INQUA Sub-Commission on Data-Handling Methods

Newsletter 16: July 1997

Graphics conversion

K.D. Bennett
Department of Plant Sciences
University of Cambridge
Downing St
Cambridge CB2 3EA, UK
k.d.bennett@queens-belfast.ac.uk

There is a plethora of different formats for graphics files, and it is frequently useful, even essential, to be able to convert one form to another, preferably without loss of quality or resolution. Often, conversion is needed between formats on different computing systems. The aim of this note is to describe two packages that allow a wide range of useful conversions, including their availability and use.

The pbmplus package

pbmplus is a toolkit for interconverting various image formats via a single internal format. The idea is that to convert among N image formats, only 2*N conversion filters are needed (into and out of the internal format), instead of the N2 that would be needed if a conversion filter was written separately for each possible pairwise combination of formats. It was developed by Jef Poskanzer for Unix systems, but has been ported to MS-DOS.

The system handles a very wide range of formats, including GIF, MacIntosh PICT, and TIFF. It does not handle PostScript, for which see below. pbmplus has four internal formats for distinct types of graphics files:

  1. PBM (Portable Bitmap) for the conversion of black-and-white images
  2. PGM (Portable Graymap) for the conversion of gray-scale images
  3. PPM (Portable Pixmap) for the conversion of full-colour images
  4. PNM (Portable anymap) for the manipulation of images of any type.
On Unix systems, the system consists of a large number of conversion programs each of which takes a filename as input, and sends the converted output to `standard output'. This normally means that it would be sent to the screen as raw data, by default. In practice, the output is redirected to file, or `piped' on to another utility. For example: This is a rather handy way to convert images scanned on a Macintosh system to the GIF images beloved by WWW pages. Similarly, I used the programs bmptoppm and ppmtogif to convert Windows BMP files that Adam Walanus sent me for his article in this issue (Walanus & Nalepka 1997) into the GIF images that you will find there.

As well as the conversion filters, the package also includes programs for graphics file manipulation, including rotation and colour reduction, for example. I frequently need to convert images from Kodak Photo CD format to GIF format for WWW use, and I do this with the command sequence:
hpcdtoppm -1 IMAGE.PCD IMAGE.ppm
ppmquant -fs 256 IMAGE.ppm | ppmtogif >IMAGE.gif
 (hpcdtoppm, to convert Kodak Photo CD to ppm, is distributed with pbmplus, but is written by Hadmut Danisch, and works slightly differently.) ppmquant reduces the number of colours in the file to (in this case) 256 for the GIF format, as Photo CD images may have many thousands of colours.

So-called `transparent' GIF files for use in WWW pages (with no background colour) can be obtained by using the option `-tranparent white' (or whatever the background colour is) with ppmtogif.

pbmplus is so close to being a standard utility that it is normally installed by administrators on Unix systems. If not, it can be obtained by anonymous ftp to many sites (use archie to find your nearest). A version for MS-DOS is available, also by anonymous ftp, and included on a CD-ROM with the book on graphics files by Murray & vanRyper (1994). On a Unix system, you can find out what conversion filters are available and how to use them from the man pages (start with man pbmplus).

The Ghostscript / Ghostview package

Aladdin Ghostscript is a program that `interprets' PostScript for non-PostScript output devices. In practice, this means that it can display PostScript files on screens, and it can convert it into other formats, for non-PostScript printers, and other graphics formats (notably ppm, discussed above). It can be used directly, on the command line in MS-DOS and Unix, and this is a handy way to carry out most file conversions, but it is also invoked indirectly by a PostScript viewer called ghostview (on Unix) or GSview (on MS-Windows and Macintosh). This viewer also allows some conversions, especially in the MS-Windows version, but exists primarily to enable viewing of PostScript files before printing them out. The Macintosh version is more recent than the others, and still lacks some of their features. Notably, it cannot rotate diagrams, but this can always be solved by rotating the terminal instead.

Ghostscript, used directly, is complex, and not very user friendly, but it does work. To give a flavour of it, I use the command sequence below to convert PostScript files to GIF files for WWW pages, combining use of Ghostscript with pbmplus:
gs -sDEVICE=ppm -sOutputFile=diagram.ppm diagram.ps
ppmtogif diagram.ppm >diagram.gif
 (The Ghostscript program is called `gs' on both Unix and MS-DOS).

Aladdin Ghostscript and the viewer, like pbmplus, is normally already installed on Unix systems. All versions can be obtained from by WWW to Aladdin Enterprises. Installation is reasonably straightforward, but the files are very large, and there are many of them (ghostscript, the viewer, fonts, at least). It would be difficult and frustrating to attempt this without direct internet access.

Murray, J.D. & vanRyper, W. 1994. Graphics File Formats. O'Reilly, Sebastopol, CA.

Walanus, A. & Nalepka, D. 1997. Palynological diagram drawing in Polish POLPAL for Windows. INQUA Working Group on Data-handling Methods, Newsletter 16.

Copyright © 1997 K.D. Bennett
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