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

[Paul Waldo]

Has anyone in the Digikam community had good results with Highlight Rebuild?  I just can't get it to work without destroying my image :-(

Paul
----- "Paul Waldo" <paul at waldoware.com> wrote:

> Hi Alex,
> 
> I'm a bit confused, but maybe I'm comparing apples to oranges.  Let's
> say that I have an image that covers the entire dynamic range of my
> camera; I've recorded pixels that are slightly too dark to register,
> slightly too bright to be accurately captured and all brightnesses in
> between.  I have a fully populated histogram that looks like this:
> 
> |
> |   +---------------\
> |  /                 \
> | |                   +
> |-/                    |
> +----------------------
> 
> Sorry, my ASCII art skills are a bit rusty these days :-)
> 
> If I use UFRaw to convert this image, I get an image that has a fully
> populated histogram, and the slightly blown highlights are recovered. 
> If I use digikam, what I get is a dark image with much of its
> information gone.  The entire image has moved down to the left, so
> that the brightest pixel is around 127, like this:
> 
> |
> |---------\
> |          \
> |           +
> |            |
> +----------------------
> 
> 
> If I then manually shift the histogram back to the right (gamma?) all
> of the pixels that should be at 0-127 are gone, like this:
> 
> |
> |          |--------\
> |          |         \
> |          |          +
> |          |           |
> +----------------------
> 
> I've lost about half of the image's data.
> 
> Hopefully I'm just doing something wrong, but it doesn't make sense
> that I have to give up half of my image just to recover a few blown
> highlights :-)
> 
> Thanks for your thoughts!
> 
> Paul
> 
> ----- "Alex Tutubalin" <lexa at lexa.ru> wrote:
> 
> > Paul,
> > 
> > to be precise: all LibRaw postprocessing code is derived from dcraw
> 
> > sources. The only change is an ability to produce 16-bit
> > gamma-corrected 
> > data (dcraw's 16 bit output is linear).
> > 
> >  From my point of view, this code is non-perfect in terms of image 
> > quality. On the other side, LibRaw is focused on RAW/metadata
> > extraction 
> > and pre-processing (before demosaic stage), there is no (near)
> plans
> > to 
> > change postprocessing stage.
> > 
> > > 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.  
> > Not so easy. Different color channels on (every) digikam have
> > different 
> > sensitivity. So, when green channel is already clipped, there is
> some
> > 
> > details in red/blue ones (for daylight and neutral highlights, just
> an
> > 
> > example). So highlight recovery tries to recover clipped green
> channel
> > 
> > data via correlation with other channels.
> > 
> > So, we need to rescale data (for example from 0...16383 to 0..8191
> to
> > 
> > get 1 extra stop), then try to reconstruct green pixels above 8191.
> > 
> > > * What happens to the shadow data that was on the left?  Does it
> > drop off and get clipped to 0?
> > >   
> > Shadows are compressed
> > > * Why not fix the highlights and shift the histogram back to
> where
> > it was?
> > >   
> > Imagine some clipped and unclipped highlights. E.g. clipped cloud
> part
> > 
> > in range 255-255 (8bit values) and unclipped in range, say,
> 240-254.
> > If 
> > you recover clipped part to, say, 245-255, you need to shift
> unclipped
> > 
> > part to something like 230-245. If not, you'll get incorrect tonal 
> > relations.
> > 
> > > * 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.
> > >   
> > before any processing, RAW data converted to working RGB and scaled
> to
> > 
> > full range.
> > 
> > 
> > -- 
> > Alex Tutubalin
> > Web: http://blog.lexa.ru
> > mailto:lexa at lexa.ru
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