Files and formats 1321 TIFF

The TIFF file format is often the most convenient way to store digital images. Compression is optional and, if used, is completely lossless; you always get exactly the pixels that you saved.

There are several varieties of TIFF files. The pixels may be 8 or 16 bits; the color can be grayscale, RGB, or CMYK; the compression may be none, LZW, or RLE (Packbits); and there may or may not be multiple layers.

Because LZW (Lempel-Ziv-Welch) compression was protected by a patent until late 2006, low-cost software usually does not support it. For maximum interchangeability, use single-layer RGB color images with Packbits compression or no compression at all.

Uncompressed TIFF files are large:

Bit depth

File size (bytes) = Width (pixels) x Height (pixels) x Colors (usually 3) x -—^—

(plus a few hundred bytes for the header). More concisely: 8-bit uncompressed TIFF = 3 megabytes per megapixel

16-bit uncompressed TIFF = 6 megabytes per megapixel

13.2.2 JPEG

Digital cameras normally deliver their output as JPEG files which have undergone gamma correction and other automatic adjustments in the camera. JPEG files are compressed by discarding some low-contrast detail. The degree of compression is adjustable, and if it is too high, there will be ripples around edges in the decoded image. Minimally compressed (highest-quality) JPEG files approach TIFF quality.

RGB JPEG is the usual format for photographs on the World Wide Web. If you encounter a JPEG file that will not display in your web browser, check whether its color encoding is CMYK.

13.2.3 FITS

The Flexible Image Transport System (FITS) is the standard file format for astronomical images. Unfortunately, non-astronomers rarely use it, and non-astronomical software seldom supports it. You can enable Photoshop and Photoshop Elements to open (not save) some types of FITS files by downloading and installing NASA's FITS Liberator from www.spacetelescope.org. This is intended mainly for image files from the Hubble Space Telescope and other observatories, and at present, FITS Liberator does not make Photoshop fully FITS-compatible, although features are being added with every release.

FITS is the most adaptable graphics file format. It allows you to use 8-bit, 16bit, 32-bit, 64-bit, or floating-point pixels, with or without lossless compression. In addition, the FITS file header has room for copious metadata (information about the file's contents), such as the date, time, telescope, observatory, exposure time, camera settings, and filters. The header consists of ASCII text and can be read with any editor.

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