Introduction to Javascript HTML5 Canvas API

HTML5 Canvas
Some examples of Canvas:

1. HTML5 Canvas

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Before HTML5 was created, when web developers wanted to use graphics or animation on the website, a product of the thirt party such as Flash, Java Applet had to be embedded. Although this may solve the problem, the website becomes heavier and there is something that has not satisfied you.

When HTML5 was launched, it introduced people to many new features and tags including the <canvas> element. The <canvas> element creates a rectangular area and you need to use Javascript to draw on that area.

Canvas is available as an API and it is supported by modern browsers. Moreover, it is compatible with multiple platforms. Therefore, you can create applications once and deploy to many different platforms - PCs or mobile devices.

As a result, you can create a game like this with Javascript only:

Browsers support the <canvas> element:

2. Some examples of Canvas:

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In this section, I'm going to introduce you to several examples using HTML5 Canvas,the main purpose of which is to help you visualize what you can do with HTML5 Canvas.

By default, the <canvas> element creates a rectangular area without border and content. You can set the width and height for the <canvas> by using the width and height attributes. If the width and height attributes are not specified, they will have a default value of 300px and 150px respectively.

first-canvas-example.html

<!DOCTYPE html>
<html>
   <head>
      <title>Canvas</title>
      <meta charset="UTF-8">
   </head>
   <body>
       <h1>HTML5 Canvas</h1>
       <canvas style="border:1px solid black" width=280 height=150>
            Your browser does not support the HTML5 Canvas.
       </canvas>
   </body>
</html>

An example of drawing a circle on Canvas:

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

See full source code of the example here:

draw-circle-example.html

<!DOCTYPE html>
<html>
   <head>
      <title>Canvas</title>
      <meta charset="UTF-8">
      <script>
          function drawCircle()  {
              var c = document.getElementById("myCanvas");
              var ctx = c.getContext("2d");
              ctx.beginPath();
              ctx.arc(95, 50, 40, 0, 2 * Math.PI);
              ctx.stroke();
          }
      </script>
   </head>
   <body>
       <h2>Draw Circle</h2>
       <button onclick="drawCircle()">Draw Circle</button>
       <a href="">Reset</a>
       <br/><br/>
       <canvas id="myCanvas" style="border:1px solid black" width=280 height=150>
            Your browser does not support the HTML5 Canvas.
       </canvas>
   </body>
</html>

HTML 5 Clock

The example below uses Javascript and Canvas to create a clock that displays the current time. This shows that the Canvas API can create animation effects.

clock.js

function drawClock() {
    var canvas = document.getElementById('myClock');
    if (canvas.getContext) {
        var c2d = canvas.getContext('2d');
        c2d.clearRect(0, 0, 300, 300);
        //Define gradients for 3D / shadow effect
        var grad1 = c2d.createLinearGradient(0, 0, 300, 300);
        grad1.addColorStop(0, "#D83040");
        grad1.addColorStop(1, "#801020");
        var grad2 = c2d.createLinearGradient(0, 0, 300, 300);
        grad2.addColorStop(0, "#801020");
        grad2.addColorStop(1, "#D83040");
        c2d.font = "Bold 20px Arial";
        c2d.textBaseline = "middle";
        c2d.textAlign = "center";
        c2d.lineWidth = 1;
        c2d.save();
        //Outer bezel
        c2d.strokeStyle = grad1;
        c2d.lineWidth = 10;
        c2d.beginPath();
        c2d.arc(150, 150, 138, 0, Math.PI * 2, true);
        c2d.shadowOffsetX = 4;
        c2d.shadowOffsetY = 4;
        c2d.shadowColor = "rgba(0,0,0,0.6)";
        c2d.shadowBlur = 6;
        c2d.stroke();
        //Inner bezel
        c2d.restore();
        c2d.strokeStyle = grad2;
        c2d.lineWidth = 10;
        c2d.beginPath();
        c2d.arc(150, 150, 129, 0, Math.PI * 2, true);
        c2d.stroke();
        c2d.strokeStyle = "#222";
        c2d.save();
        c2d.translate(150, 150);
        //Markings/Numerals
        for (i = 1; i <= 60; i++) {
            ang = Math.PI / 30 * i;
            sang = Math.sin(ang);
            cang = Math.cos(ang);
            //If modulus of divide by 5 is zero then draw an hour marker/numeral
            if (i % 5 == 0) {
                c2d.lineWidth = 8;
                sx = sang * 95;
                sy = cang * -95;
                ex = sang * 120;
                ey = cang * -120;
                nx = sang * 80;
                ny = cang * -80;
                c2d.fillText(i / 5, nx, ny);
                //Else this is a minute marker
            } else {
                c2d.lineWidth = 2;
                sx = sang * 110;
                sy = cang * 110;
                ex = sang * 120;
                ey = cang * 120;
            }
            c2d.beginPath();
            c2d.moveTo(sx, sy);
            c2d.lineTo(ex, ey);
            c2d.stroke();
        }
        //Fetch the current time
        var ampm = "AM";
        var now = new Date();
        var hrs = now.getHours();
        var min = now.getMinutes();
        var sec = now.getSeconds();
        c2d.strokeStyle = "#000";
        //Draw AM/PM indicator
        if (hrs >= 12) ampm = "PM";
        c2d.lineWidth = 1;
        c2d.strokeRect(21, -14, 44, 27);
        c2d.fillText(ampm, 43, 0);

        c2d.lineWidth = 6;
        c2d.save();
        //Draw clock pointers but this time rotate the canvas rather than
        //calculate x/y start/end positions.
        //
        //Draw hour hand
        c2d.rotate(Math.PI / 6 * (hrs + (min / 60) + (sec / 3600)));
        c2d.beginPath();
        c2d.moveTo(0, 10);
        c2d.lineTo(0, -60);
        c2d.stroke();
        c2d.restore();
        c2d.save();
        //Draw minute hand
        c2d.rotate(Math.PI / 30 * (min + (sec / 60)));
        c2d.beginPath();
        c2d.moveTo(0, 20);
        c2d.lineTo(0, -110);
        c2d.stroke();
        c2d.restore();
        c2d.save();
        //Draw second hand
        c2d.rotate(Math.PI / 30 * sec);
        c2d.strokeStyle = "#E33";
        c2d.beginPath();
        c2d.moveTo(0, 20);
        c2d.lineTo(0, -110);
        c2d.stroke();
        c2d.restore();

        //Additional restore to go back to state before translate
        //Alternative would be to simply reverse the original translate
        c2d.restore();
        setTimeout(drawClock, 1000);
    }
}

clock.html

<!DOCTYPE html>
<html>
   <head>
      <title>Clock</title>
      <meta charset="UTF-8">
      <script src="clock.js"></script>
   </head>
   <body onload="drawClock()">
       <h1>HTML 5 Clock</h1>
       <canvas id="myClock" width=320 height=320>
            Your browser does not support the HTML5 Canvas.
       </canvas>
   </body>
</html>

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Introduction to Javascript HTML5 Canvas API

1. HTML5 Canvas

2. Some examples of Canvas:

ECMAScript, Javascript Tutorials

ECMAScript, Javascript Tutorials

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