OpenGL-Explaination.md

Explaination of all the functions and what they do in an opengl program. Wish you best of luck.

-- Amin Ullah Khan

Program Here

Include all the headers required.

#include <gl/glut.h>
#include <math.h>

Define a structure that caries x and y coordinates of a point in 2d plane.

struct Point {
	GLint x;
	GLint y;
};

This function draws a line from p1 to p2

void draw_dda(Point p1, Point p2) {

Calculate the change between the two points

	GLfloat dx = p2.x - p1.x;
	GLfloat dy = p2.y - p1.y;

Initial point from which the drawing will be started

	GLfloat x1 = p1.x;
	GLfloat y1 = p1.y;

How much to increment on each iteration

	GLfloat step = 0;

if |dx| is more than |dy|, then step = |dx| else step = |dy|

	if(abs(dx) > abs(dy)) {
		step = abs(dx);
	} else {
		step = abs(dy);
	}

Increments in x and y, which the line is being drawn

	GLfloat xInc = dx/step;
	GLfloat yInc = dy/step;

The loop that will draw the line

	for(float i = 1; i <= step; i++) {
		glVertex2i(x1, y1);
		x1 += xInc;
		y1 += yInc;
	}
}

Initialize OpenGL

void init() {

Clear the screen with with white, 1.0, 1.0, 1.0 is for white color

	glClearColor(1.0, 1.0, 1.0, 0.0);

Set the drawing color to black, 0.0, 0.0, 0.0 is for black color

	glColor3f(0.0, 0.0, 0.0);

The point size is 1 pixel

	glPointSize(1.0);

Current drawing matrix

	glMatrixMode(GL_PROJECTION);

Load the identity matrix to draw on

	glLoadIdentity();

Set drawing canvas

1. starting x left
2. ending x right
3. starting y bottom
4. ending y top

	gluOrtho2D(0, 640, 0, 480);
}

Function that draws a square Point a: bottom-left vectex of a square GLint length: Length of each side

void draw_square(Point a, GLint length) {

Point a is provided, the other three will be calculated.

	Point b = {a.x + length, a.y},
		c = {b.x,	b.y+length},
		d = {c.x-length, c.y};

Draw Points

	draw_dda(a, b);
	draw_dda(b, c);
	draw_dda(c, d);
	draw_dda(d, a);	
}

Display function will be called by glutDisplayFunc()

void display(void) {

Point p1: The starting bottom-left vertex of the square

	Point p1 = {100, 100};

Length of each side

	GLint length = 100; // size of square

Clear the frame buffer

	glClear(GL_COLOR_BUFFER_BIT);

Tell opengl, we are drawing in points

	glBegin(GL_POINTS);

Draw your square

		draw_square(p1, length);

End your drawing

	glEnd();

Flush the output to the display

	glFlush();
}

Your main() function

int main(int argc, char** argv)
{

Initialize GLUT (OpenGL Utility Kit)

	glutInit(&argc, argv);

Set display mode of the output. We are drawing in black and white so GLUT_SINGLE is ok, we can also draw in RGB using GLUT_RGB

	glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);

Width and height of the window respectively

	glutInitWindowSize(640, 480);

Margins of your glut window from left and top respectively

	glutInitWindowPosition(200, 200);

Create your window with a title

	glutCreateWindow("Open GL");

Call your init() function

	init();

Provide GLUT your display() function

	glutDisplayFunc(display);

Run a main loop, this acts like system("pause") in console application, it doesn't let a program quit.

	glutMainLoop();

Return 0 as your program output

	return 0;
}