Compressing and downsampling images, Compression methods – Adobe Acrobat 8 3D User Manual
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ADOBE ACROBAT 3D VERSION 8
User Guide
Compressing and downsampling images
When converting PostScript files to PDF, you can compress vector objects (such as text and line art) and compress
and downsample images. Line art is described with a mathematical equation and is usually created with a drawing
program such as Adobe Illustrator. Images—whether color, monochrome, or grayscale—are described as pixels and
are created with applications like Adobe Photoshop or by scanning. Monochrome images include most black-and
white illustrations made by paint programs and any images scanned with an image depth of 1 bit.
When you downsample (or decrease the number of pixels), information is deleted from the image. With Distiller,
you specify an interpolation method—average downsampling, bicubic downsampling, or subsampling—to
determine how pixels are deleted. Depending on the settings you choose, compression and downsampling can signif
icantly reduce the size of a PDF with little or no loss of detail and precision.
When Distiller processes a file, it normally applies the compression settings to images throughout the file. However,
you can assign different compression and downsampling methods to individual images.
Varying the compression and downsampling methods within a PDF
Before you create a PDF, you can take various approaches to applying different compression and downsampling
options to the individual images that will go into that PDF:
•
Use Adobe Photoshop to resample and compress existing image files before using Distiller. When you are ready
to create the PDF in Distiller, be careful to deselect the compression and downsampling or subsampling options.
•
Create separate PostScript files for each part of the document that you want to process differently, and use different
compression options to distill each part. Then use Distiller to merge the files into a single PDF.
•
When you create color, grayscale, and monochrome images in an art application (such as Adobe Photoshop),
select the compression and downsampling settings that you want when you save each image from within that
application.
•
Insert Distiller parameters before images in a PostScript file. You can use this technique to process every image in
a document differently. This technique is the most difficult, because it requires knowledge of PostScript
programming. For more information on using parameters, see the documentation available for download on the
page (English only) of the Adobe website.
Note: To apply the inserted Distiller parameters, select Allow PostScript File To Override Adobe PDF Settings on the
Advanced panel of the Adobe PDF Settings dialog box in Distiller. This option overrides settings you selected in the
Adobe PDF dialog box.
Compression methods
Distiller 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.