[Digikam-users] Dark Images for 16-bit raw and reconstructed highlights

[Paul Waldo]

Thanks for the reply, Alex.  So, as I understand your comment, Highlight Recovery shifts the entire histogram to the left, allowing extra capture data in the highlights to roll in.  At that point it is up to the user to decide what to do with the extra data.  This brings up some questions:

* What happens to the shadow data that was on the left?  Does it drop off and get clipped to 0?
* Why not fix the highlights and shift the histogram back to where it was?
* In 16 bit mode, why do we need to shift and darken at all?  The image is 12 bits, so all operations can certainly be performed in a 16 bit space.

The second question is the key, I suppose.  Maybe I have unrealistic expectations, but I would have thought that the extra highlight data would be used to figure out what to do with 8 bit highlights that would have been blown out.  By having a little more data above the 255 brightness level, one could apply a bit of a film-like shoulder to the highlights to keep them from blowing out.  This, I believe is what UFRaw does.

Gilles, the technical stuff is fun, but the bottom line is that when I use Highlight Restoration in 16 or 8 bit mode, I get a dark image.  I have tried to take this dark image and apply any number of Digikam tools to get it back to where it looks OK, but it always looks significantly worse that if I had done no highlight restoration.  Maybe you can shed some light on a good workflow for Resoration and raw files?

BTW, I am using Digikam and ShowFoto 0.10.0, trying to convert a raw file from my Digital Rebel.  Thanks for any help!

Paul

PS
I have confirmed that in Digikam, in an 8 bit mode conversion, the shadows do get clipped to 0, which is probably one of the reasons I am having trouble.  I took a well-toned raw image and 8-bit converted with no Highlight Recovery.  The resulting histogram looks good from 0-255.  When I return to the raw, but choose Highlight Recovery and convert to 8 bit, only approximately half of the histogram is there! 0-128 (approx.) has good values, but 126-255 is all zero!!!
  
----- "Alex Tutubalin" <lexa at lexa.ru> wrote:

> Hi,
> 
> I'm not  digikam-users@ member, so this message will be rejected from
> 
> list. Hope, you'll receive personal copies.
> 
> LibRaw do _all_ processing in 16 bit. 16->8-bit conversion is made on
> 
> final output stage. So, you should not see any difference (plus-minus
> 
> 8/16 bit quality issues) for 8/16 bit modes.
> 
> On the other side, highlights recovery WILL darker entire image:  you 
> 
> lower highlights values (to see details instead of clipped pixels), so
> 
> you need to lower all other tones to preserve tonal correlations
> 
> 
> Gilles Caulier wrote:
> > This is a technical question for libraw athor. I CC him (:=)))
> >
> > Paul please, you forget to mention which camera raw file you use...
> >
> > Gilles Caulier
> >
> > 2009/5/6 Paul Waldo <paul at waldoware.com>:
> >   
> >> Hi all,
> >>
> >> I have been trying to figure out why my 16-bit raw image
> conversions are
> >> going dark on me: if I use "Highlights: Solid White", I get a
> decent image.
> >>  If I use "Highlights: Reconstruct", the image is significantly
> darker and
> >> the details are not as sharp.
> >>
> >> I have looked at the dcraw man page, Digikam documentation and the
> Oracle of
> >> Google, but I can't find any mention of why highlights would cause
> an image
> >> to be dark.  If this is documented somewhere, please help me with a
> link.
> >> How do you process raw files where you want to rebuild the
> highlights?  Any
> >> help would be appreciated!
> >>
> >> Paul
> >>
> >> _______________________________________________
> >> Digikam-users mailing list
> >> Digikam-users at kde.org
> >> https://mail.kde.org/mailman/listinfo/digikam-users
> >>
> >>
> >>     
> >
> >
> >
> >   
> 
> 
> -- 
> Alex Tutubalin
> Web: http://blog.lexa.ru
> mailto:lexa at lexa.ru