Using SAC

SAC Graphics File


Each SAC Graphics File (SGF) contains all the information needed to describe a single picture, called a frame. Prior to the mid90s when the C-based SAC2000 replaced the Fortran-based SAC, several utilities programs, external to SAC, were written to perform various function on an SGF file: a program to merge up to several SGF files, a program that directly displayed the SGF plots on the console, a program that listed the commands encoded in an SGF file, ond sgftops, which converted an SGF file to a postscript file. Most of these programs were never converted to C, and currently, sgftops is the only program that is maintained. A description of program sgftops is given at the end of this section.

SGF Format


Each SGF contains all the information needed to describe a single picture (called a frame.) The filenames are normally of the form "f nnn.sgf" where nnn is the three digit frame number. A translation program must be written to convert these files to the format needed for any specific graphics device.

Physical Format

A SGF contains variable length records with a maximum record size of 2500 32-bit words. The first 32-bit word of each record contains the length of that record, including this word count. They are written in binary format for faster i/o. To keep them small and portable between different computer systems, all commands and data are stored in 16-bit integer format, 2 bytes.


The draw command (draw a line from the previous location to the new location) is the most common command. This command is simply a pair of integers giving the new x and y locations. These integers are in the range 0 to 32000 in the x direction and 0 to 24000 in the y direction. (This produces an aspect ratio of 3:4 which maps well to most output devices.)

Other Commands

The rest of the commands (with one exception) consist of a command identification number, a data count, and zero or more data words. The identification number is a negative integer and tells the translation program what operation is to be performed. The use of negative integers makes it easy to distinguish these commands from the draw commands. The data count is the number of 16-bit data words, 2 bytes, contained in this command. This format allows for the future addition of new commands. Also it allows each translation program to quickly skip over commands that it cannot process. The one exception to this format is the null or no-op command. This has an identification number of -1 and contains no data count and no data words. It is used to fill out a record to an even number of 32-bit words. The table on the next page summarizes the current commands. A plot produced from a simple SGF is also included, along with a table describing the contents of that simple SGF.

SGF Commands Table

ID Count Command Description
-1 0 No-op No Operation
-2 0 End End of picture.
-3 2 Move Move to the location contained in the two (x,y) data words.
-4 1 Color Change color to value contained in data word.
Text Write hardware text at current location. Data count contains number of 16-bit words of text plus one. First data word is the number of characters in the text. Rest of data words contain the text, two characters per word. Last byte of last word is not significant if character count is odd.
-6 2 Text Size Change hardware text size. Data words contain the text width and height as integer fractions of the maximum coordinate system size (32000). For example a value of 320 would set text size to 0.01 or one percent of the full plot size.
-7 1 Line Style Change linestyle to value contained in data word.
-8 1 Plot Size Change the physical size of the plot. Data word is the desired length in the x direction in 0.001 inch increments. Default value is 10000 which is equivalent to 10.0 inches. None of the SGF conversion programs currently make use of this option.
-9 1 Line Width Change the linewidth to value contained in data word.
-10 3 Polygon Fill Fill a polygon with a gray value in the first data word by moving to data words 2 and 3.
-11 5 Plot Filled Rectangle Fill a rectangle defined at data word 1 and 2 (x, y) with a width and height data words 3 and 4 and a color at data word 5.
-12 1 Text Angle Set the Angle of the Text
-13 4 Color Image Plot a color image of width data word 1 and height data word 2 at data word 3 and 4 (x, y).
-14 1 color Fill flag Fill polygon with current color. Can be used to fill positive and negative parts of waveform with different colors


Plots from SAC can be saved to a file as a Sac Graphics Format (SGF) file. Program sgfops converts a binary .sgf file to a postscript file. As of version 101.4, sgftops can handle .sgf files with eithr endian.

The source code can be found in sac/utils/sgftops.c, and it is built and put in sac/bin at the time SAC is built and installed.

Entering sgftops with no arguments produces:

Usage: sgftops sgf_file ps_file [line_width scale_id]


line_width = 1, 1.5, 2, 3, etc.

scale_id = i (landscape mode plus id); scale_id = s (shift,rotate & scale); scale_id = si (s plus id).

time/date in id is file creation date for the .sgf file.

Example: sgftops foo.sgf 2 si

Produces a plot with line thick=2 and ID at the bottom. Prompts further for translation, rotation and scale.

The origin of plot is lower left corner of portrait mode and angle is Counter Clockwise (CCW)

The reason that sgftops default is landscape is that when it was first written (20+ years ago), the major use of a postscript file was to be ported to a postscript printr.


There are two scripts in sac/bin that call sgftops and produce a screen display (sgftox.csh) or an EPS file (sgftoeps.csh). Enter the script names with no arguments to get further information. Both, as written, require that the program "gs" is in your path. Script sgftox.csh uses gs to display the image on the screen, but it is easily modified to use other available postscript file viewers, such as gv, ggv, or evince. Script sgftoeps.csh can be modified to produce a PDF file if script epstopdf is on the system and in the path. Note that the output file for both sgftoeps.csh and sgftox.csh is in portrait format.