Understanding file compression

Many image file formats use compression techniques to reduce the storage space required by bitmap image data. Compression techniques are distinguished by whether they remove detail and color from the image. Lossless techniques compress image data without removing detail; lossy techniques compress images by removing detail. The following are commonly used compression techniques:

RLE Run Length Encoding is a lossless compression technique supported by Photoshop and some common Windows file formats. In images with multiple layers containing transparency, RLE compression will compress the transparent portions of each layer.

LZW Lemple-Zif-Welch is a lossless compression technique that provides the best results in compressing images that contain large areas of single color, such as screenshots or simple paint images.

JPEG Joint Photographic Experts Group is a lossy compression technique that provides the best results with continuous-tone images, such as photographs. When you choose JPEG compression, you specify the image quality by choosing an option from the Quality menu, dragging the Quality pop-up slider, or entering a value between 1 and 12 in the Quality text box. For the best printed results, choose maximum-quality compression.

CCITT CCITT encoding is a family of lossless compression techniques for black-and-white images. CCITT is an abbreviation for the French spelling of International Telegraph and Telekeyed Consultive Committee.

ZIP ZIP encoding is a lossless compression technique. Like LZW, ZIP compression is most effective for images that contain large areas of a single color.

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