State Management and Drawing Geometry Objects

1

• State Management and Drawing Geometry Objects
• (OpenGL Book Ch 2)

2
Objective

• Clear the window to an arbitrary color
• Force any pending drawing to complete
• Draw with any geometric primitive
• Turn states on and off and query state variables
• Control the display of those primitives
• Specify normal vectors
• Use vertex arrays
• Save and restore state variables

3
Clearing the Window

• glClearColor(0.0, 0.0, 0.0, 0.0)
• glClearDepth(1.0)
• glClear(GL_COLOR_BUFFER_BIT
• GL_DEPTH_BUFFER_BIT)
• // or run more slowly with,
• // glClear(GL_COLOR_BUFFER_BIT)
• // glClear(GL_DEPTH_BUFFER_BIT)

4
Specifying a Color

• Pseudocode
• set_current_color(red)
• draw_object(A)
• draw_object(B)
• set_current_color(green) // wasted
• set_current_color(blue)
• draw_object(C)
• glColor3f(1.0, 0.0, 0.0)

5
Forcing Completion of Drawing

• glFlush()
• Forces previously issued OpenGL commands to begin
  execution.
• glFinish()
• Forces all previously issued OpenGL command s to
  complete. This command doesnt return until all
  effects from previous commands are fully realized.

6
glutReshapeFunc(reshape)

• Example code
• void reshape (int w, int h)
• glViewport (0, 0, w, h)
• glMatixMode (GL_PROJECTION)
• glLoadIdentity()
• glOrtho2D(0, w, 0, h)

7
What are Points, Lines, and Polygon?

• Points
• represented by a vertex
• Lines
• refer to line segments
• Polygons
• must be simple, convex, and planar
• Rectangles
• glRectf(x1, y1, x2, y2)
• glRectfv(pt1, pt2)

8
Curves and Curved Surface
Approximating Curves
9
Specifying Vertices

• glVertex2s(2, 3)
• glVertex3d(0.0, 0.0, 3.14)
• glVertex4f(2.4, 1.0, -2.2, 2.0)
• GLdouble v3 1.0, 9.0, 8.0
• glVertex3dv(v)

10
Drawing Geometric Primitives

• glBegin(GL_POLYGON)
• glVertex2f(0.0, 0.0)
• glVertex2f(4.0, 3.0)
• glVertex2f(6.0, 1.5)
• glVertex2f(4.0, 0.0)
• glEnd()

11
OpenGL Geometric Primitives

• GL_POINTS
• GL_LINES
• GL_LINE_STRIP
• GL_LINE_LOOP
• GL_TRIANGLES
• GL_TRIANGLE_STRIP
• GL_TRIANGLE_FAN
• GL_QUADS
• GL_QUAD_STRIP
• GL_POLYGON

12
Geometric Primitive Types
13
Valid Commands between glBegin(), glEnd()

• glVertex()
• glColor(), glIndex()
• glNormal()
• glTexCoord()
• glEdgeFlag()
• glMaterial()
• glArrayElement()
• glEvalCoord(), glEvalPoint()
• glCallList(), glCallLists()
• and any C or C codes

14
Basic State Management

• glEnable(GL_DEPTH_TEST)
• glDisable(GL_FOG)
• if (glIsEnabled(GL_FOG)) ...
• glGetBooleanv()
• glGetIntegerv()
• glGetFloatv()
• glGetDoublev(GL_CURRENT_COLOR,x)
• glGetPointerv()

15
Point and Line Details

• glPointSize(2.0)
• glLineWidth(2.0)
• glLineStipple(1,0xAAAA)
• glLineStipple(2,0xAAAA)
• glEnable(GL_LINE_STIPPLE)

16
Stippled Lines
17
Polygon Details

• Drawing polygons as points, outlines, or solids
• glPolygonMode(GL_FRONT, GL_FILL)
• glPolygonMode(GL_BACK, GL_LINE)
• glPolygonMode(GL_FRONT_AND_BACK, GL_POINT)

18
Reversing and Culling Polygon Faces

• glFrontFace(GL_CCW)
• glFrontFace(GL_CW)
• glCullFace(GL_BACK)
• glCullFace(GL_FRONT)
• glCullFace(GL_FRONT_AND_BACK)

19
Stippling Polygons

• glEnable(GL_POLYGON_STIPPLE)
• // Define stipple pattern fly here...
• glPolygonStipple(fly)

20
Marking Polygon Boundary Edges

• glPolygonMode(GL_FRONT_AND_BACK, GL_LINE)
• glBegin(GL_POLYGON)
• glEdgeFlag(GL_TRUE)
• glVertex3fv(V0)
• glEdgeFlag(GL_FALSE)
• glVertex3fv(V1)
• glEdgeFlag(GL_TRUE)
• glVertex3fv(V2)
• glEnd()

21
Normal Vectors

• glBegin (GL_POLYGON)
• glNormal3fv(n0)
• glVertex3fv(v0)
• glNormal3fv(n1)
• glVertex3fv(v1) glVertex3fv(v2)
• glEnd()
• Provide Unit Normals!
• glEnable(GL_NORMALIZE) can be expensive

22
Example Drawing a unit cube

• Static GLfloat vdata83
• 0.0,0.0,0.0,1.0,0.0,0.0,
• 1.0,1.0,0.0,0.0,1.0,0.0,
• 0.0,0.0,1.0,1.0,0.0,1.0,
• 1.0,1.0,1.0,0.0,1.0,1.0 //global!!
• Static GLint allIndx64
• 4,5,6,7,1,2,6,5,0,1,5,4,
• 0,3,2,1,0,4,5,4,2,3,7,6
• for (i0 ilt6 i)
• glBegin(GL_QUADS)
• glVertex3fv(vdataallIndxi00)
• glVertex3fv(vdataallIndxi10)
• glVertex3fv(vdataallIndxi20)
• glVertex3fv(vdataallIndxi30)
• glEnd()

23
Vertex Arrays

• To reduce the number of function calls
• Six sides eight shared vertices
• Step 1 Enabling arrays
• Step 2 Specifying data for the arrays
• Step 3 Dereferencing and rendering

24
Step 1 Enabling Arrays

• glEnableClientState(GL_NORMAL_ARRAY)
• glEnableClientState(GL_VERTEX_ARRAY)
• glEnableClientState(GL_COLOR_ARRAY)
• glEnableClientState(GL_INDEX_ARRAY)
• glEnableClientState(GL_EDGE_FLAG_ARRAY)
• glEnableClientState(GL_TEXTURE_COORD_ARRAY)
• glDisableClientState(GL_NORMAL_ARRAY)

25
Step 2 Specifying Data for the Arrays

• glVertexPointer(size, type, stride, pointer)
• glColorPointer(size, type, stride, pointer)
• glIndexPointer(type, stride, pointer)
• glNormalPointer(type, stride, pointer)
• glTexCoordPointer(size, type, stride, pointer)
• glEdgeFlagPointer(stride, pointer)

26
Step 2 Specifying Data for the Arrays

• glVertexPointer(size, type, stride, ptr)
• static GLfloat v
• 0.0,0.0,0.0, 1.0,0.0,0.0, 1.0,1.0,0.0,
  0.0,1.0,0.0, 0.0,0.0,1.0, 1.0,0.0,1.0,
  1.0,1.0,1.0), 0.0,1.0,1.0
• glVertexPointer(3, GL_FLOAT, 0, v)

27
Stride

• static GLfloat intertwined
• 1.0, 0.2, 1.0, 100.0, 100.0, 0.0,
• / /
• 0.2, 0.2, 1.0, 200.0, 100.0, 0.0
• glColorPointer(3, GL_FLOAT, 6sizeof(GLfloat),
  intertwined)
• glVertexPointer(3, GL_FLOAT, 6sizeof(GLfloat),
  intertwined3)

28
Step 3 Dereferencing and Rendering

• Alternative 1
• dereferencing a single array element
• glVertexPointer(3, GL_FLOAT, 0, v)
• glBegin(GL_QUADS)
• glArrayElement(4)
• glArrayElement(5)
• glArrayElement(6)
• glArrayElement(7)
• glEnd()

29
Dereferencing a List of Array Elements

• glVertexPointer(3, GL_FLOAT, 0, v)
• Static GLint allIndx244,5,6,7, 1,2,6,5,
  0,1,5,4, 0,3,2,1, 0,4,5,4, 2,3,7,6
• Alternative 2
• glBegin(GL_QUADS)
• for(int i 0 i lt 24 i)
• glArrayElement(allIndxi)
• glEnd()
• Alternative 3 Better still
• glDrawElements(GL_QUADS,24,GL_UNSIGNED_INT,allIndx
  )

30
Hints for Building Polygonal Models of Surfaces

• Keep polygon orientations consistent.
• all clockwise or all counterclockwise
• Watch out for non-triangular polygons.
• Trade-off between speed and quality.
• Avoid T- intersections
• There are more Read the book.

31

• Next
• Vectors, Matrices and Homogeneous coordinate
  system
• Transformations