of the HDR image file the color temperature used for this
white balancing. So, HDR image viewers will show the colors under the  "As
Shot" white balance, but tone mapping applications will still be able to
recover the unprocessed color data, and let the user process the HDR image
with the white balance of choice.

One could go even further by recording in the metadata of the HDR image the
color matrix of the camera (or even two color matrices depending on two
different calibration illuminants like the DNG format allows) in addition to
the white balance information. The conversion from camera to output color
space would then entirely be controlled -and adjustable- in the tone mapping step.

Further thoughts on that are welcome.

Geraldine Joffre