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PGPLOT: CGM file format

CGM (Computer Graphics Metafile) is a standard, device-independent graphics file format (ANSI/ISO 8632, 1991). It is a vector format, so files can be scaled without loss of resolution, and some editing is possible.

Two PGPLOT device types are supported by this driver: CGM and CGMD. The two differ in the way colors are represented. CGM uses the indexed method, representing colors using index values and a color table. CGMD on the other hand, uses direct color representation, whereby each color reference is stored as a combination of RGB values. Generally both CGM and CGMD should produce identical output, with CGM producing slightly smaller files. However not all software supports the indexed color selection method. Therefore if color problems occur when displaying CGM output CGMD should be used instead.

Computer Graphics Metafiles offer many advantages over other file formats currently supported by PGPLOT:

Device type code
/CGM (indexed color mode).
/CGMD (direct color mode).

Default file name
pgplot.cgm
The standard extension for CGM files is .cgm.

Default view surface dimensions
Width 7.8 inch by height 10.5 inch. These defaults can be overridden by calling PGPAP.

Resolution
PGPLOT uses an addressing resolution of 1000 units per inch. The actual resolution is set by the display device. Metric scaling is included. If this is supported by the application then imported pictures appear exactly as created, i.e. a 2-inch plot will be exactly 2inch on screen. (Microsoft Office programs do not recognize this information.)

Color capability
Color indices 0-255 are accepted, with standard defaults for color indices 0-15 (white background, color index 1 = black). For /CGM, if the color representation of a color index is changed, it affects all elements drawn in that color on the current page. For /CGMD, changes in color representation affect only elements drawn after the change.

Input capability
None.

File format
PGPLOT uses the binary encoding for CGM files, and conforms to version 1 of the CGM specification.

Known Problems
Plots look chunky and of a low resolution when imported into an application. This is because the application uses abstract scaling. To overcome this problem make the plot larger and it will then appear at a higher resolution, but at the same physical size.
Colors look wrong or the picture simply is not visible. This is because you have used the /CGM device to create the file, but the application does not understand indexed color selection. To overcome this use /CGMD instead.
A file takes a long time to load or causes the application to crash. This is caused by files which have become extremely large, usually when cell-arrays are present. The only solution to this problem is to consider using a different file format, e.g. a bitmap format such as GIF.
A picture cannot be edited by an application. This is due to the fact that large files, containing many separate drawing elements, are too large for applications like Word or PowerPoint to convert completely. They can still be re-sized and included normally, but cannot be edited.

Author
Robin Sergeant, Rutherford Appleton Laboratory (Oxfordshire, UK), (rsergeant@clara.net).

Tim Pearson, California Institute of Technology, tjp·astro.caltech.edu
Copyright © 1999 California Institute of Technology