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Channelmixer not accurate?

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  • Channelmixer not accurate?

    Hello RetouchPRO users, i am a little confused about the Channelmixer in Photoshop. In its options, when i chose Monochrome and set BLUE to 100%, the other channels to 0%, i expected it to result in the same picture i get when looking at the normal Blue channel from the Channels tab. But those two are different. Why is that? I thought by chosing those options, i ordered Channelmixer to create a greyscale image by taking all greyscale values from BLUE (100%) and ignoring RED and GREEN (0%), which should normally result in a clone of BLUE channel. Please enlighten me!

  • #2
    Because what you're really doing is setting the red and green channels to equal the blue channel. If you take a copy of your image and copy the blue channel onto the red and green channel it will look identical to what you get with the channel mixer when you click on the composite channel. In that same vein, if you take your channel mixed image and examine the red and green channels you'll find they're identical to the blue channel, but once you click on the composite channel the change you noticed will be visible again.
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    • #3
      Ah i see, thanks for your explanation Doug Nelson!

      says: There is no way to achieve this effect (visible BLUE channel) with adjustment layers.


      • #4
        Originally posted by limaze
        says: There is no way to achieve this effect (visible BLUE channel) with adjustment layers.
        Levels or Curves: just pull the output of the Red and Green channels to zero.


        • #5
          Hello byRo, I can follow your advice logically, both R+G black means no infos on these channels, so far so good. But optically it doesnt give the result i am looking for, which is a clone of what you see when only activating BLUE channel. I tried desaturating the resulting blueish picture but it then creates a too dark pic. Its not like I cant live without such a feature (it just can come in handy sometimes), so if some of you feel annoyed by my question please just ignore me

          to make things clear: one could do: CTRL+3 to activate blue channel, then copy it, then create new layer and paste it. i only wondered if a clever adjusted Adjustmentlayer could give the same result.


          • #6
            Originally posted by limaze
            says: There is no way to achieve this effect (visible BLUE channel) with adjustment layers.
            This is going to sound perverse. If you want to do this with the channel mixer, you need to do exactly what you said. This creates the RGB equivalent of the Blue channel. It looks different, but it is the same.

            Is there a difference in the extraction? Indeed there is. You are looking at the RGB representation of the blue channel. The grayscale representation is indeed different--and at the same time it is not the right version of the separation to use in RGB layers.

            Try these two things:

            1. Extract the Blue channel using your Channel Mixer solution.
            2. Create a new layer and stamp visible (locking in the channel mixer version of the Blue channel to the new layer). Name the layer Blue Channel.
            3. Copy that layer to a new image and shut off the Green and Red channels. You will see the original Blue channel.

            1. Do steps 1 and 2 above (I am assuming you will do this on a flattened image for the sake of these instructions). Shut off the view for the Channel Mixer and the Blue Channel layer.
            2. Duplicate the Background. Shut the Background layer view off. Name the new layer Green and Red. Double-click the thumbnail for the layer in the layers palette and UNCHECK the Blue Channels box. This will show what the image looks like without a blue component.
            3. Turn on the view for the Blue Channel layer. Create a new blank layer above the Blue Channel layer, name the new layer Blue Color, fill it with pure blue (RGB: 0,0,255), and group it with the Blue Channel layer. Change the Mode of the Blue Color layer to multiply. This is the image blue component only.
            4. Double-click the Blue Channel thumbnail and UNCHECK the Red and Green Channels.

            Voila, the image appears in the original color, using your separation generated via the Channel Mixer.

            Channels are pure grayscale representations of image components...layer equivalents are RGB representations. These will not be the same, even in a grayscale image.

            That help?


            • #7
              Ack! I need some numbers 'cause I'm going nuts on this one.

              Let's take some color: RGB = 50,40,30. If I use the channel mixer in monochrome mode and coefficients of 0,0,100%, then I get the color 30,30,30 as expected.

              If I do what byRo talked about with the levels adjust, I get 0,0,30--that's also what I expect.

              Here's where I go to la-la land:

              If I copy the blue channel and paste it into a layer, I get 23,23,23.

              If I copy and paste the green channel, I get 35,35,35.

              If I copy and paste the red channel, I get 46,46,46.

              So now explain (perferably with a formula) how these values arose.



              • #8
                Bart...I wouldn't assume there is a brief explanation. I used to think that formulas were going to help, but the amount of quality time you can put into reverse-engineering a calculation proves to be less valuable than you'd think. After having done this in the past, I've been left with more-or-less what the numbers describe as a relationship, and caring not about the calculation at all. More recently I've found the relationship is more important than the exact calculation--you know what to expect from a relationship when you view an image, and not from a calculation.

                I would be lying if I told you I knew exactly what the channels representation displayed. I also don't want to spend any considerable amount of time trying to figure it out because I just don't think it is that important. But what seems to be happening between copying from the grayscale channel representation to the RGB layers is a conversion (Grayscale>RGB). This will actually probably change based on your color settings. I would assume that different people with different color settings than yours will actually not achieve the same result. Try toggling/changing dithering, color space, dot gain, profile handling, etc., and copy paste between grayscale and RGB images with different settings...You will learn that conversions create a change (you probably already knew). You are trying to get the program to work predictably in two color modes at the same time.

                So again, it is my postulation that the result comes down to the great mystery that is color management (even if you understand it, there are things lurking in the corners--and that change from version to version of the program). It would be my guess that you'll not have Adobe telling you what the actual series of events or equations are for conversion, as these are proprietary (there are much lesser things I have asked and been refused). If you are overly concerned about the numbers changing on you, you will want to do everything you can to make color management less of a factor in your imaging.

                I still believe that the examples I posted show that the representation you see in layers using the Channel Mixer is the right RGB representation of the grayscale channel. I guess my real question is: what is the goal that you are trying to achieve by having the representations match between conversion number to number from one mode to the other?


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