Notes

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

/**
*
* @author sajib
*/
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, d;
private void DrawMPL(GL2 gl, float x1, float y1, float x2, float y2) {
//write your own code
gl.glPointSize(1.0f);
gl.glColor3d(1, 0, 0);

gl.glBegin(GL2.GL_POINTS);


int zonei = findZone(x1,y1,x2,y2);
float x10 = convertX0(x1, y1,zonei);
float y10 = convertY0(x1, y1,zonei);
float x20 = convertX0(x2, y2,zonei);
float y20 = convertY0(x2, y2,zonei);
dx = x20 - x10;
dy = y20 - y10;
d = dy + 2*dx;
float dse = 2*dy + 2*dx;
float ds = 2*dx;

gl.glVertex2f(x1, y1);

while(y10>y20) {
if(d>=0) {
x10 +=1;
y10 -=1;
float xt= convertXi(x10, y10, zonei);
float yt= convertYi(x10, y10, zonei);
gl.glVertex2f(xt, yt);
d = d+dse;
}
else {
y10 -=1;
float xt= convertXi(x10, y10, zonei);
float yt= convertYi(x10, y10, zonei);
gl.glVertex2f(xt, yt);
d = d+ds;
}
}


gl.glEnd();
}

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


int 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;
}
else if ((dx<0) && (dy>=0)) {
zone=3;
}
else if ((dx<0) && (dy<0)) {
zone=4;
}
else {
zone=7;
}
}
else {
if((dx>=0) && (dy>=0)) {
zone=1;
}
else if ((dx<0) && (dy>=0)) {
zone=2;
}
else if ((dx<0) && (dy<0)) {
zone=5;
}
else {
zone=6;
}
}

return (int) zone ;
}

float convertX0(float x, float y, int zone){
float convertedX0=0;
if (zone == 0) return y;
else if (zone == 1) return x;
else if (zone == 2) return -x;
else if (zone == 3) return y;
else if (zone == 4) return -y;
else if (zone == 5) return -x;
else if (zone == 6) return x;
else if (zone == 7) return -y;
return convertedX0;
}

float convertY0(float x, float y, int zone){
float convertedY0=0;
if (zone == 0) return -x;
else if (zone == 1) return -y;
else if (zone == 2) return -y;
else if (zone == 3) return x;
else if (zone == 4) return x;
else if (zone == 5) return y;
else if (zone == 6) return y;
else if (zone == 7) return -x;
return convertedY0;
}
float convertXi(float x, float y, int zone){
float convertedXi=0;
if (zone == 0) return -y;
else if (zone == 1) return x;
else if (zone == 2) return -x;
else if (zone == 3) return y;
else if (zone == 4) return y;
else if (zone == 5) return -x;
else if (zone == 6) return x;
else if (zone == 7) return -y;
return convertedXi;
}

float convertYi(float x, float y, int zone){
float convertedYi=0;
if (zone == 0) return x;
else if (zone == 1) return -y;
else if (zone == 2) return -y;
else if (zone == 3) return x;
else if (zone == 4) return -x;
else if (zone == 5) return y;
else if (zone == 6) return y;
else if (zone == 7) return -x;
return convertedYi;
}


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