More Drawing Tools

1
More Drawing Tools

• (Viewports)
• www.cis.syr.edu/mohan/gra4.ppt

2
World vs. Screen

• Scaling and translation transformations required
  to map desired portion of world to desired
  section of screen
• World window which part of the world is to be
  drawn, specified in world coordinates
• Viewport rectangular section of screen window to
  which world window must be mapped

3
Window-to-viewport mapping

• Left, top, right, bottom borders of world window
  must be mapped to corresponding screen
  coordinates
• Proportionality forces linearity
• sx Ax C
• sy By D
• where A(V.r-V.l)/(W.r-W.l), and
• CV.l - AW.l, similarly for B,D

4
Example

• (W.l, W.r, W.t, W.b)(0, 2, 0, 1)
• (V.l, V.r, V.t, V.b) (40, 400, 60, 300)
• A180, C40, B240, D60
• sx 180x 40
• sy 240y 60

5
OpenGL commands

• World window set up gluOrtho2d (l, r, b, t)
  preceded by glMatrixMode(GL_PROJECTION)
  glLoadIdentity()
• Viewport setp glViewport(x, y, width, height)

6
Handy functions

• void setWindow (float l, r, b, t)
• glMatrixMode(GL_PROJECTION)
• glLoadIdentity()
• gluOrtho2D(l, r, b, t)
• void setViewport(int l, r, b, t)
• glViewport(l, b, r-l, t-b)

7
Example depicting sinc(x)

• void myDisplay (void)
• setWindow(-5, 5, -0.3, 1)
• setViewport(0, 640, 0, 480)
• glBegin(GL_LINE_STRIP)
• for (GLfloat x-4.0 xlt4.0 x 0.1)
• glVertex2f(x, sin(3.14 x)/(3.14 x) )
• glEnd() glFlush()

8
Ex.3.2.4b Tiling with dino motif

• setWindow(0, 640, 0, 440)
• for (int I0 Ilt5 I)
• for (int j0 jlt5 j)
• if ((Ij)2 0)
• setWindow(0, 640, 0, 440)
• else setWindow(0,640, 440,0)
• glViewport(I64, j44, 64, 44)
• drawPolylineFile(dino.dat)

9
Ex.3.2.6 zooming, animation, double buffering

• In main(), glutInitDisplayMode(GLUT_DOUBLEGLUT_RG
  B)
• In myDisplay()
• float cx0.3, cy0.2
• float H, W 1.2, aspect 0.7
• Set viewport
• for (int frame0 frame lt NF frame)
• clear screen W 0.7 H W aspect
• setWindow(cx-W, cxW, cy-H, cyH)
• hexSwirl() //draw object
• glutSwapBuffers()

10
Setting window automatically

• Compute extent of window, by finding the min. and
  max. x and y coordinates among all points.
• If the object is procedurally defined, first
  execute the procedure without drawing, just to
  compute the extent then re-execute it to perform
  drawing.

11
Setting Viewport Automatically

• Goal to draw largest possible undistorted
  version of figure that fits in screen window.
• If world window aspect ratio R gtW/H,
• setViewport(0, W, 0, W/R)
• else setViewport(0, HR, 0, H)

12
Resizing Screen Window

• glutReshapeFunc(myReshape) in main()
• void myReshape( GLsizei W, GLsizei H)
• if (RgtW/H)
• setViewport(0, W, 0, W/R)
• else setViewport(0, HR, 0, H)