05-830, User Interface Software, Spring, 2000
Lecture 11,   March 20, 2000
Copyright © 2000 - Brad Myers

Output Models, Structured Graphics &
Display PostScript

Drawing Operations

• Lots of parameters control the drawing of objects.
• X drawline has at least:
  1. Window in which to draw
  2. X1
  3. Y1
  4. X2
  5. Y2
  6. relative-p
  7. line-width
  8. draw function
  9. background-color
  10. foreground-color
  11. cap style
  12. line style (solid, dashed, double-dashed)
  13. dash pattern
  14. dash offset
  15. stipple bitmap
  16. stipple origin X
  17. stipple origin Y
  18. clip mask
  19. plane mask (for drawing on specific planes)
• Two basic models:
  • Graphics package stores the state:
    • Set Pen color, width, etc.
    • Draw object
    • Used by Macintosh, Display PostScript, etc.
    • Example (pseudo code):

      SetColor(Red)
      MoveTo(70, 30)
      DrawTo(70, 200)
      

    • + Fewer parameters to calls
    • + Don't have to set properties don't care about
    • - Interrupts, multi-processing, may result in unexpected settings
  • Each operation supplies full parameters
    • Example:

      DrawLine(70,30,70,200, Red, ......)
      

    • X uses a "graphics context" to store them all, so not regular parameters to functions
    • Amulet uses Am_Style objects
    • Issue: Am_Red used as fill style vs. line style

Structured Graphics

• Saves a list of all the graphical objects
  • Edit the screen by editing the saved list
• Also called "display list" or "retained object model"
• Provided by many toolkits and graphics packages
  • early vector displays
  • CORE (~1977), GKS (1985), PHIGS (1988)
  • Optional in InterViews, CLIM, etc.
  • Required in Amulet, Garnet, Rendezvous, etc.
• Advantages:
  • Simpler to program with: don't call "draw" and "erase"
  • Automatic refresh of windows when uncovered, etc.
  • Automatic redisplay of objects when change
    • and also of other overlapping objects
      
  • Ability to support:
    • high-level behaviors like move, grow, cut/copy/paste, etc.
    • high-level widgets like selection handles
    • constraints among objects
    • automatic layout
    • grouping: "Groups" in Garnet
    • automatic PostScript printing
    • tutors and monitors
    • external scripting, ...
• Disadvantages:
  • Significant space penalties
    • objects take up to 1000 bytes each
    • imagine a scene with 40,000 dots (200x200 fat bits)
  • Time penalties
    • redisplay doesn't take advantage of special properties of data:
    • regularity
    • non-overlapping

Redisplay Algorithms

• Redisplay everything each time
  • may add clipping region and/or double-buffering
  • most appropriate for small numbers of objects, and if drawing is really quick compared to computation
  • used on the Macintosh and many others
  • used by Amulet
• Redisplay only the affected areas of the screen
  • requires computing what areas are affected:
  • Garnet:
    • 1) keep track of objects that change any "interesting" slot
    • 2) compute the bounding box of all these changed objects in their old and new locations
    • 3) assert this as the clipping region (must not self-intersect; Garnet uses 2 regions)
    • 4) erase the area
    • 5) go through objects from top-to-bottom, back to front draw those which overlap the bounding box
    • for step 4: goes through all top level aggregates, and any children of the aggregates that intersect (recursively)
  • Other techniques: quad trees
• Special additions in Garnet; not in Amulet
  • "Fast-redraws"
    • declared by the programmer
    • objects drawn with XOR on top of other objects
    • those that have a solid color behind them (nothing in front) so can be erased with a rectangle or by redrawing with the background color
    • When change, have to be erased using old values
    • No bounding boxes, no intersections, etc.
  • "Virtual aggregates"
    • only pretend to allocate storage for elements
    • provide table and arbitrary layouts
• "Glyphs" in InterViews
  • Don't include position information, etc. so very small
  • Much of the layout retained by the aggregate (computed as needed)
  • Object can be reused in many places: e.g.: the letter "a"
  • Used for a text editor
  • Issue: why is location special? What about red vs. blue "a"s

Display PostScript

• Used by NeXT, Galaxy
• PostScript (tm) developed by Adobe Systems Inc. to describe documents for printing
• Display PostScript (tm) is PostScript adapted for drawing on the screen
• Machine independent graphics model:
  • colors are always full color
  • coordinates are always in physical units (point, inch)
• Global graphics state containing all the parameters
• Coordinates from lower-left, in points
• Drawing model:
  • Define a "path" = any arbitrary shape or collection of shapes
  • Then "stroke" or "fill" the path with current settings
  • So all the "drawing" operations, actually add a primitive to the current path
• Real PostScript is pure postfix (stack) language:

  20 40 moveto 60 80 lineto 60 80 30 45 dup dup sum arc stroke
  

• Display PostScript and Galaxy have C procedures with normal syntax:

  vdrawMoveTo(20, 40);
  vdrawLineTo(60, 80);
  vdrawArc(60, 80, 30, 45, 2*45);
  vdrawStroke();
  

• Uses 2-D "transformation matrices" to control scaling, rotation, and translation.
  • "Homogeneous coordinate matrices" allow all three to be combined in one matrix.
  • Postscript represents matrix as:

    	a	b	0
    	c	d	0    =  [a b c d tx ty]
    	tx	ty	1
    

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