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Table of Contents
Get Canvas Context
Draw basic shapes
rectangle
Path
Set styles and colors
Text drawing
Home Web Front-end HTML Tutorial HTML `canvas` Context and Drawing Basics

HTML `canvas` Context and Drawing Basics

Jul 20, 2025 am 01:18 AM

To draw using HTML , 1. First get the canvas element and get the 2D context through JavaScript; 2. You must correctly set the width and height of canvas to avoid blur; 3. Use fillRect, strokeRect and clearRect to draw rectangles; 4. Use beginPath to start the path, draw complex graphics in combination with moveTo, lineTo, arc, etc., and display it with stroke or fill; 5. Set fillStyle, strokeStyle, lineWidth and other attributes to control styles, supporting multiple color formats and transparency; 6. Use fillText and strokeText to draw text, and set font styles through the font attribute.

HTML `canvas` Context and Drawing Basics

The <canvas></canvas> element of HTML itself is just a drawing area, and what really controls the drawing is its "context". To draw with <canvas></canvas> , you first get the context and then use the API it provides to perform various graphics drawings.

HTML `canvas` Context and Drawing Basics

Get Canvas Context

After adding a <canvas></canvas> tag to HTML, the first step is to use JavaScript to get its drawing context:

 const canvas = document.getElementById(&#39;myCanvas&#39;);
const ctx = canvas.getContext(&#39;2d&#39;);

getContext(&#39;2d&#39;) is the most commonly used call method, indicating that you get the 2D rendering context. Only by getting this ctx object can you start drawing.

HTML `canvas` Context and Drawing Basics

Common errors: Forgot to set the width and height of the canvas, the default size is 300×150. If the CSS stretches the canvas without setting the actual pixel size, the image may be blurred or deformed.

Draw basic shapes

With context, you can start drawing basic graphics such as rectangles, paths, etc.

HTML `canvas` Context and Drawing Basics

rectangle

  • fillRect(x, y, width, height) : fill a solid rectangle
  • strokeRect(x, y, width, height) : stroke rectangle
  • clearRect(x, y, width, height) : Clear the contents of the specified area

for example:

 ctx.fillStyle = &#39;blue&#39;;
ctx.fillRect(10, 10, 100, 100);

Path

When drawing more complex graphics, you need to use a path:

  1. Call beginPath() to start a new path
  2. Use moveTo() to locate the starting point, lineTo() to draw lines, arc() to draw arc, etc.
  3. Finally, use stroke() or fill() to display the graphics

For example, draw a line from (10,10) to (100,100):

 ctx.beginPath();
ctx.moveTo(10, 10);
ctx.lineTo(100, 100);
ctx.stroke();

Note: Not calling beginPath() may cause multiple paths to interfere with each other.

Set styles and colors

When drawing a graphic, you can set style properties such as line color, fill color, line width, etc.:

  • fillStyle : Fill color
  • strokeStyle : stroke color
  • lineWidth : line width

Supported color formats include hexadecimal, RGB, rgba, and color names:

 ctx.fillStyle = &#39;rgba(255, 0, 0, 0.5)&#39;;
ctx.strokeStyle = &#39;#00ff00&#39;;
ctx.lineWidth = 5;

Small details: The transparency (alpha) value ranges from 0 to 1, 0 means completely transparent, and 1 means completely opaque.

In addition, the line connection and endpoint style can also be adjusted:

  • lineCap : Set the end style of the line segment (butt/round/square)
  • lineJoin : Set the style at the line connection (bevel/round/miter)

Text drawing

In addition to graphics, you can also draw text:

  • fillText(text, x, y) : fill text
  • strokeText(text, x, y) : stroke text

You can set the font style:

 ctx.font = &#39;20px Arial&#39;;
ctx.fillText(&#39;Hello World&#39;, 10, 50);

The font string format is similar to CSS, including size, thickness, font family and other information.

Basically that's it. By mastering these basic operations, you can realize most common drawing requirements on canvas.

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