Join Jan Kabili for an in-depth discussion in this video What is a selection?, part of Photoshop CS4: Selections in Depth.
Before I get into the nitty-gritty of how to make selections, I'd like to make sure that you understand what a selection is. To do that I'm going to start by making a traditional marching ants selection. I'll use one of the basic geometric selection tools, the Rectangular Marquee tool from right here in the toolbox. I'm going to select that tool and then come into the image and click somewhere near the top left corner and drag down toward the bottom right corner and then release the mouse. If you've ever made a selection in Photoshop, you've probably seen this dashed animated border that's known as the marching ants.
The marching ants aren't really the selection itself. They are just a way to visualize a selection. What a selection is under the hood is a definition of an area. The area inside the marching ants that will be affected by your edits and the area outside the marching ants will be protected from those edits. As I said, the marching ants are just one way to visualize a selection. Another way to view and work with the selection is as in Alpha Channel and that's what I'd like to show you now. To do that, I'm going to Channels panel, which is right here next to the Layers panel.
I'll click the Channels panel tab, so that you can see that it contains four channels. The Red, Green and Blue channels that make up this RGB Composite color channel. All of these are color channels and I'm not really interested in those for this lesson. Instead, I'd like to show you a completely different kind of channel called an alpha channel. You can use an alpha channel to store, to visualize and to work with a selection. Here's how. Start with the traditional marching ants selection and then either go to the Select menu at the top of screen and choose Save Selection, which I'll show you how to do in a later movie or for this movie, go to the bottom of the Channels panel and click this icon that looks like a circle inside a square.
This will create an Alpha channel from the active selection. I'm going to deselect now to show you that the Alpha channel is there regardless of whether you retain the original marching ants selection. I'll press Command+D on my keyboard, Ctrl+D on a PC keyboard, to eliminate the marching ants selection. Next, I'm going to click on that Alpha channel to show you another way to visualize that selection and that is as a mask in the Alpha channel. The white part of the mask represents the selected area and the black part of the mask represents the non-selected area.
If there had been partially selected pixels in between, maybe as a result of a soft-edged selection, those would appear gray in this mask. One of the advantages of working with the selection in the form of an Alpha channel is that you can modify it by applying a filter. So let's say that instead of a straight edge selection like this, I'd like a rough selection. To do that, I'll go up to the Filter menu and I'll go down to the Brush Strokes category of filters, and I'm going to choose the Spatter filter. That opens the Filter Gallery dialog box where I can see a preview of how that filter is going to look on the selection border and I can modify that by moving the sliders here on the right side of this dialog box.
When I have the look that I like, I'll click OK and you can see that I've modified this mask in the Alpha channel. To work with this mask, I can turn it back into a marching ants selection by going down to the Alpha channel, holding down the Command key on a Mac, the Ctrl key on a PC and clicking right on the thumbnail on that Alpha channel and that loads the selection along the modified edge of this mask. So that you can see the selection better I'm going to click off the Alpha channel and on to the RGB Composite channel and there you can see that modified selection edge.
This works just like the original selection. So for example, if I were to get my Brush tool here in the toolbox, and I have white paint as my foreground color, which you can get by pressing D and then X on your keyboard and then going to the image and paint on top of that border. As you can see, the white paint appears only inside of the selection border as you would expect, and now if I deselect by pressing Command+D on a Mac or Ctrl+D on a PC, you can see that I've taken advantage of the modified filtered edge of my selection border that I was able to achieve by viewing and working with the selection as in Alpha channel.
As you go through the rest of this course, working primarily with selections in the form of traditional marching ants, keep in mind that there is another way to view and work with selections as a mask in an Alpha channel.
- Selecting manually with the Marquee, Lasso, and Pen tools
- Saving time with automatic selection methods like the Quick Selection tool and the Magic Wand tool
- Fine-tuning selections with Refine Edge
- Working with selections in Quick Mask mode
- Moving, modifying, and combining selections
- Saving selections as alpha channels
- Making complex selections from color channels
- Using the Background Eraser tool to replace a background
- Installing and using the legacy Extract plug-in to isolate an object from its background