Methods of Compression Resampling Setting the Options When converting files, you can have Create Adobe® PDF Online compress text and line art, and compress and resample color, grayscale, and monochrome images. Depending on the settings you choose, compression and resampling can significantly reduce the size of a PDF file with little or no loss of detail and precision. Line art, or vector graphics, is described with a mathematical equation; it is usually created with a drawing program such as Adobe Illustrator®. Images are described as pixels and are created with paint programs or from scanners. Monochrome images include most black-and-white illustrations made by paint programs and any images scanned with an image depth of 1 bit. Adobe Photoshop, for example, works with images. About Methods of Compression Create Adobe PDF Online applies ZIP compression to text and line art; ZIP or JPEG compression to color and grayscale images; and ZIP, CCITT Group 3 or 4, or Run Length compression to monochrome images. ZIP is a compression method that works well on images with large areas of single colors or repeating patterns, such as screen shots and simple images created with paint programs, and for black-and-white images that contain repeating patterns. Create Adobe PDF Online provides 4-bit and 8-bit ZIP compression options. If you use 4-bit ZIP compression with 4-bit images, or 8-bit ZIP with 4-bit or 8-bit images, the ZIP method is lossless, which means it does not remove data to reduce le size and so does not affect an image's quality. However, using 4-bit ZIP compression with 8-bit data can affect the quality, since data is lost. Note: Adobe implementation of the ZIP filter is derived from the zlib package of Jean-loup Gailly and Mark Adler, whose generous assistance we gratefully acknowledge. The JPEG (Joint Photographic Experts Group) compression method is suitable for grayscale or color images, such as continuous-tone photographs that contain more detail than can be reproduced on-screen or in print. JPEG is lossy, which means that it removes image data and may reduce image quality, but it attempts to reduce file size with the minimum loss of information. Because JPEG eliminates data, it can achieve much smaller file sizes than ZIP compression. Create Adobe PDF Online provides five JPEG options, ranging from Maximum quality (the least compression and the smallest loss of data) to Minimum quality (the most compression and the greatest loss of data). The loss of detail that results from the Maximum and High quality settings are so slight that most people cannot tell an image has been compressed; at Minimum and Low, however, the image may become blocky and acquire a mosaic look. The Medium quality setting usually strikes the best balance in creating a compact file while still maintaining enough information to produce high-quality images. The CCITT (International Coordinating Committee for Telephony and Telegraphy) compression method is appropriate for black-and-white images made by paint programs and any images scanned with an image depth of 1 bit. CCITT is a lossless method. Create Adobe PDF Online provides the CCITT Group 3 and Group 4 compression options. CCITT Group 4 is a general-purpose method that produces good compression for most types of monochrome images. CCITT Group 3, used by most fax machines, compresses monochrome images one row at a time. Run Length is a lossless compression option that produces the best results for images that contain large areas of solid white or black. About Resampling Resampling refers to changing the pixel dimensions (and therefore display size) of an image. When you downsample (or decrease the number of pixels), information is deleted from the image. When you resample up (or increase the number of pixels), new pixels are added based on color values of existing pixels. You specify an interpolation method - average downsampling, bicubic downsampling, or subsampling - to determine how pixels are added or deleted. Note: Create Adobe PDF Online can only downsample; it cannot resample up. To resample an image, Create Adobe PDF Online combines pixels in a sample area to make one larger pixel. You provide the resolution of your output device in dots per inch (dpi), and Create Adobe PDF Online combines pixels as needed to reduce the image's resolution (ppi) to the specified dpi setting: Average downsampling averages the pixels in a sample area and replaces the entire area with the average pixel color at the specified resolution. Bicubic downsampling uses a weighted average to determine pixel color and usually yields better results than the simple averaging method of downsampling. Bicubic is the slowest but most precise method, resulting in the smoothest tonal gradations. Subsampling chooses a pixel in the center of the sample area and replaces the entire area with that pixel at the specified resolution. Subsampling signicantly reduces the conversion time compared with downsampling but results in images that are less smooth and continuous. Setting the Compression options The Compression job options specify compression for text and line art, and compression and resampling for images. Compression and resampling can signicantly reduce the size of a PDF file but can also degrade the quality of images. However, it does not affect the quality of text and line art. You may want to experiment with these options to find an appropriate balance between file size and image quality. The resolution setting for color and grayscale should be 1.5 to 2 times the line screen ruling at which the file will be printed. (As long as you don't go below this recommended resolution setting, images that contain no straight lines or geometric or repeating patterns won't be affected by a lower resolution.) You should also consider whether users will need to magnify a page. For example, if you are creating a PDF of a map, consider using a higher image resolution so that users can zoom in on the map. The following table shows common types of printers and their resolution measured in dots per inch (dpi), their default screen ruling measured in lines per inch (lpi), and a resampling resolution for images measured in pixels per inch (ppi). For example, using the table, if you were printing to a 600-dpi laser printer, you would enter 170 for the resolution at which to resample images. Printer resolution Default line screen Image resolution 300 dpi (laser printer) 60 lpi 120 ppi 600 dpi (laser printer) 85 lpi 170 ppi 1200 dpi (imagesetter) 120 lpi 240 ppi 2400 dpi (imagesetter) 150 lpi 300 ppi If you select the Automatic option, Create Adobe PDF Online determines the best compression method and quality for your color or grayscale images. For most PDF files, this provides satisfactory results. JPEG is applied to 8-bit grayscale images and to 8-bit, 16-bit, and 24-bit color images when the images have continuous, smooth tones. ZIP is applied to 2-bit, 4-bit, and 8-bit grayscale images; to 4-bit color images and indexed 8-bit color images; and to 16-bit and 24-bit color images when the images have sharp color changes.