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What is OpenGL ?
What is OpenGL ?
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Open gl

  1. 1. Introduction to OpenGL Programming
  2. 2. What is OpenGL  OpenGL is a software API to graphics hardware.  designed as a streamlined, hardware-independent interface to be implemented on many different hardware platforms  Intuitive, procedural interface with c binding  No windowing commands !  No high-level commands for describing models of three- dimensional objects  The OpenGL Utility Library (GLU) provides many of the modeling features, such as quadric surfaces and NURBS curves and surfaces
  3. 3. SGI and GL  Silicon Graphics (SGI) revolutionized the graphics workstation by implementing the pipeline in hardware (1982)  To access the system, application programmers used a library called GL  With GL, it was relatively simple to program three dimensional interactive applications
  4. 4. OpenGL The success of GL lead to OpenGL (1992), a platform-independent API that was  Easy to use  Close enough to the hardware to get excellent performance  Focus on rendering  Omitted windowing and input to avoid window system dependencies
  5. 5. OpenGL Evolution  Controlled by an Architectural Review Board (ARB)  Members include SGI, Microsoft, Nvidia, HP, 3DLabs, IBM,…….  Relatively stable (present version 2.0)  Evolution reflects new hardware capabilities  3D texture mapping and texture objects  Vertex and fragment programs  Allows for platform specific features through extensions
  6. 6. OpenGL Libraries  OpenGL core library  OpenGL32 on Windows  GL on most unix/linux systems (libGL.a)  OpenGL Utility Library (GLU)  Provides functionality in OpenGL core but avoids having to rewrite code  Links with window system  GLX for X window systems, WGL for Windows  Cross-platform GUI libraries: GLUT, SDL, FLTK, QT, …
  7. 7. Windowing with OpenGL  OpenGL is independent of any specific window system  OpenGL can be used with different window systems  X windows (GLX)  MFC  …  GLUTprovide a portable API for creating window and interacting with I/O devices
  8. 8. GLUT  OpenGL Utility Toolkit (GLUT)  Provides functionality common to all window systems  Open a window  Get input from mouse and keyboard  Menus  Event-driven  Code is portable but GLUT lacks the functionality of a good toolkit for a specific platform  No slide bars, buttons, …
  9. 9. Software Organization application program OpenGL Motif widget or similar GLUT GLX, AGL or WGL GLU X, Win32, Mac O/S OpenGL software and/or hardware
  10. 10. OpenGL Architecture Immediate Mode geometry pipeline Per Vertex Polynomial Operations & Evaluator Primitive Assembly Display Per Fragment Frame CPU List Rasterization Operations Buffer Texture Memory Pixel Operations
  11. 11. OpenGL as a state machine  GL State Variables- can be set and queried by OpenGL. Remains unchanged until the next change.  Projection and viewing matrix  Color and material properties  Lights and shading  Line and polygon drawing modes  …  OpenGL functions are of two types  Primitive generating  Can cause output if primitive is visible  How vertices are processed and appearance of primitive are controlled by the state  State changing  Transformation functions  Attribute functions
  12. 12. OpenGL Syntax  Functions have prefix gl and initial capital letters for each word  glClearColor(), glEnable(), glPushMatrix() …  glu for GLU functions  gluLookAt(), gluPerspective() …  Constants begin with GL_, use all capital letters  GL_COLOR_BUFFER_BIT, GL_PROJECTION, GL_MODELVIEW …  Extra letters in some commands indicate the number and type of variables  glColor3f(), glVertex3f() …  OpenGL data types  GLfloat, GLdouble, GLint, GLenum, …  Underlying storage mode is the same  Easy to create overloaded functions in C++ but issue is efficiency
  13. 13. OpenGL function format function name dimensions glVertex3f(x,y,z) x,y,z are floats belongs to GL library glVertex3fv(p) p is a pointer to an array
  14. 14. OpenGL #defines  Most constants are defined in the include files gl.h, glu.h and glut.h  Note #include <GL/glut.h> should automatically include the others  Examples  glBegin(GL_POLYGON)  glClear(GL_COLOR_BUFFER_BIT)  include files also define OpenGL data types: GLfloat, GLdouble,….
  15. 15. GLUT  Developed by Mark Kilgard  Hides the complexities of differing window system APIs  Default user interface for class projects  Glut routines have prefix glut  glutCreateWindow() …  Has very limited GUI interface  Glui is the C++ extension of glut
  16. 16. Glut Routines  Initialization: glutInit() processes (and removes) command-line arguments that may be of interest to glut and the window system and does general initialization of Glut and OpenGL  Must be called before any other glut routines  Display Mode: The next procedure, glutInitDisplayMode(), performs initializations informing OpenGL how to set up the frame buffer.  Display Mode Meaning  GLUT_RGB Use RGB colors  GLUT_RGBA Use RGB plus alpha (for transparency)  GLUT_INDEX Use indexed colors (not recommended)  GLUT_DOUBLE Use double buffering (recommended)  GLUT_SINGLE Use single buffering (not recommended)  GLUT_DEPTH Use depth-buffer (for hidden surface removal.)
  17. 17. Glut Routines  Window Setup  glutInitWindowSize(int width, int height)  glutInitWindowPosition(int x, int y)  glutCreateWindow(char* title)
  18. 18. A Simple Program Generate a square on a solid background
  19. 19. simple.c #include <GL/glut.h> 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(); } int main(int argc, char** argv){ glutInit(&argc, argv); glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB); glutInitWindowSize(500,500); glutInitWindowPosition(0,0); glutCreateWindow("simple"); glutDisplayFunc(mydisplay); init(); glutMainLoop(); }
  20. 20. Closer Look at the main() #include <GL/glut.h> includes gl.h int main(int argc, char** argv) { glutInit(&argc,argv); glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB); glutInitWindowSize(500,500); glutInitWindowPosition(0,0); glutCreateWindow("simple"); glutDisplayFunc(mydisplay); define window properties init(); display callback set OpenGL state glutMainLoop(); } enter event loop
  21. 21. init.c black clear color void init() opaque window { glClearColor (0.0, 0.0, 0.0, 1.0); glColor3f(1.0, 1.0, 1.0); fill/draw with white glMatrixMode (GL_PROJECTION); glLoadIdentity (); glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0); } viewing volume
  22. 22. Event Handling  Virtually all interactive graphics programs are even driven  GLUT uses callbacks to handle events  Windows system invokes a particular procedure when an event of particular type occurs.  MOST IMPORTANT: display event  Signaled when window first displays and whenever portions of the window reveals from blocking window  glutDisplayFunc(void (*func)(void)) registers the display callback function  Running the program: glutMainLoop()  Main event loop. Never exit()
  23. 23. More Callbacks  glutReshapeFunc(void (*func)(int w, int h)) indicates what action should be taken when the window is resized.  glutKeyboardFunc(void (*func)(unsigned char key, int x, int y)) and glutMouseFunc(void (*func)(int button, int state, int x, int y)) allow you to link a keyboard key or a mouse button with a routine that's invoked when the key or mouse button is pressed or released.  glutMotionFunc(void (*func)(int x, int y)) registers a routine to call back when the mouse is moved while a mouse button is also pressed.  glutIdleFunc(void (*func)(void)) registers a function that's to be executed if no other events are pending - for example, when the event loop would otherwise be idle
  24. 24. Compilation on Windows  See class web site on how to set up a project with OpenGL  Visual C++  Get glut.h, glut32.lib and glut32.dll from web  Create a console application  Add opengl32.lib, glut32.lib, glut32.lib to project settings (under link tab)
  25. 25. Simple Animation  Animation  Redraw + swap buffers  What looks like if using single buffer  Example program  More on the glut documentation  Chapter 2 of textbook  OpenGL redbook  Links in the class resources page
  26. 26. OpenGL Drawing  We have learned how to create a window  Steps in the display function  Clear the window  Set drawing attributes  Send drawing commands  Swap the buffers  OpenGL coordinate system has different origin from the window system  Uses lower left corner instead of upper left corner as origin
  27. 27. Clear the Window  glClear(GL_COLOR_BUFFER_BIT)  clears the frame buffer by overwriting it with the background color.  Background color is a state set by glClearColor(GLfloat r, GLfloat g, GLfloat b, GLfloat a) in the init().
  28. 28. Drawing Attributes: Color  glColor3f(GLfloat r, GLfloat g, GLfloat b) sets the drawing color  glColor3d(), glColor3ui() can also be used  Remember OpenGL is a state machine  Once set, the attribute applies to all subsequent defined objects until it is set to some other value  glColor3fv() takes a flat array as input  There are more drawing attributes than color  Point size: glPointSize()  Line width: glLinewidth()  Dash or dotted line: glLineStipple()  Polygon pattern: glPolygonStipple()  …
  29. 29. Drawing Commands  Simple Objects glRectf()  Complex Objects  Use construct glBegin(mode) and glEnd() and a list of vertices in between  glBegin(mode) glVertex(v0); glVertex(v1); ... glEnd();  Some other commands can also be used between glBegin() and glEnd(), e.g. glColor3f().  Example
  30. 30. Projection and Viewport
  31. 31. Orthographic projection  Orthographic View  glOrtho(left, right, bottom, top, front, back)  Specifies the coordinates of 3D region to be projected z=0 into the image space.  Any drawing outside the region will be automatically clipped away.
  32. 32. Viewports  Do not have use the entire window for the image: glViewport(x,y,w,h)  Values in pixels (screen coordinates)
  33. 33. Window to Viewport mapping Aspect Ratio: Height/Width If the aspect ratio of the window Is different from that of the viewport, the picture will be distorted.
  34. 34. Sierpinski Gasket (2D)  Start with a triangle  Connect bisectors of sides and remove central triangle  Repeat
  35. 35. Example  Five subdivisions
  36. 36. Gasket Program #include <GL/glut.h> /* initial triangle */ GLfloat v[3][2]={{-1.0, -0.58}, {1.0, -0.58}, {0.0, 1.15}}; int n; /* number of recursive steps */ void triangle( GLfloat *a, GLfloat *b, GLfloat *c) /* display one triangle */ { glVertex2fv(a); glVertex2fv(b); glVertex2fv(c); }
  37. 37. Triangle Subdivision void divide_triangle(GLfloat *a, GLfloat *b, GLfloat *c, int m) { /* triangle subdivision using vertex numbers */ point2 v0, v1, v2; int j; if(m>0) { for(j=0; j<2; j++) v0[j]=(a[j]+b[j])/2; for(j=0; j<2; j++) v1[j]=(a[j]+c[j])/2; for(j=0; j<2; j++) v2[j]=(b[j]+c[j])/2; divide_triangle(a, v0, v1, m-1); divide_triangle(c, v1, v2, m-1); divide_triangle(b, v2, v0, m-1); } else(triangle(a,b,c)); /* draw triangle at end of recursion */ }
  38. 38. Gasket Display Functions void display() { glClear(GL_COLOR_BUFFER_BIT); glBegin(GL_TRIANGLES); divide_triangle(v[0], v[1], v[2], n); glEnd(); glFlush(); } • By having the glBegin and glEnd in the display callback rather than in the function triangle and using GL_TRIANGLES rather than GL_POLYGON in glBegin, we call glBegin and glEnd only once for the entire gasket rather than once for each triangle
  39. 39. Example: 3D Gasket after 5 iterations
  40. 40. Tetrahedron Code void tetrahedron( int m) { glColor3f(1.0,0.0,0.0); divide_triangle(v[0], v[1], v[2], m); glColor3f(0.0,1.0,0.0); divide_triangle(v[3], v[2], v[1], m); glColor3f(0.0,0.0,1.0); divide_triangle(v[0], v[3], v[1], m); glColor3f(0.0,0.0,0.0); divide_triangle(v[0], v[2], v[3], m); }