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CS559: Computer Graphics

Learn the basics of OpenGL, a software interface to graphics hardware, and explore its capabilities in 3D graphics programming.

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CS559: Computer Graphics

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  1. CS559: Computer Graphics Lecture 12: OpenGL Li Zhang Spring 2008

  2. So far: 3D Geometry Pipeline Rotation Translation Rotation Translation Resizing Eye Space (View Space) World Space Model Space (Object Space) Image Space (pixels) Raster Space Screen Space (2D) Canonical View Space

  3. OpenGL • We have been focused on math description • We’ll move on practical graphics programming for a week

  4. Display Modern graphics systems Photoshop 3D Max Softimage Maya and ? nVidia ATI … OpenGL Direct3D … Your homework

  5. OpenGL • A software interface to graphics hardware. • 700 distinct commands • 650 in the core OpenGL • 50 in the utility library • Specify objects, viewing, lighting, surface material • Hardware independent interface • No commands for windowing tasks • No high level object models • You need to specify geometric primitives • Points, lines, polygons.

  6. What can OpenGL do? wireframe

  7. What can OpenGL do? Antialised lines

  8. What can OpenGL do? Depth cue using fog

  9. What can OpenGL do? Flat-shaded polygons

  10. What can OpenGL do? Lighting and smooth-shaded polygons

  11. What can OpenGL do? Texturemap and shadow

  12. What can OpenGL do? Motion blur

  13. What can OpenGL do? View point change

  14. What can OpenGL do? Smoke

  15. What can OpenGL do? Depth of field

  16. Hello, world #include <whateverYouNeed.h> main() {    OpenAWindowPlease();    glClearColor(0.0, 0.0, 0.0, 0.0);    glClear(GL_COLOR_BUFFER_BIT);    glColor3f(1.0, 1.0, 1.0);    glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);     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();    KeepTheWindowOnTheScreenForAWhile(); }

  17. OpenGL syntax gl prefix for all commands GL_ for constants glColor3f(1.0, 1.0, 1.0); glColor3d(1.0, 1.0, 1.0); glColor3s(1, 1, 1); glColor3i(1, 1, 1); ……

  18. OpenGL syntax glVertex2i(1, 1); glVertex2f(1.0, 1.0); glColor3f(1.0, 0.0, 0.0); glColor3ub(255, 0, 0); glColor3f(1.0, 0.0, 0.0); float color_array[] = {1.0, 0.0, 0.0}; glColor3fv(color_array);

  19. Windows management GLUT lib #include <GL/gl.h> #include <GL/glut.h> int main(int argc, char** argv){ glutInit(&argc, argv); glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB); glutInitWindowSize (250, 250); glutInitWindowPosition (100, 100); glutCreateWindow ("hello"); init (); }

  20. Windows management GLUT lib #include <GL/gl.h> #include <GL/glut.h> int main(int argc, char** argv){ glutInit(&argc, argv); glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB); glutInitWindowSize (250, 250); glutInitWindowPosition (100, 100); glutCreateWindow ("hello"); init (); glutDisplayFunc(display); } void init (void) { glClearColor (0.0, 0.0, 0.0, 0.0);  glMatrixMode(GL_PROJECTION); glLoadIdentity(); glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0); }

  21. Windows management GLUT lib #include <GL/gl.h> #include <GL/glut.h> int main(int argc, char** argv){ glutInit(&argc, argv); glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB); glutInitWindowSize (250, 250); glutInitWindowPosition (100, 100); glutCreateWindow ("hello"); init (); glutDisplayFunc(display); glutMainLoop(); return 0; } • void display(void){ • glClear (GL_COLOR_BUFFER_BIT);  • glColor3f (1.0, 1.0, 1.0); • 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 (); • }  

  22. Animation open_window(); for (i = 0; i < 1000000; i++) { clear_the_window(); draw_frame(i); wait_until_a_24th_of_a_second_is_over(); } Q: What happens when you write to the framebuffer while it is being displayed on the monitor?

  23. Animation open_window(); for (i = 0; i < 1000000; i++) { clear_the_window(); draw_frame(i); wait_until_a_24th_of_a_second_is_over(); } Q: What happens when you write to the framebuffer while it is being displayed on the monitor?

  24. Animation open_window(); for (i = 0; i < 1000000; i++) { clear_the_window(); draw_frame(i); wait_until_a_24th_of_a_second_is_over(); swap_the_buffers(); } Q: What happens when you write to the framebuffer while it is being displayed on the monitor?

  25. Animation Example • int main(int argc, char** argv){ • glutInit(&argc, argv); • glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB); glutInitWindowSize (250, 250); glutInitWindowPosition (100, 100); glutCreateWindow (argv[0]); • init (); • glutDisplayFunc(display); • } • void init(void) { • glClearColor (0.0, 0.0, 0.0, 0.0); glShadeModel (GL_FLAT); • } 

  26. Animation Example • int main(int argc, char** argv){ • glutInit(&argc, argv); • glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB); glutInitWindowSize (250, 250); glutInitWindowPosition (100, 100); glutCreateWindow (argv[0]); • init (); • glutDisplayFunc(display); • glutMouseFunc(mouse); • } • static GLfloat spin = 0.0;  • void display(void){ • glClear(GL_COLOR_BUFFER_BIT); • glPushMatrix(); • glRotatef(spin, 0.0, 0.0, 1.0); • glColor3f(1.0, 1.0, 1.0); • glRectf(-25.0, -25.0, 25.0, 25.0); glPopMatrix(); • glutSwapBuffers(); • } 

  27. Animation Example • void mouse(int button, int state, int x, int y) { • switch (button) { • case GLUT_LEFT_BUTTON: • if (state == GLUT_DOWN) glutIdleFunc(spinDisplay); • break; • case GLUT_MIDDLE_BUTTON: • if (state == GLUT_DOWN) glutIdleFunc(NULL); • break; • default: • break; • } • } • int main(int argc, char** argv){ • glutInit(&argc, argv); • glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB); glutInitWindowSize (250, 250); glutInitWindowPosition (100, 100); glutCreateWindow (argv[0]); • init (); • glutDisplayFunc(display); • glutMouseFunc(mouse); • glutReshapeFunc(reshape); • } • spinDisplay(void){ • spin = spin + 2.0; • if (spin > 360.0) spin -= 360.0; glutPostRedisplay(); • }

  28. Animation Example • void reshape(int w, int h){ • glViewport (0, 0, (GLsizei) w, (GLsizei) h); • glMatrixMode(GL_PROJECTION); glLoadIdentity(); • glOrtho(-50.0, 50.0, -50.0, 50.0, -1.0, 1.0); • glMatrixMode(GL_MODELVIEW); glLoadIdentity(); • }  • int main(intargc, char** argv){ • glutInit(&argc, argv); • glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB); glutInitWindowSize (250, 250); glutInitWindowPosition (100, 100); glutCreateWindow (argv[0]); • init (); • glutDisplayFunc(display); • glutMouseFunc(mouse); • glutReshapeFunc(reshape); • glutMainLoop(); return 0; • }

  29. Event-Driven Programming Window management library GLUT / FLTK Events: key strokes, mouse clicks Event handlers: mouse(), display(), reshape()

  30. Viewport • void reshape (int w, int h) { • glViewport (0, 0, w, h); • glMatrixMode (GL_PROJECTION); • glLoadIdentity (); • gluOrtho2D (0.0, w, 0.0, h); • }

  31. Points, lines, and polygons • Points • Lines • Polygons Simple Polygonss: Convex & planar Nonplanar Polygon Transformed to Nonsimple Polygon

  32. Drawing Primitives glBegin(GL_POLYGON); glVertex2f(0.0, 0.0); glVertex2f(0.0, 3.0); glVertex2f(4.0, 3.0); glVertex2f(6.0, 1.5); glVertex2f(4.0, 0.0); glEnd();

  33. Drawing Primitives glBegin(GL_POINTS); glVertex2f(0.0, 0.0); glVertex2f(0.0, 3.0); glVertex2f(4.0, 3.0); glVertex2f(6.0, 1.5); glVertex2f(4.0, 0.0); glEnd();

  34. Drawing Primitives

  35. Drawing Primitives

  36. Drawing Primitives

  37. Drawing Primitives

  38. Primitive Details • glPointSize(GLfloat size) • Approximate the point by squares for anti-aliasing glEnable(GL_POINT_SMOOTH);

  39. Primitive Details • glLineWidth(GLfloat width) • Approximate the line by a rectangle for anti-aliasing glEnable (GL_LINE_SMOOTH); glLineWidth (1.5);

  40. Primitive Details • glPolygonMode(GLenum face, GLenum mode); • face: GL_FRONT, GL_BACK • mode: GL_POINT, GL_LINE, GL_FILL glPolygonMode(GL_FRONT, GL_LINE); glRectf(0, 0, 100, 100); glPolygonMode(GL_FRONT, GL_FILL); glRectf(0, 0, 100, 100);

  41. Primitive Details • Determine Polygon Orientation y P3 Q3 Q4 P4 Q2 P2 P1 Q1 x Orientation == sign of the area

  42. Icosahedron Normal Vectors

  43. Icosahedron //initial icosahedron Static float t[20][3][3] = {…}; void display(void) { }

  44. Icosahedron //initial icosahedron Static float t[20][3][3] = {…}; void display(void) { //clear buffer //set up viewport and frustum if (animation) angle+=0.3; if (angle>360) angle-=360.0; glPushMatrix(); glRotatef(angle,1,0,1); }

  45. Icosahedron //initial icosahedron Static float t[20][3][3] = {…}; void display(void) { //clear buffer //set up viewport and frustum if (animation) angle+=0.3; if (angle>360) angle-=360.0; glPushMatrix(); glRotatef(angle,1,0,1); // subdivide each face of the triangle for (int i = 0; i < 20; i++) { Subdivide(t[i][0], t[i][1], t[i][2], subdiv); } }

  46. Icosahedron //initial icosahedron Static float t[20][3][3] = {…}; void display(void) { //clear buffer //set up viewport and frustum if (animation) angle+=0.3; if (angle>360) angle-=360.0; glPushMatrix(); glRotatef(angle,1,0,1); // subdivide each face of the triangle for (int i = 0; i < 20; i++) { Subdivide(t[i][0], t[i][1], t[i][2], subdiv); } glPopMatrix(); glFlush(); glutSwapBuffers(); }

  47. Icosahedron void Subdivide(GLfloat v1[3], GLfloat v2[3], GLfloat v3[3], int depth) { }

  48. Icosahedron • void Subdivide(GLfloat v1[3], GLfloat v2[3], GLfloat v3[3], int depth) • { • if (depth == 0) { • glColor3f(0.5,0.5,0.5); • glBegin(GL_TRIANGLES); • glVertex3fv(v1); glVertex3fv(v2); glVertex3fv(v3); • glEnd(); • } • }

  49. Icosahedron • void Subdivide(GLfloat v1[3], GLfloat v2[3], GLfloat v3[3], int depth) • { • if (depth == 0) { • glColor3f(0.5,0.5,0.5); • glBegin(GL_TRIANGLES); • glVertex3fv(v1); glVertex3fv(v2); glVertex3fv(v3); • glEnd(); • } • else • { • GLfloat v12[3], v23[3], v31[3]; • for (int i = 0; i < 3; i++) { • v12[i] = (v1[i]+v2[i])/2.0; Normalize(v12); • v23[i] = (v2[i]+v3[i])/2.0; Normalize(v23); • v31[i] = (v3[i]+v1[i])/2.0; Normalize(v31); • } • } • }

  50. Icosahedron • void Subdivide(GLfloat v1[3], GLfloat v2[3], GLfloat v3[3], int depth) • { • if (depth == 0) { • glColor3f(0.5,0.5,0.5); • glBegin(GL_TRIANGLES); • glVertex3fv(v1); glVertex3fv(v2); glVertex3fv(v3); • glEnd(); • } • else • { • GLfloat v12[3], v23[3], v31[3]; • for (int i = 0; i < 3; i++) { • v12[i] = (v1[i]+v2[i])/2.0; Normalize(v12); • v23[i] = (v2[i]+v3[i])/2.0; Normalize(v23); • v31[i] = (v3[i]+v1[i])/2.0; Normalize(v31); • } • Subdivide(v1, v12, v31, depth-1); • Subdivide(v2, v23, v12, depth-1); • Subdivide(v3, v31, v23, depth-1); • Subdivide(v12, v23, v31, depth-1); • } • }

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