Notes

import javax.media.opengl.GL2;
import javax.media.opengl.GLAutoDrawable;
import javax.media.opengl.GLEventListener;
import javax.media.opengl.glu.GLU;

public class MidPointLines implements GLEventListener {
/**
* Interface to the GLU library.
*/
private GLU glu;

/**
* Take care of initialization here.
* @param gld
*/
@Override
public void init(GLAutoDrawable gld) {
GL2 gl = gld.getGL().getGL2();
glu = new GLU();

gl.glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
gl.glViewport(-250, -150, 250, 150);
gl.glMatrixMode(GL2.GL_PROJECTION);
gl.glLoadIdentity();
glu.gluOrtho2D(-250.0, 250.0, -150.0, 150.0);
}

/**
* Take care of drawing here.
* @param drawable
*/
@Override
public void display(GLAutoDrawable drawable) {
GL2 gl = drawable.getGL().getGL2();

gl.glClear(GL2.GL_COLOR_BUFFER_BIT);
/*
* put your code here
*/
//points should be in the same zone
DrawMPL(gl,10,10,60,50);
DrawMPL(gl,10,-10,60,-50);
DrawMPL(gl,-30,-10,-100,-40);

}

@Override
public void reshape(GLAutoDrawable drawable, int x, int y, int width, int height) {
//do nothing
}

public void displayChanged(GLAutoDrawable drawable, boolean modeChanged, boolean deviceChanged) {
//do nothing
}

float dx, dy, b;
private void DrawMPL(GL2 gl, float x1, float y1, float x2, float y2) {
gl.glPointSize(3.0f);
gl.glColor3d(1, 0, 0);
float x,y,dx,dy,d,dE,dNE;
float zone=findZone(x1, y1, x2,y2);
float x1new=convertX0( x1, y1, zone);
float x2new=convertX0( x2, y2, zone);
float y1new=convertY0( x1, y1, zone);
float y2new=convertY0( x2, y2, zone);

dx=x2new-x1new;
dy=y2new-y1new;
dE=dy;
dNE=dy-dx;
d=dy-(dx/2);
x=x1new;
y=y1new;



gl.glBegin(GL2.GL_POINTS);


gl.glVertex2f(x1, y1);
gl.glVertex2f(x2, y2);

while(x<x2new)
{
if(d<0)
{
x++;
d+=dE;
gl.glVertex2f(convertXi( x, y, zone),convertYi( x, y, zone));
}
else {
x++;
y++;
d+=dNE;
gl.glVertex2f(convertXi( x, y, zone),convertYi( x, y, zone));
}
}


gl.glEnd();
}

//y=mx+b
private float func(float x, float y){
return (float)(dy*x - y*dx + b*dx);
}


float findZone(float x1, float y1, float x2, float y2) {
float Zone=0;
float dx=x2-x1;
float dy=y2-y1;
if (Math.abs(dx)>=Math.abs(dy)){
if(dx>0 && dy>0)
Zone=0;
if(dx<0 && dy>0)
Zone=3;
if(dx<0 && dy<0)
Zone=4;
if(dx>0 && dy<0)
Zone=7;


}
else{
if(dx>0 && dy>0)
Zone=1;
if(dx<0 && dy>0)
Zone=2;
if(dx<0 && dy<0)
Zone=5;
if(dx>0 && dy<0)
Zone=6;
}
return Zone ;

}

float convertX0(float x, float y, float zone){
float xnew=0;

if(zone==0) {
xnew=x;
}else if(zone==1) {

xnew=y;
}else if(zone==2) {

xnew=y;
}else if(zone==3) {

xnew=-x;
}else if(zone==4) {

xnew=-x;
}else if(zone==5) {

xnew=-y;
}else if(zone==6) {

xnew=-y;
}else if(zone==7) {

xnew=x;
}
return xnew;


}

float convertY0(float x, float y, float zone){
float ynew=0;

if(zone==0) {
ynew=y;
}else if(zone==1) {

ynew=x;
}else if(zone==2) {

ynew=-x;
}else if(zone==3) {

ynew=y;
}else if(zone==4) {

ynew=-y;
}else if(zone==5) {

ynew=-x;
}else if(zone==6) {

ynew=x;
}else if(zone==7) {

ynew=-y;
}
return ynew;

}

float convertXi(float x, float y, float zone){
float xnew=0;

if(zone==0) {
xnew=x;
}else if(zone==1) {

xnew=y;
}else if(zone==2) {

xnew=-y;
}else if(zone==3) {

xnew=-x;
}else if(zone==4) {

xnew=-x;
}else if(zone==5) {

xnew=-y;
}else if(zone==6) {

xnew=y;
}else if(zone==7) {

xnew=x;
}
return xnew;


}

float convertYi(float x, float y, float zone){
float ynew=0;

if(zone==0) {
ynew=y;
}else if(zone==1) {

ynew=x;
}else if(zone==2) {

ynew=x;
}else if(zone==3) {

ynew=y;
}else if(zone==4) {

ynew=-y;
}else if(zone==5) {

ynew=-x;
}else if(zone==6) {

ynew=-x;
}else if(zone==7) {

ynew=-y;
}
return ynew;

}


public void dispose(GLAutoDrawable arg0) {
//do nothing
}

}