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In some cases you may realize that the area you want to select can be defined based on brightness values. For example, with this photo I have relatively extreme example, of course. But I might want to select the sky, the brightest areas, in order to apply an adjustment there. Or I might want to select the dark areas, the foreground, in order to apply an adjustment that affects only the dark areas. This can be useful, for example, to open up shadow detail, or perhaps to create a silhouette effect of the darker areas of a photo. Let's take a look at a technique we can use to create a selection based on luminosity values or brightness values. I'll start off by creating a copy of my background image layer, because I'm going to apply a very destructive adjustment.
And I certainly don't want to harm my original pixels, so I'll drag the thumbnail from my background image layer down to the Create New Layer button. The blank sheet of paper icon at the bottom of the Layers panel. That will create my background copy, and in order to make it easy to see the effect of what I'm doing here, I'm going to change the opacity for this Background Copy layer down to about 50%. The reason for this will become evident in just a moment. Next, I'm going to adjust this image based on luminance values, so I'll go to the Image menu. And then choose Adjustments, followed by Threshold.
And the Threshold adjustment is an interesting one. It will convert my image to black and white. And I don't mean black and white like a typical black and white photograph. I mean literally only black and white. Only those two tonal values. But the key here is that I can determine where that shift between black and white occurs. So I'll choose that Threshold command and then move the dialog out of the way so I can see the image. And now I can drag upward to increase the threshold value, so that only the very brightest areas of the image are white and the rest are black. Or I can reduce the value so that only the very darkest areas of the image are black and the rest of the image is white.
And because I reduced the Opacity for my background copy layer to 50%, I'm able to see the underlying image peeking through and I can use that to determine when I've established a good value for Threshold. In this case, of course, because I'm using an image with some exaggerated tonal issues, it's relatively easy to see when I've established a good value. I want it to be a high enough value that all of the building becomes dark, but not so high that parts of the sky become dark. And in this case, the range right in between will work well.
Once I've established that value for Threshold, I can click OK in order to finalize the effect. And then I'll bring the Opacity back up to 100%, so that I can see that black and white image. That literally black or white image. And this, essentially, is my selection. I haven't made a selection just yet, but I can do so with just one click on the Channels panel. I'll switch to the Channels panel and then down at the bottom I will click on the first button. The Load Selection from Channel button, and I don't even have to choose a particular channel because all three channels are exactly the same.
White in the sky, black in the building. And so when I click, I get a selection of the white areas, in this case the sky. Of course if I actually wanted the opposite, for the buildings to be selected, I can choose Select and then Inverse from the menu. Once I have that selection I can go back to the Layers panel and turn off the Visibility of my background copy layer or even throw it away altogether just by dragging it down to the Trashcan button at the bottom of the Layers panel. But, as you can see with that simple technique, I've been able to create a selection based on specific brightness values within my photo.
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