How can I increment the rotation angle for a JS object in HTML5 canvas?

Question

My problem is that the inintialization of the red line is rotated by an angle equal to the yellow line ... I want to make this angle equal zero (red line angle ) in the begining only. ** then rotate as the mouse move. .

here how I draw the line

var x = W / 2;
var y = H / 2;
var lineLength = 900;
ctx.save();
ctx.translate(x, y);
 ctx.rotate(((0.003 * (mouse.x  - p.w))/2)) ; // how to increment the angle ??
ctx.moveTo(-lineLength / 2,0, 0);
ctx.lineTo(lineLength / 2.0, 0);
ctx.stroke();
ctx.restore();

Answer 1

A glitch in your code: Be sure you start each path drawing operation with context.beginPath();

Here's how to rotate a line around its midpoint:

    function draw(angle){
        ctx.clearRect(0,0,cw,ch);
        ctx.save();
        // draw rotated line
        ctx.translate(cx,cy);
        ctx.rotate(angle) ; 
        ctx.beginPath();
        ctx.moveTo(-lineLength/2,0)
        ctx.lineTo(lineLength/2,0);
        ctx.strokeStyle='orange';
        ctx.stroke();
        ctx.restore();
    }

Example code and a Demo: http://jsfiddle.net/m1erickson/3g0yvLov/

<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>

<style>
    body{ background-color: ivory; }
    #canvas{border:1px solid red;}
</style>

<script>
$(function(){

    var canvas=document.getElementById("canvas");
    var ctx=canvas.getContext("2d");
    var cw=canvas.width;
    var ch=canvas.height;
    var $canvas=$("#canvas");
    var canvasOffset=$canvas.offset();
    var offsetX=canvasOffset.left;
    var offsetY=canvasOffset.top;
    ctx.lineWidth=10;

    var cx=canvas.width/2;
    var cy=canvas.height/2;
    var lineLength = 90;

    $("#canvas").mousemove(function(e){handleMouseMove(e);});

    function draw(angle){
        ctx.clearRect(0,0,cw,ch);
        ctx.save();
        // draw unrotated line
        ctx.beginPath();
        ctx.moveTo(cx-lineLength/2,cy)
        ctx.lineTo(cx+lineLength/2,cy);
        ctx.strokeStyle='red';
        ctx.stroke();
        // draw rotated line
        ctx.translate(cx,cy);
        ctx.rotate(angle) ; 
        ctx.beginPath();
        ctx.moveTo(-lineLength/2,0)
        ctx.lineTo(lineLength/2,0);
        ctx.strokeStyle='orange';
        ctx.stroke();
        ctx.restore();
    }

    function handleMouseMove(e){
      e.preventDefault();
      e.stopPropagation();

      mouseX=parseInt(e.clientX-offsetX);
      mouseY=parseInt(e.clientY-offsetY);

      // Put your mousemove stuff here
      var dx=mouseX-cx;
      var dy=mouseY-cy;
      var radianAngle=Math.atan2(dy,dx);
      draw(radianAngle);

    }


}); // end $(function(){});
</script>
</head>
<body>
    <canvas id="canvas" width=300 height=300></canvas>
</body>
</html>

How can I increment the rotation angle for a JS object in HTML5 canvas?

Question

1 answers

solution1
0 ACCPTED 2014-10-05 21:35:05

How can I increment the rotation angle for a JS object in HTML5 canvas?

Question

1 answers

solution1 0 ACCPTED 2014-10-05 21:35:05

solution1
0 ACCPTED 2014-10-05 21:35:05