Computer Graphics for Java Programmers, Second Edition

Chapter 1. Elementary Concepts

This book is primarily about graphics programming and mathematics. Rather than discussing general graphics subjects for end users or how to use graphics software, we will deal with more fundamental subjects, required for graphics programming. In this chapter, we will first understand and appreciate the nature of discreteness of displayed graphics on computer screens. We will then see that x- and y- coordinates need not necessarily be pixel numbers, also known as device coordinates. In many applications logical coordinates are more convenient, provided we can convert them to device coordinates. Especially with input from a mouse, we also need the inverse conversion, as we will see at the end of this chapter.

LINES, COORDINATES AND PIXELS

The most convenient way of specifying a line segment on a computer screen is by providing the coordinates of its two endpoints. In mathematics, coordinates are real numbers, but primitive line-drawing routines may require these to be integers. This is the case, for example, in the Java language, which we will use in this book. The Java Abstract Windows Toolkit (AWT) provides the class Graphics containing the method drawLine, which we use as follows to draw the line segment connecting A and B:

g.drawLine(xA, yA, xB, yB);

The graphics context g in front of the method is normally supplied as a parameter of the paint method we are using, and the four arguments of drawLine are integers, ranging from zero to some maximum value. ...