PGraphics buf;
color pen;

int backSize = 20;

void setup() {

  size(300,300);
  buf = new PGraphics(600,600,null);  // This can be any size screen+
  buf.defaults();
  
  buf.background(255,255,255);
  background(255);

  pen = color(100,100,100,10);

  bx = buf.width/2-width;
  by = buf.height/2-height;

  ellipseMode(CENTER);
  strokeWeight(2);
  blitX = width/2-backSize/2;
  blitY = height/2-backSize/2;
  
  noSmooth();

  setCursor(Cursor.getPredefinedCursor(Cursor.CROSSHAIR_CURSOR));
}

void draw() {

  updatePos();

  // Blit buffer to screen, draw line and blit back modified section to buffer

  drawscn();

  if(mousePressed) {
  smoothLine(g,width/2,height/2,width/2+(int)xmag,height/2+(int)ymag,pen);
  backBlit();
  }
  stroke(0,255,0);
  ellipse(width/2, height/2, 2,2);
}

int blitX,blitY;

void backBlit() {

  x3=(((int)bx%buf.width)+buf.width+blitX)%buf.width;
  y3=(((int)by%buf.height)+buf.height+blitX)%buf.height;

  x1 = abs(buf.width-x3);
  x2 = abs(x1-backSize);
  y1 = abs(buf.height-y3);
  y2 = abs(y1-backSize);

  // blit X wrap around
  if(x3>=buf.width-backSize) {
    buf.copy(g,blitX+x1,blitY,blitX+x2+x1,blitY+backSize, 0,y3,x2,y3+backSize);

    // blit X and Y wrap
    if(y3>=buf.height-backSize) {
      buf.copy(g,blitX+x1,blitY+y1,blitX+x2+x1,blitY+y2+y1, 0,0,x2,y2);
    }
  }

  if(y3>=buf.height-backSize) {
    buf.copy(g,blitX,blitY+y1,blitX+backSize,blitY+y1+y2, x3,0,x3+backSize,y2);
  }

  buf.copy(g,blitX,blitY,blitX+backSize,blitY+backSize, x3,y3,x3+backSize,y3+backSize);
}

// draw large Bimage to screen as a continuous surface.
int x1,y1,x2,y2,x3,y3;

void drawscn() {

  x3=(((int)bx%buf.width)+buf.width)%buf.width;
  y3=(((int)by%buf.height)+buf.height)%buf.height;

  x1=abs(buf.width-x3);
  x2=abs(width-x1);
  y1=abs(buf.height-y3);
  y2=abs(height-y1);

  // blit X wrap around
  if(x3>=buf.width-width) {
    copy(buf,0,y3,x2,y3+y1, x1,0,x1+x2,y1);

    // blit X and Y wrap
    if(y3>=buf.height-height) {
      copy(buf,0,0,x2,y2,x1, y1,x1+x2,y1+y2);
    }
  }

  if(y3>=buf.height-height) {
    copy(buf,x3,0,x3+x1,y2, 0,y1,x1,y1+y2);
  }

  copy(buf,x3,y3,x3+x1,y3+y1, 0,0,x1,y1);
}

float bx,by;
float xmag, ymag = 0;

void updatePos() {
  xmag = (pmouseX-width/2)*0.050;
  ymag = (pmouseY-height/2)*0.050;
  bx = (bx+xmag)%buf.width;
  by = (by+ymag)%buf.height;
}

public void smoothLine(PGraphics g, int x1, int y1, int x2, int y2, color penCol) {
  g.stroke(penCol);      // Stroke on @ set weight
  g.line(x1,y1,x2,y2);     // draw the line
/*
  if(g.strokeWeight>1) {
    g.fill(penCol);
    g.noStroke();  
    g.ellipse(x1, y1, g.strokeWeight*2,g.strokeWeight*2);  // cap previous line with rounded end
  }
  */
}