now i am drawing figures with "line" function,  I can determine line coordinates like this: 

 line(mouseX, mouseY, pmouseX, pmouseY ); 

 

so i can assign mouseX, and mouseY to my variables cX and cY, and i need to send those values to serial port. Values must be between 0 and 255. Maybe this vill be more helpful if i am gonna show you my full code:

import processing.serial.*;
Serial port;
PFont f;
public void setup() {
port=new Serial(this, "COM3", 115200);
  size(255, 255);
background(0,0,0);
f = createFont("Arial",16,true);

}

void draw()

{
  int cX;
int cY;
int e;
int cPrevX = 0;
int cPrevY = 0;
int linijaX1 = 2;

int linijaY1 = 2; 
textFont(f,16);
fill(0,255,0);
  text("My tekst ",20,20);  
stroke(0,255,0);
textFont(f,16);

float linijaX = (float)height/linijaX1;
float linijaY = (float)width/linijaY1;

for(int i = 0; i < linijaX1; i++)
{
line(0, i*linijaX, width, i*linijaX);

}

for(int i = 0; i < linijaY1; i++)
{
line (i*linijaY,0,i*linijaY, height);
}

stroke(255);
if(mousePressed)
{

line(mouseX,mouseY,pmouseX, pmouseY);

cX = mouseX;
cY = mouseY;


if ((cPrevX != cX) || (cPrevY != cY))
{
cPrevX = cX;
cPrevY = cY;
  port.write (cX);
  port.write (cY);
 } 
}
}

i just want to press mouse button, draw a rectangle and send its coordinates to serial port. 

but when i do this: rect(10,10,20,20); i dont know what how to assignt cX, and cY:

cX = ????;

cY = ????;

if am writing:

cX = 10;

cY = 10;

so the serial port is getting two values. 10, and 10.