Adobe InDesign CS3 - Manual (Page 488)

INDESIGN CS3
User Guide
481
Compression and downsampling options for PDFs
When exporting documents to Adobe PDF, you can compress text and line art, and compress and downsample
bitmap images. Depending on the settings you choose, compression and downsampling can significantly reduce the
size of a PDF file with little or no loss of detail and precision.
The Compression area of the Export Adobe PDF dialog box is divided into three sections. Each section provides the
following options for compressing and resampling color, grayscale, or monochrome images in your artwork.
Downsampling If you plan to use the PDF file on the web, use downsampling to allow for higher compression. If you
plan to print the PDF file at high resolution, check with your prepress service provider before setting compression
and downsampling options.
You should also consider whether users need to magnify a page. For example, if you are creating a PDF document of
a map, consider using a higher image resolution so that users can zoom in on the map.
Downsampling refers to decreasing the number of pixels in an image. To downsample color, grayscale, or
monochrome images, choose an interpolation method—average downsampling, bicubic downsampling, or subsam-
pling—and enter the desired resolution (in pixels per inch). Then enter a resolution in the For Images Above text
box. All images with resolution above this threshold will be downsampled.
The interpolation method you choose determines how pixels are deleted:
• Average Downsampling To Averages the pixels in a sample area and replaces the entire area with the average pixel
color at the specified resolution.
• Subsampling To Chooses a pixel in the center of the sample area and replaces the entire area with that pixel color.
Subsampling significantly reduces the conversion time compared with downsampling but results in images that are
less smooth and continuous.
• Bicubic Downsampling To Uses a weighted average to determine pixel color, which 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.
Compression Determines the type of compression that is used:
• Automatic (JPEG) Determines automatically the best quality for color and grayscale images. For most files, this
option produces satisfactory results.
• JPEG Is suitable for grayscale or color images. JPEG compression is lossy, which means that it removes image data
and may reduce image quality; however, it attempts to reduce file size with a minimal loss of information. Because
JPEG compression eliminates data, it can achieve much smaller files sizes than ZIP compression.
• ZIP Works well on images with large areas of single colors or repeating patterns, and for black-and-white images
that contain repeating patterns. ZIP compression can be lossless or lossy, depending on the Image Quality setting.
• JPEG 2000 Is the international standard for the compression and packaging of image data. Like JPEG
compression, JPEG 2000 compression is suitable for grayscale or color images. It also provides additional advantages,
such as progressive display. The JPEG 2000 option is only available when Compatibility is set to Acrobat 6 (PDF 1.5)
or later.
• Automatic (JPEG 2000) Determines automatically the best quality for color and grayscale images. The Automatic
(JPEG 2000) option is only available when Compatibility is set to Acrobat 6 (PDF 1.5) or later.
• CCITT And Run Length Are only available for monochrome bitmap images. CCITT (Consultative Committee on
International Telegraphy and Telephony) compression is appropriate for black-and-white images and any images
scanned with an image depth of 1 bit. Group 4 is a general-purpose method that produces good compression for