Understand the JS technology of canvas: What are you familiar with?

Explore canvas JS technologies: Do you know which ones are there?

Introduction

In modern web development, JavaScript has become an indispensable part. As a scripting language, it can add interactivity and dynamics to web pages. In JS technology, canvas is one of the important APIs. This article will give you an in-depth understanding of canvas JS technology, and introduce some commonly used canvas-related functions and sample codes.

What is Canvas?

Canvas is an important technology in HTML5, which can draw graphics, process images, create animations, etc. through JavaScript scripts. Simply put, it is a canvas on which we can paint.

Basic steps for using Canvas

To use Canvas, we first need to create a Canvas element in the HTML file. An example is as follows:

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

Then, we need to get this Canvas element in JavaScript and create a canvas object. An example is as follows:

var canvas = document.getElementById("myCanvas");
var ctx = canvas.getContext("2d");

Here, getContext("2d") indicates that we want to draw 2D on the canvas.

Draw shapes

In Canvas, we can use a series of methods to draw various shapes, such as rectangles, circles, lines, etc. The following are some commonly used drawing methods and sample codes:

1. Drawing a rectangle

Use the ctx.rect() method to draw a rectangle. The sample code is as follows:

ctx.fillStyle = "red";
ctx.fillRect(50, 50, 100, 100);

This code will draw a red rectangle with a width and height of 100 on the canvas.

2. Draw a circle

Use the ctx.arc() method to draw a circle. The sample code is as follows:

ctx.beginPath();
ctx.arc(200, 150, 50, 0, Math.PI * 2, false);
ctx.fillStyle = "blue";
ctx.fill();

This code will draw a blue circle with a radius of 50 on the canvas.

3. Draw a straight line

Use the ctx.moveTo() and ctx.lineTo() methods to draw a straight line. The sample code is as follows:

ctx.beginPath();
ctx.moveTo(300, 50);
ctx.lineTo(400, 150);
ctx.strokeStyle = "green";
ctx.lineWidth = 3;
ctx.stroke();

This code will draw a green straight line on the canvas with the starting point coordinates (300, 50) and the end point coordinates (400, 150).

Processing images

Canvas can not only draw shapes, but also process images. The following are some commonly used image processing methods and sample codes:

1. Draw an image

Use the ctx.drawImage() method to draw an image on the canvas. The sample code is as follows:

var img = new Image();
img.src = "image.png";
img.onload = function() {
  ctx.drawImage(img, 50, 50);
};

This code will draw an image named image.png on the canvas at the position (50, 50).

2. Set image transparency

Use the ctx.globalAlpha property to set the transparency of the image. The sample code is as follows:

ctx.globalAlpha = 0.5;
ctx.drawImage(img, 50, 50);

This code will draw an image with a transparency of 0.5 on the canvas.

3. Crop the image

Use the ctx.drawImage() method and ctx.clip() method to crop the image. The sample code is as follows:

ctx.drawImage(img, 0, 0);
ctx.beginPath();
ctx.arc(150, 150, 100, 0, Math.PI * 2, false);
ctx.clip();
ctx.drawImage(img, 50, 50);

This code will draw a cropped image on the canvas, and the cropped area is a circle with a diameter of 200.

Create animation

Canvas can also be used to create animation effects. The following is a simple animation sample code:

var x = 0;

function draw() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  ctx.beginPath();
  ctx.arc(x, 150, 50, 0, Math.PI * 2, false);
  ctx.fillStyle = "red";
  ctx.fill();
  x += 2;
  requestAnimationFrame(draw); // 实现动画效果
}

draw();

This code will draw an animation from the left on the canvas. Red circle moving to the right.

Summary

This article introduces the JS technology of canvas, including basic steps, drawing shapes, processing images, and creating animations. By learning these contents, we can better use canvas to achieve various dynamic effects and add more interactivity and visual appeal to web pages. I hope this article will help you gain a deeper understanding of canvas JS technology!

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