[Digikam-users] Re: Channel Mixer Question

[Gilles Caulier]

If the result is the same than Gimp, so ,it's not a bug, because i
backported code from gimp and improved the algorithm to support 16
bits color depth

To compare gimp and digiKam, you must use 8 bits color depth image,
not 16 bits. If result are the same, there is no bug.

The fact than Green channel is a factor to increase noise is due to
the double green sensor in Color Array pixel from you camera.

A RAW pixel is RGGB. So 2 green component are there, arround double
contribution.

Because a RAW camera pixel is square, camera maker need to set a color
component is double. The color filter is named Bayer :

http://en.wikipedia.org/wiki/Bayer_filter

Why to double Green chanel : to reproduce human vision sensitivity...

The current B&W converter in digiKam work in RGB color space. To be
more sensible to green noise, the tool must perform color mixing in
another color space as HSL.

Gilles Caulier

2011/2/26 Elle Stone <l.elle.stone at gmail.com>:
> David, I just used the digikam-edit channel mixer in monochrome mode,
> put the red at +200%, green at -200%, blue at -100%, with a tiff
> rendered with digikam (saved as "raw" - no input or output profile
> applied - the only way I can get the digikam raw converter to work the
> way I want it to work) starting with a cr2 raw file.
>
> The image does look IR-ish, kinda cool, but I'm not seeing any unusual
> graininess.
>
> I tried the same settings in photoshop, got the exact same results.
> Tried again in Gimp - got the tiniest variation in mid-tone tonality,
> hardly enough to even mention.
>
> Gilles is right - the red channel is inherently noisier than the green
> or blue channel. In cr2 files, at least for my camera, the red channel
> is multiplied by a bit more than 2 to get the daylight white balance,
> which means there is less information in the red channel to begin with
> - it is underexposed compared to green or blue channel. The only way
> to reduce that source of graininess is to use a magenta filter (an
> honest to goodness physical filter in front of your lens - not a
> software filter applied after the fact) to bring down the green
> channel. Or shoot multiple exposures bracketing and take the red
> channel from the over-exposed image.
>
> Elle
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