User manual

Images options
The Images options specify compression and resampling for images. You may want to
experiment with these options to find an appropriate balance between file size and image
quality. (See Compressing and downsampling images.)
Adobe PDF Settings dialog box with the Images panel displayed
Downsample
To downsample color, grayscale, or monochrome images, Distiller 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 enter a resolution in pixels per inch (ppi) in the For Images Above
box. For all images with resolution above this threshold, Distiller combines pixels as
needed to reduce the image's resolution (ppi) to the specified dpi setting. (See Compressing
and downsampling images.)
● 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 at the specified resolution. 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 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.
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.) The resolution for monochrome images should be
the same as the output device, but be aware that saving a monochrome image at a
resolution higher than 1500 dpi increases the file size without noticeably improving image
quality.
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.
Note: Resampling monochrome images can have unexpected viewing results, such as no
image display. If this happens, turn off resampling and convert the file again. This problem
is most likely to occur with subsampling, and least likely with bicubic downsampling.
The following table shows common types of printers and their resolution measured in 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, 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
Compression/Image Quality
Sets the compression to be applied to color, grayscale, and monochrome images. For color
and grayscale images, also sets the image quality.
For color or grayscale images, choose ZIP to apply compression that works well on images
with large areas of single colors or repeating patterns, such as screen shots, simple images
created with paint programs, and black-and-white images that contain repeating patterns.
Choose JPEG, quality minimum to maximum, to apply compression that 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. Choose JPEG2000, quality Lossless, to apply
lossless compression with additional advantages, such as progressive display. Choose
Automatic (JPEG) or Automatic (JPEG2000) to determine automatically the best quality
for color and grayscale images. (JPEG2000 is the new international standard for the
compression and packaging of image data. For more information on JPEG2000, see
Conversion options for JPEG and JPEG2000 format.) To display JPEG2000 options, you
must select Acrobat 6.0 (PDF 1.5) or later from the Compatibility menu on the General
panel.
For monochrome images, choose CCITT Group 4, CCITT Group 3, ZIP, or Run Length
compression. (For more information, see Methods of compression.) Make sure that
monochrome images are scanned as monochrome and not as grayscale. Scanned text is
sometimes saved as grayscale images by default. Grayscale text compressed with the JPEG
compression method is muddy at best, and may be unreadable.
Anti-Alias To Gray
Smooths jagged edges in monochrome images. Choose 2 bit, 4 bit, or 8 bit to specify 4, 16,
or 256 levels of gray. (Anti-aliasing may cause small type or thin lines to look blurry.)
Compression of text and line art is always on. If you need to turn it off, you can do so by
setting the appropriate Distiller parameter. For details, see the Acrobat Distiller Parameters
manual at http://partners.adobe.com/links/acrobat (English only) on the Adobe website.
Policy
Specifies how to process images when they are below the resolution you define. For Color,
Grayscale, and Monochrome images, enter a resolution, and then select either Ignore, Warn
And Continue or Cancel Job from the pop-up menu.