Tài liệu Adobe Photoshop 7.0 for Photographers- P7 doc

Adobe Photoshop 7.0 for Photographers

280

Montage Techniques

Chapter Eleven

Confusion often arises when trying to understand the relationship between

alpha channels, masks, Quick mask mode and selections. Let me help ease

the learning curve by saying they are interrelated and essentially all part of

the same thing. That is to say, a selection can be viewed as a Quick mask or saved as

an alpha channel (also referred to as a mask channel). An alpha channel can be con￾verted to make a selection, which in turn can be viewed back in Quick mask mode.

Also discussed later in this chapter is the use of image layer masks and vector masks

and how to draw paths with the pen tool and convert these to selections.

Selections and channels

So when you read somewhere about masks, mask channels, image layer mask

channels, alpha channels, Quick masks and saved selections, the writer is basically

describing the same thing: either an active, semipermanent or permanently saved

selection. We will begin with defining a selection. There are several tools you can

use to do this – the marquee, lasso, magic wand and Select > Color Range. The

marquee comes in four flavors: rectangular, oval, single pixel horizontal row and

vertical column. The lasso has three modes – one for freehand, another for polygon

(point by point) drawing and a magnetic lasso tool. When you use a selection tool to

define an area within an image (see Figure 11.1), you will notice that a selection is

defined by a border of marching ants. Selections are only temporary. If you make a

selection and accidentally click outside the selected area with the selection tool, it

will disappear – although you can restore the selection with Edit > Undo (Command/

Ctrl-Z).

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Montage techniques

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During a typical Photoshop session, I will draw basic selections to define areas of

the image where I want to carry out image adjustments and afterwards deselect them.

If you end up spending any length of time preparing a selection then you will usually

want to save the selection as an alpha channel (also referred to as a mask channel). To do

this, choose Save Selection from the Select menu. The dialog box will ask if you want to

save as a new selection. Doing so creates a brand new alpha channel. If you check the

Channels palette, you will notice the selection appears there labeled as an alpha channel

(#4 in RGB mode, #5 if in CMYK mode). To reactivate this saved selection, choose

‘Load Selection’ from the Select menu and select the appropriate channel number from

the submenu, or Command/Ctrl-click the alpha channel in the Channels palette.

You don’t have to use the selection tools at all. You can also create a new alpha channel

by clicking on the Make New Channel button at the bottom of the Channels palette

and fill the empty new channel with a gradient or paint in the alpha channel with a

painting tool using the default white or black colors. This new channel can then be

converted into a selection. In between masks and selections we have what is known

as a Quick mask. To see how a selection looks as a mask, switch to Quick mask mode

(click on the right-hand icon third up from the bottom in the Tools palette). Now you

see the selection areas as a transparent colored overlay mask. If the mask color is too

similar to the subject image, double-click the Quick mask icon, click on the Color box in

the opened dialog and choose a different color with the Color Picker. In Quick mask

mode (or when working directly on the alpha

channel) you can use any combination of

Photoshop paint tools, Image adjust commands

or filters to modify the alpha channel content.

To revert from a Quick mask to a selection, click

the selection icon in the Tools palette (a quick

tip is to press ‘Q’ to toggle between the two

modes). To reload a selection from the saved

mask channel, go Select > Load Selection.

Command/Ctrl-clicking a channel is the other

shortcut for loading selection and by extension,

combining Option/Alt+Command/Ctrl-channel

# (where # equals the channel number) does

the same thing. Alternatively you can also drag

the channel icon down to the Make Selection

button in the Channels palette.

Figure 11.1 The right half of the image shows a feathered selection (feathering is discussed later in

this chapter) and the left half the Quick mask mode equivalent display.

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Adobe Photoshop 7.0 for Photographers

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Summary of channels and selections

Selections

In marching ants mode, a selection is active and available for use. All image modifi￾cations made will be effective within the selected area only. Selections are tempo￾rary and can be deselected by clicking outside the selection area with a selection tool

or choose Select > Deselect (Command/Ctrl-D).

Quick mask mode

A semipermanent selection, whereby you can view a selection as a transparent

colored mask overlay. To switch to Quick mask mode from a selection, click on the

Quick mask icon in the Tools palette or use the keyboard shortcut ‘Q’ to toggle

between selection and Quick mask mode. Quick mask modifications can be carried

out using any of the fill or paint tools.

Alpha channels

A selection can be stored as a saved selection, converting it to become a new alpha

channel (Select > Save Selection). A selection can be reactivated by loading a selec￾tion from the saved channel (Select > Load Selection). Alpha channels, like color

channels, contain 256 shades of gray, 8-bit information. An anti-aliased selection, or

one that has been modified in Quick mask mode with the fill and paint tools, will

contain graduated tonal information. An active alpha channel (click on the channel

in the Channels palette to make it active) can be manipulated any way you want in

Photoshop. A saved channel can be viewed as a colored transparent mask, overlay￾ing the composite channel image, identical in appearance to a Quick mask. To view

this way, highlight the chosen mask channel to select it and click on the eye icon next

to the composite channel.

Work paths

A work path can be created in Photoshop using the pen tool in work path mode. A path

is (among other things) an alternative method for defining an image outline. A work

path (closed or not) can be converted to a foreground fill, stroke or a selection. For

example, in the Paths palette, drag the path icon down to one of the buttons such as

the Make Selection button. An active selection can be saved as a path – choose Make

Path from the Paths palette submenu. Saving a selection as a path occupies just a few

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Montage techniques

283

Figure 11.2 The above diagram shows the relationship between selections, channels, paths and layers.

A selection can be saved as a

mask channel or as a path

(choose Make Work Path from

the Paths palette submenu).

Paths and mask channels can be

converted back to selections.

When a selection is active

(shown by the marching ants

surrounding the border), only

the pixels inside the selection

area will be modified by further

Photoshop commands.

Duplicate the background layer,

load the alpha 1 channel as a

selection and click on the Add

a mask button. This will add a

layer mask to the active layer

and reveal the selection area.

An active selection can be

viewed in Quick mask mode,

shown here as a semitransparent

red color. Double-click on the

Quick mask icon and click on

the color box, and you can choose

a different color from the picker.

Making a new ‘Layer via Copy’

based on an active selection

creates a new layer. The back￾ground is switched off and the

transparent areas are shown as

a checkerboard pattern.

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Adobe Photoshop 7.0 for Photographers

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kilobytes of file space. This is only more economical than saving as an alpha channel if

you are saving in the TIFF format. Paths cannot save graduated tone selections though. A

saved path can only generate a non-anti-aliased, anti-aliased or feathered selection, but

we’ll come to that later on in the chapter. A path can be used to define a vector mask

(which will mask a layer’s contents) or it can be used in Create Shape Layer mode to add

a filled layer which is auto masked as you define a path outline.

Modifying selections

As was mentioned in Chapter Six, to modify the content of a selection you need to

learn how to coordinate the use of the modifier keys with the dragging of the mouse

as you define a selection. To add to a selection with a selection tool, hold down the

Shift key as you drag. To subtract from a selection with a selection tool, hold down

the Option/Alt key as you drag. To intersect a selection with a selection tool, hold

down the Shift+Option/Alt keys as you drag. Placing the cursor inside the selection

and dragging moves the selection boundary position, but not the selection contents.

The magic wand is a selection tool too – click with the wand, holding down the appropri￾ate key(s) to add or subtract from a selection.

To expand or shrink a selection, choose Select > Modify > Expand/Contract. Selections

can be modified up to a maximum of 100 pixels (but produces angled corners when

expanding a rectangular marquee selection). Other options include Border and

Smooth. To see how these work, make a selection and choose Select > Modify options.

Enter various pixel amounts and inspect the results by switching from selection to

Quick mask mode. The border modification feature is rather crude and can be improved

by applying feathering or saving the selection as a channel and filtering with Gaussian

blur. An example of a border modification is featured in the Extract tutorial on page 323.

Smoothing and enlarging a selection

Selections that are made using the magic wand or Color Range method, under close

inspection are rarely complete. The Smooth option in the Select > Modify submenu

addresses this by enabling you to smooth out the pixels selected or not selected to the

level of tolerance you set in the dialog box.

The Grow and Similar options enlarge the selection using the same criteria as with

the magic wand tool, regardless of whether the original selection was created with

the wand or not. To determine the range of color levels to expand the selection by,

enter a tolerance value in the Options palette. A higher tolerance value means that a

wider range of color levels will be included in the enlarged selection.

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