Calamus has available a built-in mechanism for image compression,
which allows the storage requirement for raster images to be reduced.
The algorithm used is known as
RLE (run length encoding).
Although this is one of the simplest forms of data compression, with
certain images appreciable space saving can be achieved.
Access to compressed image data – e.g. when outputting the frame – is naturally somewhat slower than that to uncompressed data. In some cases it may therefore be preferrable to have raster images available in an uncompressed format. For this reason the module COMPRESS.CXM was developed and included with the Calamus package.
Using the module:
Please select exactly one image frame. Then call the module and click on the function icon. A small dialog box appears offering the following choices:
With these radio buttons you can determine whether the selected raster graphics are to be compressed or decompressed.
The choice made with these radio buttons tells the module which frames it should process: Either all selected image frames on the current page, or all in the current document (in this case also those that are not selected).
The run length encoding used in Calamus is most sensibly used with bitmap images (monochrome or duochrome). For this reason only these will be compressed by the image import drivers. Actually the COMPRESS module can compress other image types as well; but it may then happen – specially with TrueColor images – that due to unfavourable data organisation as well as the additional management information the compressed data takes up more space than the uncompressed data!