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Apply Image

Discussion in 'Retouching and Post Processing' started by Iliah, Nov 3, 2005.

  1. Iliah


    Jan 29, 2005
    Participants in this Forum don't need all the details about channels, but keep in mind that those with less experience will need to understand the basics before they can appreciate how Apply Image actually gets the job done. A cookbook approach, to my mind, would be worthless.

    Apply Image copies a single channel, or a composite channel and pastes it over another channel or composite channel that exists either in that same file or in a different file. More simply, it takes a picture from any open image file and pastes it into the active image file. You can do the same thing by simply copying one channel or composite channel and using Edit/Paste to paste it over another channel or composite channel. The advantage of using Apply Image is that it gives you more options and it gives them to you in a neat little package.

    The key to understanding Apply Image is to understand the basic nature of channels. All channels, regardless of which color space the image resides in, are nothing but grayscale images. And the grayscale image itself is just Photoshop's interpretation of what you can picture as a sheet of graph paper full of numbers from 0 to 255. Each little square on that graph paper contains one of those numbers. Each of those numbers represents a single shade of gray.

    But if each of those little squares represents a single shade of gray, where does the color come from? It comes from a subprogram inside of Photoshop that interprets each number in each square. But that doesn't make sense, either. How does Photoshop know which color is represented by each number? O.K. Have you ever tried moving the channels in the Channels Palette? You can't because Photoshop's interpreter wouldn't know how to interpret them if you did. As long as the Red channel is in its original, assigned position, Photoshop knows that each of the numbers in the Red channel refer to a specific shade of red. Now you can change the shades of red represented by the numbers in the Red channel, but you still get red from that channel. Just paste the numbers from another channel into that Red channel. A number is just a number to the Red channel as long as the channel stays where it belongs. Once the new values have been pasted into the Red channel the interpreter will again interpret the numbers as different shades of red, but this time the shades of red will reflect the new numbers.

    Now you can see why it is possible to paste the a channel from a LAB file into the Blue channel of an RGB file. Both channels are nothing more than a collection of numbers from 0 to 255. The Blue channel has no way of knowing that those numbers represent shades of red and green when they're safely stored in the a Channel of a LAB file. Once the numbers are blended into the Blue channel, they represent shades of blue. You can also see why both files must be exactly the same size and the same resolution. You can't copy numbers from one sheet of graph paper onto another unless they are exactly the same size. Otherwise you wouldn't be able to copy them to corresponding squares. There either wouldn't be enough numbers, or you would have some left over.

    Here we come to something that has stumped many a Photoshop user. You've got a confusing variety of blending modes from which to choose, and you run into yet another variable in the form of the Opacity slider. What to do? Should you carefully compile and memorize a set of definitions telling you the purpose of each blending mode? How do you know what opacity to use? This is the easiest problem of all to solve. You try them out on your image. There's no way to analyze your image and be 100% certain that any given blending mode is the one that will give the best result. You may believe strongly that only Darken mode will work in one specific case. But you can't be sure that something else won't work better, no matter how many definitions you memorize. What you do is to try out those that seem the most promising. When you're close to what you want, try adjusting the opacity and see what happens. Using a mask vastly increases the number of possibilities. So what do the blending modes do, anyway? They represent mathematical formulas. Photoshop is a calculator. It takes your numbers and performs mathematical operations on them. Photoshop uses the formulas to perform all the calculations called for by the individual modes. Multiply mode, for example, multiplies the numbers in the Source channel with the numbers in the Target channel. The results of the calculations overwrite the original numbers in the Target channel. It's these new numbers that give the Target channel its new appearance. When you use Blending modes with Apply Image, the results of these calculations are stored in the Target channel. Your job is to tell it to try different formulas for the computations, then you decide which one works best. Photoshop is a great calculator, but you can't make it think for you. I might add that all the thinking in the world can't predict with certainty just what will happen with a specific image. What works for one may not work at all for another.

    By this time you should be ready to go to work with Apply Image. You've got to work with it over and over until you can get a real feel for just what it can do for you. Now that you're ready to go, let's cover a few details that can trip you up if you don't watch out for them.

    Before you ever open Apply Image, you choose your intended Target, the channel or composite channel into which you want to paste a channel that will be chosen from any open image that is the same size. Make the Target layer active, then open the Channels palette and click on the channel you want as the Target. Now open Apply Image and choose a specific Source channel from any open image. Remember that you aren't limited to an individual color channel. You can also choose any composite channel as the Source and/or as the Target. When Apply Image is open, it assumes that the currently active layer and the currently active channel (or composite channel) is what you intended for the Target. You will find that your Target choice is now set in stone (can't be changed). At this point you can only choose the name of the Source (any open file) from which you're going to borrow a channel to blend with the chosen channel in the Target image. Pick out a likely blending mode (or randomly select one) and see what happens to your image. With Apply Image still open, you can choose one blending mode after another. It only takes effect when you click O.K. If you want to mask your targeted image, click the Mask box and choose your mask from any channel in any open file that is the same size as the Target image file. This can be any channel in the Channels palette, including an Alpha channel that you created from scratch. It's all grayscale images to Photoshop. You can even invert the mask if it's the opposite of what you need. See how flexible Apply Image can be? So how do you choose a Source channel, a Target channel, and possibly a Mask channel? Think about what you want to do, then try out likely combinations. One unintended result may open up a whole range of new possibilities for you. Experimentation is as important as thinking.

    To sum up the function of Apply Image, you use it to blend one channel with another, using a combination of blending modes, opacity settings, and perhaps even a mask to limit the changes to a specific part of the image. As a precaution, use Apply Image with a duplicate layer until you're satisfied with the result. Then you can always delete the layer if you don't like it, or you can blend it into the background layer by changing its opacity and even its blending mode. It may occur to you that you can even replace the contents of one channel with that of another-if the need should arise. Just use Apply Image, Normal mode at 100% opacity to make the Source channel your new Red channel (or your new Luminosity channel, or whatever). Note that I say you can replace the contents of a channel with that from another. You still can't move the channels around. Their positions are set in place with Super Glue. You can replace the contents of the Red channel with the contents of the Green channel, but you're still going to get red from the Red channel. It may turn your image into garbage, but your Red channel is still going to give you red.

    That's the basics. Hopefully you've tried to visualize all the action from the above descriptions instead of worrying about memorizing the details. If not, keep reading until you can picture what's going on. Memorization is deadly. If it still isn't clear, start working with it a step at a time until you're comfortable with it. See what you can make it do for you. Forget rules and definitions. Play with it until you are satisfied. You won't hurt anything as long as you keep a copy of your original image safely filed away.

    Howard Smith (from Color Theory List)
  2. Hello, Iliah!

    Your insightful tidbits are alway so helpful! Thank you.

    aka beaucamera
  3. Illiah - Your posts are always chocked full of information. Still trying to digest all of this but, eventually it will sink in. Or not. I do appreciate your work.
  4. Neat article...thanks Illiah :smile:
  5. Thanks Illiah for such an indepth explanation. Need several read to digest it and start to understand. Thanks again and I really appreciate your effort.
  6. Well stated Iliah. The only addition I might suggest is to not limit this to Photoshop -- the functionality/thought process applies to all decent editors, and it is the understanding/thought process that makes the difference.
  7. jfriend


    Nov 11, 2005
    SF Bay Area
    Question about "Apply Image" for Iliah

    As I've read about Apply Image in various Photoshop books, one thing I've not understood is how it's different than copy/pasting a channel and just using blend modes to achieve your desired effect? One book made it sound like the Apply Image function existed before Photoshop had layers so it would have been really valuable and unique in those days, but does it actually let you do anything new that can't be done by simply copying a channel or channels, pasting into a new layer and then setting the blend mode, opacity, blend-if parameters and/or mask?

    Thanks for any help you can provide in understanding this.
  8. Iliah


    Jan 29, 2005
    Dear John,

    With layers, how will you control the destination channel for blending?
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