Hi Raquel,<br><br>Generally agreeing with Lars, I would like to add that it should not be impossible<br>to create a useful HDR image if the input exposures have RGB values greater than<br>200 or smaller than 20. Actually, as you pointed out it should be very difficult to<br>
capture bracketed sequences where any exposure fits into these limits.<br><br>The HDR creation algorithms generally employ a weighting scheme to underplay<br>the influence of over- and under-exposed pixels. This is usually a smooth function<br>
giving higher weights to midtones and lower weights to extreme darks and lights.<br>I think any tool that employs such a weighting scheme should be able to cope<br>with input images that have pixel values close to the edges of the RGB range.<br>
<br>Cheers,<br>Oguz<br><br><br><br><div class="gmail_quote">On Wed, Sep 9, 2009 at 9:53 AM, Lars O. Grobe <span dir="ltr"><<a href="mailto:grobe@gmx.net">grobe@gmx.net</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Hi,<br>
<br>
I think there is a misunderstanding here. I am sure that the<br>
documentation you refer to is related to the source images. These are<br>
8-bit coded, and allow values from 0-255. This means that below 20 and<br>
over 200 you get into the edges of this range. As the jpg's you get from<br>
a digital camera are not linear mappings from luminance to pixel values,<br>
and a slight change in these edge pixel values is related to a huge<br>
change in the luminance of the recorded scene, it gets impossible to<br>
reconstruct luminance values from such values. Have a look at the<br>
s-shaped response curve of your camera, it should explain this problem.<br>
<br>
So hdr tools can use only this "middle" range, and this means that you<br>
must make sure that the ranges of your overlap have a sufficient<br>
overlap. Also, of course, the darkest (shortest exposure time) image<br>
should not contain any very bright pixels, and the brightest one<br>
(longest exposure time) no very dark ones. So if e.g. your brightest<br>
image was taken with 1/2sec and has significant pixel values below 20,<br>
take one more with 1sec time.<br>
<br>
Cheers, Lars.<br>
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<br></blockquote></div><br>