When optimizing a photographic image, you'll often find that you want to apply an adjustment to the dark shadow or bright highlight areas of an image. In many cases, a selection that identifies the brightest or darkest areas of an image can prove quite useful. In this lesson, we'll take a look at how you can create a selection, based on luminosity values within an image. Now this technique is certainly destructive to pixels, so the first thing we need to do, is to create a copy of our background image layer. To do so we can simply click on the thumbnail for our background layer, and drag it to the create new layer button, the blank sheet of paper icon at the bottom of the Layers panel. This will create a background copy layer that we can work with, without being concerned about any harm being done to the original background image. Because of what we're going to do to this layer, I want to reduce the opacity. You'll see why that's important in just a moment, but the important thing is to reduce the opacity for our background copy layer down to around 50%. At this point we're ready to get started creating our selection. So I'll choose Image > Adjustments and then Threshold from the menu. This will bring up the Threshold command which allows us to adjust an image so that it is absolutely black and white, and by that I don't mean shades of gray. I mean literally only black pixels or white pixels. But the beauty of it is that we can adjust the level at which that transition takes place.
For example increasing threshold will cause more of the image to be black, and only a handful of tonal values to be white, reducing the value will do the opposite. Most of the image will appear white and only small areas will appear black. As you can see, this gives us the opportunity to identify the brightest or darkest areas of the image. In this case, I'd like to create a selection that defines these snow pillows, for example. And maybe just the brightest areas of those snow pillows, so I might increase to somewhere around here. This allows me to create an image where the snow pillows are white in all other areas of the image, including the shadow areas of those snow pillows will appear in black.
Obviously we could then use that as a basis of a selection. You can continue refining the threshold as desired to produce the intended effect. Let's say I want something around there. I'll go ahead and click OK, and then I'll increase the opacity, for my background copy layer to 100%. In general this looks pretty good, but I see there are a few areas, I might like to clean up. Most notably I have a couple of pixels over here, on the left, that are still white. That's no problem at all, as it's easy to fix.
I'll simply choose my Brush tool, make sure my foreground color is set to black. I can press D on the keyboard to get the default colors as needed, and then I'll press the left or right Square Bracket keys as needed to reduce or enlarge the size of the Brush. And then I'll simply paint that area away. I could also paint other areas of the image. For example, if I want to clean up these black areas, areas that will not be selected, I can simply press X to switch by foreground color to white. And then paint over those areas, so I have a great deal of flexibility here in terms of being able to clean up the result before I create my selection.
I'll just clean up a few other areas here, and I think this will be just fine. I mostly wanted to focus on the brightest areas of this image in any event. At this point I can create a selection. So I'll go to the Channels panel and then click on the Load Channel As Selection button, the dash circle at the bottom of the Channels panel. In this case because the image is completely monochromatic I don't need to choose a particular channel. Simply using the RGB Composite Channel will work perfectly well. When I click the button, all areas that were white will be selected and all areas that were black will be deselected. I can now go back to my layers panel and I'll turn off the visibility of my background copy layer, so that you can get a better sense in the image itself exactly which areas I've selected and which areas I haven't. When you need to define a selection for the brightest or darkest areas of an image, the technique of utilizing the Threshold command to define a selection can prove invaluable.
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