Object-Oriented Programming with Java Lecture Notes

1 April 2008 • Graphic Programming

Outline

• Display hardware.

• Computer graphics.

• Computer graphics in Java.

Computer Graphics

• Computer Graphics is the visual display of non-textual information.

  • It may include text, but not optimized for it.

• Not computer graphics: terminals, printers (but they can be, depending).

Display Hardware

• The original computer-graphics hardware were blinkenlights and oscilloscopes.

• Oscilloscopes eventually morphed (early 60s) into vector displays.

Vector Display Hardware

• A vector display is a CRT that knows how to draw lines and (perhaps) simple curves (conics, say).

  • Collectively known as strokes.

  • “Knows how” means “has the electronics”.

• Data to be displayed are described in terms of the drawing strokes.

  • A compact and efficient data representation.

Driving Vector Displays

• An image descritption (a sequence of stroke descriptions) is written to a storage block called the display list.

• The vector display driver interprets the display list and drives the display electronics.

• The display electronics leaves traces on the screen, showing the data.

Vector Graphics

• There are two lasting ideas from vector displays: display lists and high-level graphical descriptions.

• Display lists have morphed into hierarchial, tree-like scene descriptors.

• High-level graphical descriptions have morphed into vector graphics.

  • Currently popular vector-graphics systems include Flash and SVG (scalable vector graphics).

Video Killed The Vector Star

• Televisions were a potential competitor to vector displays.

• Vector displays have a constant-intensity beam moving in varying patterns.

• Television displays have a varying intensity beam moving in constant patterns.

  • First tv advantage: images of varying intensity.

• The beam pattern is a raster: side-to-side scans moving down the display.

  • Hence “raster” in raster display and raster graphics.

Driving Raster Display

• Raster displays are driven from an image buffer, but not a display list.

• Raster image buffers are integer matrices (video memory).

  • Each element in the matrix describes a pixel (picture element) in the displayed image.

  • The description includes color and intensity.

• Video memory is read in sync with the raster scan, the values read determine the beam intensity.

Bit-Mapped Displays

• The main problem with raster displays is the cost of video memory.

• Technology advanced enough by the early 70s to make bit-mapped displays possible.

  • The Alto developed at Xerox PARC had a bi-level display.

Graphics State

• The graphics state defines context under which drawing and display takes place.

  • More generally known as the

    {graphics, imaging, drawing} × {state, context, environment}

• Graphics state contents include such information as

  • Drawing surface characteristics (height, width, depth, position).

  • Current drawing characteristics (color, position, transformation).

Graphics

• The abstract class Graphics is the Java implementation of a graphics context.

  • It contains abstract methods to draw arcs, images, lines, ovals, polygons, rectangles, strings, and others.

  • It contains abstract methods to manipulate clipping, font, color, drawing modes, and other graphics state.

Concrete Graphics Classes

• Graphics2D and DebugGraphics are concrete implementations of the Graphics abstract class.

• The Graphics2D class describes a 2D-drawing world.

• It extends Graphics to provide methods for

  • 2D transformations (scaling, rotation, shear).

  • Rendering hints (pixel rounding, antialiasing).

  • Device specific routines.

2D Graphics

• The Java 2D drawing surface is an infinite Cartesian integer grid.

• Each grid point is a pixel, the characteristics of which are determined by the underlying device.

• Drawing-surface coordinates are in screen orientation, a x-axis mirror image of the standard coordinate-system orientation.

  • Standard orientation: positive x to the left, positive y up.

  • Screen orientation: positive x to the left, positive y down.