[HDR-photo] Re: RAW to EXR

[Dan Reetz]

Geraldine,

I'm a little unclear on one thing. :\

In the case of a single RAW file, converted to HDR without white
balance, what would be the difference between the values stored in the
RAW file (the 10 or 12bit-per-pixel information) and the generated
HDR?

I just want to say, I'm really excited by this discussion, and I hope
you go ahead with implementing this in Photomatix. In terms of natural
images research for visual psychology, it could be a real
breakthrough, removing one (perhaps several) stages from the camera
calibration procedures we currently use.

Daniel Reetz
www.danreetz.com
www.fakeproject.com


On 5/1/07, Geraldine Joffre <hdr-photo at hdrsoft.com> wrote:
> 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
>
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