System information

To downsample an image, select Resample above and specify the threshold resolution in
dpi. Only images with a resolution higher than or equal to this threshold resolution will be
downsampled.
•
• To resample an image up, click to clear the Resample above check box.
5. Select a resampling (interpolation) method and specify the resolution (dpi) to which the image
should be resampled.
6. Click Apply.
To resample multiple images
See Global Changes on page 167
7.23 Compressing pixel images
About compression
You can compress pixel images in your PDF document to reduce the size of your PDF file. If you
do so, you select a compression method (see Compression methods on page 143) and, if necessary,
an ASCII filter (see ASCII filters on page 143).
Compression methods
You can select one of the following compression methods:
• JPEG compression on page 143
• ZIP compression on page 143
JPEG compression
The JPEG compression method is best suited for grayscale or color images. A typical example of
an image for which JPEG compression is advised is a continuous-tone photograph. This type of
image contains more information (more detail) than can be reproduced on screen or in print.
Therefore, it makes sense to remove the information that cannot be displayed anyway. This can
result in a loss of image quality, as the compression method removes information: it is called a
lossy reduction method. However, file sizes are greatly reduced by JPEG compression.
ZIP compression
ZIP compression will decrease the image size without losing information (lossless compression).
ASCII filters
When compressing images, you can select an ASCII filter to encode the image data in your PDF
document as plain (ASCII) text. You may wish to use ASCII encoding if you need to transmit your
PDF documents through channels where only ASCII characters are allowed. This may be required,
for example, in some e-mail systems although most e-mail software in use today can properly
handle non-ASCII documents. If ASCII encoding of the images in your PDF documents is necessary,
you can select either one of the following ASCII filters:
• ASCII Hex, which may double the image data size
143
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