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Copying part of a channel

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  • Copying part of a channel

    As a newbie, I was having a go at challenge #44 with Photoshop 7. I got bogged down in an issue which really shouldn't be that difficult.

    The writing at the bottom of the photo needs to be removed, and, conveniently, doesn't show in the blue channel. So, just take that part of the image from the blue channel and leave behind the messy writing in the other two. Comments suggest that's what many folks have done.

    Only no one gives details on exactly how to do it. Some say they just took the entire channel and dumped all of red and green, which strikes me as a bit heavy handed. I've tried selecting the writing, but can't get anything like an accurate selection for it.

    Can someone give me a clue? Or better, a step by step guide on how they got rid of that writing using the blue channel information?

    Much appreciated...

  • #2
    Hi Derek,
    To dump any channel, just hold it, in the channels window, whith your mouse and drag it to the litle trashcan on the bottom, or right click-delete. To copy it into it's own document, Choose the channel you want then on the picture ctrl-c or on the toolbar, edit/select/all. The open a new doc and ctrl-v or edit/paste. You can mix and match any or all channels that way. When you do however, there's a lot of new adjusting needed sometimes for color or levels. But that's a way to start.
    When you choose a channel from the pallete, the picture you see on your monitor is the one you anre now working with, so anything you do is done to that channel only.
    Hope this is a start.
    There are many tutorials on this site and if you do a forum search for channels, you should find a lot more information.
    I overlooked the blue channel, never saw it, and patched and cloned out the names, giving me more work but stll getting the job done.


    • #3
      Hi Derekfountain,
      Welcome to Retouch Pro. Your question got me thinking since I've copied entire channels, was it possible to copy only small bits. So I gave it a try. Yes, you can select just a small area on one channel and hit copy, then go to the channel you want to change and hit paste. It works just fine however in a color photo it will greatly change the coloring of that area. It may be harder to blend just a selected area rather than an entire image. I didn't venture that far to see. It does seem to be a bit of work to fix in color.

      As for an RGB black and white or sepia, I think you may have something there. It would work great. In that situation you can always desaturate the selection and and if it's sepia just desaturate everything and reapply the sepia to the entire image. You may have to adjust the lightness and darkness to that area to match up but it's alot less hastle than working out the color mismatch.

      You can also generate a duplicate of an image and change it to CYMK and if you like one of those channels you can copy one of them into your RGB original file to replace a channel. Just activate the channel you want to replace and just hold down the shift key when you move your copied CMYK channel to the original file. Hope that helps.


      • #4
        Something to keep in mind here for those new to CMYK:

        If replacing a RGB channel with it's similar 'cousin' from CMY (r=c, g=m, b=y) - the keep two things in mind:

        i) Dot gain

        ii) TAC shadow suppression

        RGB and CMY are similar, but do have their differences.

        I suggest using a Custom CMYK (one that is 'false' and that you would never use for a press conversion) which may use zero or greatly lower dot gain than a standard separation...and to make the total ink limit 400% so shadows do not get wiped out - but don't do this for a press image in CMYK). So perhaps try a GCR sep with NO K generation with a total ink limit of 300-400%. Also consider that highlight detail is often reduced with dot gain, so that if these tones are critical they may have issues with a CMY conversion.

        But part of the reason that you like the CMY channels may be due to one of these 'flaws'. With custom CMYK there are many options for creating the CMY and K channel interaction which can provide countless variatoins.

        And when we are borrowing channel data it is often a good idea to clean up hue/chroma colour artifacts before blends take place (there have been a few threads on colour blur blends or AB blurs in LAB etc).


        Stephen Marsh.


        • #5
          Here are some steps in setting that up. You can even make a profile(in P.S.) for it

          Image Optimizing

          It's on the last part of the page.

          That God for the web. That I don't have to type out those steps.
          Christmas would have come and gone. I would still be a type-in'.

          BTW, Steven. Did you get Dan Margulis' new book yet? It has Mike Russells' profile, as well as his own on this topic.


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