Hi all,

I have an SVG file with objects that have a translate(). I would like to rotate these objects but a combination of translate() and rotate() is not succesful. When I reverse engineer what Inkscape is doing, I see after rotation, around the objects center, a matrix() replace the translate(). The contraints on the 6 parameters I understand but am unable to reproducte these in correctly altering the SVG. Below are two examples of 45 degree rotations in both directions.

"node19" before rotation:


"node19" after rotation:


"node33" before rotation:


"node33" after rotation


As one can see the translate is identical to both nodes. How do I calculate the 'e' and 'f' parameters of both matrix() transformations. How do I use cy and cx for that? Since I assume I need to use these values somehow.

Thanks for your help,

Pander

Well, it has been a while since I last had to do matrix maths, or looked into how it applies to svg,

Would either of my previous answers to this question help:
1) viewtopic.php?f=16&t=10550&p=39202#p39202
2) viewtopic.php?f=16&t=5398&p=23229#p23229

I must admit that it has been some time since I have looked at those, but if they don't answer your question then feel free to ask follow up questions.

Hi Pander,

The clever folks that cooked up SVG saw that one coming. There is a very nice javascript (and possibly other languages) function for that called getCTM. No need to do all the calculation yourself. Just do your translate and rotate and then use the function getCTM.

Basically what you need to do is



Here is a good example for that
http://www.learnsvg.com/tutorials/tutor ... getCTM.svg

In this case I need/want to calculate it myself because my script is inserting different rotations for different objects. I think I need to combine the translation of (x+4,y+321.187) in matrix form with the rotation to get the final matrix.



So k = 222.490 - 4 = 218.490 and l = 327.835 - 321.178 = 6.657 If this is correct, how do I get these two values from cx and cy?

Anyone?

very rusty on matrix maths...however it might be noteworthy that numbers you plug into the transformation matrix for rotation are Sin of the angle and Cos of the angle, in radians. Any reference on matrix transformations should show the correct formula and you can confirm whether inkscape also does the same.

The rotation matrix I can follow, see also https://en.wikipedia.org/wiki/Rotation_matrix It is the cx and cy which are a mystery to me.

edit: Please read my following post, because this answer is not correct.

Hi Pander,

a translation can't be expressed as a matrix (beside the zero translation).

If you multiply a matrix with a zero vector, you'll always get a zero vector. ==> no translation


Regards

max

Hi Pander,

my answer above was not correct. Due I'm new in SVG I have not noticed that the number of columns of the transformation matrix (3x3) is higher then the number of rows of the vectors / points (2x1) and the vector v = (x y)^T is replaced by (x y 1)^T in all calculations.

By this a translation is possible (^T = transpose):

You already know how to get a-d of the (3x3) matrix:



Let t_o be the old translation vector,
c = (cx cy)^T,
R be the 2x2 rotation matrix =



E be the 2x2 identity matrix =



Then the new translation vector t_n = (e f)^T can be calculated by:

t_n = (E-R)*c + t_o


Merry Xmax

Max

Translation can be expressed as a matrix. Of course it can. Apply this matrix to point [1; 1; 1] (this represents a point 1,1 in 2d the last element is just a place holder. The first two numbers are the x and y coordinates) and you will get the answer [2; 2; 1]. The matrix should be [1, 0, 1; 0, 1, 1; 0, 0, 1]. A comma means next element in the same row and semi colon means next row. This matrix will translate any point in 2d by 1,1.

cmax wrote:edit: Please read my following post, because this answer is not correct.

Hi Pander,

a translation can't be expressed as a matrix (beside the zero translation).

If you multiply a matrix with a zero vector, you'll always get a zero vector. ==> no translation


Regards

max