Working big in Photoshop can present some real problems. First of all, the file sizes are just that — BIG—meaning slower processing times for transformations, filters and just about any other procedure. Even if the computer contains a considerable amount of memory, (2 ­GB or more) and a dual Intel microprocessors, the processing can be snail-like as the computer strains to convert all of those pixels into the numerical values of the target colors.

THANKS FOR THE MEMORY
Random access memory (RAM) is the computer’s short-term memory. That’s where all the calculations for a given operation are performed. When a process is applied to a selected area, a rotation on a 300 ppi, 36" x 72" image, for example, the math is performed in RAM to convert the RGB values of the pixels to new values within the selection outline. The more pixels there are, the more time it takes to calculate the conversion.

To check the amount of RAM available for any given Photoshop document, click the arrow on the status bar and choose Scratch Sizes to display information about the amount of available RAM (see Figure 1 at right). The number on the left represents the amount of memory currently being used by the program to display all open images. The number on the right represents the total amount of RAM available for processing images.
If at any time the procedure being performed exceeds the memory allocated to Photoshop, virtual memory kicks in. Photoshop starts using space on the hard disk as a scratch disk on which to perform it’s calculations. Virtual memory is Photoshop’s “ace in the hole” for image processing. Unfortunately, it performs much slower than RAM.

Large-format images are particularly susceptible to problems resulting from insufficient memory. One simple solution to avoid running short is to purge the memory caches. Under the Edit menu, choose Purge (see Figure 2 above). Choosing Clipboard purges the data remembered when content is copied or cut. Undo purges the memory of the last operation performed. History eliminates up to the last 20 operations performed (or as many history states as are designated in the General Preferences). All deletes the memory of the Clipboard, the Undo and the History. Purging memory caches cannot be undone, so proceed with caution!

Sometimes it’s necessary to allocate more RAM to Photoshop, particularly when virtual memory has kicked in and performance has been slowed. First, quit other running programs.

Choose Edit (Win)/Photoshop (Mac)> Preferences >Performance… to display the dialog box shown in Figure 3 above. (In versions earlier than CS3 it’s in the Memory and Image Cache Preferences.) The Memory Usage field displays the amount of available memory, the ideal range for a given document, and a slider that increases or decreases the amount of allocated RAM.

Drag the slider to the right to increase the memory allocation, but be warned, never allocate all of the computer’s memory to Photoshop or any other program as the system needs memory to function. Depending on the amount of memory the computer has, allocate a maximum of 80 percent. Then quit Photoshop and re-launch it to deploy the new allocation.
If virtual memory still kicks in after all this fuss, the performance will be compromised anyway. Keep ample room on the hard disk for processing. If the hard disk has too much data on it, Photoshop will display a warning, as in Figure 4, at right that says that the scratch disk is full.

Before you start tearing out what’s left of your hair, save the file (if you can without seeing the warning again), and do one of two things: either dump stuff from the hard disk, or go to the Preferences and choose Performance again. In the Scratch Disks field, designate a new, different or additional scratch disk. Then quit Photoshop and relaunch.
Incidentally, if memory problems are to be expected, pre-configure the Scratch Disk Preferences dialog box before Photoshop is fully opened by pressing Opt+Cmnd (Mac) or Alt+Ctrl (Win) when the program launches.

YOU SAY YOU WANT A RESOLUTION
Memory problems are not the only issues that arise when working on big images. Resolution must also be considered. Consult the printer’s documentation to determine what resolution is best for each printer and substrate.

Run a test print. Print a small strip of the image to be sure there is no pixelization (see Figure 5). Large images, like billboards, that are seen from a distance usually require smaller resolutions than highly detailed fine art prints that are viewed close up. Resolutions required for large format-images can range from 72ppi to 600ppi, depending on the application, printer type and substrate. Of course, higher resolutions produce larger file sizes, resulting in longer processing times for both imaging and printing.

Avoid using resampling to increase the physical size or resolution of an image. Resampling adds or subtracts pixels to the image as determined by one of five interpolation algorithms (see Figure 6, at right. It usually doesn’t hurt to reduce the physical size or resolution by resampling, but even with the Bicubic Smoother interpolation algorithm that Adobe recommends, image quality can deteriorate when enlarging, usually compromising sharpness and contrast.

A far better solution is to collect enough pixels at the scanner to be adequate for the intended purpose. For instance, if the image is 8" x 10", and it’s scanned at 2400 ppi, it can be blown up to 64" x 80" (a factor of eight) and still end up with a resolution of 300 ppi. Avoid exceeding the scanner’s optical resolution, though, because the scanner software resamples just as Photoshop does, and the quality of the scan can be compromised.

SIZE MATTERS
Of course, the physical size of an image is also a factor in determining its file size. Photoshop format (PSD) supports a maximum of 30,000 pixels in height and width, and up to two gigabytes of data. An uncompressed 30,000 x 30,000 (100" x 100"), 300ppi document consumes 853 megabytes of hard disk space. With the addition of layers, alpha channels or other Photoshop add­-ons, it’s pretty easy to consume the additional 1.17 GB when working on a banner or wall graphic.

Photoshop’s Large Document Format (PSB) solves this problem by supporting documents of up to 300,000 pixels in height and width, and unlimited file size. All Photoshop features, such as layers, layer styles and filters are supported by this format. It even supports high dynamic range (32 bits-per-channel images). After an image is scanned and opened in Photoshop, it can be saved directly into PSB format (see Figure 7 below).

Photoshop offers two other file formats for saving images with more than 30,000 pixels in either dimension. Photoshop Raw supports documents of any pixel dimension or file size, but does not support layers. Large documents saved in the Photoshop Raw format are automatically flattened. TIFF (Tagged Image File Format) Supports files up to 4 GB and supports most Photoshop features including layers, alpha channels, annotations and ICC profiles.

The big drawback to large-format files bigger than 2 GB or images exceeding 30,000 pixels in either dimension is that most other applications, including versions of Photoshop earlier than CS, cannot handle them.