javascript - How to get the actual x/y position of an element in SVG with transformations and matrices

Question

I need to get the x/y positions of an element that is written like this in SVG:

<g transform="scale(10,10)" id="g6558">
    <text transform="matrix(1,0,0,-1,236.532,417.253)" id="text6560">
        <tspan x="0 4.448" y="0" id="tspan6562">10</tspan>
    </text>
</g>

How do I get the current position of the tspan element in this case? I have to manually calculate them as I can't use getBBox() or other native functions because I'm using a Flash-wrapper to display the SVG (http://code.google.com/p/svgweb/) which only supports basic attributes, tag names etc.

How are the matrices and transformations caluclated on the x/y position of the element?

Answer 1

As well as i understood your problem, you need to know the x and y co-ordinates of an element, after it is transformed.

Mathematically, all transformations can be represented as 3x3 transformation matrices of the following form:

a   b   e
c   d   f
0   0   1

Since only six values are used in the above 3x3 matrix, a transformation matrix is also expressed as a vector: [a b c d e f]. a and d responsible for scaling in x and y respectively, whereas e and f gives you the translated axis in the x and y respectively. So In your code which is

     <text transform="matrix(1,0,0,-1,236.532,417.253)" id="text6560">
      <tspan x="0 4.448" y="0" id="tspan6562">10</tspan>
    </text>

Element text is translated 236.532 in the x-axis 417.253 in the y-axis. So tspan x point becomes 236.. + 4.4.. and y point 417.. + 0.