MSRI Graphics School - Lecture 2 on PostScript

More basic commands

Colours This shows you colors whose components are indexed by integers 0, 1, ... , 5. In Java or PostScript these values correspond to floating point numbers 0.0, 0.2, ..., 1.0.

Documents with several pages

The command showpage has an uncertain effect on the graphics state, and consequently it is best to start off in the default state on each page. This can be done by defining procedures something like this: 

/page-begin {
  gsave
  72 dup scale
  1 72 div setlinewidth
} def

/page-end {
  grestore
  showpage
} def

and then making each page page-begin ... page-end.

Manipulating the stack

  • pop Removes the top item from the stack
  • exch Exchanges the two top items on the stack
  • 4 1 roll Rolls up (and around) the top 4 items on the stack by one: a b c d gets changed to d a b c
  • 2 index Puts a copy of item 2 down in the stack on top (counting from 0): a b c d becomes a b c d b
Text

There are many different ways to put text in figures, some quite sophisticated. Here I'll explain the simplest way, and come back later to others.

  • /Helvetica-Bold findfont
    10 scalefont
    setfont

    sets the font in your current graphics state to be Helvetica-Bold. You have a limited choice in general. The next line scales its height to be 10 of the current units. Then finally the font at that scale is chosen to be the current font. You must choose some font in order to get text displayed.

  • 0 0 moveto
    (This is a string) show

    displays the string This is a string at the origin. Strings in PostScript are enclosed between parentheses. To get a parenthesis in the string, use \( or .

  • 0 0 moveto
    x ( ) cvs show

    places the current value of x into a string (of at most 6 characters long) and then displays it at the origin. The command cvs is convert to string.

Displaying a string moves the current point to the end of the string. So (A ) show (string) show displays A string.

Loops

  • 10 { ... } repeat
  • 0 1 10 { /i exch def ... } for
  • { ... n 10 ge { exit } if ... } loop
Arrays
  • /A [ 0 1 ] def Defines A to be the array [ 0 1 ] of two numbers.
  • A 0 get Puts A[0] on the stack
  • A 0 3 put Puts 3 in the 0th spot of A
  • A aload pop Places all the contents of A on the stack
Arrays are not just arrays of numbers, but arrays of anything, maybe all different. They are the only way to create complicated data structures in PostScript. Arrays can also be used as points in 2D or 3D, and for example can be used as packaged arguments or return values for procedures. A procedure that leaves something on the stack is effectively a function.

Conditionals

There are tests ge, le, lt, gt, eq, ne for testing inequalities. They return boolean values true/false.

  • n 10 ge { ... } if
  • n 10 ge { ... }{ ... } ifelse

Sample code