Draggable curdrag = null;
Draggable[] corners = new Draggable[5];
Draggable las = new Draggable(60,60);

float curx,cury;
float tryscaley,tryscalex,lastscalex,lastscaley;

void setup(){
 tryscaley = 0.5;
 tryscalex = 0.5;
 lastscalex = 1;
 lastscaley = 1;
 size(500,500);
 frameRate(30);
 smooth();
 corners[0] = new Draggable(50,50);
 corners[1] = new Draggable(450,50);
 corners[2] = new Draggable(400,450);
 corners[3] = new Draggable(100,450);
}

void draw(){
  fill(255);
  background(120);
  las.x = mouseX;
  las.y = mouseY;
    lines();
  for (int i=0; i<4; i++){
   corners[i].update();
  }
  fill(255,0,0);
  las.update();
  ellipse(500*tryscalex-5,500*tryscaley-5,10,10);
}

void lines(){
  line(corners[0].x,corners[0].y, corners[1].x, corners[1].y);
  line(corners[1].x,corners[1].y, corners[2].x, corners[2].y);
  line(corners[2].x,corners[2].y, corners[3].x, corners[3].y);
  line(corners[3].x,corners[3].y, corners[0].x, corners[0].y);
  float mx = mouseX;
  float my = mouseY;
  //dotx(corners[0].x,corners[0].y,corners[1].x,corners[1].y,mx/500);
  //dotx(corners[3].x,corners[3].y,corners[2].x,corners[2].y,mx/500);
  //doty(corners[0].x,corners[0].y,corners[3].x,corners[3].y,tryscale);
  //doty(corners[1].x,corners[1].y,corners[2].x,corners[2].y,tryscale);
  tryscaley = 0.5;
  tryscalex = 0.5;
  lastscalex = 1;
  lastscaley = 1;
  for(int i=0; i<12; i++){
   tryfory(corners[0].x,corners[0].y,corners[3].x,corners[3].y,corners[1].x,corners[1].y,corners[2].x,corners[2].y,tryscaley);
   tryforx(corners[0].x,corners[0].y,corners[1].x,corners[1].y,corners[3].x,corners[3].y,corners[2].x,corners[2].y,tryscalex);
  }
}

void tryfory(float ax1, float ay1, float ax2, float ay2, float bx1, float by1, float bx2, float by2,float ascale){
  float y1 = ay1+ascale*(ay2-ay1);
  float x1 = ax1+(y1-ay1)/(ay2-ay1)*(ax2-ax1);
  float y2 = by1+ascale*(by2-by1);
  float x2 = bx1+(y1-by1)/(by2-by1)*(bx2-bx1);
  //result: two points that have the same virtual y
  //Now, find where cursor y would be at a given x
  float tcy = (y2-y1)/(x2-x1)*(las.x-x1)+y1;
  line(x1,y1,x2,y2);
  if(tcy < las.y){
   float newsy = tryscaley + Math.abs((lastscaley-tryscaley)/2);
   lastscaley = tryscaley;
   tryscaley = newsy;
  } else if(tcy > las.y){
   float newsy = tryscaley - Math.abs((lastscaley-tryscaley)/2);
   lastscaley = tryscaley;
   tryscaley = newsy;
  }
}

void tryforx(float ax1, float ay1, float ax2, float ay2, float bx1, float by1, float bx2, float by2,float ascale){
  float x1 = ax1+ascale*(ax2-ax1);
  float y1 = ay1+(x1-ax1)/(ax2-ax1)*(ay2-ay1);
  float x2 = bx1+ascale*(bx2-bx1);
  float y2 = by1+(x1-bx1)/(bx2-bx1)*(by2-by1);
  //result: two points that have the same virtual y
  //Now, find where cursor y would be at a given x
  float tcx = (x2-x1)/(y2-y1)*(las.y-y1)+x1;
  line(x1,y1,x2,y2);
  if(tcx < las.x){
   float newsx = tryscalex + Math.abs((lastscalex-tryscalex)/2);
   lastscalex = tryscalex;
   tryscalex = newsx;
  } else if(tcx > las.x){
   float newsx = tryscalex - Math.abs((lastscalex-tryscalex)/2);
   lastscalex = tryscalex;
   tryscalex = newsx;
  }
}

void dotx(float ax1, float ay1, float ax2, float ay2, float ascale){
 float x = ax1+ascale*(ax2-ax1);
 float y = ay1+(x-ax1)/(ax2-ax1)*(ay2-ay1);
 ellipse(x,y,6,6);
}

void doty(float ax1, float ay1, float ax2, float ay2, float ascale){
 float y = ay1+ascale*(ay2-ay1);
 float x = ax1+(y-ay1)/(ay2-ay1)*(ax2-ax1);
 ellipse(x,y,6,6);
}

void mousePressed(){
  for (int i=0; i<4; i++){
   Draggable d = corners[i];
 if(mouseX > d.x-5 && mouseX < d.x+d.wid+5){
  if(mouseY > d.y-5 && mouseY < d.y+d.wid+5){
   d.dragging = true;
   curdrag = d;
  } 
 }
  }
}

void mouseReleased(){
 if(curdrag != null){
  curdrag.dragging = false;
  curdrag = null;
 } 
}