1 / 24

CSC345: Advanced Graphics & Virtual Environments

CSC345: Advanced Graphics & Virtual Environments. Lecture 2: Introduction to OpenGL (2) Patrick Olivier p.l.olivier@ncl.ac.uk 2 nd floor in the Devonshire Building. GL_POINTS. GL_POLYGON. GL_LINE_STRIP. GL_LINES. GL_LINE_LOOP. GL_TRIANGLES. GL_QUAD_STRIP. GL_TRIANGLE_FAN.

tamas
Download Presentation

CSC345: Advanced Graphics & Virtual Environments

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CSC345: Advanced Graphics &Virtual Environments Lecture 2: Introduction to OpenGL (2) Patrick Olivier p.l.olivier@ncl.ac.uk 2nd floor in the Devonshire Building Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  2. GL_POINTS GL_POLYGON GL_LINE_STRIP GL_LINES GL_LINE_LOOP GL_TRIANGLES GL_QUAD_STRIP GL_TRIANGLE_FAN GL_TRIANGLE_STRIP OpenGL Primitives Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  3. 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 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 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  4. Attributes • Attributes are part 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 Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  5. Colour (color!) • Each colour component stored separately in frame buffer • Usually 8 bits per component in buffer glColor3f ranges from 0.0 to 1.0 glColor3ub ranges from 0 to 255 • Colour set by glColor is part of the state (used until changed) • Colours (all attributes) not part of object - assigned when rendering • We can create (conceptual) vertex colours by: glColor; glVertex; glColor; glVertex • We can create (conceptual) vertex colours by: • 1st vertex colour:glShadeModel(GL_SMOOTH) • Interpolates:glShadeModel(GL_FLAT) Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  6. Viewports • Do not have use the entire window for the image: glViewport(x,y,w,h) • Values in pixels (screen coordinates) Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  7. OpenGL and 3D applications • In OpenGL, two-dimensional applications are a special case of three-dimensional graphics • Going to 3D • Not much changes • Use glVertex3*( ) • Worry about the order in which polygons are drawn or use hidden-surface removal • Polygons should be simple, convex, flat Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  8. Sierpinski gasket or fractal (2D) • Start with a triangle • Connect bisectors of sides and remove central triangle • Repeat Repeated 5 times Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  9. 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); } Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  10. 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); } /* draw triangle at end of recursion */ else(triangle(a,b,c)); } Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  11. void display() { glClear(GL_COLOR_BUFFER_BIT); glBegin(GL_TRIANGLES); divide_triangle(v[0], v[1], v[2], n); glEnd(); glFlush(); } void myinit() { glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluOrtho2D(-2.0, 2.0, -2.0, 2.0); glMatrixMode(GL_MODELVIEW); glClearColor (1.0, 1.0, 1.0,1.0) glColor3f(0.0,0.0,0.0); }

  12. main function int main(int argc, char **argv) { n=4; glutInit(&argc, argv); glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB); glutInitWindowSize(500, 500); glutCreateWindow(“2D Gasket"); glutDisplayFunc(display); myinit(); glutMainLoop(); } Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  13. 3D Gasket • We can easily make the program three-dimensional: GLfloat v[3][3] glVertex3f glOrtho • Not interesting - instead, start with a tetrahedron Repeated 5 times Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  14. Triangle code void triangle( GLfloat *a, GLfloat *b, GLfloat *c) { glVertex3fv(a); glVertex3fv(b); glVertex3fv(c); } Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  15. Subdivision code void divide_triangle(GLfloat *a, GLfloat *b, GLfloat *c, int m) { GLfloat v1[3], v2[3], v3[3]; int j; if(m>0) { for(j=0; j<3; j++) v1[j]=(a[j]+b[j])/2; for(j=0; j<3; j++) v2[j]=(a[j]+c[j])/2; for(j=0; j<3; j++) v3[j]=(b[j]+c[j])/2; divide_triangle(a, v1, v2, m-1); divide_triangle(c, v2, v3, m-1); divide_triangle(b, v3, v1, m-1); } else(triangle(a,b,c)); } Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  16. 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); } Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  17. Hidden surface removal • The triangles are drawn in order they are defined so the front triangles are not always rendered in front triangles behind them • We want to see only those surfaces in front of other surfaces • OpenGL uses a hidden-surface method called the z-buffer algorithm that saves depth information as objects are rendered so that only the front objects appear in the image Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  18. Using the z-buffer • Requested in main.c • glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH) • Enabled in init.c • glEnable(GL_DEPTH_TEST) • Cleared in the display callback • glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT) Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  19. Input & interaction • Most systems have more than one input device, each of which can be triggered at an arbitrary time by a user • Each trigger generates an event whose measure is put in an event queue which can be examined by the user program Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  20. Event types • Window: resize, expose, iconify • Mouse: click one or more buttons • Motion: move mouse • Keyboard: press or release a key • Idle: nonevent (define what should be done if no other event is in queue) Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  21. Callbacks • Programming interface for event-driven input • Define a callback function for each type of event the graphics system recognizes • This user-supplied function is executed when the event occurs • GLUT example: /* mouse callback function */ glutMouseFunc(mymouse) Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  22. GLUT callbacks GLUT recognizes a subset of the events recognized by any particular window system (Windows, X, Macintosh) • glutDisplayFunc • glutMouseFunc • glutReshapeFunc • glutKeyboardFunc • glutIdleFunc • glutMotionFunc • glutPassiveMotionFunc Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  23. The display callback • Recall that the last line in main.c for a program using GLUT must be glutMainLoop()which puts the program in an infinite event loop • In each pass through the event loop, GLUT • looks at the events in the queue • for each event in the queue, GLUT executes the appropriate callback function if one is defined • if no callback is defined for the event, the event is ignored Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

  24. Posting redisplays • Many events may invoke the display callback function which can lead to multiple executions of the display callback on a single pass through the event loop • We can avoid this problem by instead using glutPostRedisplay()which sets a flag. • GLUT checks to see if the flag is set at the end of the event loop • If set then the display callback function is executed Based on: Angel (4th Edition) & Akeine-Möller & Haines (2nd Edition)

More Related