Question

I have a java code for SVG drawing. It processes transforms including rotate, and does this very well, as far as I can see in numerous test pictures compared against their rendering in Chrome. Next what I need is to get actual object location, which is in many images declared via transforms. So I decided just to read X and Y from Matrix used for drawing. Unfortunately I get incorrect values for rotate transform, that is they do not correspond to real object location in the image.

The stripped down code looks like this:

Matrix matrix = new Matrix();
float cx = 1000;    // suppose this is an object X coordinate
float cy = 300;     // this is its Y coordinate
float angle = -90;  // rotate counterclockwise, got from "rotate(-90, 1000, 300)"

// shift to -X,-Y, so object is in the center
matrix.postTranslate(-cx, -cy);
// rotate actually
matrix.postRotate(angle);
// shift back
matrix.postTranslate(cx, cy);
// debug goes here
float[] values = new float[9];
matrix.getValues(values);
Log.v("HELLO", values[Matrix.MTRANS_X] + " " + values[Matrix.MTRANS_Y]);

The log outputs the values 700 and 1300 respectively. I’d expect 0 and 0, because I see the object rotated inplace in my image (that is there is no any movement), and postTranslate calls should compensate each other. Of course, I see how these values are formed from 1000 and 300, but don’t understand why. Once again, I point out that the matrix with these strange values is used for actual object drawing, and it looks correct. Could someone explain what happens here? Am I missing something? So far I have only one solution of my problem: just do not try to obtain position from rotate, do it only for explicit matrix and translate transforms. But this approach lacks generality, and anyway I thought matrix should have reasonable values (including offsets) for any transformation type.

Answer 1

The answer is that the matrix is an operator for space transformation, and should not be used for direct extraction of object position. Instead, one should get initial object coordinates, as specified in x and y attributes of an SVG tag, and apply the matrix on them:

float[] src = new float[2];
src[0] = cx;
src[1] = cy;
matrix.mapPoints(src);

After this we get proper location values in x and y variables.