Bresenham's algorithm in OpenGL - opengl

Question

This is my fist time studying Computer Graphics and i was given the code below from my professor. I think i have understood how the algorithm is implemented in general, but what i fail to understand is in what way the incx,incy,inc1,inc2 lines work. I obviously know they are there to increase something but since i haven't fully understood the algorithm i'm kinda baffled. This is the full code:
#include <gl/glut.h>
#include <stdio.h>
int xstart, ystart, xend, yend;
void myInit() {
glClear(GL_COLOR_BUFFER_BIT);
glClearColor(0.0, 0.0, 0.0, 1.0);
glMatrixMode(GL_PROJECTION);
gluOrtho2D(0, 500, 0, 500);
}
void draw_pixel(int x, int y) {
glBegin(GL_POINTS);
glVertex2i(x, y);
glEnd();
}
void draw_line(int xstart, int xend, int ystart, int yend) {
int dx, dy, i, e;
int incx, incy, inc1, inc2;
int x,y;
dx = xend-xstart;
dy = yend-ystart;
if (dx < 0) dx = -dx;
if (dy < 0) dy = -dy;
incx = 1;
if (xend < xstart) incx = -1;
incy = 1;
if (yend < ystart) incy = -1;
x = xstart;
y = ystart;
if (dx > dy) {
draw_pixel(x, y);
e = 2 * dy-dx;
inc1 = 2*(dy-dx);
inc2 = 2*dy;
for (i=0; i<dx; i++) {
if (e >= 0) {
y += incy;
e += inc1;
}
else
e += inc2;
x += incx;
draw_pixel(x, y);
}
} else {
draw_pixel(x, y);
e = 2*dx-dy;
inc1 = 2*(dx-dy);
inc2 = 2*dx;
for (i=0; i<dy; i++) {
if (e >= 0) {
x += incx;
e += inc1;
}
else
e += inc2;
y += incy;
draw_pixel(x, y);
}
}
}
void myDisplay() {
draw_line(xstart, xend, ystart, yend);
glFlush();
}
void main(int argc, char **argv) {
printf( "Enter (xstart, ystart, xend, yend)\n");
scanf("%d %d %d %d", &xstart, &ystart, &xend, &yend);
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowSize(500, 500);
glutInitWindowPosition(0, 0);
glutCreateWindow("Bresenham's Line Algorithm Visualization");
myInit();
glutDisplayFunc(myDisplay);
glutMainLoop();
}