CSC461%20Lecture%206:%202D%20Programming%20in%20OpenGL

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CSC461 Lecture 62D Programming in OpenGL

• Objectives
• Fundamental OpenGL primitives
• Attributes
• Viewport

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OpenGL Primitives
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Polygon Issues

• OpenGL will only display polygons correctly that
  are
• Simple edges cannot cross
• Convex All points on line segment between two
  points in a polygon are also in the polygon
• Flat all vertices are in the same plane
• User program must check if above true
• Triangles satisfy all conditions

nonconvex polygon
nonsimple polygon
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Drawing a Sphere

• Sphere description
• X(?,?)sin?cos?
• y(?,?)cos?cos?
• z(?,?)sin?
• Approximation
• A set of polygons
• Efficiently using
• Quad strips
• Triangle strips

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Drawing a Sphere (Cont.)

• Fix ? and change ? ? get circles of constant
  longitude
• Fix ? and change ? ? get circles of constant
  latitude
• Generate points at fixed increments of ? and ? ?
  quadrilaterals
• Color corresponds to increments of 20 degrees in
  ? and 20 degrees in ?

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Source code Drawing quadrilaterals

• cM_PI/180
• for (phi-80.0 philt80.0 phi20.0)
• glBegin(GL_QUAD_STRIP)
• for (theta-180.0 thetalt180.0 theta20.0)
• x sin(ctheta)cos(c.phi)
• y cos(ctheta)cos(cphi)
• z sin(ctheta)
• glVertex3d(x,y,x)
• x sin(ctheta)cos(c(phi20.0))
• y cos(ctheta)cos(c(phi20.0))
• z sin(c(phi20.0))
• glVertex3d(x,y,z)
• glEnd()

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Drawing poles

• cM_PI/180.0
• glBegin(GL_TRANGLE_FAN)
• glVertex3d(x,y,z)
• zsin(c80.0)
• for (theta-180.0 thetalt180.0 theta20)
• x sin(ctheta)cos(c.80.0)
• y cos(ctheta)cos(c80.0)
• glVertex3d(x,y,z)
• glEnd()
• Xy0
• z-1
• glBegin(GL_TRIANGLE_FAN)
• glVertex3d(x,y,z)
• z-sin(c80.0)
• for (theta-180.0 thetalt180.0 theta20)
• x sin(ctheta)cos(c.80.0)
• y cos(ctheta)cos(c80.0)
• glVertex3d(x,y,z)

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Attributes

• Attributes are part of the OpenGL and determine
  the appearance of objects
• Color (points, lines, polygons)
• Size and width (points, lines)
• Stipple pattern (lines, polygons)
• Polygon mode
• Display as filled solid color or stipple pattern
• Display edges

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RGB color

• Each color component stored separately in the
  frame buffer
• Usually 8 bits per component in buffer
• Note in glColor3f the color values range from 0.0
  (none) to 1.0 (all), while in glColor3ub the
  values range from 0 to 255

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Indexed Color

• Colors are indices into tables of RGB values
• Selecting color from table
• glIndexi(element)
• Setting color into table
• glutSetColor(int color, GLfloat r, GLfloat b,
  GLfloat g)
• Requires less memory
• indices usually 8 bits
• not as important now
• Memory inexpensive
• Need more colors for shading

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Color and State

• The color as set by glColor becomes part of the
  state and will be used until changed
• Colors and other attributes are not part of the
  object but are assigned when the object is
  rendered
• We can create conceptual vertex colors by code
  such as
• glColor()
• glVertex()
• glColor()
• glVertex()

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Smooth Color

• Default is smooth shading
• OpenGL interpolates vertex colors across visible
  polygons
• Alternative is flat shading
• Color of first vertex
• determines fill color
• Code
• glShadeModel(GL_SMOOTH)
• glShadeModel(GL_FLAT)

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Viewports

• Do not have use the entire window for the image
  glViewport(x,y,w,h)
• Values in pixels (screen coordinates)