Drawing Figures contd'

1
Drawing Figures (contd.)

• (elementary OpenGL programming)
• www.cis.syr.edu/mohan/gra3.ppt

2
Plotting a function

• To ensure that the plot covers a reasonably large
  part of the screen, it is necessary to scale and
  shift the coordinate system from (x,f(x) to
  (AxB, Cf(x)D), choosing the coefficients so
  that AxB ranges from 0 to screenWidth and
  Cf(x)D ranges from 0 to screenHeight. (Example
  Figure 2.16)

3
Drawing Lines

• Set line color using glColor3f, and line width
  using glLineWidth.
• Enclose a list of vertices between glBegin
  (GL_LINES/LINE_STRIP/LINE_LOOP/POLYGON)glEnd().
• Vertices can be stored in a file and read by the
  OpenGL program (e.g., fig.2.22, with file
  dino.dat), or in an array (fig.2.27, 2.28).

4
Parameterized Figures

• Families of objects can be drawn, distinguished
  by parameter values, e.g., houses parameterized
  by peak position, width and height (Fig. 2.25)

5
moveto() and lineto()

• Manipulate a hypothetical pen at current position
  (cp)
• cp is modified by moveto() and lineto() the
  latter also draws a line from the previous cp to
  the new one.

6
Drawing aligned rectangles

• glRecti(,,,) draws a rectangle whose diagonally
  opposite vertex coordinates are supplied as
  arguments.
• Exercise generate a checkerboard
• Aspect ratio width/height

7
Other Primitives in OpenGL

• GL_TRIANGLES
• GL_TRIANGLE_STRIP
• GL_TRIANGLE_FAN
• GL_QUADS
• GL_QUAD_STRIP

8
Mouse Interaction(1)

• glutMouseFunc(f) registers f with the event that
  occurs with button press/release f must be
  designed to take four arguments
• GLUT_LEFT/MIDDLE/RIGHT_BUTTON
• GLUT_UP/DOWN
• x (int)
• y (int)
• E.g., specifying rectangles/polylines, fig.2.38,
  2.40

9
Mouse Interaction(2)

• glutMotionFunc(f) registers f with the event that
  occurs when the mouse is moved while a button is
  pressed f must be defined to take x and y
  coordinates as parameters
• Example useful in freehand drawing with a fat
  brush, p.66

10
Keyboard Interaction

• glutKeyboardFunc(f) registers f with keyboard key
  press event f must be defined to take three
  parameters key id, mouseX and mouseY
  coordinates.
• E.g., fig.2.41
• The y coordinates must be flipped (subtracted
  from screenHeight) in some of these routines.