How to rotate a SVG transformation matrix around its center point

CSS

.square {
  background-color: green;
  height: 40px;
  width: 40px;
}

JS

var square = 
{  
    sizeReal   : { "width": 40, "height": 40 }                   
,   position   : { "x": 100, "y": 100 } 
};

$(".square")[0].style.transform = 'translate(' + square.position.x + 'px,' + square.position.y + 'px)';

The code above modifies the CSS of a square element by moving it using transform property. To achieve the same effect but with different approach involving matrix multiplication, Sylvester library is utilized in the provided example fiddle for clarification.

http://jsfiddle.net/xYsHZ/3/

Your goal is to make the red rectangle behave like the green one. Review your matrices and ensure correct operations are being performed.

Answer №1

Success! I was able to identify and correct the incorrect sequence of matrix elements :)

$("#red")[0].setAttribute( 'transform', 'matrix(' + layer.matrix[ 0 ][ 0 ] + ',' + layer.matrix[ 1 ][ 0 ] + ',' + layer.matrix[ 0 ][ 1 ] + ',' + layer.matrix[ 1 ][ 1 ] + ',' + layer.matrix[ 0 ][ 2 ] + ',' + layer.matrix[ 1 ][ 2 ] + ')' );
},50);

http://jsfiddle.net/6DR3D/2/

Answer №2

The issue lies with the rotate(angle, x, y) function being used. It is meant to rotate around a specific point (x,y) at the given angle. However, in this case, the matrix is being built to rotate around layer.position instead.

To properly rotate around a specified point (x,y), relative to the object, the process involves first translating to (-x,-y), rotating, and then translating back to (x,y).

If there was a matrix multiplication function called Mul, it would be implemented as follows:

var m1 = GetMatrix(0, 0, {"x":-layer.sizeScaled.width/2, "y":-layer.sizeScaled.height/2});
var m2 = GetMatrix(layer.rotation, layer.scale, layer.position);
var m3 = GetMatrix(0, 0, {"x":layer.sizeScaled.width/2, "y":layer.sizeScaled.height/2});

var m = Mul(Mul(m3,m2), m1); //applying the matrices in sequence: m1, then m2, then m3

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