[Digikam-users] Re: digiKam raw converter default gamma curve

[Elle Stone]

Pentii, not knowing what a Nikonian gamma curve is (in Canon there are
"picture style" curves, but they are applied post-processing), I would
like to ask, what it is that you want to have happen when you decode a
raw file, that perhaps isn't happening with the digikam raw converter?

Gilles, when you say "gamma curve" do you mean the gamma curve on the
post-processing tab? because that is not the gamma that I'm talking
about.

I'm talking about the "gamma curve" in the code below ("ts" stands for
"toe slope" - if it isn't 0, it isn't really a gamma curve):

dcraw code:
void CLASS gamma_curve (double pwr, double ts, int mode, int imax)

Code from libkdcraw (line starting with double gamm[6]):
(this section of code appears in another document, but it's the same
both places)

LibRaw:: LibRaw(unsigned int flags)
{
    double aber[4] = {1,1,1,1};
    double gamm[6] = { 0.45,4.5,0,0,0,0 };
    unsigned greybox[4] =  { 0, 0, UINT_MAX, UINT_MAX };
    unsigned cropbox[4] =  { 0, 0, UINT_MAX, UINT_MAX };

In the above libraw code gamm[6] has 0.45 as the "power" value and 4.5
as the "toe slope" value.

dcraw uses the same defaults as  what is hard-coded into the LibRaw
code snippet above. UFRaw defaults are close visually, but I don't
know what scale or math-model UFRaw uses in the gui for allowing the
user to set the toe slope (toe-slope is called "linearity" in UFRaw).

In UFRAW and in dcraw the end user, that is, the person using the
programs to decode raw files, can specify OTHER values, such as "0"
for the toe slope, to get a true gamma curve DURING interpolation.

Gamma applied post-processing canNOT make up for whatever gamma was
applied during raw rendering. Especially not if a non-zero toe-slope
was used during raw rendering.

Are you saying that the libraw api doesn't allow the digiKam raw
converter programmer ( the person writing the code behind the digiKam
raw converter - I think that is you, Gilles, yes?), to give the end
user (the person actually using the program to decode a raw file) a
choice of power and toe slope? That the libraw api ONLY lets the
programmer provide the hard-coded defaults to the end user?

The reason I am asking, is all my camera input profiles use a true
gamma curve of either 1 (linear gamma) or 2.2. So the respective
gamm[6] toe-slope value needs to be 0. I don't mind altering the
libkdcrw source code and recompiling libkdcraw for my own personal
use.

But if the programmer gave the end-user access to setting their own
power and toe-slope, like dcraw and UFRaw do, it would be really much
nicer.

Elle

On 2/28/11, Gilles Caulier <caulier.gilles at gmail.com> wrote:
> The gamma curve is computed automatically by libraw. Ther is no tune
> available from libraw api to adjust curve.
>
> Only some gamma adjustments are possible from libraw, not a fulll curve.
>
> This is why; i added a full curve correction in post processing
> settings. There is 2 way to use it :
>
> - let's post processing computed by libraw to adjust gamma
> automatically, and process fine adjustment from digiKam postprocessing
> curves.
> - disable libraw gamma processing and adjust whole gamma curve from
> digiKam post processing.
>
> Gilles Caulier
>
> 2011/2/27 Pentti Rautio <pentti.j.rautio at googlemail.com>:
>> Hi Elle, hope that you tell what are your conclusions about gamma
>> correction (when and where and how?)
>>
>> By the way : could there be a manufacturer specific correction
>> somewhere in the code so for example nikon NEF uses a "nikonian" gamma
>> etc.
>>
>> Pentti
>>
>> 2011/2/26 Elle Stone <l.elle.stone at gmail.com>:
>>> Never mind, kfind and geany to the rescue. I found the code section.
>>> Elle
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