Get the rotate amount in animate a div through a SVG path

Question

From my last post helped to animate a div through a SVG path. Success and div "#car" animating through my path.

Now the problem is how to rotate/ transform the div according to the curve ? ie if it moving through a slanding path need to rotate the div according to the curve of line.

HTML Part

<div class="movepath">

        <div class="car" id="car"></div>


<svg id="canvas" width="1000px" class="content" xmlns="http://www.w3.org/2000/svg" height="370px">
<style type="text/css"><![CDATA[
    .lineC {
        fill:#fff;stroke:#000;
    }
]]></style>


</svg>

    </div>

CSS Part

.movepath {
width:"1000px";
height:350px;
position:relative;
float:left;
}

.car {
width:100px;
height:50px;
background-color:red;
position:absolute;
left:0;top:0;
}

JS Part

$(document).ready(function(){

    timer=setInterval('rotateIt()',20);

    var width=getDocWidth();
    $('#canvas').width(width+'px');
    $('.carpath').width(width+'px');


    var shape = document.createElementNS(svgNS, "path");

    var points = 'M0 10 Q -27 10, 95 80 T'+width+' 40';
    shape.setAttributeNS(null, "d", points);
    shape.setAttributeNS(null, "class", "lineC");
    shape.setAttributeNS(null, "id", 'road');
    document.getElementById("canvas").appendChild(shape);

});

function rotateIt(){

    rot += 10;
    left+=1;
    $('.right_t').css(prefix, 'rotate(' + rot + 'deg)');
    $('.left_t').css(prefix, 'rotate(' + rot + 'deg)');

    var length=$('#road')[0].getTotalLength() * i;
    var point = $('#road')[0].getPointAtLength( length );


    $('#car').css('left',point.x+'px');
    $('#car').css('top',(point.y-62)+'px');
    i+=.0005;
}

Can any one please give me an idea for rotate "#car" according to the path ?

Answer 1

You need to calculate the angle between two points along the curve (say, the lower edges of your "car"), the formula would be this:

function getAngle(p1, p2) {
  var deltaY = p2.y - p1.y;
  var deltaX = p2.x - p1.x;
  return Math.atan2(deltaY, deltaX) * 180 / Math.PI
}

Here is a rough working example with your code:

http://jsfiddle.net/5CB4u/