HTML Canvas Basics & DPR Drawing Techniques

JS Syntax

Canvas

2025 Aug 06480 words|Estimated reading time: 3 minutes

What is Canvas

Canvas is an HTML5 element that lets you draw graphics on web pages using JavaScript. Think of it as a "drawing board" where you can create visuals with JS.

Basic Canvas Usage

Using Canvas is simple. Just add a <canvas> tag in HTML, then get its drawing context in JS.

<canvas id="myCanvas" width="500" height="300"></canvas>

const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');

// Draw a red rectangle
ctx.fillStyle = 'red';
ctx.fillRect(10, 10, 100, 100);

The fillRect method helps you draw rectangles easily.

Drawing Images on Canvas

You can also draw images on Canvas:

const canvas = document.getElementById('myCanvas');
const ctx = canvas.getContext('2d');

const img = new Image();
img.src = 'path/to/image.jpg';

img.onload = function() {
  // Draw image at original size
  ctx.drawImage(img, 0, 0);
  
  // Draw scaled image (200px width)
  ctx.drawImage(img, 0, 0, 200, img.height * (200/img.width));
};

Sometimes images may appear blurry. This happens when Canvas size is too small for the screen resolution. Let's fix this.

Canvas Size: width vs clientWidth

width and height

These properties control Canvas's actual pixel size for drawing.

canvas.width = 800;  // Set width to 800px
canvas.height = 600; // Set height to 600px

clientWidth and clientHeight

These show Canvas's display size on the webpage (set by CSS).

#myCanvas {
  width: 400px;
  height: 300px;
}

Key Differences

Feature	`width`/`height`	`clientWidth`/`clientHeight`
Type	Read/write	Read-only
Unit	Pixels	Pixels
Effect	Drawing area	Display size
Default	300x150px	CSS determines

Note: When canvas.width < CSS width × DPR, browsers stretch pixels, causing blurriness.

What is DPR?

DPR (Device Pixel Ratio) means how many physical pixels make one CSS pixel. High-DPR screens (like Retina) use more pixels per CSS pixel.

const dpr = window.devicePixelRatio || 1;

Without adjustment, Canvas drawings may look blurry on high-DPR screens.

How to Prevent Blurriness

Adjust Canvas size to match DPR:

function setupCanvas(canvas) {
  const dpr = window.devicePixelRatio || 1;
  const rect = canvas.getBoundingClientRect();
  
  // Set actual drawing size
  canvas.width = rect.width * dpr;
  canvas.height = rect.height * dpr;
  
  // Set display size
  canvas.style.width = rect.width + 'px';
  canvas.style.height = rect.height + 'px';
  
  // Scale the drawing context
  const ctx = canvas.getContext('2d');
  ctx.scale(dpr, dpr);
  
  return ctx;
}

// Example
const canvas = document.getElementById('myCanvas');
const ctx = setupCanvas(canvas);

// Now drawings will be sharp
ctx.fillRect(10, 10, 100, 100);

For best results:

When canvas.width === CSS width × DPR: 1 Canvas pixel = 1 physical pixel (sharpest)
When canvas.width < CSS width × DPR: Canvas pixels stretch (blurry)
When canvas.width > CSS width × DPR: Canvas pixels compress (sharp but may waste resources)

This ensures clear graphics on all screens.