Episode 033: The Channel Mixer – Monochrome conversion (2)

The changing Parrot
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I promised a better way to convert coloured images to monochrome. And here is the Channel Mixer. It allows you to mix the amount of the red, green and blue channel of an RGB image and change the appearance dramatically. The image to the left is an example that I stole from a terrific website Cambridge in Colour. You see the original (in colour) and some outputs of the channel mixer.

Double Face revisitedThen I tackle the “double face” from episode 31 again. I convert the new passport image with the channel mixer and get a better skin tone. Adding the same colour tint to both halves of the image and getting rid of the grain by blurring give a better result than before.

But the channel mixer is not the final stop on the road to the “perfect” monochrome image. Next week I’ll use layers and the curves tool to get an even better result. And then there are duotones and sepia….

Finally I start a new challenge for you. Do a monochrome conversion with GIMP, post the image in the meetthegimp.org photogroup at 23 and be sure to use the tag “mtg-monochrome”. The challenge ends March 31 1600GMT and I’ll draw a winner by random choice. No interview partner as a random generator- I’ll go high tech this time. ;-)

Some links from the video


00:34 Site statistics and plugging
04:12 Parrot example image
05:06 The problem with simple desaturation
06:00 Cambridge in Colour tutorials
09:56 Channel Mixer
11:20 – Average settings
11:47 – Preserve luminosity
14:32 Double face re-visited
15:00 – Convert to monochrome using the channel mixer
17:57 – Colour matching
19:00 – Curves adjustment
20:01 – Blur the grain
21:52 The Gimp web-site
23:10 Next episode – a better way
23:44 The Black and White Challenge
26:59 The End

TOC made by paynekj

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17 thoughts on “Episode 033: The Channel Mixer – Monochrome conversion (2)

  1. Thanks for the link to Cambridge in Colour. Stunning pictures. I have to explore that site in more detail.

    Good to see a challenge again. Do you only accept a conversion to B&W or is post-processing also allowed?

  2. you could also improve the quality of the grayscale conversion by making sure the picture is in linear RGB colorspace rather than sRGB. That is doable by:
    1. get a linear RGB ICC profile (see http://www.aim-dtp.net/aim/techniques/linear_raw/index.htm — search ‘D65 Gamma’)
    2. get an sRGB ICC profile (the one available at http://www.color.org/profiles.xalter is excellent)

    3. set your display profile to the sRGB profile
    4. (hopefully, if you think the benefits of linear working space are worth it) set your working space to the linear profile.
    5. convert any sRGB images to linear upon loading (assign the sRGB profile if no profile was assigned, then convert to linear profile.)
    6. convert linear RGB images to sRGB before saving a copy for print or display.

    I realize that with the limited 8-bit image format of GIMP, some degradation may occur upon conversion. However I suspect the subsequent ease and quality of grayscale conversion, channel mixing, and other manipulations are worth it.
    As ‘Meet the GIMP’ seems largely focused on photography, I’m surprised you haven’t covered working in a linear space already. Editing in sRGB can certainly cause some serious problems (-> http://www.4p8.com/eric.brasseur/gamma.html)

  3. Thanks for that info and links, David; all of it is sort of ‘never heard before’ stuff to me…

    I think the article on the last link you provided (ericbrasseur) is a very good one.

    And seems like photo edition is a very large topic, plenty of things to learn :-)

  4. Not me :) I’d like to be able to make videos, but my computer just slows down to unusability when I’m trying to.
    It’s about as simple as I said, though.

    Each image can have a ICC profile: this indicates the parameters of it’s colorspace (ie what does RGB 0 0 0 mean; what does RGB 45 100 64 mean, .. what does every possible value in the colorspace mean in objective terms.)

    A display has a ICC profile: this indicates the parameters of it’s colorspace. If you have no display profile assigned, a builtin sRGB profile is used as the display profile. Before display, the image pixels are transformed from the image colorspace to the display colorspace.

    sRGB is an acceptable default profile (actual displays vary — they tend to expect sRGB, most displays distort the output somewhat due to imperfections or user misadjustment)

    Most programs, unfortunately, disregard ICC profiles (which can be embedded in PNGs, .. or XCFs of course:)
    This is why it is necessary to transform back to sRGB before saving the image; otherwise, most programs just assume that your data is sRGB, so your results will not look as you intended.
    See the second link I provided before — it gives an example of this.

    This is changing though — eg. recent development versions of Firefox 3 have ICC support.

  5. oh, and of course those things can be set up in the Preferences dialog, under ‘display’ (or is it ‘color management’/)

    ‘Assign profile’ and ‘Convert to profile’ can be found in the image->mode menu

    You can toggle between a color-managed and non-color-managed display by bringing up the ‘view->Display filters’ dialog and toggling ‘color management’. This is not terribly useful except as a visual aid.

    Image properties dialog includes a ‘color profile’ tab which can show you the description of the attached color profile.

  6. By the way, I use a panasonic dmc-fz30, but I haven’t been able to find a icc profile file for it.
    But ufraw is said to be able to manage this camera’ pictures. What does that mean? Using ufraw I can’t choose a different icc other that the built-in (sRGB)…

    Is there a place in the inet where I could be looking for my camera icc file?

  7. @David.
    Finally I had time to look at this. Up till now I used sRGB and Adobe RGB colour spaces. I did experiments with CIE D65 and sRGB working spaces following the examples of Eric Brasseur and that was an eye opener. I was never aware of this gamma error. I have to adjust my workflow.


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  10. There is no such thing as “The perfect monochrome image” and monochrome images are not necessarily black and white. Surely, a negative image produced by chemically processing the latent image stored in an emulsion containing minute particles of silver halide, layered onto a transparent substrate will have a vast range of grey from black to white. The content of this negative image will inevitably be changed when converting it to a positive image on paper and much of the detail will be lost. The impressive images that are published are inevitably the result of considerable work in the darkroom and the retouching studio.

    So, Sergio, I challenge you to describe the process whereby the so called real BW negative is obtained and how it is converted into the so called perfect monochrome image.

  11. Norman, going to this modern stuff with emulsions was a big mistake.

    We should have stayed with the big invention of M. Daguerre. So much detail is lost with these modern techniques!

    On the other hand I am really not sure if we should have started with this picture stuff 50000 years ago. Decorating walls of caves with animals – what a waste of time!

    (I had once a chance to look at a Daguerreotype with a microscope – the detail is indeed incredible – better than any film or sensor. But there are other drawbacks – including a nasty and early death by mercury poisoning. ;-) )

Anything to add from your side of the computer?