PGPLOT: PPM file format
The PPM (Portable Pixel Map) raster format was defined by Jef
Poskanzer, whose Portable Bitmap Utilities (available on the Internet)
can be used to translate it into a variety of other formats. The
program xv
(by John Bradley) can
be used to display PPM files on UNIX and VMS workstations. PGPLOT uses
the P6 variant, with 24 bits (8 each for R, G, B) for each pixel. The
format is not efficient in use of storage, so for most purposes the
GIF format is preferred. However, the PPM format allows more than 256
colors.
- Device type code
-
/PPM
(landscape orientation),
/VPPM
(portrait orientation).
- Default file name
-
pgplot.ppm
A PPM file can contain only a single image, so for multi-page plots
PGPLOT creates additional files. If the supplied file name contains
the character #, the file name is derived by replacing this
character with the current page number. If the supplied file name does not
contain a #, PGPLOT appends an underscore and the page number for the
second and subsequent pages, e.g.,
PGOPEN file name: File names used:
pgplot#.ppm pgplot1.ppm, pgplot2.ppm, pgplot3.ppm, ...
pgplot.ppm pgplot.ppm, pgplot.ppm_2, pgplot.ppm_3, ...
If the supplied file name is ``-'', the output is sent to the standard
output stream, so that it can, for example, be piped into a viewing
program. This will only work for single-page plots.
- Default view surface dimensions
-
/PPM: 850 by 680 pixels (nominally 10.0 by 8.0 inches)
/VPPM: 680
by 850 pixels (nominally 8.0 by 10.0 inches)
These defaults can be
overridden by specifying environment variables, or by calling routine
PGPAP. The maximum size is limited
only by available memory.
- Resolution
-
PGPLOT assumes that the device resolution is 85 pixels/inch, but
the actual resolution will vary depending on the display device.
- Color capability
-
Color indices 0--255 are accepted, with standard defaults for color
indices 0--15. If the color representation of a color index is changed,
it affects only pixels drawn subsequently, so the number of different
colors that can be used in an image is not limited to 256.
- Input capability
-
None.
- Environment variables
-
PGPLOT_PPM_WIDTH
: width of image in pixels (default 850)
PGPLOT_PPM_HEIGHT
: height of image in pixels (default 680)
- File format
-
PPM files are binary files.