On Mon, Apr 9, 2018 at 7:09 AM, Matthieu Moy <li...@matthieu-moy.fr> wrote:

> ----- Original Message -----
> > From: "Chris" <bydersjo...@gmail.com>
>
> > Seems as though the pink colours appear in my image when I disable
> 'highlight
> > reconstruction'.
>
> Yes, this is expected. The module is precisely here to remove this pink
> color.
>
> > With this disabled the Green & Blue channels have a lot less
> > data in the highlights than the red. Enable highlight recon, and it
> seems to
> > scale all the RGB to a simple range.
>
> The problem is the following:
>
> * All channels (R, G, B) are initially coded on a range, say [0, 2^14-1]
> for a 14-bits sensor RAW image. Channels that clip have the value 2^14-1
> (well, I'm slightly oversimplifying because saturation of the DAC comes a
> bit before this value, but ...).
>
> * The white balance module scales each channels, and usually scales the
> green channel less than the other. So the clipped pixels won't have the
> same values on R, G and B anymore after white balance. So, blown out areas
> are not white/grey anymore, they have color.
>
> The "highlight reconstruction" module does two things after that:
>
> * Desaturate the blown-out areas (done for all modes of the module)
>
> * Optionally, try to recover details. The default mode does not try that,
> it just clips highlights and turn them into plain white (which may become
> grey if you apply a negative exposure compensation). The "Reconstruct in
> LCh" is quite good to recover details when at least one channel is not
> clipped (e.g R and B are clipped, you can't expect much on colors, but the
> G channels can be used to recover an L channel). "Reconstruct color" does
> more or less the same, but works well on different images.
>
> In any case, setting the "clipping threshold" to 2.0 means the module will
> only work on pixels that have an L value 2 times greater than the clipping
> threshold. Which in practice means that the module will have no effect. The
> manual says "The default is usually satisfactory without any need for
> additional adjustments", which in dt is usually to be read as "There's no
> reason for you to change this value, but it's a parameter that appears in
> the internal algorithms so we've exposed it to the user just in case".
>

My experience with clipping threshold was different, at least on this dng.
Setting it to 2.0 allowed me to recover the cloud data and come up with a
pretty close match to what Chris achieved using LR.  Result is here
https://www.dropbox.com/s/lrg8wix996e6k2f/_MG_1496.jpg?dl=0

>
> > Odd, because I do not get this pink appear when I reduce the highlights
> in
> > Lightroom... and I am still using this same .dng file.
>
> I never used Lightroom, but from what I've read, LR's philosophy is to do
> a bit of processing automatically (or "behind your back", depending on the
> point of view). OTOH, dt makes almost everything explicit, and gives you
> control on everything, including settings that are not really meant to be
> changed. You may or may not like it, but it's unlikely to change ;-).
>
> The pink was there in the RAW (after white balance), LR just removed it
> for you.
>
> --
> Matthieu Moy
> https://matthieu-moy.fr/
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>
> Bill

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