Algorithm for drawing lines with 90-degree angles

Question

To get straight to the point, what I hope to achieve is to be able to create a connecting line between two elements with this shape:

DBDIAGRAM.IO

When the elements move, the line resets but always maintains a 90 degree angle, instead of being a straight or diagonal line between [x,y] to [x,y].

Is there some kind of algorithm for this? Maybe a grid with some kind of A* implementation?

Answer 1

I don't know how to make rounded corners easy, but the easiest example will be this:

const canvas = document.querySelector('canvas')
const ctx = canvas.getContext('2d')

// define the points
const p1 = {
  x: 30,
  y: 50
}

const p2 = {
  x: 150,
  y: 130
}

ctx.strokeStyle = 'red'

// draw the points
ctx.beginPath()
ctx.arc(p1.x, p1.y, 5, 0, Math.PI * 2)
ctx.stroke()

ctx.beginPath()
ctx.arc(p2.x, p2.y, 5, 0, Math.PI * 2)
ctx.stroke()

// get distance between
const horizontalDistance = p2.x - p1.x

ctx.strokeStyle = 'black'


// draw left part
ctx.beginPath()
ctx.moveTo(p1.x, p1.y)
ctx.lineTo(p1.x + horizontalDistance / 2, p1.y)
ctx.stroke()

// draw vertical part
ctx.beginPath()
ctx.moveTo(p1.x + horizontalDistance / 2, p1.y)
ctx.lineTo(p1.x + horizontalDistance / 2, p2.y)
ctx.stroke()

// draw right part
ctx.beginPath()
ctx.moveTo(p1.x + horizontalDistance / 2, p2.y)
ctx.lineTo(p2.x, p2.y)
ctx.stroke()

canvas {
  border: 1px solid black;
}

<canvas></canvas>

Real-time version:

const canvas = document.querySelector('canvas')
const ctx = canvas.getContext('2d')

const p1 = {
  x: canvas.width / 2,
  y: canvas.height / 2
}

const p2 = {
  x: 150,
  y: 130
}

canvas.addEventListener('mousemove', e => {
  const mousePos = getMousePos(canvas, e)
  p2.x = mousePos.x
  p2.y = mousePos.y
})

loop()

function loop() {
  draw()
  requestAnimationFrame(loop)
}

function draw() {
  ctx.clearRect(0, 0, canvas.width, canvas.height)
  ctx.strokeStyle = 'red'

  ctx.beginPath()
  ctx.arc(p1.x, p1.y, 5, 0, Math.PI * 2)
  ctx.stroke()

  ctx.beginPath()
  ctx.arc(p2.x, p2.y, 5, 0, Math.PI * 2)
  ctx.stroke()

  const horizontalDistance = p2.x - p1.x

  ctx.strokeStyle = 'black'


  ctx.beginPath()
  ctx.moveTo(p1.x, p1.y)
  ctx.lineTo(p1.x + horizontalDistance / 2, p1.y)
  ctx.stroke()

  ctx.beginPath()
  ctx.moveTo(p1.x + horizontalDistance / 2, p1.y)
  ctx.lineTo(p1.x + horizontalDistance / 2, p2.y)
  ctx.stroke()

  ctx.beginPath()
  ctx.moveTo(p1.x + horizontalDistance / 2, p2.y)
  ctx.lineTo(p2.x, p2.y)
  ctx.stroke()

}

function getMousePos(canvas, evt) {
  const rect = canvas.getBoundingClientRect();
  return {
    x: evt.clientX - rect.left,
    y: evt.clientY - rect.top
  };
}

canvas {
  border: 1px solid black;
}

<canvas></canvas>