OpenGL Computer Graphics

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OpenGL Computer Graphics

• Programming with Transformations

2
Topics

• Transformations in OpenGL
• Saving Current Transformation
• Drawing 3D Scenes with OpenGL
• OpenGL Functions for Modeling and Viewing

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Transformations in OpenGL

• CT current transformation
• Simplified graphics pipeline
• OpenGL maintains so-called modelview matrix
• Every vertex passed down the graphics pipeline is
  multiplied by this matrix

Window-to-Viewport Transformation
V
S
Q
CT
S
V
Q
Viewport
World Window
Screen Coordinate System
Model (Master) Coordinate System
World Coordinate System
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Transformations in OpenGL

• OpenGL is a 3D graphics package
• Transformations are 3D
• How does it work in 2D?
• 2D drawing is done in the xy-plane, z coordinate
  is 0.
• Translation dz 0
• Scaling Sz 1
• Rotation z-roll

y
x
z
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Transformations in OpenGL

• Fundamental Transformations
• Translation glTranslated(dx, dy, dz)
• for 2D glTranslated(dx, dy, 0)
• Scaling glScaled(sx, sy, sz)
• for 2D glScaled(sx, sy, 1.0)
• Rotation glRotated(angle, ux, uy, uz)
• for 2D glRotated(angle, 0, 0, 1)
• Transformations does not set CT directly, a
  matrix is postmultiplied to CT
• CT CT ? M

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Transformations in OpenGL

• Canvas functions
• void Canvas initCT(void)
• glMatrixMode(GL_MODELVIEW)
• glLoadIdentity()
• void Canvas scale2D(double sx, double sy)
• glMatrixMode(GL_MODELVIEW)
• glScaled(dx, dy, 1.0)

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Transformations in OpenGL

• Canvas functions
• void Canvas translate2D(double dx, double dy)
• glMatrixMode(GL_MODELVIEW)
• glTranslated(dx, dy, 0)
• void Canvas rotate2D(double angle)
• glMatrixMode(GL_MODELVIEW)
• glRotated(angle, 0.0, 0.0, 1.0)

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Transformations Example

• Draw a house. Draw another house by rotating it
  through -30 and then translating it through (32,
  25)
• cvs.initCT()
• house()
• cvs.translate2D(32, 25)
• cvs.rotate2D(-30.0)
• house()

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Transformations Example
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Transformations Example

• Think of it in two different ways
• Q T(32, 25)R(-30)P
• ? CT CT ? T(32, 25) ? R(-30)
• Translate the coordinate system through (32, 25)
  and then rotate it through 30
• The code generated by these two ways is identical.

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Saving Current Transformation

• We can save and restore CTs using glPushMatrix()
  and glPopMatrix()
• Manipulation of a stack of CT

After popCT()
After rotate2D()
Before
After pushCT()
CT4 CT3 ? Rot
CT4
CT3
CT3
CT3
CT3
CT2
CT2
CT2
CT2
CT1
CT1
CT1
CT1
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Saving Current Transformation

• Canvas functions
• void Canvas pushCT(void)
• glMatrixMode(GL_MODELVIEW)
• glPushMatrix()
• void Canvas popCT(void)
• glMatrixMode(GL_MODELVIEW)
• glPopMatrix()

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Saving CT Examples

• Master coordinate system where an object is
  defined
• Modeling transformation transforms an object
  from its master coordinate system to world
  coordinate system to produce an instance
• Instance a picture of an object in the scene

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Drawing 3D Scenes with OpenGL

• The concept of camera (eye) is used for 3D
  viewing
• Our 2D drawing is a special case of 3D drawing

far plane
y
view volume
near plane
x
z
eye
Viewport
window
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Drawing 3D Scenes with OpenGL

• Camera to produce parallel view of a 3D scene

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Drawing 3D Scenes with OpenGL

• Simplified OpenGL graphics pipeline

VM
P
Vp
clip
viewport matrix
modelview matrix
projection matrix
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Drawing 3D Scenes with OpenGL

• Modelview matrix CT
• Object transformation camera transformation
• Applying model matrix M then viewing matrix V

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Drawing 3D Scenes with OpenGL

• Projection matrix
• Shifts and scales view volume into a standard
  cube (extension from 1 to 1)
• Distortion can be compensated by viewport
  transformation later

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Drawing 3D Scenes with OpenGL

• Viewport matrix
• Maps surviving portion of objects into a 3D
  viewport after clipping is performed
• Standard cube ?
• block w/ x and y extending across viewport and z
  from 0 to 1

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OpenGL Modeling and Viewing Functions

• Modeling transformation
• Translation glTranslated(dx, dy, dz)
• Scaling glScaled(sx, sy, sz)
• Rotation glRotated(angle, ux, uy, uz)
• Camera for parallel projection
• glMatrixMode(GL_PROJECTION)
• glLoadIdentity()
• glOrtho(left, right, bottom, top, near, far)
• Example
• near2 near plane is 2 units in front of eye
• far20 far plane is 20 units in front of eye

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OpenGL Modeling and Viewing Functions

• Positioning and aiming camera
• glMatrixMode(GL_MODELVIEW)
• glLoadIdentity()
• glutLookAt(eye.x, eye.y, eye.z, // eye
  position
• look.x, look.y, look.z, // look at
  point
• up.x, up.y, up.z) // up
  vector
• Up vector is often set to (0, 1, 0)
• glutLookAt() builds a matrix that converts world
  coordinates into eye coordinates.

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Set up a Typical Camera - Example

• glMatrixMode(GL_PROJECTION)
• glLoadIdentity()
• glOrtho(-3.2, 3.2, -2.4, 2.4, 1, 50)
• glMatrixMode(GL_MODELVIEW)
• glLoadIdentity()
• glutLookAt(4, 4, 4,
• 0, 1, 0,
• 0, 1, 0)

(4, 4, 4)
(0, 1, 0)
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Transformation Matrix for LookAt

• Camera coordinate system
• Axes u, v, n
• n eye look
• u up ? n
• v n ? u
• Origin eye (looking in the direction n)
• Transformation matrix

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Transformation Matrix for LookAt
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Elementary 3D Shapes Provided by OpenGL

• Cube
• glutWireCube(GLdouble size)
• size length of a side
• Sphere
• glutWireSphere(GLdouble radius, GLint nSlices,
  GLint nStacks)
• Approximated by polygonal faces
• nSlices polygons around z-axis
• nStacks bands along z-axis

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Elementary 3D Shapes Provided by OpenGL

• Torus
• glutWireTorus(GLdouble inRad, GLdouble outRad,
  GLint nSlices, GLint nStacks)
• Approximated by polygonal faces
• Teapots
• glutWireTeapot(GLdouble size)
• There are solid counterparts of the wire objects

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Plantonic Solids Provided by OpenGL

• Tetrahedron
• glutWireTetrahedron()
• Octahedron
• glutWireOctahedron()
• Dodecahedron
• glutWireDodecahedron()
• Icosahedron
• glutWireIcosahedron()
• All of them are centered at the origin

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Plantonic Solids Provided by OpenGL
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Cone Provided by OpenGL

• Cone
• glutWireCone(GLdouble baseRad, GLdouble height,
  GLint nSlices, GLint nStacks)
• Axis coincides with the z-axis
• Base rests on xy-plane and extends to z height
• baseRad radius at z 0

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Tapered Cylinder Provided by OpenGL

• Tapered cylinder
• gluCylinder(GLUquadricObj qobj, GLdouble
  baseRad, GLdouble topRad, GLdouble height, GLint
  nSlices, GLint nStacks)
• Axis coincides with the z-axis
• Base rests on xy-plane and extends to z height
• baseRad radius at z 0
• topRad radius at z height

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Tapered Cylinder Provided by OpenGL

• A family of shapes distinguished by the value of
  topRad
• To draw, we have to
• Deifne a new quadric object
• Set drawing style
• GLU_LINE wire frame
• GLU_FILL solid rendering
• Draw the object

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Tapered Cylinder Provided by OpenGL

• Example wire frame cylinder
• GLUquadricObj qobj
• qobj gluNewQuadric()
• gluQuadricDrawStyle(qobj, GLU_LINE)
• gluCylinder(qobj, baseRad, topRad, height,
  nSlices, nStacks)

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• include ltgl/glut.hgt
• //ltltltltltltltltltltltltltltltltltltlt axis gtgtgtgtgtgtgtgtgtgtgtgtgtgt
• void axis(double length)
• // draw a z-axis, with cone at end
• glPushMatrix()
• glBegin(GL_LINES)
• glVertex3d(0, 0, 0) glVertex3d(0,0,length)
  // along the z-axis
• glEnd()
• glTranslated(0, 0,length -0.2)
• glutWireCone(0.04, 0.2, 12, 9)
• glPopMatrix()

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• //ltltltltltltltltltltltltltlt displayWire gtgtgtgtgtgtgtgtgtgtgtgtgtgt
• void displayWire(void)
• glMatrixMode(GL_PROJECTION) // set the view
  volume shape
• glLoadIdentity()
• glOrtho(-2.064/48.0, 2.064/48.0, -2.0, 2.0,
  0.1, 100)
• glMatrixMode(GL_MODELVIEW) // position and aim
  the camera
• glLoadIdentity()
• gluLookAt(2.0, 2.0, 2.0, 0.0, 0.0, 0.0, 0.0,
  1.0, 0.0)
• // to obtain the picture shown in Figure 5.59 we
  have to
• // change the eye location as follows
• // gluLookAt(1.0, 1.0, 2.0, 0.0, 0.0, 0.0, 0.0,
  1.0, 0.0)

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• glClear(GL_COLOR_BUFFER_BIT) // clear the
  screen
• glColor3d(0,0,0) // draw black lines
• axis(0.5) // z-axis
• glPushMatrix()
• glRotated(90, 0, 1, 0)
• axis(0.5) // x-axis
• glRotated(-90, 1, 0, 0)
• axis(0.5) // y-axis
• glPopMatrix()
• glPushMatrix()
• glTranslated(0.5, 0.5, 0.5) // big cube at
  (0.5, 0.5, 0.5)
• glutWireCube(1.0)
• glPopMatrix()

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• glPushMatrix()
• glTranslated(1.0,1.0,0) // sphere at (1,1,0)
• glutWireSphere(0.25, 10, 8)
• glPopMatrix()
• glPushMatrix()
• glTranslated(1.0,0,1.0) // cone at (1,0,1)
• glutWireCone(0.2, 0.5, 10, 8)
• glPopMatrix()
• glPushMatrix()
• glTranslated(1,1,1)
• glutWireTeapot(0.2) // teapot at (1,1,1)
• glPopMatrix()

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• glPushMatrix()
• glTranslated(0, 1.0 ,0) // torus at (0,1,0)
• glRotated(90.0, 1,0,0)
• glutWireTorus(0.1, 0.3, 10,10)
• glPopMatrix()
• glPushMatrix()
• glTranslated(1.0, 0 ,0) // dodecahedron at
  (1,0,0)
• glScaled(0.15, 0.15, 0.15)
• glutWireDodecahedron()
• glPopMatrix()

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• glPushMatrix()
• glTranslated(0, 1.0 ,1.0) // small cube at
  (0,1,1)
• glutWireCube(0.25)
• glPopMatrix()
• glPushMatrix()
• glTranslated(0, 0 ,1.0) // cylinder at (0,0,1)
• GLUquadricObj qobj
• qobj gluNewQuadric()
• gluQuadricDrawStyle(qobj,GLU_LINE)
• gluCylinder(qobj, 0.2, 0.2, 0.4, 8,8)
• glPopMatrix()
• glFlush()

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• //ltltltltltltltltltltltltltltltlt main gtgtgtgtgtgtgtgtgtgtgtgtgtgtgtgt
• void main(int argc, char argv)
• glutInit(argc, argv)
• glutInitDisplayMode(GLUT_SINGLE GLUT_RGB )
• glutInitWindowSize(640,480)
• glutInitWindowPosition(100, 100)
• glutCreateWindow("Transformation testbed -
  wireframes")
• glutDisplayFunc(displayWire)
• glClearColor(1.0f, 1.0f, 1.0f,0.0f) //
  background is white
• glViewport(0, 0, 640, 480)
• glutMainLoop()