4.5.6 Plots of compressed 3d volume data with AVS

Files containing 3d volume data usually need large amounts of disc space. For parallel runs, the disc space requirement can be clearly reduced if the data are output in compressed format (note: currently a data compression is permitted only for parallel runs on Cray-T3E). Data compression can switched on by setting the initialization parameter do3d_compress = .TRUE. and by assigning the desired accuracy (number of significant digits) via do3d_precision. A separate accuracy must be assigned for each individual quantity. Using uncompressed data, each grid point value is represented by a 32-bit real number, in case of compression the grid point values are represented by integers whose length depends on the desired accuracy (in general clearly smaller than 32 bits). The individual grid point values are packed into 32-bit integers and output. To plot these compressed data the following output command must be inserted into the mrun configuration file (in addition to the steps described in chapter 4.5.5):

If a file named PLOT3D_COMPRESSED exists in the local working directory, the files produced by the individual processors are packed into one file (PLOT3D_DATA) using the "tar" command. The flag file PLOT3D_COMPRESSED is created by PALM whenever data in compressed form is to be output. For uncompressed output, the individual files have to be joined via the program combine_plot_fields.x . In this case, an additional output command is also necessary (see chapter 4.5.5). For compressed output, this output command may remain unchanged in the configuration file, since combine_plot_fields.x recognizes the existence of PLOT3D_COMPRESSED and carries out no actions in such cases.

The file connection statements for the local file PLOT3D_DATA and all further necessary statements to create plots of 3d volume data can be found in chapter 4.5.5.

After been transferred to the local computer of the user, the tar file with the compressed data cannot be read with the usual AVS module “read field”. Alternatively, the module “read_compressed_field” particularly developed for this purpose must be used. This module is located in a module library under:


This module must be loaded within AVS (network editor - > module tools - > read module library) and then it can be used like all other modules. Further information about using this module is available (e.g. for accelerated treatment of compressed data with AVS the module write_compressed_field is available).

 Last change:  20/04/05 (SR)