The primary PIRL enhancement provides a module (xvPVL.c) that implements the LoadPVL function. This module is designed to load any PVL (Parameter Value Language) formatted image file, but does not provide a save/write function. PVL is used by the Planetary Data System as a "standard" for the distribution of products (primarily image files, but there are other types) associated with spacecraft science missions. Though there is a formal specification for PVL to be used with PDS products, it has not been implemented consistently in the software that produces the products for each mission. This has resulted in the proliferation of software programs that are only able to handle the PDS products associated with their mission. The xv program is very popular in the Planetary and Space Sciences communities (e.g. there is a FITS module for astronomical files) and so a module - xvPDS - was developed to load PDS product imagery into xv. Over time, as modifications were added to deal with the peculiarities of each PVL implementation, this module came to be a cacophony of conditionals and cruft.

In the summer of 1998 I accepted the task of adding support for yet another PVL implementation to the xvPDS module. I found that the xvPDS module was employing an ad hoc file label parsing mechanism and had hard-wired parameter names infused throughout the code. My efforts to find a PVL library that could replace this with an easier to manage API were unsuccessful: no PVL library that I found was able to handle all of the PVL flavors.

So I decided to implement a PVL library that would be resilient enough to handle any PVL implementation with minimal (and non-modal) knowledge of the peculiarities of any particular flavor, be completely independent of the contents of the PVL, take its input from an external file or an internal buffer, output formal PVL according to the language specification document, provide a simple yet complete and extensible API, and offer tunable environment controls to the application. The result was the PIRL Parameter Value Logic (PPVL) library. PPVL has been proven successful at reading all PVL files it encounters. This includes the Pascal-like binary sized (a.k.a. variable length) records of the Voyager products, VICAR labels that lack a (required) END statement, etc. It will even provide a PVL interpretation of this file (up to the word "END" in the previous sentence :-). The design philosophy of the PPVL module is to provide the application with whatever can be provided by a tolerant (or strict, if desired) PVL interpretation of the input in a form, and with the tools, to make it easy to determine what is useful to the application.

The PPVL library, included in the xv-PIRL distribution, is used by the xvPVL module. Because the burden of dealing with the peculiarities of various PVL implementations was removed, the xvPVL module was able to grow much more sophisticated in its handling of the PVL file contents. A table lists the parameter names that contain data values that are converted into internal binary variables used by the program. Multiple parameter names, from different products, may be associated with a single variable (it's also possible to have the same parameter name associate with more than one variable). When a new image file is encountered with unexpected parameter names these names can be simply entered into the table in association with the appropriate program variable. Usually that is all that is required to teach xvPVL about a new product.

The xvPVL module is able to handle a very wide variety of image data structures. It can handle certain specialized compressed data (currently vdcomp, clemdcmp and readmoc); signed and unsigned integer data up to long precision, and floating point data up to double precision (but only IEEE format, not VAX [until there is a demand]); byte swapped data; selective data scaling of multi-byte data (xv only uses 8-bit data for display); and band selection for multi-band data. Various descriptive data is provided in the "image comments" window. However, this may not have the info that you are looking for. The PPVL_report utility can provide anything and everything that is contained in a PVL label. Documentation for this utility, along with documentation on the entire PPVL distribution, can be found in the PPVL/doc subdirectory.

Because the PVL in an image file primarily specifies scientific meta-data associated with the image data, this format is used in research application contexts that provide all of the parameter names and their values. Therefore there has been no interest in providing the capability to write image files using this format in the context of a generalized image display program.

As of the PIRL v2.1 enhancement, USGS Digital Elevation Model (DEM) input capability has been added. The USGS_DEM.{c.h} files are standalone files for a USGS DEM file input API. These are used in the implementation of the xvDEM module.

As of the PIRL v2.3.4 enhancement the Makefile/config.h structure has been revised somewhat (what is really needed is proper autoconf support). The config.h file no longer needs to be edited as all of its options can be set in the Makefile. The config.h file was primarily used to specify the command lines for the gunzip or uncompress utilities for decompressing files to be displayed and the ghostscript (gs) PostScript interpreter to render into a generic raster image format. To facilitate changes to the xv operating environment (e.g. when using a binary distribution of xv-PIRL) the main xv code has been slightly modified to allow these utility command lines to be specified at run time in several ways (see the config.h file for details). If the utility commands are not specified their corresponding capabilities will simply not be available; a capability can be turned off at run time, even if a command line had been set when xv was compiled, by specifying the empty string ("") for the corresponding utility command. The Makefiles now have sections that can be used to build xv with system supplied JPEG, TIFF, PPVL, PNG and zlib libraries rather than using the distributions for these libraries that are bundled with xv (the latter are static libraries, while the former may be shared dynamic versions). The xvpng add-on module has also been incorporated for PNG image file support.

N.B.: The xv-PIRL distribution includes several operating system targeted Makefile files using gmake syntax that have been used at PIRL to build xv for use on the corresponding operating systems. However, there is certainly no certainty that any Makefile will work on any system. While some effort has been made to ease the configuration specification process in the Makefiles, they do not provide the ease of an autoconf setup (it would be great if someone would do this). The original Makefile.std and Imakefile files are available along with John Bradley's INSTALL description text. All of these are very dated (as is indicated by the last entry in John's CHANGELOG dated 12/29/94) and unsupported at PIRL.

Caveat: The xv software must be registered for all but personal use (see the README by John Bradley in the distribution). All the software in this distribution is provided to you as-is with no warranty of fitness whatsoever. If it works for you, great! If not, you can complain to the author who may try to help. To encourage the author's help it is strongly advised that a detailed problem description be included with your complaint and that you be willing to work with the author in coming to some resolution.

Source: The latest xv-PIRL distribution is available from the PIRL software distribution site.

Suggestions for improvements, actual bug fixes, and other contributions are welcome and much appreciated.

Enjoy!

Bradford Castalia
Senior Systems Analyst	Castalia@Arizona.edu
Planetary Image Research Laboratory	520-621-2197
Department of Planetary Sciences	1629 E. University Blvd.
University of Arizona	Tucson, Arizona 85721-0092

"Build an image in your mind, fit yourself into it."