Real World Adobe® Photoshop® CS2 [Electronic resources] : Industrial-Strength Production Techniques نسخه متنی

File Formats for Multimedia and the Web

Where TIFF and EPS (and the DCS variant) file formats are designed for print, there are plenty of other formats that you can use for multimedia and Web publishing, including PICT, JPEG, and GIF. These file formats all have some form of compression built in, and in the case of GIF and JPEG, the results can degrade your image significantlybut with the upside of much smaller file sizes.

PICT

The PICT format (pronounced just as it looks) is a Mac-standard object-oriented file format. A PICT graphic can contain a bitmap as one of the objects in the file, or as its only object ("bitmap-only PICT"). Bitmap-only PICTs can be any size or resolution, though they cannot include 16-bit data. While Photoshop for Windows can open a PICT (.pct) image, few other programs on the PC can. If the PC is your final destination, you'd be well advised to use some other file format.

You should especially not use PICTs in a page-layout package; Adobe InDesign, PageMaker, and QuarkXPress are all prone to produce unpredictable results from PICTs.

PICT files can't handle multiple layers, CMYK data, or more than four channels (that's RGB plus one channel total). Nonetheless, PICT is the primary format when you're printing to non-PostScript devices (like most film recorders) or for non-Web multimedia work (like working with Macromedia Director); in these cases, you rarely need to move out of RGB mode. The PICT file format is not appropriate for Web publishing.

When you save a file in the PICT format, Photoshop asks whether you want to use JPEG (and if so, what quality you wantas we'll see in the next section, the lower the quality, the smaller the result, but the more artifacts are introduced). You can pick "None" compression, but Photoshop will still use lossless RLE compression.

CompuServe GIF

Compressing Images," later in this chapter). But we never save a GIF file with the Save As dialog box; rather, we use the Save For Web feature (see Chapter 14, Multimedia and the Web).

JPEG

Earlier in the chapter we talked about JPEG as a compression method within another file formatlike JPEG DCSbut these days when most people say "JPEG" they're referring to the JPEG file format itself. While plenty of people use JPEG images for prepress work, the vast majority of JPEG images are found on the Web. The only problem with using the JPEG format for printing is that it's lossy (see "Compressing Images," later in this chapter). We recommend only using JPEG files in a prepress workflow if it's essential that you drastically limit your file sizes (perhaps you have limited RAM or hard drive space, or your weekly rag has 800 images). Note that if you do save your files in a JPEG format, it's important not to open them and save them repeatedlyeach time you save a JPEG file it further degrades.

Note that neither XPress nor PageMaker nor InDesign actually sends the JPEG information to the printer for decompression (as they do with JPEG-encoded EPSs). Instead, they decompress it and send it down just as they would a TIFF file. So you get the hard disk savings, but it actually takes longer every time you print the file because the printing program has to decompress the JPEG image each time.

JPEGs on the Web are a different matter. JPEGs are ubiquitous on the Internet because they're the only good way to display full color (24-bit) Chapter 14, Multimedia and the Web.