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is a tool witch display the histogram and blink the over exposed pixels using 
black color and the under exposed using white color.

Look on this review, at "Play Display" section. There is example of the panel 
in action :

http://www.dpreview.com/reviews/konicaminolta7d/page7.asp

I have tried to reproduce this feature in editor. The blinking way is not 
possible to do, but build and show a mask to overcolor the pixels is easy :

http://digikam3rdparty.free.fr/Screenshots/underexposureindicator.png
http://digikam3rdparty.free.fr/Screenshots/overexposureindicator.png

If you want to try it, there is a patch against svn here : 

http://digikam3rdparty.free.fr/misc.tarballs/exposureindicator.patch

Let's me hear if you like these features...

Gilles