Channel Mixer Question

[david-vj]

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[Gilles Caulier-4]

It's not a bug. It's typically a grainess and artefact amplification
due to green chanel, especially with JPEG.

Shot in RAW and try to see if effect already exist.

Gilles Caulier

2011/2/24 davidvj <[hidden email]>:

>
> If I open up an image with the 'Channel Mixer' with the Monochrome box
> checked there appears to be substantial and unnaturally significant
> pixelation/grain  showing at the 100% view. This is seen with all of the
> settings set in default positions prior to any changes being made.
>
> When I switch away from the monochrome view (back to colour) the image looks
> normal again.
>
> Is there a simple explanation or is this a bug?
>
> David
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[david-vj]

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[Elle Stone]

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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[Gilles Caulier-4]

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 <[hidden email]>:

> 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
> _______________________________________________
> Digikam-users mailing list
> [hidden email]
> https://mail.kde.org/mailman/listinfo/digikam-users
>

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[david-vj]

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[Elle Stone]

>The settings that Elle sugested (using all 9
>sliders) unfortunately did not achieve the desired result.

Hi, David. Sometimes I think English must not be my native language. I
didn't suggest using all nine sliders. I didn't even notice that the
green or blue channel TABS at the top were available. What I meant was
the red slider in the red tab, the green slider in the red tab, the
blue slider in the red tab.

In monochrome mode, the red slider on the red channel TAB (top
setting) IS affecting the red channel of the image - it is how much
red gets put into the black and white conversion. And etc. The green
and blue channel TABS (at the top) don't have any effect whatsoever.
In fact, until you mentioned them, I didn't even notice they were
there. Sorry to have been unclear! And you are right, now that I look
at it, that gui is confusing.

Elle
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[david-vj]

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[Elle Stone]

I use a custom camera profile made with argyllcms and don't usually
convert to sRGB. I'm having trouble getting the raw converter to
convert correctly using color spaces other than in-built camera matrix
and sRGB. I haven't experimented around enough to be able to state
more clearly exactly where the problem is - maybe I'm just pushing the
wrong buttons. But the option to output without applying an input
profile/converting to a working space works perfectly. Camera profile
and conversion to working space can be done at a later point.

On 2/26/11, davidvj <[hidden email]> wrote:

> OK .. all talking the same language now.
>
> I was interested to note your use of decoding from raw without applying a
> colour profile .. any special reason for doing that?
>
> David
>
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> View this message in context:
> http://digikam.1695700.n4.nabble.com/Channel-Mixer-Question-tp3323437p3326379.html
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