About file formats
You can use various file formats to get images into and out of various imaging applications. Graphic file formats differ in the way they represent graphic information— either as vector drawings or as bitmap images. Some formats contain only vector drawings or bitmap images, but many include both in the same file.
JPEG format
The Joint Photographic Experts Group (JPEG) format is commonly used to display photographs and other continuous-tone images in hypertext markup language (HTML) documents over the World Wide Web and other online services. The JPEG format supports CMYK, RGB, and grayscale color modes, and does not support alpha channels. Unlike the GIF format, JPEG retains all color information in an RGB image but compresses file size by selectively discarding data. A JPEG image is automatically decompressed when opened. A higher level of compression results in lower image quality, and a lower level of compression results in better image quality. In most cases, the Maximum quality option produces a result indistinguishable from the original. (See Saving files in JPEG format.)
GIF
The Graphics Interchange Format (GIF) is the file format commonly used to display indexed-color graphics and images in hypertext markup language (HTML) documents over the World Wide Web and other online services. GIF is a LZW-compressed format designed to minimize file size and electronic transfer time. The GIF format does not support alpha channels. You can convert files to GIF format using one of the following methods: • The File > Save As command lets you save a Bitmap-mode, grayscale, or indexed-color image in GIF format and specify an interlaced display. An interlaced image appears gradually in increasing detail as it is downloaded from the Web but can increase the file size. You cannot save alpha channels with the image. • The GIF89a Export command lets you convert an RGB or indexed-color image to GIF89a format, specify interlacing, and define background transparency. (See Exporting images.) The GIF89a format supports single alpha channels.
Portable Document Format (PDF) is used by Adobe Acrobat, Adobe’s electronic publishing software for Windows, Mac OS, UNIX® , and DOS. You can view PDF files using the Acrobat Reader® software included on the application CD-ROM. Identical to PostScript pages, PDF files can represent both vector and bitmap graphics, and can contain electronic document search and navigation features such as electronic links. For more information on PDF and Adobe Acrobat, see the Electronic Publishing Guide included on the Tour & Training CD-ROM. The Photoshop PDF format supports RGB, indexed-color, CMYK, grayscale, Bitmap, and Lab color modes, and does not support alpha channels. The format supports JPEG and ZIP compression, except for Bitmap-mode files, which use CCITT Group 4 compression when saved as Photoshop PDF. When you open a PDF created by another application, Photoshop rasterizes the file (see Importing Adobe Illustrator, PDF, and EPS files).
PNG
Developed as a patent-free alternative to GIF, the Portable Network Graphics (PNG) format is used for losslessly compressing and displaying images on the World Wide Web. Unlike GIF, PNG supports 24-bit images and produces background transparency without jagged edges; however, some older versions of Web browsers may not support PNG images. The PNG format supports grayscale and RGB color modes with a single alpha channel, and Bitmap and indexed-color modes without alpha channels. PNG uses the saved alpha channel to define transparency in the file; be sure to delete all but the desired alpha channel from your image before saving as PNG. When saving in PNG format, you can select Adam7 to display the image in increasing detail as it is downloaded. You can also run the image through a filtering algorithm that prepares the image data for optimal compression:
• None compresses the image without a filter, and is recommended for indexed-color and Bitmap-mode images.
• Sub optimizes the compression of images with even horizontal patterns or blends.
• Up optimizes the compression of images with even vertical patterns.
• Average optimizes the compression of low-level noise by averaging the color values of adjacent pixels.
• Paeth optimizes the compression of low-level noise by reassigning adjacent color values.
• Adaptive applies the filter—Sub, Up, Average, or Paeth—best-suited for the image. Select Adaptive if you are unsure of which filter to use.
TIFF
The Tagged-Image File Format (TIFF) is used to exchange files between applications and computer platforms. TIFF is a flexible bitmap image format supported by virtually all paint, image-editing, and page-layout applications. Also, virtually all desktop scanners can produce TIFF images. The TIFF format supports CMYK, RGB, and grayscale files with alpha channels, and Lab, indexed-color, and Bitmap files without alpha channels. TIFF also supports LZW compression. When you save an Adobe Photoshop image in TIFF, you can choose to save in a format that can be read either by IBM PC-compatible or Macintosh computers. To compress the file automatically, click the LZW Compression check box; compressing a TIFF file decreases its file size but increases the time required to open and save the file. Adobe Photoshop also reads and saves captions in TIFF files. For information on using captions, see Adding file information.
Photoshop EPS
The Encapsulated PostScript (EPS) language file format can contain both vector and bitmap graphics and is supported by virtually all graphic, illustration, and page-layout programs. The EPS format is used to transfer PostScript language artwork between applications. When you open an EPS file containing vector graphics created in another application, Photoshop rasterizes the file, converting the vector graphics to pixels (see Importing Adobe Illustrator, PDF, and EPS files). The EPS format supports Lab, CMYK, RGB, indexed-color, duotone, grayscale, and Bitmap color modes, and does not support alpha channels. EPS does support clipping paths. For more information on EPS options, see Saving files in Photoshop EPS or DCS format.
Filmstrip
The Filmstrip format is used for RGB animation or movie files created by Adobe Premiere®. If you resize, resample, remove alpha channels, or change the color mode or file format of a Filmstrip file in Photoshop, you won’t be able to save it back to Filmstrip format. For further guidelines, see the Adobe Premiere User Guide.
PICT File
The PICT format is widely used among Macintosh graphics and page-layout applications as an intermediary file format for transferring files between applications. The PICT format supports RGB files with a single alpha channel, and indexed-color, grayscale, and Bitmap files without alpha channels. The PICT format is especially effective at compressing images with large areas of solid color. This compression can be dramatic for alpha channels with their large areas of white and black. When saving an RGB image in PICT format, you can choose either a 16-bit or 32-bit pixel resolution. For a grayscale image, you can choose from 2, 4, or 8 bits per pixel. In Mac OS with QuickTime installed, four JPEG compression options are available.
BMP
BMP is the standard Windows image format on DOS and Windows-compatible computers. The BMP format supports RGB, indexed-color, grayscale, and Bitmap color modes, and does not support alpha channels. You can specify either Microsoft® Windows or OS/2® format and a bit depth for the image. For 4-bit and 8-bit images using Windows format, you can also specify RLE compression.
Raw
The Raw format is a flexible file format for transferring files between applications and computer platforms. This format supports CMYK, RGB, and grayscale files with alpha channels, and multichannel, Lab, indexed-color, and duotone files without alpha channels. Raw format consists of a stream of bytes describing the color information in the file. Each pixel is described in binary format, with 0 equaling black and 255 white (for images with 16-bit channels, the white value is 65535). Adobe Photoshop designates the number of channels needed to describe the image, plus any additional channels in the file. You can specify the file extension (Windows), file type (Mac OS), file creator (MacOS), and header information. In Mac OS, the file type is generally a four-character ID that identifies the file—for example, TEXT identifies the file as an ASCII text file. The file creator is also generally a four-character ID. Most Macintosh applications have a unique file creator ID that is registered with the Apple Computer Developer Services group. The header parameter specifies how many bytes of information appear in the file before actual image information begins. This value determines the number of zeroes inserted at the beginning of the file as placeholders. By default, there is no header (header size = 0). You can enter a header when you open the file in the Raw format. You can also save the file without a header and then use a file-editing program, such as HEdit (Windows) or Norton Utilities (MacOS), to replace the zeros with header information. You can save the image in an interleaved or noninterleaved format. If you choose interleaved, the color values (red, green, and blue, for example) are stored sequentially. Your choice depends on requirements of the application that will open the file.