quad-by-clicking

int srcX, srcY, dstX, dstY;

void setup()
{
    srcX = srcY = dstX = dstY = 0;
    size(640, 480);
}

void draw()
{
    background(125);
    //line(srcX, srcY, dstX, dstY);

    if (srcX != dstX && srcY != dstY) {
    	drawSquare();
    }
}

void drawSquare()
{
    float size = dist(srcX, srcY, dstX, dstY);
    PVector src = new PVector(srcX, srcY);
    PVector dst = new PVector(dstX, dstY); 
    PVector dir = PVector.sub(dst, src);
    dir.rotate(HALF_PI);
    dir.limit(size/2);
    PVector v1 = dir.get();
    PVector v2 = v1.get();
    v2.rotate(PI);
    PVector v3 = v2.get();
    PVector v4 = v1.get();
    v1.add(src);
    v2.add(src);
    v3.add(dst);
    v4.add(dst);
    quad(v1.x, v1.y, v2.x, v2.y, v3.x, v3.y, v4.x, v4.y);
}

void mousePressed()
{
    srcX = mouseX;
    srcY = mouseY;
    dstX = mouseX;
    dstY = mouseY;
}

void mouseReleased()
{
    dstX = mouseX;
    dstY = mouseY;
}