The preview box is Photoshop’s way of passing you a memo marked FYI. No biggie, nothing to fret about, just a little bit of info you might want to know. As an unusually obliging piece of software, Photoshop likes to keep its human masters informed on the latest developments.

By default, the preview box contains two numbers divided by a slash. The first number is the size of the base image in memory. The second number takes into account any additional layers in your image.

Photoshop calculates the first value by multiplying the height and width of the image (both in pixels) by the bit depth of the image, which is the size of each pixel in memory.

Consider a typical full-color, 640 × 480-pixel image. A full-color image takes up 24 bits of memory per pixel (which is why it’s called a 24-bit image). There are 8 bits in a byte, so 24 bits translates to 3 bytes. Multiply that by the number of pixels and you get 640 ×480 × 3 = 921,600 bytes.

Because there are 1,024 bytes in a kilobyte, 921,600 bytes is exactly 900K. Try it yourself open a 640 × 480-pixel RGB image and you’ll see that the first number in the preview box reads 900K. Now you know why. But it’s the second value, the one that factors in the layers, that represents the real amount of memory that Photoshop needs.

If the image contains one layer only, the numbers before and after the slash are the same. Otherwise, Photoshop measures the opaque pixels in each layer and adds approximately 1 byte of overhead per pixel to calculate the transparency.

The second number also grows to accommodate paths, masks, spot-color channels, undoable operations, and miscellaneous data required by the image cache. Now obviously, it’s not necessary that you be able to predict these values (which is lucky, because predicting the second value is virtually impossible).

Photoshop asks no help when calculating the values in the preview box and will summarily ignore any help you might care to offer. But it’s a good idea to know what’s going on as you start piling layers on top of an image. The larger the preview numbers grow, the more work Photoshop has to do and the slower it’s likely to perform.

Image position

A welcome new print feature, called Print Options, enables you to position a picture precisely on a page before printing. You can find Print Options on the File menu, near the other printing commands. To get a rough idea of the current image position, however, click and hold on the preview box.

Photoshop displays a pop-up window showing the size and placement of the image in relation to the paper. The preview also shows the approximate placement of crop marks and other elements requested in the Page Setup dialog box (File>Page Setup). Press Alt and mouse down on the preview box to view the size and resolution of the image.

You can also Ctrl-click the preview box to see the tile sizes. Photoshop uses tiles to calculate pixel manipulations. If you confine your work to a single tile, it will probably go faster than if you slop a little over into a second tile. But who cares? Unless you’re some kind of tile-reading robot, this technical information is rarely of any practical use.

Click the right-pointing arrowhead next to the preview box to display a pop-up menu of six options. The first option Document Sizes is selected by default. This option displays the image-size values described in the previous section. You can find out what information the other choices provide in the next few sections.

The prefix displayed before the values in the preview box indicates which of the options is active: Doc shows that Document Sizes is selected; Scr, Scratch Sizes; and Eff, Efficiency. When the Timing option is active, an s appears after the numerical value.

If a tool name appears in the preview box, you know the final option, Current Tool, is active. Similarly, if you see a color profile statement, such as “untagged RBG,” the Document Profile setting, new to Version 6, has the floor.

Color Profile

If you work regularly with many different color profiles, you may find the new Document Profile option handy. When you select this option, the name of the current color profile appears in the preview box. Adobe changed several other features related to color profiles, too.

Memory Consumption and Availability

When you select Scratch Sizes, Photoshop changes the values in the preview box to represent memory consumption and availability. The first value is the amount of room required to hold the currently open images in RAM. The second value indicates the total amount of RAM that Photoshop has to work with.

For the program to run at top efficiency, the first number must be smaller than the second. In the old days, the number before the slash was generally equal to between three and five times the size of all open images, including layers.

But thanks to the advent of multiple undos, this value can grow to more than one hundred times as big as any one image. This is because Photoshop has to store each operation in memory on the off chance that you may want to undo to a previous point in time.

For each and every action, Photoshop nudges the first value upward until you reach the ceiling of undoable operations. The second value is simply equal to the amount of memory available to your images after the Photoshop application itself has loaded. For example, suppose you’ve assigned 100MB of RAM to Photoshop.

The code that makes up the Photoshop application consumes about 15MB, so that leaves 85MB to hold and edit images. If the second value is bigger than the first, then all is happiness and Photoshop is running as fast as your particular brand of computer permits.

But if the first value is larger, Photoshop has to dig into its supply of virtual memory, a disk-bound adjunct to RAM. Virtual memory makes Photoshop run more slowly because the program must swap portions of the image on and off your hard disk. The simple fact is, disks have moving parts and RAM does not.

That means disk-bound “virtual” memory is slower than real memory. To increase the size of the value after the slash, you have to get more RAM to your images in one of the following ways:

• Purchase more RAM. Installing an adequate supply of memory is the single best way to make Photoshop run more quickly.

• Quit other applications so that only Photoshop is running.

• Quit Photoshop and remove any filters that you don’t need from the Plug-Ins folder (which resides in the same folder as the Photoshop application). Don’t throw the filters away, just move them to a location outside the Plug-Ins folder so they won’t load into RAM when you launch Photoshop.

• Choose Edit>Preferences>Memory and Image Cache and increase the Physical Memory Usage value.

Operating efficiency

When you select the Efficiency option, Photoshop lists the amount of time it spends running operations in RAM compared with swapping data back and forth between the hard disk. A value of 100 percent is the best-case scenario.

It means Photoshop never has to rely on scratch files. Low values indicate higher reliance on the hard disk and, as a result, slower operations. Adobe recommends that if the value falls below 75 percent, you should either assign more memory to Photoshop or purchase more RAM for your computer.

The Efficiency option is a reality check. If it seems Photoshop is dragging its feet, and you hear it writing a little too often, you can refer to the Efficiency rating to see if performance is as bad as you suspect. Keep in mind, hearing Photoshop occasionally write to disk is not, in and of itself, cause for concern.

All versions of Photoshop since 3.0 automatically copy open images to a disk buffer in case virtual memory is later warranted. In fact, this is the reason Adobe added the Efficiency option to Version 3.0.1 to quash fears that a few sparks from your hard drive indicated anything less than peak performance.

Operations Timing

If you select Timing, the preview box tells how long Photoshop took to perform the last operation (including background tasks, such as transferring an image to the system Clipboard). Adobe may have added this option to help testing facilities run their Photoshop tests. But built-in timing helps you as well.

For example, suppose you’re trying to decide whether to purchase a new computer. You read a magazine article comparing the newest super-fast system. You can run the same filters with the same settings on your computer and see how much slower your results are, all without picking up a stopwatch.

At the risk of starting interoffice feuding, the Timing option also provides you with a mechanism for testing your computer against those of coworkers and friends. The Timing option serves as a neutral arbitrator, enabling you and an associate to test identical operations over the phone.

Like Efficiency, Timing is a reality check. If you and your associate own similarly configured computers and your Timing values are vastly different, something’s wrong.

Active Tool

Choose Current Tool, and Photoshop displays the name of the active tool. Why do you need such a condescending option? Surely you’re not so far gone that you need Photoshop telling you what you already know.

Adobe’s intention is not to drum you over the head with redundant information, but to offer a helping hand if you find the tool configuration confusing. Also, the tool name serves as a companion to the tool description to the right of it in the status bar. Now you see not just what the tool does, but what the tool is.

Still, my guess is that this option will prove as rarely useful to everyday image editing as Timing. Use it if you’re having problems when first using Photoshop 6 and then set it back to Document Sizes, Scratch Sizes, or Efficiency. The original three options continue to be the best.