If you want to draw pictures, make animations or even develop mini-games on web pages, you can use the
If you want to draw pictures, make animations or even develop mini-games on web pages, the HTML5 <canvas></canvas> tag is a very practical tool. It itself is just a blank canvas, and the real magic lies in using JavaScript to control how and what it draws.
Create Canvas elements
First, add a <canvas></canvas> tag to HTML:
<canvas id="myCanvas" width="500" height="300"></canvas>
This tag is transparent by default, has no content, and does not come with any drawing capabilities. You can define its dimensions by setting width and height properties. If not set, the default size (usually 300x150) is used.
Then get the element in JavaScript and get the drawing context:
const canvas = document.getElementById('myCanvas'); const ctx = canvas.getContext('2d');
ctx is the object you use to "draw", and almost all operations revolve around it.
Draw basic graphics
Once you have the context, you can start drawing things. For example, drawing a simple rectangle:
ctx.fillStyle = 'blue'; ctx.fillRect(50, 50, 100, 100);
This code sets the fill color to blue, and then starts from coordinates (50, 50), and draws a rectangle of 100 pixels in width and height.
You can also draw borders, circles, lines, etc. For example, draw a red circle:
ctx.beginPath(); ctx.arc(200, 150, 40, 0, Math.PI * 2); ctx.fillStyle = 'red'; ctx.fill(); ctx.closePath();
-
beginPath()means starting a new path; -
arc(x, y, radius, startAngle, endAngle)is the method of drawing circles; -
fill()fills the path; -
closePath()closes the path.
How to do animation and interaction?
Canvas can not only draw statically, but also create animation effects. A common practice is to constantly clear the canvas and repaint the contents of the painting. For example, do an animation of a small ball moving:
let x = 0;
function draw() {
ctx.clearRect(0, 0, canvas.width, canvas.height); // Clear canvas ctx.beginPath();
ctx.arc(x, 150, 20, 0, Math.PI * 2);
ctx.fillStyle = 'green';
ctx.fill();
x = 2;
if (x > canvas.width) x = 0; // loop requestAnimationFrame(draw); // continuously draw}
draw();This will enable the effect of a small ball moving back and forth on the screen. You can also interact with mouse events or touch events, such as clicking on the canvas to change the color, dragging animals, etc.
Notes and FAQs
- Resolution problem : If the image is blurred after the page is scaled, it may be because the CSS sets the canvas size but does not adjust it to the actual pixels. It is recommended to keep
canvas.widthandcanvas.heightmatching the display size. - Performance optimization : Try to update only the changing part when repainting frequently, not the entire canvas.
- Browser compatibility : Modern browsers basically support Canvas, but there may be problems on older versions of IE.
- Export picture : You can use
canvas.toDataURL()method to save the current picture as a picture.
Basically that's it. Canvas doesn't seem complicated, but there are many details, especially when it comes to animation, interaction or a lot of graphics processing, you need to practice more.
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