CS349 - Implementing User Interfaces - Spring 2010

Public Service Annoucements

1. Assignment 4.

Lecture 32 - PostScript

PostScript was created as a `page description language', which means that it was designed to have the ability to put onto the output of a printer anything that the printer can print. `Anything that a printer can print' has an easy definition:

• writePixel( colour, x, y ).

What does this mean?

• 600 x 600 x 8.5 x 11 ~ 300 MColours

Once upon a time this was a big deal, but not any more

• at 100 Mbps this is just 3 x (bits/Colour) seconds
• and gigabit ethernet is increasingly common,
  • including inside high end printers.

Actually, it means something much more serious

• knowing all the pecularities of the printer that is printing - how do printers actually work
  • start with a bit-map (pixel-map)
  • write the bit-map to a charged cylinder using a laser and a mirror
  • drag the cylinder through the toner
  • roll the cylinder over the paper
  • heat the paper to fuse the toner
• every printer is different, and needs a different set of pixels written
  • We used to say

    lpr -TLexmark_T520 -Prubble

    when we wanted to print.

  • The point is that the lpr command needs to know the peculiarities of the printer.

In practice this is a serious problem for the printer maker

1. All kinds of `secret' information about your printer has to be made available to the maker of the operating system.
2. The quality of the operating system implementation determines the quality that users obtain from your printer.

Printer makers promoted a different model

• Application writes a program in a `printer command language'
• lpr sends the program to the printer
• The printer executes the program to produce the page, factoring in its own peculiarities.

Very attractive feature of this model is

• The same print engine produces different quality output on different printers, and is sold at different prices.

Obviously, there's a `natural monopoly' among competing `printer command languages'.

• John Warnock: Xerox -> Adobe, followed by the Interpress fiasco.

General principles

• Stack-based language: why "stack"? why "language"?
• Transformation-based geometry: why "geometry"? why "transformation"?

Pen-based drawing model: why "drawing model"? why "pen"?

1. current page (drawing surface)
2. current coordinate system
   • moved around by a collection of transformation operators
   • actually `nothing but' a transformation matrix
     • which is applied to any point handed to the system
   • important difference between moving coordinate systems and moving objects within a coordinate system
     • What is it?
3. current clipping region
   • used to determine which geometry actually gets to the page
4. current path
   • accumulator for geometry
   • put onto the current page by explicit requests
     • stroke
     • fill
5. current position
   • used when adding things to the path
6. current coordinate system
   • used when interpreting numbers as positions
7. current graphics context
   • consulted at the point when stroke or fill is used.
8. current dictionary
   • contains a collection of definitions that are the current context of the computation
   • the top of a stack of dictionaries
   • used to change the current context, just like in other programming languages. What does this mean?

Two distinct coordinate systems

1. User coordinate system
   • what was called above the current coordinate system
   • rooted in the page
   • units are 1/72 of an inch
     • Why?
2. Device coordinate system
   • depends on the display device
   • suitable transformation is performed in the device driver

Examples

1. Draw a square

   newpath
     0 0 moveto
     1 0 lineto
     1 1 lineto
     0 1 lineto
     closepath
     stroke
   showpage

   Comments

   • newpath zeros the current path
   • closepath fills in the last edge, guarantees that the path is closed, fixes up the corners
   • stroke adds the current path to the current page
   • showpage outputs the current page and clears it.
2. Define a filled square

   /square
   { newpath
       0 0 moveto
       1 0 lineto
       1 1 lineto
       0 1 lineto
       closepath
       fill } def
       

   Comments

   • fill paints the pixels enclosed by the path
   • def puts the key square into the dictionary, defined as the (executable) array of characters enclosed in braces.
     • Nothing is executed
3. Draw many squares

   /square
   { newpath
       0 0 moveto
       1 0 lineto
       1 1 lineto
       0 1 lineto
       closepath
       fill } def
   square
   100 100 translate
   square
   100 100 translate
   square
   showpage
       

   Comments

   • square puts its definition on top of the stack
   • translate moves the user corrdinate system to a new position on the page

Giving arguments to a procedure

/square 
{ newpath
     0 0 moveto
     1 0 lineto 
     1 1 lineto
     0 1 lineto
     closepath
     fill } def
 /translatedSquare
 { translate
   square } def
0 0 translatedSquare
100 100 translatedSquare
100 100 translatedSquare
    

Comments:

• 0 is put on the stack
• 0 is put on the stack
• {translate square} is put on the stack and executed as if it is a program
• translate is executed, consuming the two arguments

square is executed

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• Bill Cowan's home page.