Html5 canvas

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Tuesday, April 11, 2023 2

HTML5◦ <canvas> Element◦ Examples ◦ Assignment◦ Practical Tasks

Outlines


The <canvas> element is used to draw graphics, on the fly, on a web page.

The example shows a red rectangle, a gradient rectangle, a multicolor rectangle, and some multicolor text that is drawn onto the canvas.

HTML5- <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.

What is Canvas?


A canvas is a rectangular area on an HTML page, and it is specified with the <canvas> element.

The markup looks like this: <canvas id="myCanvas" width="200"

height="100"></canvas>

Note: By default, the <canvas> element has no border and no content.

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.

Create a Canvas


To add a border, use the style attribute:<canvas id="myCanvas" width="200“ height="100“ style="border:1px solid #000000;"></canvas>

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

Create a Canvas


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);

Example explained:


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

Example explained:


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

Draw Onto The Canvas With JavaScript<script>var c=document.getElementById("myCanvas");var ctx=c.getContext("2d");ctx. strokeStyle="#FF0000";ctx. strokeRect(20,20,150,100);</script>

strokeStyle Property


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.

Canvas Coordinates


To draw straight lines on a canvas, we will use the following two methods:1. moveTo(x,y) defines the starting point of

the line2. lineTo(x,y) defines the ending point of the

lineTo actually draw the line, we must use one of the "ink" methods, like stroke().

Canvas - Paths


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:

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

Example


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().

Canvas- Circle


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

Canvas- Circle-Stroke


Canvas- Circle-Stroke / Filled

Stroke Fill

<script>var c=document.getElementById("myCanvas");var ctx=c.getContext("2d");ctx.beginPath();ctx.arc(95,50,40,0,2*Math.PI);ctx.strokeStyle="#FF0000";ctx.stroke();</script>

<script>var c=document.getElementById("myCanvas");var ctx=c.getContext("2d");ctx.beginPath();ctx.arc(95,50,40,0,2*Math.PI);ctx.fillStyle="#FF0000";ctx.fill();</script>


Canvas Circle Explanation


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 (x,y Shows the position where to display text)

strokeText(text,x,y) - Draws text on the canvas (no fill)

Canvas - Text


Write a 30px high filled text on the canvas, using the font "Arial": var

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

Canvas – Text Using fillText():


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

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

Canvas – Text Using strokeText():


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: 1.createLinearGradient(x,y,x1,y1) – Creates a linear gradient 2.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.

Canvas - Gradients


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.

Canvas - Gradients


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

// Create gradientvar grd=ctx.createLinearGradient(0,0,200,0);grd.addColorStop(0,"red");grd.addColorStop(1,"white");

// Fill with gradientctx.fillStyle=grd;ctx.fillRect(10,10,150,80);

Using createLinearGradient():Horizontal Gradient Effect



// Create gradientvar grd=ctx.createLinearGradient(0,0,0,200);grd.addColorStop(0,"red");grd.addColorStop(1,"white");


Using createLinearGradient():Vertical Gradient Effect



// Create gradientvar grd=ctx.createLinearGradient(0,0,200,200);grd.addColorStop(0,"red");grd.addColorStop(1,“black");


Using createLinearGradient():Diagonal Gradient Effect


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


// Create gradientvar grd=ctx.createRadialGradient(75,50,5,90,60,100);grd.addColorStop(0,"red");grd.addColorStop(1,"white");


Using createRadialGradient():


<script>var c=document.getElementById("myCanvas");var ctx=c.getContext("2d");

var gradient=ctx.createLinearGradient(0,0,170,0);gradient.addColorStop("0","magenta");gradient.addColorStop("0.5","blue");gradient.addColorStop("1.0","red");// Fill with gradientctx.strokeStyle=gradient;ctx.lineWidth=5;ctx.strokeRect(20,20,150,100);</script>

Stroke Gradient Rectangle


<script>var c=document.getElementById("myCanvas");var ctx=c.getContext("2d");ctx.font="30px Verdana";// Create gradientvar gradient=ctx.createLinearGradient(0,0,c.width,0);gradient.addColorStop("0","magenta");gradient.addColorStop("0.5","blue");gradient.addColorStop("1.0","red");// Fill with gradientctx.strokeStyle=gradient;ctx.strokeText("QUEST Nawabshah !",5,80);</script>

Stroke Gradient on Text


<script>var c=document.getElementById("myCanvas");var ctx=c.getContext("2d");ctx.font="30px Verdana";// Create gradientvar gradient=ctx.createLinearGradient(0,0,c.width,0);gradient.addColorStop("0","magenta");gradient.addColorStop("0.5","blue");gradient.addColorStop("1.0","red");// Fill with gradientctx.fillStyle=gradient;ctx.fillText("QUEST Nawabshah !",5,80);</script>

Stroke Gradient on Text


What is Canvas? What is purpose of canvas? What are advantages and disadvantages of

Canvas?

Assignments


Draw a Rectangle with following effect1. stroke 2. Fill

Draw a Rectangle with following effect 1. Stroke Gradient

2. Stroke Fill Gradient

Practical Tasks


Draw a Circle with following effect1. Stroke 2. Fill

Write AnyText with following effect 1. Stroke Gradient

2. Stroke Fill Gradient3. Stroke4. Fill

Practical Tasks


Draw a Rectangle with Fill and stroke having following effects1. Linear Gradient

1. Horizontal, vertical and Diagonal Effect

2. Radial Gradient

Practical Tasks


Creative Tasks


Creative Tasks


The shadowColor property sets or returns the color to use for shadows.

Note: Use the shadowColor property together with the shadowBlur property to create a shadow.Tip: Adjust the shadow by using the shadowOffsetX and shadowOffsetY properties.

Canvas-Shadow Color

http://www.w3schools.com/tags/canvas_shadowblur.asp

http://www.w3schools.com/tags/canvas_shadowoffsetx.asp

http://www.w3schools.com/tags/canvas_shadowoffsety.asp


<script>var c=document.getElementById("myCanvas");var ctx=c.getContext("2d");ctx.shadowBlur=20;ctx.shadowColor="black";ctx.fillStyle="red";ctx.fillRect(20,20,100,80);</script>

Canvas-Shadow Color Example


<script>var c=document.getElementById("myCanvas");var ctx=c.getContext("2d");ctx.shadowBlur=35;ctx.shadowColor="black";ctx.fillStyle="white";ctx.fillRect(20,20,100,80);</script>



<script>var c=document.getElementById("myCanvas");var ctx=c.getContext("2d");ctx.shadowBlur=20;ctx.fillStyle="red";ctx.shadowColor="black";ctx.fillRect(20,20,100,80);ctx.shadowColor="blue";ctx.fillRect(140,20,100,80);</script>



The shadowOffsetX property sets or returns the horizontal distance of the shadow from the shape.

shadowOffsetX=0 indicates that the shadow is right behind the shape.

shadowOffsetX=20 indicates that the shadow starts 20 pixels to the right (from the shape's left position).

shadowOffsetX=-20 indicates that the shadow starts 20 pixels to the left (from the shape's left position).

canvas shadowOffsetX Property


<script>var c=document.getElementById("myCanvas");var ctx=c.getContext("2d");ctx.shadowBlur=10;ctx.shadowOffsetX=10;ctx.shadowColor="black";ctx.fillStyle="red";ctx.fillRect(20,20,100,80);</script>

canvas shadowOffsetX PropertyExample


ctx.shadowOffsetX=0;

ctx.shadowOffsetX=-10;



canvas shadowOffsetX PropertyExample


The shadowOffsetY property sets or returns the vertical distance of the shadow from the shape.

shadowOffsety=0 indicates that the shadow is right behind the shape.

shadowOffsetY=20 indicates that the shadow starts 20 pixels below the shape's top position.

shadowOffsetY=-20 indicates that the shadow starts 20 pixels above the shape's top position.

canvas shadowOffsetY Property


<script>var c=document.getElementById("myCanvas");var ctx=c.getContext("2d");ctx.shadowBlur=10;ctx.shadowOffsetY=20;ctx.shadowColor="black";ctx.fillStyle="red";ctx.fillRect(20,20,100,80);</script>

canvas shadowOffsetY PropertyExample


ctx.shadowOffsetY=0;

ctx.shadowOffsetY=-10;



canvas shadowOffsetY PropertyExample


Apply Horizontal and Vertical shadow properties on the shapes which are given in practical task of <canvas>

Practical Tasks


http://www.w3schools.com/tags/ref_canvas.asp

http://www.w3schools.com/html/default.asp

References





Education

Html5 canvas