CSCE 441 Lecture 2

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OpenGL Command Formats

• prefix: gl
• name: Vertex
• number of components: 2, 3, or 4
• Data types
  • byte: b
  • unsigned byte: ub
  • short: s
  • unsigned short: us
  • integer: i
  • float: f
• Optional "Vector form" (arguments specified as array): v

All geometric primitves are specified by vertices:

• GL_POINTS draws dots on the screen
• GL_LINES draws lines between consecutive points on the screen
• GL_LINE_STRIP connect lines between all consecutive points
• GL_LINE_LOOP same as GL_LINE_STRIP, except it connects the first and last line
• GL_POLYGON similar to GL_LINE_LOOP, except that it fills the loop
• GL_TRIANGLES makes triangle polygons from consecutive triplets of points
• GL_TRIANGLES_STRIP draws triangles from all consecutive points
• GL_TRIANGLES_FAN keep first point as vertex, and draw triangles between all remaining pairs of points
• GL_QUADS take consecutive quadruples of points
• GL_QUADS_STRIP take all "doubly-consecutive" quadruples of points (pairs of lines)

Related Libraries

GLU (OpenGL Utility Library)

• Part of OpenGL
• Provides higher-level drawing routines like spheres, NURBS, tesselators, quadric shapes, etc.

GLUT (OpenGL Utility Toolkit)

• perform system-level I/O with OS (windows, user events)
• cross-platform for most purposes
• portable windowing API
• not officially part of OpenGL

#include <GLUT/glut.h>

void init();

void display();

void idle();

void keyboard(unsigned char key, int x, int y)
{
    switch (key) {
    }

    // tell GLUT that something has changed and it should redraw the screen
    // calling display() won't do anything
    glutPostRedisplay();
}

void mouse();

void mouseMove(int x, int y)
{
    glutPostRedisplay();
}

int main(int argc, char *argv[])
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(250, 250);
    glutInitWindowPosition(100, 100);
    glutCreateWindow("HELLO"); // title of window

    init(); // my initialization callback

    glutDisplayFunc(display);
    glutIdleFunc(idle);
    glutKeyboardFunc(keyboard);
    glutMouseFunc(mouse);
    glutMotionFunc(mouseMove);

    glutMainLoop();

    return 0;
}

Assignment 1

Build a simple OpenGL/GLUT application

Full description

Scan Conversion of Lines

All display devices are abstracted to discrete grids of pixels

• Integer coordinates
• Color information associated

Drawing lines

Draw a line between 2 points... which pixels should be on?

Obviously endpoints and any points that lie on the line between.

We'll assume slope $m$ is between $0$ and $1$. Other lines can be drawn by negating $m$ or swapping roles of $x$ and $y$.

Simple Algorithm

int xL, yL, xH, yH;

int x = xL;
float y = (float)yL;

float m = ((float)yH - (float)yL) / ((float)xH - (float)xL);

for (int i = 0; i <= xH - xL; i++) {
    drawPixel(x, round(y));
    x++;
    y += m;
}

Problems:

• Floating point operations are expensive and have roundoff errors
• Round, Ceil, or Floor?