How to create missing semicircle graphics in javascript

JavaScript is a popular and powerful programming language that is often used for the development of websites, applications, and games. When making a website, graphic elements are an integral part. Many websites require the use of graphic elements in various shapes, such as lines, rectangles, circles, and semicircles. In this article, we will discuss how to make a half-circle graphic using JavaScript.

1. Understand the method of realizing a semicircle

Semicircular graphics are usually composed of three elements: circle, cutting line and cutting area. If you want to make a circle with half of it missing, you need to delete a semicircle from the entire circle and add a line for the cutout. Then, the remaining semicircle needs to be filled with the appropriate color.

2. Making a circle with half missing

In JavaScript, making a circle with half missing requires the following steps:

1. Create an HTML Canvas element.

2. Create a drawing context object.

3. Use the beginPath() method to start creating the path.

4. Use the arc() method to create a circular path.

5. Use the lineTo() method to add a cutting line path.

6. Use the closePath() method to end the path.

7. Use fill() or stroke() method to fill or stroke.

8. Set styles and add graphics to the page.

The following is a JavaScript code example to implement a circle with half of it missing:

let canvas = document.createElement('canvas');
let ctx = canvas.getContext('2d');

canvas.width = 200;
canvas.height = 200;

ctx.beginPath();
ctx.moveTo(canvas.width / 2, canvas.height / 2);
ctx.arc(
  canvas.width / 2,
  canvas.height / 2,
  100,
  (Math.PI / 2),
  -(Math.PI / 2),
  true
);
ctx.lineTo(canvas.width / 2, canvas.height / 2);
ctx.closePath();
ctx.fillStyle = 'red';
ctx.fill();

document.body.appendChild(canvas);

In this code, we first create a canvas element and use the getContext() method to obtain the context object. Then, we set the width and height of the canvas and start creating the path using the beginPath() method. Next, use the arc() method to create a circular path starting at 90 degrees (i.e. 1/2π) and ending at -90 degrees (i.e. -1/2π), indicating that we cut the top half. Finally, we use the lineTo() method to create a line connecting the circle and use the fillStyle property to fill the remaining circle portion with red.

3. Use SVG to realize a circle missing half of it

When using JavaScript to make a semicircle, there is another way to use SVG to achieve it. SVG (Scalable Vector Graphics) is an XML format used to describe vector-based graphics. Using SVG, you can easily create and edit various types of graphics, including semicircles and circles with half of them missing.

Here is a code example using SVG to create a missing half of a circle:

<svg>
  <path></path>
</svg>

In this code, we first create a 200x200 pixel SVG element. Next, we create a path using its path element. The d attribute of a path describes the shape of the path. "M 100,100" means creating a path starting from coordinates (100,100). "a 100,100 0 1 1 -2,0 z" means using an elliptical arc path to draw a circle with half of it missing.

4. Summary

Semicircles are a common graphic element when making websites and applications, and JavaScript and SVG can easily implement circles with half of them missing. With the advancement of technology, web developers have many powerful tools to achieve various complex graphics and animation effects. However, for beginners, it is still very important to understand the basics and techniques. As you learn to make graphic elements, remember to stay focused on every step of the code and always strive for perfection.

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