Chapter 03: Graphics Primitives

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Chapter 03 Graphics Primitives

• Course web page
• http//www.gomezconsultants.com

Chapter 3
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Outline

• OpenGL GLUT basics
• User interaction
• 2-D drawing
• Graphics Primitives

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OpenGL What is It?

• GL (Graphics Library) Library of 2-D, 3-D
  drawing primitives and operations
• API for 3-D hardware acceleration
• GLU (GL Utilities) Miscellaneous functions
  dealing with camera set-up and higher-level shape
  descriptions
• GLUT (GL Utility Toolkit) Window-system
  independent toolkit with numerous utility
  functions, mostly dealing with user interface
• Course web page has links to online function
  references (functions from each library start
  with library prefixi.e., gl, glu, glut)

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Event-driven GLUT program structure

• Configure and open window
• Initialize OpenGL state, program variables
• Register callback functions
• Display (where rendering occurs)
• Resize
• User input keyboard, mouse clicks, motion, etc.
• Enter event processing loop

Portions of some slides adapted from An
Interactive Introduction to OpenGL
Programming, D. Shreiner, E. Angel, V. Shreiner,
SIGGRAPH 2001 course
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OpenGL Architecture
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OpenGL as a Renderer

• Geometric primitives
• points, lines and polygons
• Image Primitives
• images and bitmaps
• separate pipeline for images and geometry
• linked through texture mapping
• Rendering depends on state
• colors, materials, light sources, etc.

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Related APIs

• AGL, GLX, WGL
• glue between OpenGL and windowing systems
• GLU (OpenGL Utility Library)
• part of OpenGL
• NURBS, tessellators, quadric shapes, etc.
• GLUT (OpenGL Utility Toolkit)
• portable windowing API
• not officially part of OpenGL

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OpenGL and Related APIs
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Simple OpenGL program

• include ltstdio.hgt
• include ltGL/glut.hgt
• void main(int argc, char argv)
• glutInit(argc, argv)
• glutInitDisplayMode(GLUT_RGB GLUT_DOUBLE)
• glutInitWindowSize(100, 100)
• glutCreateWindow(hello)
• init() // set OpenGL states, variables
• glutDisplayFunc(display) // register
  callback routines
• glutKeyboardFunc(keyboard)
• glutIdleFunc(idle)
• glutMainLoop() // enter event-driven loop

adapted from E. Angel
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Simple JOGL program

• import java.awt.
• import java.awt.event.
• import net.java.games.jogl.
• public class Simple
• public static void main(String args)
• Frame simpleFrame new Frame("simple")
• GLCanvas drawable GLDrawableFactory.getFactory()
  .
• createGLCanvas(new GLCapabilities())
• drawable.addGLEventListener(new
  simpleRenderer())
• simpleFrame.add(drawable)
• simpleFrame.show()

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Simple JOGL program - continued

• static class simpleRenderer implements
  GLEventListener
• public void displayChanged(GLDrawable drawable,
  )
• public void reshape(GLDrawable drawable, )
• public void display(GLDrawable drawable)
• private GL gl drawable.getGL()
• private GLU glu drawable.getGLU()
• gl.glClear(GL.GL_COLOR_BUFFER_BIT) // clear
  window
• gl.glBegin(GL.GL_POLYGON) // draw
  unit square polygon
• gl.glVertex2f(-0.5f, -0.5f)
• gl.glVertex2f(-0.5f, 0.5f)
• gl.glVertex2f(0.5f, 0.5f)
• gl.glVertex2f(0.5f, -0.5f)
• gl.glEnd()
• gl.glFlush()
  // flush GL buffers
• public void init(GLDrawable drawable)

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JOGL Programming

• public class Simple
• public static void main(String args)
• Frame simpleFrame new Frame("simple") // AWT
  Frame or Window
• GLCapabilities glc new GLCapabilities() //
  Configure OpenGL
• glc.setDoubleBuffered(false)
• GLCanvas drawable GLDrawableFactory.getFactory()
  .
• createGLCanvas(glc) // Create OpenGL
  drawing area
• drawable.addGLEventListener(new
  simpleRenderer())
• simpleFrame.add(drawable) // Insert Drawing
  Routines
• simpleFrame.setLocation(100,100) // Define
  Window properties
• simpleFrame.setSize(500, 500)
• simpleFrame.setVisibility(true) // Show
  window

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Simple OpenGL program

• include ltstdio.hgt
• include ltGL/glut.hgt
• void main(int argc, char argv)
• glutInit(argc, argv)
• glutInitDisplayMode(GLUT_RGB GLUT_DOUBLE)
• glutInitWindowSize(100, 100)
• glutCreateWindow(hello)
• init() // set OpenGL states, variables
• glutDisplayFunc(display) // register
  callback routines
• glutKeyboardFunc(keyboard)
• glutIdleFunc(idle)
• glutMainLoop() // enter event-driven loop

adapted from E. Angel
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Initialization

• glutInit Pass command-line flags on to GLUT
• glutInitDisplayMode OR together bit masks to set
  modes on pixel type (indexed vs. true color),
  buffering, etc.
• glutInitWindowSize, glutCreateWindow Set drawing
  window attributes, then make it
• init() Set OpenGL state, program variables
• Use GL types/typedefs GLfloat, GLint, GL_TRUE,
  GL_FALSE, etc. for cross-platform compatibility
• void init()
• glClearColor(0.0, 0.0, 0.0, 0.0)
• glMatrixMode(GL_PROJECTION)
• glLoadIdentity()
• gluOrtho2D(0, right, 0, top)

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OpenGL screen coordinates

• Bottom left corner is origin
• gluOrtho2D() sets the units of the
  screen coordinate system
• gluOrtho2D(0, w, 0, h) means the coordinates are
  in units of pixels
• gluOrtho2D(0, 1, 0, 1) means the coordinates are
  in units of fractions of window size
  (regardless of actual window size)

from Hill
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Example Specifying the center of a square
gluOrtho2D(0, 640, 0, 480)
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Example Specifying the center of a square
gluOrtho2D(0, 1, 0, 1)
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(0.5, 0.5)
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This is the method used in hello.cpp on the
course web page
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Simple OpenGL program

• include ltstdio.hgt
• include ltGL/glut.hgt
• void main(int argc, char argv)
• glutInit(argc, argv)
• glutInitDisplayMode(GLUT_RGB GLUT_DOUBLE)
• glutInitWindowSize(100, 100)
• glutCreateWindow(hello)
• init() // set OpenGL states, variables
• glutDisplayFunc(display) // register
  callback routines
• glutKeyboardFunc(keyboard)
• glutIdleFunc(idle)
• glutMainLoop() // enter event-driven loop

adapted from E. Angel
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Callback registration functions

• Keypress in window glutKeyboardFunc()
• Button press/release glutMouseFunc()
• Mouse movement with button down glutMotionFunc()
• Window reshaping glutResizeFunc()
• Nothing happening glutIdleFunc()
• Passive motion, other input devices, etc. See
  GLUT reference pages linked on course page
• Render glutDisplayFunc()

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Example Keyboard callback

• What key has been pressed and where the cursor
  is
• glutKeyboardFunc(keyboard)
• void keyboard(char key, int x, int y)
• switch(key)
• case q case Q
• exit(1)
• break
• case r case R
• rotate_mode GL_TRUE
• break

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Example Mouse button callback

• Which mouse button, where, and has it been
  pressed or released
• glutMouseFunc(mouse)
• void mouse(int button, int state, int x, int y)
• if (button GLUT_LEFT_BUTTON)
• if (state GLUT_DOWN)
• left_down TRUE
• mouse_x x
• mouse_y y
• else if (state GLUT_UP)
• left_down FALSE

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Example Idle callback

• Use for animation and continuous update
• glutIdleFunc(idle)
• void idle(void)
• // change position variables, etc.
• t dt
• // call glutDisplayFunc() callback ASAP
• glutPostRedisplay()

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Rendering Steps

• In function registered with glutDisplayFunc()
• Clear window
• glClear(GL_COLOR_BUFFER_BIT)
• Draw shapes
• Set colors, patterns, point/line sizes
• Specify type of geometric primitive(s) and list
  vertices
• Swap buffers if display mode is GLUT_DOUBLE
• Optionally force all operations to complete with
  glFlush()

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Single- vs. double-buffering

• Single-buffering Draw directly to screen buffer
• Double-buffering Draw to offscreen buffer, then
  make that the screen buffer when done
• For animation, double-buffering is better because
  it eliminates flickering

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OpenGL Geometric Primitives
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Specifying Geometric Primitives

• Primitives are specified using
• glBegin(primType)
• ...
• glEnd()
• primType determines how vertices are combined

GLfloat red, green, blue GLfloat x,
y glBegin(primType) for (i 0 i lt nVerts
i) glColor3f(red, green, blue)
glVertex2f(x, y) ... // change coord.
values glEnd()
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OpenGL Command Formats
glVertex3fv( v )
glColor3fv( v )
Number of components
Data Type
Vector
b - byte ub - unsigned byte s - short us -
unsigned short i - int ui - unsigned int f -
float d - double
omit v for scalar form e.g., glVertex2f(x,
y) glColor3f(r, g, b)
2 - (x,y) 3 - (x,y,z), (r,g,b) 4 -
(x,y,z,w), (r,g,b,a)
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Drawing Miscellaneous

• glColor() Range is 0, 1 for each color channel
• glRect(x1, y1, x2, y2) specifying opposite
  corners of rectangle is equivalent to GL_POLYGON
  with four vertices listed (i.e., filled)
• Can set persistent attributes outside of
  glBegin()/ glEnd()
• glPointSize(GLfloat size)
• glLineWidth(GLfloat width)

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Questions ?