Professional Information Technology-Programming Book part 29 doc

So you really want your selections to do one of two things:  Mask an adjustment layer so that it will only have an effect on targeted portions of the image.. Checking Alpha Channels A

Chapter 7 Making Targeted Adjustments

There's not a big difference in the procedure for making adjustments when you're making targeted adjustments The difference is primarily in how you adjust a specific area without losing or jeopardizing the totally nondestructive work that you've done up to this point

Much of the success of regional adjustments depends on your ability to make the mask that restricts your adjustments to one or more very specific areas of the image Of course, you could make an adjustment directly onto the image by using any of the adjustment commands after you'd made a selection But then that

adjustment becomes a permanent part of the image, destroying whatever was previously in that area So you really want your selections to do one of two things:

 Mask an adjustment layer so that it will only have an effect on targeted portions of the image This is the option you should useexcept when it is impossible

 Create a separate layer for adjustments that have no adjustment layer

equivalent

Direct Your Adjustment to a Particular Spot

This chapter isn't about how to make an adjustment You learned about

that in Chapter 6 It's about the various ways to "aim" those techniques at

enhancing a particular part of the subject in such a way that those

adjustments stay both intact and isolated You'll also learn about making

selections to create masks for those layers And you'll learn a lot more

about how to modify a layer mask to change the exact area and blending

of a targeted adjustment

7.1 Tips for Using Selections

Of course, a big part of targeting specific areas of the image for adjustment is

knowing how to most effectively and credibly isolate that portion of the image It's important that the isolated portion blend smoothly with the rest of the image so that your adjustments look natural in all respects It's also important to make sure that the adjustment is accomplished in a nondestructive way

NOTE

I assume you have some knowledge of Photoshop and the basic selection tools such as the Marquee, Magic Wand, and Lasso tools

7.1.1 Checking Alpha Channels

Any time you spend more than 10 minutes making a selection, it's a good idea to save that selection so that you can recall it if you decide to make more changes to that same image (choose Select Save Selection) The saved selection takes the form of an alpha (transparency) channel, which will be bundled with the file only if the Alpha Channels checkbox in the Save As dialog is checked The downside of saving each selection is that it increases the file size by approximately one-third from the original RGB file So you don't want to save alpha channels if you won't

be using them again for a final version of a file or for a file that has to be optimized for the Web So it's a good idea if, before you save the final version of a file, you open the Channels palette and take a look at the alpha channels If you won't need them in the future, drag them out of the palette and into the trash at the bottom of the palette

7.1.2 Making Accurate Selections Easier with Brightness/Contrast

You can use the Image Adjustments Brightness/Contrast control to make your selections easier Duplicate the layer you're selecting, boost the contrast so that the Magnetic Lasso can see the edge clearly, and make the selection Then you can save the selection as a mask (alpha channel) and throw away the duplicate

"contrasty" layer

Sometimes you'll have to select an object whose edges are a lot less contrasty

along different segments of the profile you want to trace In that case:

1 Make two or three duplicate layers, as seen in Figure 7-1

Figure 7-1 The selection as it looks after tracing one adjusted layer and then selecting another

2 Turn off all but the lowest layer

3 Choose the Magnetic Lasso and trace the edge that has the most contrast between it and the background When you get to the end of that portion of the edge, double-click so the selection is closed from the beginning of that portion of the edge to the end Save the selection, but leave it active so if you accidentally drop it, you can retrieve it

4 Throw away the layer you just traced and turn on the (or one of the) other layers and repeat the step above, only this time trace the edge with the most contrast in this rendition of the image It shouldn't take you more than two or three of these layers to end up with a fully selected image

5 Use the Selection Save Selection command each time you trace a layer

6 Throw away all the duplicated layers after you to trace them

7 Use the selection to target your image; an extreme example of this is shown

in Figure 7-2

Figure 7-2 The targeted selection was raised to a new layer and a new

background was substituted

NOTE

To easily duplicate a layer, select the layer you want to duplicate in the layers palette, press Cmd/Ctrl-A to select the whole layer, and then press Cmd/Ctrl-J to lift it to a new layer

7.1.3 Making a Mask from the Image

There will be times when you want to use the image itself as a mask, especially if its edges are really complex There are a couple of ways you can do this:

 Convert the image to black and white and then make what you don't want included in your selection black and what you do want gray or white

 Knock out the image using a program or plug-in made for the purpose You can then make the mask by locking the transparency on its layer, filling it with black, then cutting and pasting the image into an alpha channel in the

Channels palette

7.1.3.1 Making a selection from a black-and-white image

If you're going to make a mask from a photo, you will want to photograph the subject against a background that contrasts with it as much as possible This is one

of the best techniques for flying hair or skylines full of trees and trellises The image in Figure 7-3 is a pretty good example

Figure 7-3 The selection as it looks after tracing one adjusted layer and then

selecting another

Once you've made a mask from a black and white image using the Threshold

adjustment, you might want to use it to darken or equalize the sky You could also use it for isolating a color effect or for compositing in a sky from another day or location You could also use the same technique for a fashion shot or head shot that

is crying for a new or inaccessible background (For those specific techniques, see Chapter 9.) What we're going to do here is nondestructively create a mask from the photo, then add interest to the sky using an adjustment layer made from that mask Here's the procedure for making a mask from a black and white image using the Threshold adjustment to pick a hairline or a skyline

1 Open the image you want to make a mask from Make sure it's one in which there's a distinct difference in brightness and colors from the part of the image you want to keep and the part you want to mask

2 Choose Image Duplicate You are simply going to use this image as a mask so there's no need to save or rename this duplicate since you will put the whole thing back into the original image In the meantime, we can run some commands on it that we don't want to keep with the original for space-saving reasons

3 Pick a color channel that shows the greatest contrast between the foreground and the mask Open the Channels palette and click on each of the color channels You'll see how each of the channels handles brightness for that area of the image Figure 7-4 shows this image's Red, Green, and Blue

channels in that order

Figure 7-4 Red, Green, and Blue color channels for the tree image

4 Choose the channel that makes the foreground the darkest and the

background area the lightest The closer to white, the better In Figure 7-4,

that would be the Blue channel

5 With the Blue channel selected, double-click the Zoom tool to get 100

percent magnification Press and hold the spacebar and use the resulting hand tool to pan around the image until you find the smallest and lightest detail that you want to leave unmasked

6 Choose Image Adjustments Threshold You'll see the dialog shown in Figure 7-5 Drag the slider from side to side until that small detail is black but absolutely no larger than it was in the original image Now you have your mask Click OK Now be sure to pan around the mask, keeping it side

by side with the open image There's a small chance you incorrectly judged from the wrong section of the image when you had it magnified That

usually happens in the area where the background is lightest If there's a discrepancy, just repeat the procedure until your mask looks like a perfect match It's easier than it sounds

Figure 7-5 The Threshold dialog adjusted exactly as it should be for our mask

7 Press Cmd/Ctrl-A to select all, then Cmd/Ctrl-C to copy the mask to the clipboard

8 Open the original image and then open the Channels palette (you may have

it already open) Click the New Channel icon at the bottom of the palette The new channel will be named Alpha Make sure it's the selected channel and the press Cmd/Ctrl-V to paste your mask into it The mask is now in your image and it has been saved Be sure to now choose the RGB channel

in the Channels palette

You can now do anything you like to the sky without affecting the rest of the

image at all One of the most useful things you can do with the mask (if you're going to use it for a targeted adjustment layer) is to load the selection (you may want to name its channel something like Skyline Mask), then choose the

adjustment you want to make from the Adjustment Layers menu in the layers

palette I chose the Hue/Saturation adjustment layer I used the sliders to darken the sky, increase saturation, and change the Hue of the sky The result is shown in Figure 7-6

Figure 7-6 The result of using the Hue/Saturation adjustment layer on the masked sky

Another useful trick in a situation like this is to simply darken the sky You can

raise the selected sky to a new layer and then put the new layer in Multiply Blend Mode You can keep darkening the sky by duplicating the Multiply layer multiple times

7.1.3.2 Making a selection from a knockout

In a sense, a knockout (something extracted from the rest of the image) is a

selection That is because knockouts always rest on their own layers and

everything else on that layer is transparent So anything you do to the knockout will be separate and distinct from the rest of the image Still, there are reasons you may want to knock out a portion of the image just to use it as a selection:

 To create a shadow, glow, or lighting effect that outlines the selection

 To make more instances of the same object without having to extract them all You can simply use the selection made from the knockout to raise the image to another layer

 To make one object appear to cast an effect, such as a reflection, onto

another

It's pretty easy to make a mask from a knockout The trick is to make the best possible knockout (see the "Homemade Backgrounds" section in Chapter 10) The only difference here is our purpose in making the knockout Once it's been made, you have an objectoften with very complex edgesfloating on a transparent layer Figure 7-7 shows a knockout of a thistle on the lefthand side The right of the same figure shows a mask made from that knockout

Figure 7-7 A knockout (left) and the mask made from it (right)

To make the mask from the knockout:

1 Duplicate its layer

2 Click the Lock Transparency icon (the one with the checkerboard

background) in the layers palette

3 Choose Edit Fill from the main menu In the Fill dialog, choose Black from the Use menu and leave the other settings at their 100 percent defaults Click OK

4 Create a blank layer behind the black silhouette, fill it with white, and then select both layers From the layers palette menu, choose Merge Layers Figure 7-7 shows your results If there are any artifacts left over from the

knockout, simply use the Brush tool with black or white paint to get rid of them Figure 7-8 shows how two separate adjustment layers were used to dramatically reinterpret the image's subject and background, thanks to the use of one simple mask

Figure 7-8 The thistle's colors have been dramatized and the background's hue and lightness have been changed completely by using a mask made from a knockout