[HDRI] RGB values out of range

[Oguz Akyuz]

Hi Raquel,

Generally agreeing with Lars, I would like to add that it should not be
impossible
to create a useful HDR image if the input exposures have RGB values greater
than
200 or smaller than 20. Actually, as you pointed out it should be very
difficult to
capture bracketed sequences where any exposure fits into these limits.

The HDR creation algorithms generally employ a weighting scheme to underplay
the influence of over- and under-exposed pixels. This is usually a smooth
function
giving higher weights to midtones and lower weights to extreme darks and
lights.
I think any tool that employs such a weighting scheme should be able to cope
with input images that have pixel values close to the edges of the RGB
range.

Cheers,
Oguz



On Wed, Sep 9, 2009 at 9:53 AM, Lars O. Grobe <grobe at gmx.net> wrote:

> Hi,
>
> I think there is a misunderstanding here. I am sure that the
> documentation you refer to is related to the source images. These are
> 8-bit coded, and allow values from 0-255. This means that below 20 and
> over 200 you get into the edges of this range. As the jpg's you get from
> a digital camera are not linear mappings from luminance to pixel values,
> and a slight change in these edge pixel values is related to a huge
> change in the luminance of the recorded scene, it gets impossible to
> reconstruct luminance values from such values. Have a look at the
> s-shaped response curve of your camera, it should explain this problem.
>
> So hdr tools can use only this "middle" range, and this means that you
> must make sure that the ranges of your overlap have a sufficient
> overlap. Also, of course, the darkest (shortest exposure time) image
> should not contain any very bright pixels, and the brightest one
> (longest exposure time) no very dark ones. So if e.g. your brightest
> image was taken with 1/2sec and has significant pixel values below 20,
> take one more with 1sec time.
>
> Cheers, Lars.
>
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