The web Programming

Friday, 2 May 2014

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:

<canvas id="myCanvas" width="200" height="100"></canvas>

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

<canvas id="myCanvas" width="200" height="100"
style="border:1px solid #000000;">
</canvas>

Try it yourself »

Draw Onto The Canvas With JavaScript

All drawing on the canvas must be done inside a JavaScript:

Example

<script>
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.fillStyle = "#FF0000";
ctx.fillRect(0,0,150,75);
</script>

Try it yourself »

Example explained:
First, find the <canvas> element:

var c = document.getElementById("myCanvas");

Then, call its getContext() method (you must pass the string "2d" to the getContext() method):

var ctx = c.getContext("2d");

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.fillStyle = "#FF0000";
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 150x75 pixels rectangle.
Coordinates Example
Mouse over the rectangle below to see its x and y coordinates:

X

Y

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 c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.moveTo(0,0);
ctx.lineTo(200,100);
ctx.stroke();

Try it yourself »

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 c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.beginPath();
ctx.arc(95,50,40,0,2*Math.PI);
ctx.stroke();

Try it yourself »

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 c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.font = "30px Arial";
ctx.fillText("Hello World",10,50);

Try it yourself »

Using strokeText():

Example

Write a 30px high text (no fill) on the canvas, using the font "Arial":
JavaScript:

var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.font = "30px Arial";
ctx.strokeText("Hello World",10,50);

Try it yourself »

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 c = document.getElementById("myCanvas");
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);

Try it yourself »

Using createRadialGradient():

Example

Create a radial/circular gradient. Fill rectangle with the gradient:
JavaScript:

var c = document.getElementById("myCanvas");
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);

Try it yourself »

Canvas - Images

To draw an image on a canvas, we will use the following method:

drawImage(image,x,y)

Image to use:

The Scream

Example

Draw the image onto the canvas:
JavaScript:

var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
var img = document.getElementById("scream");
ctx.drawImage(img,10,10);

Try it yourself »

HTML Canvas Reference

For a complete reference of all the properties and methods that can be used with the Canvas object (with try-it examples on every property and method), go to our Canvas Reference.

The HTML <canvas> Tag

Tag	Description
<canvas>	Used to draw graphics, on the fly, via scripting (usually JavaScript)

HTML5 Form Attributes

HTML5 New Form Attributes

HTML5 has several new attributes for <form> and <input>.
New attributes for <form>:

autocomplete
novalidate

New attributes for <input>:

autocomplete
autofocus
form
formaction
formenctype
formmethod
formnovalidate
formtarget
height and width
list
min and max
multiple
pattern (regexp)
placeholder
required
step

<form> / <input> autocomplete Attribute

The autocomplete attribute specifies whether a form or input field should have autocomplete on or off.
When autocomplete is on, the browser automatically complete values based on values that the user has entered before.
Tip: It is possible to have autocomplete "on" for the form, and "off" for specific input fields, or vice versa.
Note: The autocomplete attribute works with <form> and the following <input> types: text, search, url, tel, email, password, datepickers, range, and color.

Example

An HTML form with autocomplete on (and off for one input field):

<form action="demo_form.asp" autocomplete="on">
First name:<input type="text" name="fname"><br>
Last name: <input type="text" name="lname"><br>
E-mail: <input type="email" name="email" autocomplete="off"><br>
<input type="submit">
</form>

Try it yourself »

Tip: In some browsers you may need to activate the autocomplete function for this to work.

<form> novalidate Attribute

The novalidate attribute is a boolean attribute.
When present, it specifies that the form-data (input) should not be validated when submitted.

Example

Indicates that the form is not to be validated on submit:

<form action="demo_form.asp" novalidate>
E-mail: <input type="email" name="user_email">
<input type="submit">
</form>

Try it yourself »

<input> autofocus Attribute

The autofocus attribute is a boolean attribute.
When present, it specifies that an <input> element should automatically get focus when the page loads.

Example

Let the "First name" input field automatically get focus when the page loads:

First name:<input type="text" name="fname" autofocus>

Try it yourself »

<input> form Attribute

The form attribute specifies one or more forms an <input> element belongs to.
Tip: To refer to more than one form, use a space-separated list of form ids.

Example

An input field located outside the HTML form (but still a part of the form):

<form action="demo_form.asp" id="form1">
First name: <input type="text" name="fname"><br>
<input type="submit" value="Submit">
</form>

Last name: <input type="text" name="lname" form="form1">

Try it yourself »

<input> formaction Attribute

The formaction attribute specifies the URL of a file that will process the input control when the form is submitted.
The formaction attribute overrides the action attribute of the <form> element.
Note: The formaction attribute is used with type="submit" and type="image".

Example

An HTML form with two submit buttons, with different actions:

<form action="demo_form.asp">
First name: <input type="text" name="fname"><br>
Last name: <input type="text" name="lname"><br>
<input type="submit" value="Submit"><br>
<input type="submit" formaction="demo_admin.asp"
value="Submit as admin">
</form>

Try it yourself »

<input> formenctype Attribute

The formenctype attribute specifies how the form-data should be encoded when submitting it to the server (only for forms with method="post")
The formenctype attribute overrides the enctype attribute of the <form> element.
Note: The formenctype attribute is used with type="submit" and type="image".

Example

Send form-data that is default encoded (the first submit button), and encoded as "multipart/form-data" (the second submit button):

<form action="demo_post_enctype.asp" method="post">
First name: <input type="text" name="fname"><br>
<input type="submit" value="Submit">
<input type="submit" formenctype="multipart/form-data"
value="Submit as Multipart/form-data">
</form>

Try it yourself »

<input> formmethod Attribute

The formmethod attribute defines the HTTP method for sending form-data to the action URL.
The formmethod attribute overrides the method attribute of the <form> element.
Note: The formmethod attribute can be used with type="submit" and type="image".

Example

The second submit button overrides the HTTP method of the form:

<form action="demo_form.asp" method="get">
First name: <input type="text" name="fname"><br>
Last name: <input type="text" name="lname"><br>
<input type="submit" value="Submit">
<input type="submit" formmethod="post" formaction="demo_post.asp"
value="Submit using POST">
</form>

Try it yourself »

<input> formnovalidate Attribute

The novalidate attribute is a boolean attribute.
When present, it specifies that the <input> element should not be validated when submitted.
The formnovalidate attribute overrides the novalidate attribute of the <form> element.
Note: The formnovalidate attribute can be used with type="submit".

Example

A form with two submit buttons (with and without validation):

<form action="demo_form.asp">
E-mail: <input type="email" name="userid"><br>
<input type="submit" value="Submit"><br>
<input type="submit" formnovalidate value="Submit without validation">
</form>

Try it yourself »

<input> formtarget Attribute

The formtarget attribute specifies a name or a keyword that indicates where to display the response that is received after submitting the form.
The formtarget attribute overrides the target attribute of the <form> element.
Note: The formtarget attribute can be used with type="submit" and type="image".

Example

A form with two submit buttons, with different target windows:

<form action="demo_form.asp">
First name: <input type="text" name="fname"><br>
Last name: <input type="text" name="lname"><br>
<input type="submit" value="Submit as normal">
<input type="submit" formtarget="_blank"
value="Submit to a new window">
</form>

Try it yourself »

<input> height and width Attributes

The height and width attributes specify the height and width of an <input> element.
Note: The height and width attributes are only used with <input type="image">.
Tip: Always specify both the height and width attributes for images. If height and width are set, the space required for the image is reserved when the page is loaded. However, without these attributes, the browser does not know the size of the image, and cannot reserve the appropriate space to it. The effect will be that the page layout will change during loading (while the images load).

Example

Define an image as the submit button, with height and width attributes:

<input type="image" src="img_submit.gif" alt="Submit" width="48" height="48">

Try it yourself »

<input> list Attribute

The list attribute refers to a <datalist> element that contains pre-defined options for an <input> element.

Example

An <input> element with pre-defined values in a <datalist>:

<input list="browsers">

<datalist id="browsers">
<option value="Internet Explorer">
<option value="Firefox">
<option value="Chrome">
<option value="Opera">
<option value="Safari">
</datalist>

Try it yourself »

<input> min and max Attributes

The min and max attributes specify the minimum and maximum value for an <input> element.
Note: The min and max attributes works with the following input types: number, range, date, datetime, datetime-local, month, time and week.

Example

<input> elements with min and max values:

Enter a date before 1980-01-01:
<input type="date" name="bday" max="1979-12-31">

Enter a date after 2000-01-01:
<input type="date" name="bday" min="2000-01-02">

Quantity (between 1 and 5):
<input type="number" name="quantity" min="1" max="5">

Try it yourself »

<input> multiple Attribute

The multiple attribute is a boolean attribute.
When present, it specifies that the user is allowed to enter more than one value in the <input> element.
Note: The multiple attribute works with the following input types: email, and file.

Example

A file upload field that accepts multiple values:

Select images: <input type="file" name="img" multiple>

Try it yourself »

<input> pattern Attribute

The pattern attribute specifies a regular expression that the <input> element's value is checked against.
Note: The pattern attribute works with the following input types: text, search, url, tel, email, and password.
Tip: Use the global title attribute to describe the pattern to help the user.
Tip: Learn more about regular expressions in our JavaScript tutorial.

Example

An input field that can contain only three letters (no numbers or special characters):

Country code: <input type="text" name="country_code" pattern="[A-Za-z]{3}" title="Three letter country code">

Try it yourself »

<input> placeholder Attribute

The placeholder attribute specifies a short hint that describes the expected value of an input field (e.g. a sample value or a short description of the expected format).
The short hint is displayed in the input field before the user enters a value.
Note: The placeholder attribute works with the following input types: text, search, url, tel, email, and password.

Example

An input field with a placeholder text:

<input type="text" name="fname" placeholder="First name">

Try it yourself »

<input> required Attribute

The required attribute is a boolean attribute.
When present, it specifies that an input field must be filled out before submitting the form.
Note: The required attribute works with the following input types: text, search, url, tel, email, password, date pickers, number, checkbox, radio, and file.

Example

A required input field:

Username: <input type="text" name="usrname" required>

Try it yourself »

<input> step Attribute

The step attribute specifies the legal number intervals for an <input> element.
Example: if step="3", legal numbers could be -3, 0, 3, 6, etc.
Tip: The step attribute can be used together with the max and min attributes to create a range of legal values.
Note: The step attribute works with the following input types: number, range, date, datetime, datetime-local, month, time and week.

Example

An input field with a specified legal number intervals:

<input type="number" name="points" step="3">

Try it yourself »

HTML5 <input> Tag

Tag	Description
<form>	Defines an HTML form for user input
<input>	Defines an input control

Subscribe to: Posts (Atom)