Chapter 8: Selections and Paths - Photoshop.CS.Bible [Electronic resources] نسخه متنی

Chapter 8: Selections and Paths

Selection Fundamentals

Chapter 9.

That's why this chapter and the one that follows are the most important chapters in this book.

Pretty cool, huh? You put a provocative sentence like that on a line by itself and it resonates with authority. Granted, it's a little overstated, but can you blame me? I mean, I can't have a sentence like, "If you want my opinion, I think these are some pretty doggone important chapters — at least, that's the way it seems to me; certainly, you might have a different opinion," on a line by itself. The other paragraphs would laugh at it.

At any rate, I invite you to pay close attention to the fundamental concepts and approaches documented throughout this chapter. Although I wouldn't characterize each and every technique as essential — lots of artists get by without paying much attention to paths, for example, while other artists swear by them — a working knowledge of selection outlines is key to using Photoshop successfully.

How selections work

If you want to edit a portion of an image without fear that you might accidentally muck up another portion of the image, you must first select it, which is computerese for indicating the boundaries of the area you want to edit. To select part of an image in a painting program, you surround it with a selection outline or a marquee, which tells Photoshop where to apply your editing instructions. The selection outline appears as a moving pattern of dash marks, lovingly termed marching ants by doughheads who've been using computers too long. (See Figure 8-1 for the inside story.)

Figure 8-1: A magnified view of a dash mark in a selection outline reveals a startling discovery.

Visible selection outlines can be helpful sometimes, but they can just as readily impede your view of an image. When they annoy you, you can press Ctrl+H (z -H on the Mac) to shoo them away. Pressing Ctrl+H (Win) or z -H (Mac) toggles the View Extras command, which hides and displays all on-screen aids, not just those pesky ants. So you also lose guides, the grid, note icons, slices, and target paths. If you want to hide just the ants, choose View Show Selection Edges to toggle the command off. Choose the command again to toggle the ants back on. You also can control which items disappear when you press Ctrl+H (z -H on the Mac) by choosing View Show Show Extras Options. In the resulting dialog box, check the items that you want Photoshop to display at all times.

As for creating selections, you have at your disposal a plethora of tools, all shown in Figure 8-2 and described briefly in the following list. You can access most of the tools by using keyboard shortcuts, which appear in parentheses.

Figure 8-2: Photoshop offers a bounty of selection tools.

Rectangular marquee (M): Long a staple of painting programs, this tool enables you to select rectangular or square portions of an image.

Elliptical marquee (M): The elliptical marquee tool works like the rectangular marquee except it selects elliptical or circular portions of an image.

Single-row and single-column: The single-row and single-column tools enable you to select a single row or column of pixels that stretches the entire width or height of the image. These tools are so seldom used that Adobe didn't give them keyboard shortcuts.

Lasso (L): Drag with the lasso tool to select a free-form portion of an image. Unlike the lasso tools in most painting programs, which shrink selection outlines to disqualify pixels in the background color, Photoshop's lasso tool selects the exact portion of the image you enclose in your drag.

Polygonal lasso (L): Click different points in your image to set corners in a straight-sided selection outline. This is a great way to select free-form areas if you're not good at wielding the mouse or your wrists are a tad sore. (You can achieve the same effect by Alt-clicking or Option-clicking with the lasso tool; I explain this more in the "Free-form outlines" section later in this chapter.)

Magnetic lasso (L): Click with the magnetic lasso along the edge of an image element that you want to select independently from its background. Then move (you don't have to drag) the magnetic lasso around the edge of the element. It's a tricky tool to use, so you can be sure I describe it in excruciating detail in the coming pages.

Magic wand (W): First introduced by Photoshop, this tool lets you select a contiguous region of similarly colored pixels by clicking inside the region. For example, you might click inside the boundaries of a face to isolate it from the hair and background elements. Novices tend to gravitate toward the magic wand because it seems like such a miracle tool, but, in fact, it's the least predictable and ultimately the least useful of the bunch.

Pen (P): The pen tool is difficult to master, but it's the most accurate and versatile of the selection tools. You use the pen tool to create a path, which is an object-oriented breed of selection outline. You click and drag to create individual points in the path. You can edit the path after the fact by moving, adding, and deleting points. You can even transfer a path by dragging and dropping between Photoshop, Illustrator, and FreeHand. For a discussion of the pen tool, read the "How to Draw and Edit Paths" section later in this chapter.

Freeform pen and magnetic pen (P): If you hate setting points but you need to create a clipping path, the freeform pen is the tool for you. You just drag with the tool as if you were selecting with the lasso tool and let Photoshop define the points automatically. Obviously, you can't expect the same level of accuracy that you get from the standard pen tool, but it's child's play to use.

Selecting the Magnetic check box in the Options bar transforms the freeform pen into the magnetic pen, which once upon a time was a tool in its own right. The magnetic pen is basically an object-oriented version of the magnetic lasso tool. Click to set the first point and then move your mouse and watch Photoshop create the other points automatically. It's not a great tool, but it can prove handy when selecting image elements that stand out very clearly from their backgrounds.

Shape tools (U): To draw paths in simple geometric shapes — rectangles, polygons, and so on — give the shape tools a whirl. First, put the tools in the path mode by clicking the Paths icon at the left end of the Options bar. Then simply drag to create the path. To find out more about working with these tools, visit Chapter 14.

Path and shape selection tools: How to Draw and Edit Paths."

If this were all you needed to know to use the selection tools in Photoshop, the application would be on par with the average paint program. Part of what makes Photoshop exceptional, however, is that it provides literally hundreds of little tricks to increase the functionality of every selection tool. Furthermore, all of Photoshop's selection tools work together in perfect harmony. You can exploit the specialized capabilities of the selection tools to create a single selection boundary. After you understand which tool best serves which purpose, you can isolate any element in an image, no matter how complex or how delicate its outline.

Geometric selection outlines

Tools for creating simple geometric selection outlines occupy the first slot in the Photoshop toolbox. By default, the rectangular marquee tool has the stage. You select the elliptical, single-row, and single-column marquee tools from the flyout menu that appears when you drag from the marquee tool icon.

The marquee tools are more versatile than they may appear at first glance. You can adjust the performance of each tool as follows:

Constraining to a square or circle: Press and hold Shift after beginning your drag to draw a perfect square with the rectangular marquee tool or a perfect circle with the elliptical marquee tool. (Pressing Shift before dragging also works if no other selection is active; otherwise, this adds to a selection, as I explain later in the "Ways to Change Selection Outlines" section.)

Drawing a circular marquee: When I was perusing an online forum a while back, someone asked how to create a perfect circular marquee. Despite more than a month of helpful suggestions — some highly imaginative — no one offered the easiest suggestion of all (well, I ultimately did, but I'm a know-it-all). So remember to press Shift after you begin to drag, and you'll be one step ahead of the game.

Drawing out from the center: Press and hold Alt (Option on the Mac) after you begin dragging to draw the marquee from the center outward instead of from corner to corner. (Again, pressing Alt or Option before dragging works if no selection outline is active; otherwise, this subtracts from the selection.) This technique is especially useful when you draw an elliptical marquee. Locating the center of the area you want to select is frequently easier than locating one of its corners — particularly because ellipses don't have corners.

TipMoving the marquee on-the-fly: While drawing a marquee, press and hold the spacebar to move the marquee rather than resize it. When you get the marquee in place, release the spacebar and keep dragging to modify the size. The spacebar is most helpful when drawing elliptical selections or when drawing a marquee out from the center — this eliminates the guesswork, so you can position your marquees exactly on target.

Selecting a single-pixel line: Use the single-row or single-column tools to select a single row or column (respectively) of pixels stretching across the width or length of your image. I use these tools to fix screwups such as a missing line of pixels in a screen shot, to delete random pixels around the perim-eter of an image, or to create perpendicular lines in a fixed space.

Constraining the aspect ratio: If you want to create an image that conforms to a certain aspect ratio, you can constrain either a rectangular or an elliptical marquee so that the ratio between height and width remains fixed, no matter how large or small a marquee you create. To accomplish this, select Fixed Aspect Ratio from the Style pop-up menu in the Options bar, as shown in Figure 8-3. Enter the desired ratio values in the Width and Height option boxes.

Figure 8-3: Select Fixed Aspect Ratio from the Style pop-up menu in the Options bar to constrain the width and height of a rectangular selection outline.

Remember that you're just establishing the image aspect ratio here, not setting the output width and height. So you could just as easily enter 2 and 3 in the Width and Height option boxes. The size of the final, cropped image depends on how large you draw the marquee and the Resolution value you set in the Image Size dialog box.

Sizing the marquee numerically: If you're editing a screen shot or some other form of regular or schematic image, you may find it helpful to specify the size of the marquee numerically. To do so, select Fixed Size from the Style pop-up menu and enter size values in the Width and Height option boxes. To match the selection to a 640480-pixel screen, for example, change the Width and Height values to 640 and 480, respectively. Then click in the image to create the marquee.

Drawing feathered selections: A Feather option box is available in the Options bar when you use any of the marquee tools. To feather a selection is to blur its edges beyond the automatic antialiasing afforded by most tools. For more information on feathering, refer to the "Softening selection outlines" section later in this chapter.

Creating jagged ellipses: By default, elliptical selection outlines are antialiased. If you don't want antialiasing — you might prefer harsh edges when editing screen shots or designing screen interfaces — deselect the Anti-aliased check box. (This option is dimmed when you use the rectangular marquee because antialiasing is always off for this tool.)

Free-form outlines

In comparison to the rectangular and elliptical marquee tools, the lasso tool provides a rather limited range of options. Generally speaking, you drag in a free-form path around the image you want to select. The few special considerations are as follows:

Feathering and antialiasing: Just as you can feather rectangular and elliptical marquees, you can feather selections drawn with the lasso tool by first selecting the Feather check box in the Options bar. To soften the edges of a lasso outline, select the Anti-aliased check box.

Drawing polygons: When you press and hold Alt (Option on the Mac), the lasso tool functions like (and the icon even looks like) the polygonal lasso tool. (Polygon, incidentally, means a shape with multiple straight sides.) With the Alt or Option key down, click to specify corners in a free-form polygon, as shown in Figure 8-4. If you want to add curves to the selection outline, drag with the tool while still pressing Alt or Option. Photoshop closes the selection outline the moment you release both the mouse button and the Alt or Option key.

Figure 8-4: Alt-click (Win) or Option-click (Mac) with the lasso tool to create corners in a selection outline, shown as black squares in the bottom image. Drag to create free-form curves. Surprisingly, you can Alt-click or Option-click anywhere in the image window, even on the scroll bars, to add corners outside the boundaries of the image.

The polygonal lasso tool: If you don't want to bother with pressing Alt (Win) or Option (Mac), select the polygonal lasso. When the lasso is active, you can switch to the polygonal lasso by pressing L or Shift+L, depending on your Use Shift Key for Tool Switch setting in the Preferences dialog box. Or drag from the lasso tool icon to display the lasso flyout menu and select the polygonal lasso that way. Then click inside the image to set corners in the selection. Click the first point in the selection or double-click with the tool to complete the selection outline.

To create free-form curves with the polygonal lasso tool, press Alt (Win) or Option (Mac) and drag.

Adobe added the polygonal lasso for those times when Alt-clicking (Option-clicking on the Mac) isn't convenient. If no portion of the image is selected, it's no trick to Alt-click (Win) or Option-click (Mac) with the standard lasso to draw a straight-sided selection. But if some area in the image is selected, pressing Alt or Option tells Photoshop that you want to subtract from the selection outline. For this reason, it's often easier to use the polygonal lasso — although you still can make it work by pressing Alt or Option after you click with the lasso tool, as I explain in the "Using Shift and Alt (or Shift and Option) like a pro" section later in this chapter.

Magnetic selections

In the old days of black-and-white painting programs — most notably MacPaint on the Mac — black pixels were considered foreground elements and white pixels were the background. To select a black element, you had only to vaguely drag around it with the lasso tool and the program would automatically omit the white pixels and "shrink" the selection around the black ones.

The magnetic lasso tool is Adobe's attempt to transfer shrinking into the world of color. Under ideal conditions — very ideal conditions, I might add — a selection drawn with the magnetic lasso automatically shrinks around the foreground element and omits the background. Naturally, it rarely works this miraculously, but it does produce halfway decent selection outlines with very little effort — provided that you know what you're doing.

Using the magnetic lasso tool

Typically, when people have a problem using the magnetic lasso tool, it's because they're trying to make the process too complex. Work less, and the tool works better. Here are the basic steps for using this unusual tool:

STEPS: Making Sense of the Magnetic Lasso Tool

Select an image with very definite contrast between the foreground image and its background. The skull in Figure 8-5 is a good example: a light gray skull against a dark gray background. Here's something that Photoshop can really sink its teeth into.

Figure 8-5: After clicking to set the start point (top), I moved the magnetic lasso cursor along the edge of the skull. Then I reversed the completed selection by pressing Ctrl+Shift+I (z -Shift-I on the Mac) and pressed Backspace (Delete on the Mac) to fill it with white (bottom).

Select the magnetic lasso. If any tool but a lasso tool is active, press L to grab the lasso that's showing in the toolbox. Then press L as necessary to cycle to the magnetic lasso.

Click anywhere along the edge of the foreground element. I clicked at the top of the skull, as labeled in Figure 8-5.

Move the cursor around the edge of the foreground element. Just move the mouse, don't drag — that is, there's no need to press the mouse button. As your cursor passes over the image, Photoshop lays down a line along the edge of the element, as Figure 8-5 shows. If you don't like the placement of the line, back up the cursor and try moving along the edge again. The magnetic lasso also lays down anchor points at significant locations around the image. If you don't like where the program puts a point, press Backspace (Win) or Delete (Mac). Each time you press Backspace or Delete, Photoshop gets rid of the most recent point along the line. To set your own anchor points, just click.

When you make it all the way around to the beginning of the shape, click the first point in the outline to close the selection. Or just double-click to close with a straight edge.

As I mentioned, the magnetic lasso does not perform miracles. It almost never selects an image exactly the way you would like it to. After moving the cursor around the skull, I reversed the selection by choosing Select Inverse, and then I pressed Backspace (Delete on the Mac) to fill the background with white. The result appears in the second example in Figure 8-5. As you can see, the magnetic lasso did a very nice job of isolating the skull — much better than I could have achieved with the lasso alone — but the selection isn't perfect. Notice the gap on the right side of the skull and the clumsy treatment of the tip of the pointy lower jaw on left. Okay, no automated selection tool is perfect, but the magnetic lasso makes as few mistakes as any I've seen.

Modifying the magnetic lasso options

You modify the performance of the magnetic lasso tool by adjusting the values in the Options bar. The Feather and Anti-aliased options define the softness of the final selection outline, just as they do for the standard lasso tool. The others control how the magnetic lasso positions lines and lays down points:

Width: I might have named this option Sloppiness Factor. It determines how close to an edge you have to move the cursor for Photoshop to accurately see the image element. Large values are great for smooth elements that stand out clearly from their backgrounds. If I raise the Width to 20 when selecting the top of the skull, for example, I can move the cursor 20 pixels away from the skull and Photoshop still shrinks the selection tight around the skull's edge. That's a lot of wiggle room and makes my life easier. But when you're selecting narrow passageways, you need a low value to keep Photoshop from veering off to the wrong edge. The spot where the pointy jaw meets with the snout is a good example of a place where I need to set a small Width and move very carefully around the edge.

If you have a pressure-sensitive tablet and select the Pen Pressure check box, you can control the sloppiness factor dynamically according to how hard you press on the pen. Bear down to be careful; let up to be sloppy. Because this is the way you probably work naturally, you'll be able to adjust the width as needed without even thinking much about it.

Edge Contrast: This is the simplest of the options. It tells Photoshop how much contrast there has to be between the element you're trying to select and its background to even be recognized. If the foreground element stands out clearly, you may want to raise the Edge Contrast value to avoid selecting random flack around the edges. If the contrast between foreground and background is subtle, lower the value.

Frequency: This option tells the magnetic lasso when to lay down points. As you drag with the tool, the line around the image changes to keep up with your movements. When some point in the line stays still for a few moments, Photoshop decides it must be on target and anchors it down with a point. If you want Photoshop to anchor points more frequently, raise the value. For less frequent anchoring, lower the option. High values tend to be better for rough edges; lower values are better for smooth edges.

Most of the time, you can rely on the bracket keys to adjust the Width and leave the Frequency and Edge Contrast values set to their defaults. When dealing with a low-contrast image, lower the Edge Contrast value to 5 percent or so. And when selecting unusually rough edges, raise the Frequency to 70 or more. But careful movements with the magnetic lasso tool go further than adjusting any of these settings.

The world of the wand

Using the magic wand tool is a no-brainer, right? You just click with the tool and it selects all colors that fall within a selected range. The problem is getting the wand to recognize the same range of colors that you see on screen. For example, if you're editing a photo of a red plate against a pink tablecloth, how do you tell the magic wand to select the plate and leave the tablecloth alone?

Sadly, adjusting the wand is pretty tricky and frequently unsatisfying. When the magic wand is active, you'll see the following four controls in the Options bar:

Tolerance: This option determines the range of colors the tool selects when you click with it in the image window.

Anti-aliased: This check box softens the selection, just as it does for the lasso tools.

Contiguous: When selected, this option tells Photoshop to select a contiguous region of pixels emanating from the pixel on which you click. If you're trying to select landmasses on a globe, for example, clicking on St. Louis selects everything from Juneau to Mexico City. It doesn't select London, though, because an ocean of water that doesn't fall within the tolerance range separates the cities. To select all similarly colored pixels throughout the picture, deselect the option.

Use All Layers: Turn this option on to take all visible layers into account when defining a selection.

You now know all you need to know about the Anti-aliased and Contiguous options; the next two sections explain Tolerance and Use All Layers.

Adjusting the tolerance

You may have heard the standard explanation for adjusting the Tolerance value: You can enter any number from 0 to 255 in the Tolerance option box. Enter a low number to select a small range of colors; increase the value to select a wider range of colors.

Nothing is wrong with this explanation — it's accurate, in its own small way — but it doesn't provide one iota of information you couldn't glean on your own. If you really want to understand this option, you have to dig a little deeper.

When you click a pixel with the magic wand tool, Photoshop first reads the brightness value that each color channel assigned to that pixel. If you're working with a grayscale image, Photoshop reads a single brightness value from the one channel only; if you're working with an RGB image, it reads three brightness values, one each from the red, green, and blue channels; and so on. Because each color channel permits 8 bits of data, brightness values range from 0 to 255.

Next, Photoshop applies the Tolerance value, or simply tolerance, to the pixel. The tolerance describes a range that extends in both directions — lighter and darker — from each brightness value.

Suppose you're editing a standard RGB image. The tolerance is set to 32 (as it is by default). You click with the magic wand on a turquoise pixel, whose brightness values are 40 red, 210 green, and 170 blue. Photoshop subtracts and adds 32 from each brightness value to calculate the magic wand range that, in this case, is 8 to 72 red, 178 to 242 green, and 138 to 202 blue. Photoshop selects any pixel that both falls in this range and can be traced back to the original pixel through an uninterrupted line of other pixels, which also fall within the range.

From this information, you can draw the following basic conclusions about the magic wand tool:

Clicking on midtones maintains a higher range: Because the tolerance range extends in two directions, you cut off the range when you click a light or dark pixel, as demonstrated in Figure 8-6. Consider the two middle gradations: In both cases, I selected the Contiguous check box and set the Tolerance value to 60. In the top gradation, I clicked on a pixel with a brightness value of 140, so Photoshop calculated a range from 80 to 200. But when I clicked on a pixel with a brightness value of 10, as in the bottom gradation, the range shrank to 0 to 70. Clicking on a medium-brightness pixel, therefore, permits the most generous range.

Figure 8-6: Note the results of clicking on a pixel with a brightness value of 140 (top row) and a brightness value of 10 (bottom row) with the tolerance set to three different values.

Selecting brightness ranges: Many people have the impression that the magic wand selects color ranges. The magic wand, in fact, selects brightness ranges within color channels. So if you want to select a flesh-colored region — regardless of shade — set against an orange or a red background that is roughly equivalent in terms of brightness values, you probably should use a different tool.

Selecting from a single channel: If the magic wand repeatedly fails to select a region of color that appears unique from its background, try isolating that region inside a single-color channel. You'll probably have the most luck isolating a color on the channel that least resembles it. For example, to select the sky shown in Figure 8-7, I examined each color channel closely to see which provided the greatest contrast between the sky and the lighthouse. Although at first glance the red and green channels both looked promising, on closer inspection I noted that only in the red channel does the red flag at the top of the lighthouse stand out against the sky. If I use the red channel, the magic wand can distinguish between the lighthouse and the sky easily. Experiment with this technique and it will prove even more useful over time.

Figure 8-7: Because the sky contains almost no red, it appears most clearly distinguished from the lighthouse in the red channel. So the red channel is the easiest channel in which to select the sky with the magic wand.

Making the wand see beyond a single layer

The Use All Layers option enables you to create a selection based on pixels from different layers (see Chapter 12 for more about layers). Returning to my previous landmass example, suppose you set Europe on one layer and North America on the layer behind it so that the two continents overlap. Normally, if you clicked inside Europe with the magic wand, it would select an area inside Europe without extending out into the area occupied by North America on the other layer. Because the wand doesn't even see the contents of other layers, anything outside Europe is an empty void. We're talking pre-Columbus Europe here.

If you select Use All Layers, though, the situation changes. Suddenly, the wand can see all the layers you can see. If you click on Europe, and if North America and Europe contain similar colors, the wand selects across both shapes.

Mind you, while the Use All Layers option enables the wand to consider pixels on different layers when creating a selection, it does not permit the wand to actually select images on two separate layers. Strange as this may sound, no selection tool can pull off this feat. Every one of the techniques explained in this chapter is applicable to only a single layer at a time. Use All Layers merely allows the wand to draw selection outlines that appear to encompass colors on many layers.

What good is this? Well, suppose you want to apply an effect to both Europe and North America. With the help of Use All Layers, you can draw a selection outline that encompasses both continents. After you apply the effect to Europe, you can switch to the North America layer — the selection outline remains intact — and then reapply the effect.