CS 468 OpenGL

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CS 468OpenGL

• Jon Moon
• Spring 2004
• Computer Science
• Cornell University

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Who am I?

• Jon Moon
• University of Minnesota 03
• First year PhD
• jmoon_at_cs.cornell.edu
• Office 4143 Upson

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The Plan

• What is OpenGL?
• OpenGL basic interactions
• OpenGL with Java JOGL
• Sample code
• General framework
• 3D
• Transformations
• Lighting

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What is OpenGL?

• Its a graphics API
• Software layer that sits between the programmer
  and the CPU, GPU
• Its window system and platform independent
• Its fast and easy to use

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What is OpenGL?

• Its a renderer
• Primitives are points, lines, polygons
• Its a State Machine
• The output of the renderer depends on the state,
  which includes
• Current color, normal, etc.
• Shading, lighting
• Viewing
• All sorts of other stuff

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Viewing in OpenGL

• OpenGL maintains two viewing matrices
• Model view matrix for object positions
• Projection matrix for camera properties
• Projection can be perspective or orthographic
• Camera is always at the origin, looking toward -z

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OpenGL with Java enter JOGL

• JOGL is a java binding for the OpenGL API.
• JOGL is not very object oriented
• Make a single GL object, which then has 1800
  methods, 2400 fields
• Basically a wrapped c header file gl.h
• Simple to port C code to java this way

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Enough talk, wheres the code?

• import ...
• import net.java.games.jogl.
• public class OpenGLDemo01 extends JFrame
  implements GLEventListener
• public OpenGLDemo01() ...
• public static void main(String args) new
  OpenGLDemo01()
• public void init(...)
• public void reshape(...) ...
• public void display(...) ...
• public void displayChanged(...)

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Demo 1, high level

• Constructor handles window canvas, JPanel
• Main simply creates an instance
• Init sets some start state stuff
• Reshape sets the modelview and projection
  matrices and viewport
• Display specifies geometry and transformation

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Demo 1 constructor

• public OpenGLDemo01()
• super("OpenGL Demo 01")
• setLocation(100, 100)
• setSize(VIEWPORT_WIDTH, VIEWPORT_HEIGHT)
• canvas GLDrawableFactory.getFactory().
  createGLCanvas(new GLCapabilities())
• canvas.addGLEventListener(this)
• JPanel glPanel new JPanel(new
  BorderLayout())
• glPanel.add(canvas, BorderLayout.CENTER)
• glPanel.setSize(VIEWPORT_WIDTH,
  VIEWPORT_HEIGHT)
• getContentPane().add(glPanel)
• setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)
• show()

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Demo1 reshape

• public void reshape(GLDrawable gLDrawable, int
  x, int y, int width, int height)
• final GL gl gLDrawable.getGL()
• final GLU glu gLDrawable.getGLU()
• gl.glViewport(0, 0, width, height)
• gl.glMatrixMode(gl.GL_PROJECTION)
• gl.glLoadIdentity()
• glu.gluOrtho2D(0, VIEWPORT_WIDTH, 0,
  VIEWPORT_HEIGHT)
• gl.glMatrixMode(gl.GL_MODELVIEW)
• gl.glLoadIdentity()

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Demo1 display

• public void display(GLDrawable gLDrawable)
• final GL gl gLDrawable.getGL()
• gl.glClear(GL.GL_COLOR_BUFFER_BIT)//
• gl.glBegin(gl.GL_TRIANGLES)
• gl.glColor3d(1.0, 0.0, 0.0)
• gl.glVertex2i(250, 450)
• gl.glColor3d(0.0, 1.0, 0.0)
• gl.glVertex2i(50, 50)
• gl.glColor3d(0.0, 0.0, 1.0)
• gl.glVertex2i(450, 50)
• gl.glEnd()

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That wasnt so bad, was it?

• But that was just 2D drawing a triangle isnt
  exactly amazing
• Suppose we wanted to draw a perspective (3D)
  cube
• What would have to change?
• Camera type
• Geometry
• Hidden Surface removal!

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Demo2 Init

• public void init(GLDrawable gLDrawable)
• final GL gl gLDrawable.getGL()
• gl.glEnable(gl.GL_DEPTH_TEST)

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Demo2 reshape changes

• ...
• gl.glMatrixMode(gl.GL_PROJECTION)
• gl.glLoadIdentity()
• glu.gluPerspective(45.0f, (float) width /
  (float) height, 0.2f, 100.0f)
• glu.gluLookAt(3.0f, 5.0f, 5.0f,
• 0.0f, 0.0f, 0.0f,
• 0.0f, 1.0f, 0.0f)
• ...

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Demo2 display changes

• ...
• gl.glClear(GL.GL_COLOR_BUFFER_BIT
  GL.GL_DEPTH_BUFFER_BIT)
• gl.glLoadIdentity()
• gl.glBegin(gl.GL_QUADS)
• // Front Face
• gl.glColor3d(0.0, 0.0, 1.0)
• gl.glVertex3f(-1.0f, -1.0f, 1.0f)
• gl.glVertex3f(1.0f, -1.0f, 1.0f)
• gl.glVertex3f(1.0f, 1.0f, 1.0f)
• gl.glVertex3f(-1.0f, 1.0f, 1.0f)
• ...

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Demo2 Thats it!

• Booya -- perspective 3D cube.
• glu.gluPerspective and glu.gluLookAt
• HSR accomplished by enabling z-buffer,
  gl.glEnable(gl.GL_DEPTH_TEST) and clearing it for
  each display
• Use gl.glBegin(GL_QUADS) list vertices in groups
  of 4, changing color/normals if desired

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Demo3 Using the Model View Matrix

• Even 3D perspective cubes get boring what if we
  wanted it to spin?
• Recall that the Model View matrix is part of the
  state of OpenGL
• Changing it in one display call will persist
  until it is changed again.
• Lets call the display method regularly, and each
  time tweak model view matrix

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Demo3 Using the Model View Matrix

• In display, we make this call
• gl.glRotatef(0.5f, 1.0f, 1.6f, 0.7f)
• rotate .5 degrees around the vector 1,1.6,.7
• More accurately, calculate the matrix for that
  rotation, and multiply the Model View matrix by
  it
• Other nifty matrix changing calls
• gl.glTranslatef(x,y,z)
• gl.glScalef(x,y,z)

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Matrix stacks

• OpenGL maintains multiple matrix stacks
• gl.glPushMatrix pushes a copy of the current
  matrix onto the stack
• gl.glPopMatrix discards the top of the stack
• What is this useful for?

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Demo4 Hierarchical transforms

• Goal box with two small boxes on top
• Non-hierarchical method
• Draw big base box
• Translate, rotate, scale into desired position of
  2nd box, draw it
• Translate, rotate, scale over to desired 3rd box
  position, draw it? (or)
• But 3rd should be relative to 1st

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Demo4 Hierarchical transforms

• Hierarchical method
• Draw big base box
• Push current matrix onto stack
• Transform to draw 2nd box
• Pop matrix from stack
• Back in coordinates of base box!
• Push matrix again
• Transform to draw 3rd box
• Pop matrix

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Demo4 Do it in OpenGL

• gl.glPushMatrix()
• gl.glScalef(1,.3f,1)
• drawCube(gl) //just draws standard cube
• gl.glPushMatrix()
• gl.glTranslatef(.5f,2f,0)
• gl.glScalef(0.3f,1f,0.3f)
• gl.glRotatef(15,0,1,0)
• drawCube(gl)
• gl.glPopMatrix()
• gl.glPushMatrix()
• gl.glTranslatef(-.5f,2f,0)
• gl.glScalef(0.2f,1f,0.2f)
• drawCube(gl)
• gl.glPopMatrix()
• gl.glPopMatrix()

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Demo5 The last one, I promise

• What about lighting?
• You can specify lights as part of the OpenGL
  state position, color, emission, type, and more
• First define a light
• float lightColor 1.0f, 1.0f, 1.0f
• gl.glLightfv(gl.GL_LIGHT0, gl.GL_COLOR,
  lightColor ) (similar for position, etc)
• Then, enable it
• glEnable(gl.GL_LIGHTING)
• glEnable(gl.GL_LIGHT0)

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Demo5 Materials too!

• Can also define a material as part of the state
• float materialColor 1.0f, 1.0f, 0.0f
• gl.glMaterialfv(gl.GL_FRONT, gl.GL_DIFFUSE,
  materialColor)
• Dont worry much about lighting and materials in
  OpenGL we can do much cooler stuff with Cg.

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Other resources

• JOGL home https//jogl.dev.java.net/
• GL, GLU function descriptions http//tc1.chemie.u
  ni-bielefeld.de/doc/OpenGL/hp/Reference.html
• Java API spec http//java.sun.com/j2se/1.4.2/docs
  /api/
• Swing guides http//java.sun.com/docs/books/tutor
  ial/uiswing/mini/index.html