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At the end of the previous exercise I promised to show you how to create marquees of specific sizes, and I will do that, it's a very useful technique. But it dawns on me in the meantime that I've made a dreadful mistake. I've gone ahead and permanently modified the color of pixels here on the Background layer, which is such a rookie error, especially for somebody who has been using the program as long as I have. However, I want to show you a great trick for retaining your changes, popping them onto a new layer, without having to redo all that work. So if you've made that same mistake I did, if I led you down the garden path, then here's what you do.
You start by going up to the Window menu and choosing the History command, which brings up the History panel. I want you to go down to this last state, which in my case is called Deselect, because the last thing I did was deselect that circle, and I want you to click in front of it in order to set Deselect as the Source History State. Then, go up to the File menu and choose the Revert command or press F12, and Photoshop will go ahead and restore the original undamaged version of the frog. Of course, we lose our changes as well, but they are still waiting for us here in the Deselect state, in case we want to come back to them, and that's what we're going to do.
So leave Revert selected there in the History panel, and then I want you to go to the Layers panel, make sure the Background is selected, and press Ctrl+Alt+J or Command+Option+J on the Mac in order to jump the layer and name it at the same time, and I'm going to call this guy original frog. And then click OK to create that new layer, and now turn it off, so we can see what we're doing, and click in the Background layer once again. Then, go up to the Edit menu and choose the Fill command. Now, just so you're aware, in case you're wondering why I gave Stroke a keyboard shortcut but I didn't give one to Fill.
Fill already has a keyboard shortcut, it's just not listed there. It's Shift+Backspace on the PC or Shift+Delete on the Mac. And that brings up the Fill dialog box. Go ahead and change Use from Foreground Color or whatever it's set to, to History. Then, make sure that mode is set to Normal, Opacity is set to 100% in order to fill that Background layer with the history state. Now we have both the damaged and undamaged versions of this image. All right! Let's combine them together in order to separate out those lines, and you do so like this.
Go ahead and double-click on the Background layer in order to bring up the New layer dialog box, and let's call this one damaged layer, and I'll click OK. Now, we'll move damaged layer in front of original frog and turn the original frog on. Now let's find the differences between the damaged layer and the undamaged one by selecting damaged layer and changing the Blend mode from Normal to Difference. And that's going to go ahead and send all undamaged pixels to black, as you see here, and leave just the damaged pixels in color. All right! Now we want to go ahead and merge these two layers together onto a new layer, and you do that by first, if you're working along with me on the PC, you first press the Escape key so that Difference is no longer active, and then you press the keyboard shortcut, Ctrl+Shift+Alt+ E or Command+Shift+Option+E on the Mac.
So you have to press mash your fist E, because there is no corresponding command for merging the visible layers onto a new one. And now let's call this one differences. Now, notice if you Zoom in here that the lines are not absolutely white, they're various shades of inverted frog colors. So we've got these purples at the top, blues at the bottom and so forth. Let's go ahead and make them as white as possible by switching over to the Channels panel. This is a really common masking trick, by the way,. Go ahead and click on the various Channels to find out which of them provides the lightest lines.
And what you'll find, again, if you're working along with me, is that the Blue Channel provides us with the brightness we need. Go ahead and grab that Blue Channel and drag it to the little Page icon at the bottom of the Channels panel in order to create a copy, and I'll go ahead and rename this new Alpha Channel lines, like so. Now let's brighten things up just a little bit more by going up to the Image menu, choosing Adjustments, and choosing the Levels command, or you can press Ctrl+L or Command+L on the Mac. And I'm going to take this White slider Triangle to the left there.
Ultimately, I decided a White Point value of 128 will work out pretty well for me. That gives us some nice bright lines, click OK. Now we just need to load these white lines as a selection. By pressing the Ctrl key or the Command key on the Mac and clicking on that Lines Channel. Now, switch back to RGB, go ahead and switch back to the Layers panel, turn off both difference and damaged layer, because we don't need them, and we need to create a New layer by pressing Ctrl+ Shift+N or Command+Shift+N on the Mac, and we'll go ahead and call this New layer lines, and then click OK.
Now we want to fill the selection with white; white is my background color, so I'll press Ctrl+Backspace or Command+Delete on the Mac in order to fill that selection, and now you press Ctrl+D or Command+D on the Mac in order to deselect the image. Now, check it out. We've got the white lines restored to their own independent layer, and you can turn it off if you want to, turn it back on as well, and we've got the original frog completely undamaged. Now, if you want to, you can go ahead and grab damaged layer and differences and throw them away, because we don't need them anymore. Just go ahead and select those two layers and press the Backspace key or the Delete key on the Mac and we have miraculously taken a brain dead, destructive modification and turned it into a nondestructive layer, using what is essentially a compositing trick, the Difference mode to find the differences inside the image, and then masking those differences here inside Photoshop.
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