Book Image

Canvas Cookbook

Book Image

Canvas Cookbook

Overview of this book

With the growing popularity of HTML5 Canvas, this book offers tailored recipes to help you develop portable applications, presentations, and games. The recipes are simple yet creative and build on each other. At every step, the book inspires the reader to develop his/her own recipe. From basic to advanced, every aspect of Canvas API has been covered to guide readers to develop their own application, presentation, or game.
Table of Contents (16 chapters)
Canvas Cookbook
Credits
About the Author
About the Reviewer
www.PacktPub.com
Preface
Index

Drawing a flag


Using the knowledge of Bezier curves you can develop various examples of your own. Here is a recipe whose output looks like this:

How to do it...

If you observe the drawing of a flag, you can easily understand that it's drawn using a combination of a line, a Bezier curve, and a circle. Here is our recipe:

<html>
<head>
<title>A Flag</title>
<script>
  function init()
  {
    can = document.getElementById("MyCanvasArea"); 
    ctx = can.getContext("2d");
    var xstart = 50;   var ystart = 25;
    var xctrl1 = 100;  var yctrl1 = 10;
    var xctrl2 = 100;  var yctrl2 = 70;
    var xend = 180;  var yend = ystart;

    //10 curves having orange color
    for(i=1;i<=10;i++)
    {
      drawBezierCurve(xstart,ystart,xctrl1,yctrl1,xctrl2,yctrl2,xend, yend,"orange");
      ystart+=5;yend+=5;yctrl1+=5;yctrl2+=5;
    }
    //10 curves having white color
    for(j=1;j<=10;j++)
    {
      drawBezierCurve(xstart,ystart,xctrl1,yctrl1,xctrl2,yctrl2,xend,yend,"white");
      ystart+=5;yend+=5;yctrl1+=5;yctrl2+=5;
    }
    //10 curves having green color
    for(j=1;j<=10;j++)
    {
      drawBezierCurve(xstart,ystart,xctrl1,yctrl1,xctrl2,yctrl2,xend,yend,"green");
      ystart+=5;yend+=5;yctrl1+=5;yctrl2+=5;
    }
    //the ashoka wheel we draw two arcs and lines
    x1=120;
    y1=113;
    drawArc(x1,y1,22,0,360,false,"navy");
    //draw a stand for the flag
    drawLine(50,350,50,20,10,"brown");
  }
function drawBezierCurve(xstart,ystart,xctrl1,yctrl1,xctrl2,yctrl2,xend,yend,color,width)
{   
    ctx.strokeStyle=color;
    ctx.lineWidth=6;
    ctx.beginPath();
    ctx.moveTo(xstart,ystart);
    ctx.bezierCurveTo(xctrl1,yctrl1,xctrl2,yctrl2,xend,yend);
    ctx.stroke();
}
function drawArc(xPos,yPos,radius,startAngle,endAngle,anticlockwise,lineColor)
{
    var startAngle = startAngle * (Math.PI/180);
    var endAngle   = endAngle   * (Math.PI/180);
    var radius = radius;
    ctx.strokeStyle = lineColor;
    ctx.fillStyle="navy";
    ctx.lineWidth   = 4;
    ctx.beginPath();
    ctx.arc(xPos,yPos,radius,startAngle,endAngle,anticlockwise);
    ctx.fill();
    ctx.stroke();  
}
function drawLine(xstart,ystart,xend,yend,width,color)
{
    ctx.beginPath();
    ctx.strokeStyle=color;
    ctx.lineWidth=width;
    ctx.moveTo(xstart,ystart);  
    ctx.lineTo(xend,yend);
    ctx.stroke();
    ctx.closePath();
}
</script>
</head>
<body onload="init()">
  <canvas id="MyCanvasArea" width ="300"  height="400" style="border:2px solid black">
    Your browser doesn't currently support HTML5 Canvas.
  </canvas>
</body>
</html>

How it works...

In the recipe, you will notice that the function to draw a Bezier curve is called 10 times through a loop for different coordinates and colors. Other than this, a vertical line with a thickness of 10 is drawn. For this, the function drawLine() is called specifying the appropriate parameters.