[Digikam-users] color cast in extreme highlights in DNG files

[Remco Viëtor]

on Tuesday 11 May 2010, Rei Shinozuka wrote: 
> 
> In DNG images with large areas of overexposure, entering the Image 
> Editor renders a color cast, often magenta or yellow.  It almost 
> resembles a stain on the image.  Color casts do not appear when looking 
> at the DNG in album mode, nor in the simultaneously-created in-camera 
> JPGs, but only in the digikam Image Editor. 

This can happen depending on how dcraw treats overexposed areas:
apparently one channel saturates faster/earlier than the others, so in 
unclipped mode there is a cast of the colour of the remaining channels.
album mode uses a camera-generated preview jpeg, where the in-camera software 
prevents this (by clipping the other channels as well? no idea)

> If the image is saved from 
> Image Editor as a jpg, the color cast is present in the output file.  
Of course, if in the image editor you see a color cast, that SHOULD be 
preserved in the output...
> The color cast only seems to happen in areas which appear to be 100% 
> white, absolutely burnt out highlights, though it does not precisely 
> coincide by areas marked by the overexposure indicator.

It is an over-exposure phenomenon.

> 
> I can provide sample images demonstrating this phenomenon.
> 
> digiKam Version 1.2.0  Using KDE Development Platform 4.4.2 (KDE 4.4.2)
> 2.6.32-22-generic #33-Ubuntu SMP

I suppose image editor opens in the 'Raw Import' mode. In that case, check 
under 'White Balance - Highlights:' if it shows 'unclipped' then there's the 
cause, try changing to one of the others (Solid White is a good start to 
check, see manuals for other options)

Remco