CODEWITHSUNDEEP
javascripthtmlcanvas
user3491862
user3491862

Reputation: 382

What does beginPath() and closePath() do exactly?

The question that I have is with context.beginPath() and context.closePath(). The code below will draw a circle in an arc around the screen until it disappears followed by a small dot which I would comment out because it is a .jpg which I do not know how to include.

My question is exactly what does the beginPath() do and also the closePath() do?

If I comment them out I get a wild result other than what I am expecting. I have looked on the internet but I have not seen the results like this.

Code with a question:

function drawTheBall() {
    context.fillStyle = "#00AB0F"; //sets the color of the ball
    context.beginPath();
        context.arc(ball.x,ball.y,10,0,Math.PI*2,true); //draws the ball
    context.closePath();
    context.fill();
}

Working code below

HTML-Javascript

<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>CH5EX10: Moving In Simple Geometric Spiral </title>
<script src="modernizr.js"></script>
<script type="text/javascript">
window.addEventListener('load', eventWindowLoaded, false);  
function eventWindowLoaded() {
    canvasApp();
}

function canvasSupport () {
    return Modernizr.canvas;
}

function canvasApp() {
    var radiusInc = 2;
    var circle = {centerX:250, centerY:250, radius:2, angle:0, radiusInc:2}
    var ball = {x:0, y:0,speed:.1};
    var points = new Array();

    theCanvas = document.getElementById('canvasOne');
    context = theCanvas.getContext('2d');

    var pointImage = new Image();
    pointImage.src = "point.png";   <-- Comment this line out

    if (!canvasSupport()) {
        return;
    }

  function erraseCanvas() {
     context.clearRect(0,0,theCanvas.width,theCanvas.height);
  }

  function drawPathPoints() {
    //Draw points to illustrate path
    points.push({x:ball.x,y:ball.y});
    for (var i= 0; i< points.length; i++) {
        context.drawImage(pointImage, points[i].x, points[i].y,1,1);
    }
  }

  function drawTheBall() {
    context.fillStyle = "#00AB0F"; //sets the color of the ball
    context.beginPath();
        context.arc(ball.x,ball.y,10,0,Math.PI*2,true); //draws the ball
    context.closePath();
    context.fill();
  }

  function buildBall() {
    ball.x = circle.centerX + Math.cos(circle.angle) * circle.radius;
    ball.y = circle.centerY + Math.sin(circle.angle) * circle.radius;
    circle.angle += ball.speed;
    circle.radius += radiusInc; 
  }

  function  drawScreen () {
    erraseCanvas();
    buildBall();
    drawPathPoints();
    drawTheBall();
  }

    function gameLoop() {
        window.setTimeout(gameLoop, 20);
        drawScreen()    
    }

    gameLoop();
}


</script>

</head>
<body>  
<div style="position: absolute; top: 50px; left: 50px;">

<canvas id="canvasOne" width="500" height="500">
 Your browser does not support the HTML 5 Canvas. 
</canvas>
</div>

</body>
</html>

Upvotes: 21

Views: 16534

Answers (2)

user1693593
user1693593

Reputation:

beginPath()

beginPath() clears the current internal path object and its sub-paths, which accumulates path operations such as line, rect, arc, arcTo and so on, regardless of if they are filled or stroked.

closePath()

closePath() connects the current path, or sub-path, position with the first point on that path created either with beginPath() or moveTo() if that was used. The latter creates a sub-path on the current main path and only this sub-path gets closed.

Some methods do a closePath() implicit and temporary for you such as fill() and clip() which means it's not needed for those calls. In any case, it must be called before a stroke() (or fill(), if you chose to use that) is called.

One can perhaps understand this method better if one think of it as "closing the loop" rather than ending or closing the path [object] which it doesn't.

Upvotes: 22

Nick Beeuwsaert
Nick Beeuwsaert

Reputation: 1638

BeginPath will start a new path, unlike moveTo which will begin a new subpath

Close path will close the path. it probably isn't needed unless you want stroking to stroke the entire path without a gap

Close Path:

//An Open Box
ctx.moveTo(0  ,   0);
ctx.lineTo(0  , 100);
ctx.lineTo(100, 100);
ctx.lineTo(100,   0);
ctx.stroke();

ctx.translate(150, 0);

//A closed box
ctx.moveTo(0  ,   0);
ctx.lineTo(0  , 100);
ctx.lineTo(100, 100);
ctx.lineTo(100,   0);
ctx.closePath();
ctx.stroke();

Upvotes: 0

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