The clearCanvas() function presented in this page can be used to clear propely the drawing and text contexts from canvas.
// function to clear the canvas ( https://coursesweb.net/ )
// cnv = the object with the canvas element
function clearCanvas(cnv) {
  var ctx = cnv.getContext('2d');     // gets reference to canvas context
  ctx.beginPath();    // clear existing drawing paths
  ctx.save();         // store the current transformation matrix

  // Use the identity matrix while clearing the canvas
  ctx.setTransform(1, 0, 0, 1, 0, 0);
  ctx.clearRect(0, 0, cnv.width, cnv.height);

  ctx.restore();        // restore the transform
}
- This function is useful when you want to remove all the canvas context to can add new drawings and texts on an empty canvas.

• Here is an example, when a "Set Nr" button is clicked, a random number between 1 and 100 is displayed in canvas. A "Clear" button will clear the canvas context, so to not display the numbers one over the other; that will happen if you click the "Set Nr" button again (see the comments in code).
<h4>Canvas</h4><canvas width="200" height="100" id="cnv1"></canvas><br/>
<button id="clear_cnv">Clear</button> - <button id="setnr_cnv">Set Nr</button>
<script type="text/javascript">
// <![CDATA[
// function to clear the canvas ( https://coursesweb.net/ )
// cnv = the object with the canvas element
function clearCanvas(cnv) {
  var ctx = cnv.getContext('2d');     // gets reference to canvas context
  ctx.beginPath();    // clear existing drawing paths
  ctx.save();         // store the current transformation matrix

  // Use the identity matrix while clearing the canvas
  ctx.setTransform(1, 0, 0, 1, 0, 0);
  ctx.clearRect(0, 0, cnv.width, cnv.height);

  ctx.restore();        // restore the transform
}

// sets and adds a random number in canvas
// cnv = the object with the canvas element
function  addNrCnv(cnv) {
  // gets a random number between 1 and 100
  var nr =  Math.floor(Math.random() * 100 + 1);

  var ctx = cnv.getContext('2d');     // gets reference to canvas context

  // create text with the number in canvas (sets text color, font type and size)
  ctx.fillStyle = '#00f';
  ctx.font = 'italic 38px sans-serif';
  ctx.fillText(nr, 80, 64);
}

// get a reference to the <canvas> tag
var cnv1 = document.getElementById('cnv1');

// register onclick event for #clear_cnv button to call the clearCanvas()
document.getElementById('clear_cnv').onclick = function() { clearCanvas(cnv1); }

// register onclick event for #setnr_cnv button to call the addNrCnv()
document.getElementById('setnr_cnv').onclick = function() { addNrCnv(cnv1); }
// ]]>
</script>
- Demo (Click "Set Nr" to add a random number, click "Clear" to clear the canvas):

Canvas


-

Daily Test with Code Example

HTML
CSS
JavaScript
PHP-MySQL
Which HTML5 tag can be used to embed an external application (SWF, PDF) in web page?
<mark> <embed> <canvas>
<embed src="flash_game.swf" width="450" height="350" />
Which CSS pseudo-element adds a special style to the first line of a text?
:first-letter :before :first-line
#id:first-line {
  font-weight: bold;
  color: blue;
}
Click on the window object property which gets or sets the URL of current page.
window.location window.self window.status
var url = window.location;
alert(url);
Indicate the PHP function used to get the contents of a file or page and store it into a string.
fopen() file_put_contents() file_get_contents()
$homepage = file_get_contents("http://coursesweb.net/");
echo $homepage;
Clear Canvas Context

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