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HTML5 Canvas - Save and Restore States

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HTML5 canvas provides two important methods to save and restore the canvas states. The canvas drawing state is basically a snapshot of all the styles and transformations that have been applied and consists of the followings:

  1. The transformations such as translate, rotate and scale etc.

  2. The current clipping region.

  3. The current values of the following attributes: strokeStyle, fillStyle, globalAlpha, lineWidth, lineCap, lineJoin, miterLimit, shadowOffsetX, shadowOffsetY, shadowBlur, shadowColor, globalCompositeOperation, font, textAlign, textBaseline.

Canvas states are stored on a stack every time the save method is called, and the last saved state is returned from the stack every time the restore method is called.

SNMethod and Description
1save()
This method pushes the current state onto the stack..
2restore()
This method pops the top state on the stack, restoring the context to that state.

Example:

Following is a simple example which makes use of above mentioned methods to show how the restore is called, to restore the original state and the last rectangle is once again drawn in black.

<!DOCTYPE HTML>
<html>
<head>
<script type="text/javascript">
function drawShape(){
  // get the canvas element using the DOM
  var canvas = document.getElementById('mycanvas');
 
  // Make sure we don't execute when canvas isn't supported
  if (canvas.getContext){
 
     // use getContext to use the canvas for drawing
     var ctx = canvas.getContext('2d');

     //  draw a rectangle with default settings
     ctx.fillRect(0,0,150,150);
     //  Save the default state
     ctx.save();      
            
     // Make changes to the settings
     ctx.fillStyle = '#66FFFF'       
     ctx.fillRect( 15,15,120,120); 
     // Save the current state
     ctx.save();                  
    
     // Make the new changes to the settings
     ctx.fillStyle = '#993333'       
     ctx.globalAlpha = 0.5;    
     ctx.fillRect(30,30,90,90);

     // Restore previous state
     ctx.restore();
     // Draw a rectangle with restored settings               
     ctx.fillRect(45,45,60,60);   

     // Restore original state
     ctx.restore();
     // Draw a rectangle with restored settings          
     ctx.fillRect(40,40,90,90);  
  
  } else {
    alert('You need Safari or Firefox 1.5+ to see this demo.');
  }
}
</script>
</head>
<body onload="drawShape();">
   <canvas id="mycanvas"></canvas>
</body>
</html>

The above example would produce following result:

Canvas States

To learn above concept - Do Online Practice using latest version of either Safari or Opera.


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