SAMS Teach Yourself Adobe® Photoshop® CS2 in 24 Hours [Electronic resources] نسخه متنی

For Macintosh users only: If you have to work cross-platform, that is, on both a Macintosh and PC, you should always choose to include file extensions with your files. This option is found in the Preferences dialog box. Also, always check Use Lowercase to be sure that your file is Windows and UNIX compatible.Hour 24.

Reducing File Size

As you start to work with different Photoshop files, you'll notice that your hard drive is starting to fill up. Photoshop files can become large very quickly. You can make your files smaller in several ways:

• Reduce the resolution or the physical size of the image

• Reduce the number of colors in the image palette

• Use a format that compresses the file

• Use a compression utility after the file is saved

• Merge layers in native Photoshop files

• Delete any alpha channels that are no longer needed

Reducing the resolution is not a good idea if you're going to print the image. If the picture is going to be viewed only on your computer screen or on the Web, reduce the resolution to 72 dpi. Be aware, though, that increasing the resolution again, if you change your mind, reduces image quality. You can change the resolution in the ImageImage size dialog box. Don't make it less than 72 dpi.

Reducing the number of colors means reducing the bit depth. This can make your colors look blotchy onscreen and in print. If you are working in grayscale (no color in the picture), reduce the bit depth to 8 bits by choosing Grayscale in the Image Mode submenu. This gives you 256 shades of gray, which is more than a printer can print.

Using a compressed format means choosing a file format, such as TIFF with LZW compression, that automatically shrinks the file down as small as possible when it saves. It does this by a means called

lossless compression , so there's no image degradation or blotchy color. LZW compression (named for its inventors, Lempel, Ziv, and Welch) is also used by GIF and PDF formats.

There are also formats, such as JPEG, that use

lossy compression . Lossy means, as you might guess from the name, that some of the data that makes up the image is lost in the compression process. Instead of 20 shades of blue in the sky in a TIFF file, the same image in a JPEG file might have only 5 shades of blue. And, yes, you

can see the difference. Unfortunately, compression is necessary when you are placing images on the Web, in a multimedia presentation, or in another situation where upload time or storage space is limited. JPEG saves files in the least possible amount of disk space. One point to remember is that if you save a JPEG image a second time as a JPEG, it is compressed again and loses more information. If you work on a JPEG file a lot, you can end up with an unreadable picture. If you're going to work on a picture, save it as a Photoshop file. Don't make it a JPEG until you're done with it and ready to post it on your Web page. If you have to change the image, trash the JPEG file and go back to the Photoshop version.

When you have files you want to save for future reference, you can save them in the normal way as Photoshop documents (with a .psd filename extension) or in whatever format you prefer to work in, and then compress the files with a utility such as StuffIt or PKZip or WinZip. All these file compression utilities use lossless compression algorithms and shrink your image files by anywhere from 20% to 50%. That said, compressing JPEG or GIF files is relatively useless because they're already compressed. You won't gain more than a few percent of additional compression.

Figure 2.6 shows a typical digital photograph that I saved in a number of different file types. (The original is about 5 inches wide, and saved at 200 dpi.) Table 2.1 shows the more common file types and the sizes of the files that this picture required. The version shown is a PSD, the native Photoshop format.

Choosing a Format

With so many possible formats, how can you decide which one to use? It's really not so difficult. As long as you are working on an image, keep saving it as a Photoshop document (.psd). This makes sense, especially after you learn to work in layers, because Photoshop's native format can save the layers, whereas most other formats require that you merge the layers into one. After you have flattened the layers, you can't split them apart again. So, bottom line, as long as you think you'll want to go back to a picture and modify it, save a copy as a Photoshop document.

When you finish working on the picture and are ready to place it into another document for printing, save a copy as an EPS file if it's going to a PostScript-compatible printer. If you aren't sure how it will be printed, save it as a TIFF because TIFF is compatible with most printers and page-layout programs. If you're going to place your picture on a Web page, choose GIF if the picture is line art, has large areas of solid color, or uses a limited color palette. Choose JPEG or PNG if the picture is a photograph or continuous tone art (lots of colors). If you want to import the picture into some other graphics program for additional work, choose Photoshop format (PSD) if the other program supports it, and TIFF if the other program doesn't recognize Photoshop files.