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This chapter is from the book

Setting Photoshop's Preferences

Much of how Photoshop looks and acts depends on a file called Adobe Photoshop 7.0 Prefs. This file contains such information as how you last arranged the palettes, the dimensions and color mode of the most recent new document, what style of cursors you prefer, and how you like to name files when saving.

NOTE

The preferences are updated and the file rewritten every time you quit Photoshop. Because this file is rewritten so often, it is subject to corruption. Fixing problems that stem from a bad Prefs file is discussed later in this chapter in the section "Photoshop Quick Fix—Replacing the Preferences."

Photoshop organizes the preferences in eight groups, each of which is displayed and changed in a separate pane of the Preferences dialog box. Windows and Mac OS 9 users can open the Preferences by using the menu command Edit, Preferences. Mac OS X users will find Preferences under the Photoshop menu. The first pane of the dialog box, General, can be opened with the keyboard shortcut (Command-K) [Ctrl+K]. You can move among the eight panes by using the Next and Prev buttons, which appear in all eight panes.

Preferences, General

The General preferences include a variety of settings that affect the way Photoshop operates. The General preferences are shown with their default settings in Figure 3.2.

Figure 3.2 In Windows, the preference Keyboard Zoom Resizes Window is off by default; in Macintosh, it is on. The Windows Preferences also lack the Use System Shortcut Keys check box.

Here are the options available in the General preferences:

  • Color Picker—You have the choice of using Adobe's Color Picker, the Photoshop standard, or the color picker supplied by your operating system.

For full information on the color pickers, including the Windows and Macintosh system color pickers, see "Working with Color Pickers," p. 209 in Chapter 8, "Defining and Choosing Colors."

  • Interpolation—When you transform a selection, Photoshop must add or subtract pixels to make the new size conform to the image's resolution. Bicubic interpolation looks at a range of surrounding pixel color values to determine what color to make new pixels. This option produces the highest quality for photographic and continuous tone images. Bilinear interpolation compares the adjacent pixels. It is faster than bicubic, but generally produces less satisfactory results. Nearest Neighbor interpolation is the fastest of the three, but should be avoided for photographic images. On the other hand, the Nearest Neighbor option is most appropriate for images that have large areas of solid color, especially in linear and rectangular shapes.

  • Redo Key—Photoshop gives you the option of assigning a key for the menu command Edit, Redo (the opposite of Undo). After the Undo command or keystroke has been invoked, you can reverse that action by using Redo. The default, (Command-Z) [Ctrl+Z], is the same shortcut assigned to Undo. The shortcut, therefore, toggles between Undo and Redo. In effect, this is the same as clicking alternately on the two most recent states in the History palette. If the shortcut is assigned to either of the other choices, (Command-Shift-Z) [Ctrl+Shift+Z] or (Command-Y) [Ctrl+Y], the Undo and Redo shortcuts can be used to go backward and forward through all the available history states.

  • History States—The history states, available through the Undo/Redo commands and the History palette, are saved in the memory allocated to Photoshop. Having more history states gives you increased flexibility and more "Undo's," and having fewer frees up memory for other Photoshop tasks.

  • Print Keys—Print with Preview, comparable to Photoshop 6's Print Options dialog box, opens by default when you use the shortcut (Command-P) [Ctrl+P]. You can bypass this dialog box and go directly to your printer's Print dialog box by using the shortcut (Command-Option-P) [Ctrl+Alt+P]. This preference gives you the option of reversing that behavior so that (Command-P) [Ctrl+P] bypasses Print with Preview.

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Changing the default Redo key changes the Step Forward and Step Backward shortcuts in the Edit menu. If you accept the default, you can still move forward and back through all history states by adding the Shift and (Option) [Alt] keys to the Undo shortcut (Command-Z) [Ctrl+Z].

  • Export Clipboard—If you'll need to use something you copied from Photoshop in another program after exiting Photoshop, leave this option selected. If you deselect it, the contents of the Clipboard will not be saved upon quitting Photoshop (and Photoshop will shut down somewhat faster.) This option has no effect on Photoshop while it is running.

  • Show Tool Tips—When on, Tool Tips show you the name of any tool or button—simply leave the cursor over the tool icon or button for a second or two. If you find these little reminders annoying, deselect the option.

  • Keyboard Zoom Resizes Windows—When selected, zooming with keyboard shortcuts resizes the image window. If you use the keyboard to zoom out, the image window shrinks. If you zoom in, the window expands (to a maximum of the space available onscreen). The window resizes so that the image fills it.

  • Auto-Update Open Documents—If a document that is open in Photoshop is also open in another program and a change is made there, this option allows the Photoshop version to be updated. When it is deselected, changes are not seen in Photoshop until the image is closed and reopened.

  • Show Asian Text Options—This check box controls whether the options for Chinese, Japanese, and Korean (CJK) text are visible in the Character and Paragraph palettes. Figure 3.3 shows the difference. If you don't work with CJK fonts, there's no need to show these options.

Figure 3.3 On the left, the palettes without the Asian options. On the right, the same palettes with the options visible.

  • Use System Shortcut Keys (Mac Only)—When selected, this option overrides Photoshop's keyboard shortcuts when there's a conflict with the operating system's shortcuts. For example, if the Mac is set to use the F-keys, this option forces Photoshop to recognize the system's commands rather than the program's commands. In Mac OS X, the system's keyboard shortcuts are controlled through the System Prefs, Keyboard pane. Mac OS 9 uses the Keyboard control panel. (Windows also has a control panel named Keyboard, but the individual F-keys are not programmable through it.)

  • Beep When Done—Photoshop can be set to give an audible announcement when it finishes a task. If Photoshop runs slowly on your system or if you work with extremely large files, this option may be of use (especially if you do other things while waiting for Photoshop). If the program runs quickly and efficiently, however, the beeps might be an annoyance.

  • Dynamic Color Sliders—This option allows the color bars above the sliders in the Color palette to be updated as you drag. Without this option, the color bars retain their basic color, no matter how the sliders are arranged. Dynamic sliders, the default, enable you to see how changes to a component color will affect the overall color.

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With Photoshop's capability to save workspaces (Window, Workspace, Save Workspace), the Save Palette Locations option is not nearly as important as it once was. You can record multiple sets of palette locations and easily switch between them.

  • Save Palette Locations—When this option is selected, Photoshop writes the location and visibility of each palette to the Prefs file, which is updated every time you quit Photoshop. With this option selected, the palettes will be restored to their most recently recorded positions at Photoshop's next startup. When the option's not selected, the palettes automatically return to their default locations when Photoshop is reopened.

  • Show Font Names in English—If you use non-Roman fonts, this option shows their name in English in the Font menu. If it's not selected, such fonts are listed only with their actual names. This preference is designed for use with the Chinese, Japanese, and Korean versions of Photoshop.

  • Use Shift Key for Tool Switch—When this option is active, you can rotate among tools that share a keyboard shortcut by holding down the Shift key and typing the assigned letter shortcut. When inactive, the Shift key is not required. For example, to change from the Lasso tool to the Magnetic Lasso tool by default requires pressing Shift+L twice. (The first keypress changes from the Lasso to the Polygon Lasso, and the second to the Magnetic Lasso.) With this preference box deselected, you need only press L twice.

  • Use Smart Quotes—Smart quotes is the term used for quotation marks that differ at the beginning and end of the quoted material, pointing inward toward the quote (see Figure 3.4). You'll also find the term curly quotes used to describe them. When this option is not selected, Photoshop uses the same character for the quotation marks in both locations.

NOTE

You can manually add curly quotes, even when Smart Quotes is not selected in the Preferences dialog box. The keyboard shortcuts (for most fonts) are (Option-[) [Alt+[] and (Shift-Option-[) [Shift+Alt+[]. When Smart Quotes is active, straight quotes are not available.

  • Reset All Warning Dialogs—Photoshop offers you the opportunity to skip certain warning dialog boxes (see Figure 3.5). If you select the Don't Show Again check box in a warning, you don't have to see it again. However, you also don't get the reminder that you're making a substantial change, one that could affect your ability to edit or make changes to the image later. Clicking this button in the General preferences resets all warnings. If you share a computer or are using Photoshop on a particular computer for the first time, resetting the warnings is a good idea.

Figure 3.4 Changing the Smart Quotes preference does not affect type that has already been set.

Figure 3.5 These are a couple of the warning messages that Photoshop allows you to turn off.

Preferences, File Handling

Because of differences in the way that Windows and Macintosh handle previews, Photoshop offers several more options on the Mac for saving files (see Figure 3.6).

Although the Image Preview options differ, the bottom portion of the File Handling pane is the same for Macintosh and Windows:

  • Image Previews—Windows users have the option of always saving previews, never saving previews, or making the decision at the time the image is saved (Ask When Saving). The difference is simply a check box in the Save dialog box. The Thumbnail check box will be deselected and grayed out (Never Save), selected and grayed out (Always Save), or available for you to select or deselect (Ask When Saving).

Figure 3.6 The upper portion of the Windows version of this pane has only the Image Previews pop-up menu and a choice of lowercase or uppercase letters for the file extension.

Macintosh users, on the other hand, have a variety of options and choices available in the Preferences (see Figure 3.7).

Figure 3.7 In addition to Always, Never, and Ask When Saving, Macintosh allows a choice of previews.

  • File Extension (Append File Extension)—This option also differs in Windows and Macintosh. Windows users have the option of including the file extension in lowercase or uppercase (but it must always be included). Macintosh users can choose among Always, Never, and Ask When Saving, and if the extension is added, have the option of lowercase or uppercase. For maximum file compatibility, append the file extension in lowercase.

  • Ask Before Saving Layered TIFF Files—The so-called Enhanced TIFF format (also known as Advanced TIFF) supports layers as well as annotations and transparency. If a layered TIFF file is open in a program that doesn't support Enhanced TIFF, a flattened version is used.

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At this time, only Photoshop takes advantage of the TIFF standard's more sophisticated capabilities. For that reason, there is little use for this option. If layers are important, save the file as .psd (Photoshop's native format) and skip layered TIFF.

  • Always Maximize Compatibility for Photoshop (PSD) Files—When saving a file with maximum compatibility, a full-size, full-resolution flattened version is saved along with the actual Photoshop (.psd) file. The flattened version ensures that other programs that can use the Photoshop format show the image properly.

Image Previews

Full-sized previews are used by some page layout programs. The Macintosh and Windows previews are used in the Open dialog boxes (and sometimes elsewhere). Many programs can generate previews from the image file itself, depending on file format, instead of relying on the presence of an embedded preview.

The embedded previews can have an effect on the size of a file. Using the file Eagle.psd from the Photoshop 7 Samples folder, the information in Table 3.3 was generated. (File size is listed in bytes.)

Table 3.3 Sample File Sizes

Icon

Full Size

Mac Thumbnail

Win Thumbnail

File Size

O

O

O

O

1,622,000

X

O

O

O

1,663,786

O

X

O

O

1,714,744

O

O

X

O

1,669,504

O

O

O

X

1,665,386

X

X

O

O

1,716,072

X

O

X

O

1,670,824

X

O

O

X

1,668,250

O

X

X

O

1,721,854

O

X

O

X

1,718,124

O

O

X

X

1,675,304

X

O

X

X

1,676,630

O

X

X

X

1,722,626

X

X

X

X

1,728,948


The difference between no previews and all optional previews is some 100KB in this instance. Keep in mind, however, that the size and complexity of the original image play a large part in how much the file size increases. The artwork for a simple Web button, 64x16 pixels, was saved without previews and with all previews. The file size increased from 35,556 bytes to 42,920 bytes.

Although embedded previews can increase a file's size, such factors as the file's format, compression (if any), and even the formatting of the disk can play far more important roles in file size.

  • Enable Workgroup Functionality—Workgroup functionality is designed for use with a WebDAV server. Images stored on the server can be "checked out" (opened) by any member of a team working on a project. With workgroup management, you reduce the risk of different people trying to make conflicting changes to a single image. Like a book from the library, a second person can't check out a file until it's been checked back in by the first person. Deselecting this option in the File Handling preferences grays out the submenu command File, Workgroup.

  • Check Out from Server—The options Always, Never, and Ask pertain to opening a managed document. If you will not be working on the document (opening only to look, for example), you need not check it out. When the document is checked out, no other member of the team can make changes to it.

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If you aren't part of a workgroup or your workgroup doesn't use a WebDAV server, you can disable the Enable Workgroup Functionality option.

  • Update from Server—When opening a managed document, select this option to have Photoshop check the server to see whether the image requires updating.

  • Recent file list contains __ files—You can specify a number from 0 to 30 for this list. The files can be opened by selecting them from the menu File, Open Recent. Tracking recent documents allows you to quickly locate and reopen an image on which you recently worked. However, making the list too long can reduce its usability.

Preferences, Display & Cursors

The options in this pane of Photoshop's Preferences are identical for Windows and Macintosh. They are shown in Figure 3.8.

Figure 3.8 All tools that use brushes are considered painting tools for the purposes of this dialog box.

The choices you make in this pane can have a major impact on your interaction with Photoshop:

  • Color Channels in Color—When this option is selected, Photoshop shows each of the color channels in its own color, rather than as a grayscale image. Although this display option can ensure that the correct channel is being edited, particularly with RGB images, the lack of contrast in yellow and cyan channels could be a problem. Showing color channels in color is often effective when multiple (but not all) channels are visible. The option includes spot channels but has no effect on alpha channels.

  • Use Diffusion Dither—For use only with monitors showing 8-bit color (a total of 256 colors), diffusion dither allows Photoshop to simulate a higher color depth.

  • Use Pixel Doubling—When dragging selected pixels onscreen, pixel doubling can speed screen redraw. By reducing the selection's resolution, this option reduces the amount of data required to refresh the screen. It can be of use with high-resolution images on low-power systems.

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The Caps Lock key switches between Precise and Brush Size cursors. When either option is active, pressing Caps Lock swaps to the other. When Standard is selected, pressing Caps Lock switches between the tool icon and Precise cursors.

  • Painting Cursors—Photoshop allows you to show the cursors for the painting tools in one of three ways: Standard, Precise, or Brush Size. Standard cursors show the tool's icon, which could be useful if you often change tools by using the keyboard shortcuts. Precise cursors use a crosshair to indicate the center of the tool's brush. Brush Size shows a circle the diameter of the selected brush for that tool.

  • When a hard round brush is selected, the Brush Size option shows the cursor as a circle; the painting tool can affect pixels within that area. When a feathered (soft) brush is active, the affected area extends some distance beyond the circular cursor.

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If keeping track of which tool is in use is critical, remember that the Status Bar can be set to Current Tool, leaving you free to use Precise (or Brush Size) for your cursors.

Remember, too, that non-circular cursors are included. When Brush Size is selected, you'll also see the cursor as irregularly shaped brushes.

  • Other Cursors—Like the painting tool cursors, Photoshop's other tools can show the tool's icon or a crosshair. (Because non-painting tools don't use brushes, the third option—Brush Size—is not available.) The Caps Lock key toggles between the Standard and Precise options.

Preferences, Transparency & Gamut

Think of transparency as being able to see through a layer to the layer(s) below. When an area of an image has no opaque pixels on any layer, you see through to the next layer. That's where the transparency grid comes into play. Photoshop arranges a pattern of squares below an image to indicate where there is nothing opaque in an image. The transparency grid is nonprinting and does not appear in any final artwork. With reduced opacity (partial transparency), the grid is partially visible. You can set both size and color for the grid (see Figure 3.9).

Figure 3.9 The default values, shown here, are the same for Macintosh and Windows.

The transparency grid can be customized to make it most appropriate for the image with which you're working:

  • Grid Size—The options are Small, Medium, Large, and None, which turns off the transparency grid.

  • Grid Colors—The pop-up menu offers grids of light gray/white, medium gray/light gray, and dark gray/medium gray. If gray interferes with your work—for example, when working on a grayscale image—you can also choose grids of white with one of five other colors. In addition, you can use the Color Picker to select the two grid colors; simply click on the swatches to open the Color Picker.

  • When this pane of the Preferences dialog box is open, you can also change the colors of the transparency grid by clicking and (Option-clicking) [Alt+clicking] in any open image. Click to change the first transparency color, and then (Option-click) [Alt-click] to change the second transparency color. Remember that the change affects all images, not just the one(s) in which you click.

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Although gray transparency grids are the least likely to interfere with color perception, it can sometimes be difficult to judge partial opacity with them. In those situations, a colored grid may make more sense.

Transparency grid not visible? See "Transparency and Background Layers" in the NAPP Help Desk section at the end of this chapter.

  • Use Video Alpha (Requires Hardware Support)—If your computer includes a video board that supports chroma-key, such hardware allows the video board to overlay an image from Photoshop onto a live video signal.

  • Gamut Warning—Click on the color swatch to choose a color; drag the slider or type a percentage to set the opacity. The gamut warning is used to identify colors in an image that cannot be reproduced in the selected CMYK gamut. To toggle this option, use the menu command View, Gamut Warning.

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If you have two prominent colors in an image, you can maximize the transparency grid's contrast like this: (Command-I) [Ctrl+I] to invert the image's colors. Open the Transparency & Gamut Preferences. Click and then (Option-click) [Alt+click] to set the grid colors, and then click OK. Press (Command-I) [Ctrl+I] again. The resulting transparency grid will have extreme contrast with your image's principle colors.

Preferences, Units & Rulers

This pane of the Preferences dialog box (see Figure 3.10) governs the units of measure used generally throughout Photoshop and for type.

Figure 3.10 The default values of the Units & Rulers pane are all print-oriented.

In addition, there are several settings that affect the dimensions of new documents:

  • Rulers—In addition to Photoshop's rulers, this setting governs most units of measure, from dialog boxes to palettes to the Options Bar. The available units are pixels, inches, centimeters, millimeters, points, picas, and percent. (When percent is selected for rulers, absolute dimensions, such as those required when creating a new document, use inches.) You can also change this setting without opening the Preferences. (Control-click) [Right+click] on the ruler in any image, and select the new unit of measure from the pop-up list.

  • Type—Type in Photoshop is governed separately from other units of measure. Type can be measured in points, pixels, or millimeters. Points are the standard unit of measure for type in print.

  • Column Size—Photoshop's New dialog box allows you to specify the dimensions of a new document. One of the options is Columns (see Figure 3.11), which uses the width established in this setting.

  • Columns can be used effectively when preparing an image to be placed into a multi-column page layout. The New dialog box calculates the width of such a document by multiplying the specified number of columns by this Width value and adding the amount specified by the Gutter value between the columns.

  • New Document Preset Resolutions—Photoshop's New dialog box offers 19 preset sizes (see Figure 3.12). Some are designed for print, and some for Web or video. This pair of preferences allows you to specify which resolution is associated with each of the presets. It is simply a convenience—the resolutions can still be changed in the New dialog box.

  • Point/Pica Size—With the advent of PostScript, the traditional measurement of a point was rounded down to 1/72 of an inch. In some press environments, it's important to maintain the conventional measurement by changing this preference to the Traditional option.

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Remember that you can enter abbreviations for units of measure in most numeric fields. For example, enter 36 px when the units are set to inches, and Photoshop recognizes it as 36 pixels.

Figure 3.11 The width of a column is determined in the Units & Rulers Preferences.

Figure 3.12 The Screen Resolution setting includes Web, DVD, and video presets.

Preferences, Guides, Grid & Slices

In addition to setting the style and color for guides and grids, Photoshop now allows you to specify slice color globally. These preferences (see Figure 3.13) affect only the onscreen appearance; guides, grid, and slice lines are nonprinting. All three can be shown and hidden through the View menu.

The default values might not be appropriate for the images with which you work. These settings allow you to choose colors that may be more visible or perhaps less distracting:

  • Guides—A pop-up menu offers nine preset color values, or you can choose Custom. You can also open the Color Picker by clicking on the swatch to the right. The Style menu allows you to set guides as lines or dashed lines.

Figure 3.13 Photoshop's defaults show both guides and slice lines as Light Blue. You can customize one or the other (or both) to avoid visual confusion.

  • Grid—The grid can also be set to one of nine presets or to a custom color. In addition to lines and dashed lines, the grid can be shown as a series of dots. This pane of the Preferences dialog box also enables you to specify the grid's spacing. Both major units and subunits can be entered, using any of Photoshop's units of measure. The gridlines are thicker than the subdivisions to differentiate between them onscreen.

  • Slices—Used to define subdivisions of Web graphics, slices can be shown in one of the nine preset colors. You also have the option of hiding slice numbers while leaving the slice lines visible.

Preferences, Plug-Ins & Scratch Disks

Plug-ins are supplements to Photoshop's capabilities that you can run from within the program. Scratch disks are hard drive space used by Photoshop to supplement the allocated memory. This pane of the Preferences dialog box (see Figure 3.14) allows you to regulate what folders Photoshop uses for plug-ins and what hard drives are used for scratch disks.

Figure 3.14 If you want Photoshop to load only the default plug-ins folder, leave the upper check box deselected. If you have only one hard drive, Startup is the appropriate scratch disk choice.

You can have several folders for plug-ins, but only one additional folder can be used at a time.

  • Additional Plug-Ins Folder/Directory—If desired, you can have Photoshop load plug-ins from a second source when the program starts. (Macintosh uses "folder," and Windows uses "directory.") Click the check box, click the Choose button, and then navigate to the target folder. Select the folder in the window, and click the (Choose) [OK] button.

  • Legacy Photoshop Serial Number—Some plug-ins require a valid Photoshop serial number. Older plug-ins won't recognize Photoshop 7's serial number. If you're loading plug-ins that need a previous version of Photoshop's old-style serial number, enter that number in this field.

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Changes to the scratch disk preferences don't take effect until the next time Photoshop is started. To save time, you can change them while starting Photoshop. Hold down the (Option-Command) [Alt+Ctrl] keys during Photoshop startup to select scratch disks.

  • Scratch Disks—Scratch disks are hard drive space that Photoshop uses much as you might use a scratch pad—someplace to scribble and keep track of things. The assigned scratch disk or disks are used to support Photoshop's memory. If there's more data than Photoshop can hold in memory, it writes some to the disk for later retrieval. When things are slow, Photoshop can write the entire contents of memory to the scratch disks.

CAUTION

Never use removable media, such as Zip or Jaz drives, as scratch disks. In addition, network drives should never be assigned as scratch disks.

For more information on scratch disks, see "Dedicated Scratch Disks," p. 76.

NOTE

Remember the difference between a partition and a drive. A single hard drive can be partitioned into separate volumes, each of which your OS sees as a drive. Partitions are still physically part of the same drive, however, so the computer cannot write to multiple partitions at the same time.

  • You can assign up to four different disks and partitions as scratch disks. When multiple hard drives are available, you might see an increase in performance if the primary scratch disk is other than the startup disk. (Windows and Macintosh both use the startup disk to support memory. When the OS and Photoshop are both trying to write to the same disk, slowdowns are possible.) All available drives appear in each of the four pop-up menus.

Preferences, Memory & Image Cache

The image cache stores low-resolution versions of an image that can be used to speed screen redraw. The increased monitor response can come at the price of accurate detail, however. Photoshop also enables you to manage how the cache is used.

Windows and Mac OS X dynamically allocate memory to active programs as they need it. Mac OS 9 uses set memory allocations for each program. The more advanced operating systems allow you to use this Preferences pane (see Figure 3.15) to specify what percent of the available memory should be allocated to Photoshop.

Figure 3.15 Note that because of the way Mac OS 9 handles memory, the OS does not show the Memory portion of the preferences, so the name of the pane is simply Image Cache.

Plug-Ins and Photoshop

Plug-ins can greatly expand Photoshop's capabilities, but some care must be taken. Here are some notes on using plug-ins:

Remember that plug-ins load with Photoshop when you start the program. An excessive number of plug-ins can drastically increase the time required to start the program.

A number of other programs use Photoshop-compatible plug-ins. You can use the Additional Plug-Ins Folder preference to load plug-ins from Painter, After Effects, Illustrator, and other programs' plug-ins folders.

If you hold down the (Shift-Command) [Shift+Ctrl} keys while Photoshop is starting up, you can choose an additional plug-ins folder to load. With this capability, you can have several additional plug-ins folders and load only the set you'll need for that work session. This speeds loading and reduces the memory overhead required to run Photoshop.

Plug-ins folders can have subfolders to keep plug-ins organized.

Hundreds of plug-ins from a variety of sources are available for Photoshop. Commercial third-party software companies generally provide high-quality products. However, some shareware and freeware plug-ins can cause stability problems or crashes. It's always a good idea to keep track of which plug-ins were installed when so that if problems develop, you can try to trace them to a specific plug-in. Likewise, it's a good idea to load new plug-ins singly or in small groups, and test them immediately.

Check with the manufacturers of legacy plug-ins for Photoshop 7 compatibility information.

These settings have an impact on how Photoshop performs:

  • Cache Settings—When set to the default of four, Photoshop stores four low-resolution versions of a high-resolution image in memory. These four are at increasing zoom levels from 100%. When zoomed in, Photoshop uses the low-resolution images to redraw the image on the monitor more quickly. The cache, which uses memory allocated to Photoshop, can be set from one to eight.

    Using the cache for histograms allows Photoshop to more quickly generate the histogram for such operations as Levels. Although using the low-resolution cached version of the image produces a less accurate histogram, the differences are rarely significant. If you need to check for posterization, it's best to use a histogram based on the entire image rather than a sampling. No matter what the cache setting is, you can view an accurate histogram by holding down the Shift key while selecting the menu command Image, Histogram.

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If the cache is not set to one, always return to 100% zoom before making critical decisions on high-resolution images—the low-resolution cached image might not be accurate.

  • Memory Usage—This section of the Preferences, which is not used in Mac OS 9, shows you how much RAM is available and allows you to specify a preferred percentage. This setting, which can be changed by typing a value in the numeric field or by dragging the slider, puts a limit on the amount of memory you allow Photoshop to use. Changes are not put into effect until Photoshop is restarted.

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