Computer Programming Concepts and Visual Basic David I. Schneider

(c) The point (–2, 0) is the left-hand end point of the x axis
and the point (10, 0) is the right-hand end point; therefore,
the line joining them is just the portion of the x axis
we have already drawn.
We draw these graphs on the screen with the same three steps we use with paper, pencil,
and ruler. The only difference is that we first do Step 2 and then Steps 1 and 3. The Visual
Basic method Scale is used to specify the range of values for the axes and the method Line
serves as the ruler.
The statement
picBox.Scale (a, d)-(b, c)
specifies that numbers on the x axis range from a to b and that numbers on the y axis range
from c to d. See Figure 9-2. The ordered pair (a, d) gives the coordinates of the top left corner
of the picture box, and the ordered pair (b, c) gives the coordinates of the bottom right corner
of the picture box.
FIGURE 9-2 Result of Scale Method FIGURE 9-3 Result of the Line Method
■ GRAPHICS METHODS FOR DRAWING LINES, POINTS, AND CIRCLES
After an appropriate coordinate system has been specified by a picBox.Scale statement, graphics
can be drawn in the picture box using the Line and Circle methods. The statement
picBox.Line (x1, y1)-(x2, y2)
draws the line segment from the point with coordinates (x1, y1) to the point with coordinates
(x2, y2) in the picture box (see Figure 9-3). In particular, the statement picBox.Line (a, 0)–(b,
0) draws the x axis and the statement picBox. Line (0, c)–(0, d) draws the y axis.
The following event procedure produces the graph of Example 1, part (b):
Private Sub cmdDraw_Click()
picOutput.Cls picOutput.Scale (-2, 18)-(10, -3) ‘Specify coordinate system
picOutput.Line (-2, 0)-(10, 0) ‘Draw x-axis
picOutput.Line (0, -3)-(0, 18) ‘Draw y-axis
picOutput.Line (1, 15)-(8, 6) ‘Draw the straight line
End Sub
Introduction to Graphics 261