Notes

#include <cmath>
#include <stdlib.h>
#include <GL/glut.h>
struct Point {
GLint x;
GLint y;
};
struct Color {
GLfloat r;
GLfloat g;
GLfloat b;
};
void draw_dda(Point p1, Point p2) {
GLfloat dx = p2.x - p1.x;
GLfloat dy = p2.y - p1.y;
GLfloat x1 = p1.x;
GLfloat y1 = p1.y;
GLfloat step = 0;
if(abs(dx) > abs(dy)) {
step = abs(dx);
} else {
step = abs(dy);
}
GLfloat xInc = dx/step;
GLfloat yInc = dy/step;
for(float i = 1; i <= step; i++) {
glVertex2i(x1, y1);
x1 += xInc;
y1 += yInc;
}
}
void init() {
glClearColor(1.0, 1.0, 1.0, 0.0);
glColor3f(0.0, 0.0, 0.0);
glPointSize(1.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0, 640, 0, 480);
}

Color getPixelColor(GLint x, GLint y) {
Color color;
glReadPixels(x, y, 1, 1, GL_RGB, GL_FLOAT, &color);
return color;
}

void setPixelColor(GLint x, GLint y, Color color) {
glColor3f(color.r, color.g, color.b);
glBegin(GL_POINTS);
glVertex2i(x, y);
glEnd();
glFlush();
}

void floodFill(GLint x, GLint y, Color oldColor, Color newColor) {
Color color;
color = getPixelColor(x, y);
if(color.r == oldColor.r && color.g == oldColor.g && color.b == oldColor.b)
{
setPixelColor(x, y, newColor);
floodFill(x+1, y, oldColor, newColor);
floodFill(x, y+1, oldColor, newColor);
floodFill(x-1, y, oldColor, newColor);
floodFill(x, y-1, oldColor, newColor);
}
return;
}

void onMouseClick(int button, int state, int x, int y)
{
Color newColor = {1.0f, 0.0f, 0.0f};
Color oldColor = {1.0f, 1.0f, 1.0f};
floodFill(101, 199, oldColor, newColor);
}

void display(void) {
Point p1 = {100, 100}, // bottom-right
p2 = {200, 100}, // bottom-left
p3 = {200, 200}, // top-right
p4 = {100, 200}; // top-left
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_POINTS);
draw_dda(p1, p2);
draw_dda(p2, p3);
draw_dda(p3, p4);
draw_dda(p4, p1);
glEnd();
glFlush();
}

int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowSize(640, 480);
glutInitWindowPosition(200, 200);
glutCreateWindow("Open GL");
init();
glutDisplayFunc(display);
glutMouseFunc(onMouseClick);
glutMainLoop();
return 0;
}