Programming with OpenGL

Programming with OpenGL

• 1.
  Programming with OpenGL Part2: Complete Programs 1
• 2.
  Objectives • Refine thefirst program • Alter the default values • Introduce a standard program structure • Simple viewing • Two-dimensional viewing as a special case of three-dimensional viewing • Fundamental OpenGL primitives • Attributes 2
• 3.
  Program Structure •Most OpenGLprograms have a similar structure that consists of the following functions • main(): • defines the callback functions • opens one or more windows with the required properties • enters event loop (last executable statement) • init(): sets the state variables • Viewing • Attributes • callbacks • Display function • Input and window functions 3
• 4.
  simple.c revisited • Inthis version, we shall see the same output but we have defined all the relevant state values through function calls using the default values • In particular, we set • Colors • Viewing conditions • Window properties 4
• 5.
  main.c #include <GL/glut.h> int main(intargc, char** argv) { glutInit(&argc,argv); glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB); glutInitWindowSize(500,500); glutInitWindowPosition(0,0); glutCreateWindow("simple"); glutDisplayFunc(mydisplay); init(); glutMainLoop(); } 5 includes gl.h define window properties set OpenGL state enter event loop display callback
• 6.
  GLUT functions • glutInitallows application to get command line arguments and initializes system • gluInitDisplayMode requests properties for the window (the rendering context) • RGB color • Single buffering • Properties logically ORed together • glutWindowSize in pixels • glutWindowPosition from top-left corner of display • glutCreateWindow create window with title “simple” • glutDisplayFunc display callback • glutMainLoop enter infinite event loop 6
• 7.
  init.c void init() { glClearColor (0.0,0.0, 0.0, 1.0); glColor3f(1.0, 1.0, 1.0); glMatrixMode (GL_PROJECTION); glLoadIdentity (); glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0); } 7 black clear color opaque window fill/draw with white viewing volume
• 8.
  Coordinate Systems •The unitsin glVertex are determined by the application and are called object or problem coordinates •The viewing specifications are also in object coordinates and it is the size of the viewing volume that determines what will appear in the image •Internally, OpenGL will convert to camera (eye) coordinates and later to screen coordinates •OpenGL also uses some internal representations that usually are not visible to the application 8
• 9.
  OpenGL Camera • OpenGLplaces a camera at the origin in object space pointing in the negative z direction • The default viewing volume is a box centered at the origin with a side of length 2 9
• 10.
  Orthographic Viewing 10 z=0 z=0 In thedefault orthographic view, points are projected forward along the z axis onto the plane z=0
• 11.
  Transformations and Viewing •InOpenGL, projection is carried out by a projection matrix (transformation) •There is only one set of transformation functions so we must set the matrix mode first glMatrixMode (GL_PROJECTION) • Transformation functions are incremental so we start with an identity matrix and alter it with a projection matrix that gives the view volume glLoadIdentity(); glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0); 11
• 12.
  Two- and three-dimensionalviewing • In glOrtho(left, right, bottom, top, near, far) the near and far distances are measured from the camera • Two-dimensional vertex commands place all vertices in the plane z=0 • If the application is in two dimensions, we can use the function gluOrtho2D(left, right,bottom,top) • In two dimensions, the view or clipping volume becomes a clipping window 12
• 13.
  mydisplay.c void mydisplay() { glClear(GL_COLOR_BUFFER_BIT); glBegin(GL_POLYGON); glVertex2f(-0.5, -0.5); glVertex2f(-0.5,0.5); glVertex2f(0.5, 0.5); glVertex2f(0.5, -0.5); glEnd(); glFlush(); } 13
• 14.
  OpenGL Primitives 14 GL_QUAD_STRIPGL_QUAD_STRIP GL_POLYGONGL_POLYGON GL_TRIANGLE_STRIPGL_TRIANGLE_STRIP GL_TRIANGLE_FANGL_TRIANGLE_FAN GL_POINTSGL_POINTS GL_LINESGL_LINES GL_LINE_LOOPGL_LINE_LOOP GL_LINE_STRIPGL_LINE_STRIP GL_TRIANGLESGL_TRIANGLES
• 15.
  Polygon Issues •OpenGL willonly 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 can check if above true • OpenGL will produce output if these conditions are violated but it may not be what is desired •Triangles satisfy all conditions 15 nonsimple polygon nonconvex polygon
• 16.
  Attributes • Attributes arepart of the OpenGL state 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 • Display vertices 16
• 17.
  RGB color •Each colorcomponent is 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), whereas in glColor3ub the values range from 0 to 255 17
• 18.
  Indexed Color • Colorsare indices into tables of RGB values • Requires less memory • indices usually 8 bits • not as important now • Memory inexpensive • Need more colors for shading 18
• 19.
  Color and State •Thecolor 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 19
• 20.
  Smooth Color •Default issmooth shading • OpenGL interpolates vertex colors across visible polygons •Alternative is flat shading • Color of first vertex determines fill color • glShadeModel (GL_SMOOTH) or GL_FLAT 20
• 21.
  Viewports • Do nothave use the entire window for the image: glViewport(x,y,w,h) • Values in pixels (screen coordinates) 21