[Canvas] Make Canvas Responsive to Pixel Ratio

Canvas is great for high performance graphics rendering but by default the results look blocky on phones tablets and laptops with high pixel density or Retina displays. By using canvas width and height attributes and style props we can use window.devicePixelRatio to create a canvas that is responsive to pixel ratio.

It's much faster to paint graphics to the screen using canvas and DOM. But on high pixel density displays, like phones and Macbooks, the results look pixelated. This is because by default one canvas pixel equals one CSS pixel, not one screen pixel.

To fix this, let's create a scale factor from the device pixel ratio. We use a scale factors to create a canvas, use width and height attributes, so double the CSS pixel width and height sizes we require. We make all our painting calls relative to the scale factor.

const canvas = document.querySelector('canvas')
const sf = window.devicePixelRatio
const elWidth = canvas.width
const elHeight = canvas.height
canvas.width = elWidth * sf
canvas.height = elHeight * sf
const ctx = canvas.getContext('2d')
ctx.arc(
  canvas.width / 2,
  canvas.height / 2,
  canvas.width / 2,
  0,
  Math.PI * 2
)
ctx.fill()
canvas.style.width = `${elWidth}px`

To recap, the width and height attributes on the canvas element determine the number of pixels in the canvas, while the CSS width and height properties determine the size it will display on the screen. By setting an element's attributes different to the CSS properties.