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 Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle m} is between $0$ and $1$. Other lines can be drawn by negating Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle m} or swapping roles of Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle x} and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle 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?