javascript - How to extract position, rotation and scale from matrix SVG

Question

I have a simple problem: I want to extract translation (tx, ty), rotation (r) and scale (sx, sy) values form a transform matrix applied to my svg element.

Let's use this example:

<g
  id="myElement"
  transform="matrix(0.93893241,0.34410162,-0.34410162,0.93893241,363.88475,-76.125919)"
  >... </g>

If, in javascript I do

document.getElementById("myElement").getCTM()

I can access to a, b, c, d, e, f values. How can I get tx, ty, sx, sy and r from there? Thanks

Answer 1

Inspired by this ActionScript version: https://gist.github.com/fwextensions/2052247, I wrote a JavaScript port:

    function deltaTransformPoint(matrix, point)  {

        var dx = point.x * matrix.a + point.y * matrix.c + 0;
        var dy = point.x * matrix.b + point.y * matrix.d + 0;
        return { x: dx, y: dy };
    }


    function decomposeMatrix(matrix) {

        // @see https://gist.github.com/2052247

        // calculate delta transform point
        var px = deltaTransformPoint(matrix, { x: 0, y: 1 });
        var py = deltaTransformPoint(matrix, { x: 1, y: 0 });

        // calculate skew
        var skewX = ((180 / Math.PI) * Math.atan2(px.y, px.x) - 90);
        var skewY = ((180 / Math.PI) * Math.atan2(py.y, py.x));

        return {

            translateX: matrix.e,
            translateY: matrix.f,
            scaleX: Math.sqrt(matrix.a * matrix.a + matrix.b * matrix.b),
            scaleY: Math.sqrt(matrix.c * matrix.c + matrix.d * matrix.d),
            skewX: skewX,
            skewY: skewY,
            rotation: skewX // rotation is the same as skew x
        };        
    }

Usage: decomposeMatrix(document.getElementById('myElement').getCTM())