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HTML5 Canvas: How to make a loading spinner by rotating the image in degrees?

Answer 1

+1 A:

rotate turns the canvas(?) around your current position, which is 0, 0 to start. you need to "move" to your desired center point, which you can accomplish with

context.translate(x,y);

after you move your reference point, you want to center your image over that point. you can do this by calling

context.drawImage(myImage, -(27/2), -(27/2), 27, 27);

this tells the browser to start drawing the image from above and to the left of your current reference point, by have the size of the image, whereas before you were starting at your reference point and drawing entirely below and to the right (all directions relative to the rotation of the canvas).

since your canvas is the size of your image, your call to translate will use the same measurement, (27/2), for x and y coordinates.

so, to put it all together

// initialization:
context.translate(27/2, 27/2);

// onload: 
context.rotate(Math.PI * 45 / 180);
context.drawImage(myImage, -(27/2), -(27/2), 27, 27);

edit: also, rotation units are radians, so you'll need to translate degrees to radians in your code.

edits for rearranging stuff.

lincolnk 2010-05-03 21:15:10

Answer 2

A:

I'm closer. This will rotate the image in the top left corner. How can I center it up? Also it looks like it is redrawing the image each time overlapping the previous image. How can I just have on instance of the image on the canvas? Do I need to remove it each time?

    <!DOCTYPE html>
    <html>
     <head>
      <title>Canvas test</title>
      <script type="text/javascript">
         setInterval(draw, 150);
          var degrees = 0.0;
          function draw() {
        var drawingCanvas = document.getElementById('myDrawing');
        // Check the element is in the DOM and the browser supports canvas
        if(drawingCanvas && drawingCanvas.getContext) {

         // Initaliase a 2-dimensional drawing context
         var context = drawingCanvas.getContext('2d');

        context.translate(0, 0);
        context.rotate(Math.PI * 45 / 180);

         //Load the image object in JS, then apply to canvas onload     
         var myImage = new Image();
         myImage.onload = function() {
          context.drawImage(myImage, -(50/2), -(50/2), 50, 50);
         }

         myImage.src = "spin.png";

        }
       }
      </script>
     </head>
     <body>
      <center><canvas id="myDrawing" width="50" height="50"></center>
      </canvas>
     </body>
    </html>

bill 2010-05-04 03:25:45

Here is the image I am using in this example..http://devthought.com/wp-content/projects/mootools/APNG/Demo/images/zoom-spin-12.png

bill 2010-05-04 03:28:24

Answer 3

+4 A:

Here is the complete working example:)

<!DOCTYPE html>
<html>
    <head>
        <title>Canvas Cog</title>
        <script type="text/javascript">
            var cog = new Image();
            function init() {
                cog.src = 'data:image/png;base64,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'; // Set source path
                setInterval(draw,10);
            }
            var rotation = 0;
            function draw(){
                var ctx = document.getElementById('myCanvas').getContext('2d');
                ctx.globalCompositeOperation = 'destination-over';
                ctx.save();
                ctx.clearRect(0,0,27,27);
                ctx.translate(13.5,13.5); // to get it in the origin
                rotation +=1;
                ctx.rotate(rotation*Math.PI/64); //rotate in origin
                ctx.translate(-13.5,-13.5); //put it back
                ctx.drawImage(cog,0,0);
                ctx.restore();
            }
            init();
        </script>
    </head>
    <body>
        <canvas width="27" height="27" id="myCanvas"></canvas>
    </body>
</html>

Bill 2010-05-04 17:34:48

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HTML5 Canvas: How to make a loading spinner by rotating the image in degrees?

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