HTML5 Canvas
The <canvas> element is used to draw graphics, on the fly, on a web page.
The example at the left shows a red rectangle, a gradient rectangle, a multicolor rectangle, and some multicolor text that is drawn onto the canvas.
What is Canvas?
The HTML5 <canvas> element is used to draw graphics, on the fly, via scripting (usually JavaScript).
The <canvas> element is only a container for graphics. You must use a script to actually draw the graphics.
Canvas has several methods for drawing paths, boxes, circles, text, and adding images.
Browser Support
Internet Explorer 9+, Firefox, Opera, Chrome, and Safari support the <canvas> element.
Note: Internet Explorer 8 and earlier versions, do not support the <canvas> element.
Create a Canvas
A canvas is a rectangular area on an HTML page, and it is specified with the <canvas> element.
Note: By default, the <canvas> element has no border and no content.
The markup looks like this:
Note: Always specify an id attribute (to be referred to in a script), and a width and height attribute to define the size of the canvas.
Tip: You can have multiple <canvas> elements on one HTML page.
To add a border, use the style attribute:
Example
style=”border:1px solid #000000;”>
</canvas>
Draw Onto The Canvas With JavaScript
All drawing on the canvas must be done inside a JavaScript:
Example
var c = document.getElementById(“myCanvas”);
var ctx = c.getContext(“2d”);
ctx.fillStyle = “#FF0000”;
ctx.fillRect(0,0,150,75);
</script>
Example explained:
First, find the <canvas> element:
Then, call its getContext() method (you must pass the string “2d” to the getContext() method):
The getContext(“2d”) object is a built-in HTML5 object, with many properties and methods for drawing paths, boxes, circles, text, images, and more.
The next two lines draw a red rectangle:
ctx.fillRect(0,0,150,75);
The fillStyle property can be a CSS color, a gradient, or a pattern. The default fillStyle is #000000 (black).
The fillRect(x,y,width,height) method draws a rectangle filled with the current fill style.
Canvas Coordinates
The canvas is a two-dimensional grid.
The upper-left corner of the canvas has coordinate (0,0)
So, the fillRect() method above had the parameters (0,0,150,75).
This means: Start at the upper-left corner (0,0) and draw a 150×75 pixels rectangle.
Coordinates Example
Mouse over the rectangle below to see its x and y coordinates:
Canvas – Paths
To draw straight lines on a canvas, we will use the following two methods:
- moveTo(x,y) defines the starting point of the line
- lineTo(x,y) defines the ending point of the line
To actually draw the line, we must use one of the “ink” methods, like stroke().
Example
Define a starting point in position (0,0), and an ending point in position (200,100). Then use the stroke() method to actually draw the line:
JavaScript:
var ctx = c.getContext(“2d”);
ctx.moveTo(0,0);
ctx.lineTo(200,100);
ctx.stroke();
To draw a circle on a canvas, we will use the following method:
- arc(x,y,r,start,stop)
To actually draw the circle, we must use one of the “ink” methods, like stroke() or fill().
Example
Create a circle with the arc() method:
JavaScript:
var ctx = c.getContext(“2d”);
ctx.beginPath();
ctx.arc(95,50,40,0,2*Math.PI);
ctx.stroke();
Canvas – Text
To draw text on a canvas, the most important property and methods are:
- font – defines the font properties for text
- fillText(text,x,y) – Draws “filled” text on the canvas
- strokeText(text,x,y) – Draws text on the canvas (no fill)
Using fillText():
Example
Write a 30px high filled text on the canvas, using the font “Arial”:
JavaScript:
var ctx = c.getContext(“2d”);
ctx.font = “30px Arial”;
ctx.fillText(“Hello World”,10,50);
Using strokeText():
Example
Write a 30px high text (no fill) on the canvas, using the font “Arial”:
JavaScript:
var ctx = c.getContext(“2d”);
ctx.font = “30px Arial”;
ctx.strokeText(“Hello World”,10,50);
Canvas – Gradients
Gradients can be used to fill rectangles, circles, lines, text, etc. Shapes on the canvas are not limited to solid colors.
There are two different types of gradients:
- createLinearGradient(x,y,x1,y1) – Creates a linear gradient
- createRadialGradient(x,y,r,x1,y1,r1) – Creates a radial/circular gradient
Once we have a gradient object, we must add two or more color stops.
The addColorStop() method specifies the color stops, and its position along the gradient. Gradient positions can be anywhere between 0 to 1.
To use the gradient, set the fillStyle or strokeStyle property to the gradient, and then draw the shape, like a rectangle, text, or a line.
Using createLinearGradient():
Example
Create a linear gradient. Fill rectangle with the gradient:
JavaScript:
var ctx = c.getContext(“2d”);// Create gradient
var grd = ctx.createLinearGradient(0,0,200,0);
grd.addColorStop(0,”red”);
grd.addColorStop(1,”white”);// Fill with gradient
ctx.fillStyle = grd;
ctx.fillRect(10,10,150,80);
Using createRadialGradient():
Example
Create a radial/circular gradient. Fill rectangle with the gradient:
JavaScript:
var ctx = c.getContext(“2d”);// Create gradient
var grd = ctx.createRadialGradient(75,50,5,90,60,100);
grd.addColorStop(0,”red”);
grd.addColorStop(1,”white”);// Fill with gradient
ctx.fillStyle = grd;
ctx.fillRect(10,10,150,80);
Canvas – Images
To draw an image on a canvas, we will use the following method:
- drawImage(image,x,y)
Image to use:
Example
Draw the image onto the canvas:
JavaScript:
var ctx = c.getContext(“2d”);
var img = document.getElementById(“scream”);
ctx.drawImage(img,10,10);
The HTML <canvas> Tag
| Tag | Description |
|---|---|
| <canvas> | Used to draw graphics, on the fly, via scripting (usually JavaScript) |
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