[osg-users] 【NEWBIE】Question about osg::MatrixTransform::getMatrix().getTrans() and getRotate()

Wojciech Lewandowski w.p.lewandowski at gmail.com
Fri May 5 06:17:51 PDT 2017

Hi Jiechang,

Few observations:

1. You write you want rotation around Y axis (0,1,0). But you rotate around
Z axis (0,0,1). Btw there are osg::X_AXIS = (1,0,0) , osg::Y_AXIS =
(0,1,0), osg::Z_AXIS = (0,0,1) constants defined in OSG which you may
2. What is the origin variable in your example ? This is probably the other
matrix which you premultiply and it influences your results.
3. Values stored in quaternion fields are rather non intuitive. I suggest
you just make simpler experiment. Set quaternion variable directly with
osg::Quat quat( Angle, Axis ). For example as osg::Quat quat( osg::PI_4,
osg::Z_AXIS) as you do in your example. And then examine under debugger
what constructor does and whats actually stored on 4 fields of Quat. Those
numbers won't be the same as the ones you passed to constructor. And thats
correct. You will find a plenty of info on Quaternions on the web. Look for
them if you need to learn more.
4. This line probably has wrong order of transformations -->
osg::Matrix::rotate(osg::DegreesToRadians(45.0), 0, 0,
I suppose you rather want to make it like this --> model1->setMatrix(osg::
0, 0, 1)*osg::Matrix::translate(Center)*origin);
Reason for this is OSG uses row major matrices, so if you want to transform
vertex by matrix you do it like this: result = vert * matrix. Thus your
origin transform should be multiplied as last transform.

Hope this helps,
Wojtek Lewandowski

2017-05-05 13:54 GMT+02:00 Jiechang Guo <osgforum at tevs.eu>:

> Hi,
>      I'm a newbie and not good at math.
>      Please....
>      I'm so confused with osg::MatrixTransform::getMatrix().getTrans()
> and getRotate().
>      I use the code below to rotate an object around y axis about 45
> degrees.
> model1->setMatrix(origin*osg::Matrix::rotate(osg::DegreesToRadians(45.0),
> 0, 0, 1));
>       I want to get the rotation of the model, so I used the function to
> get a quat:
> osg::Quat rotation = model1->getMatrix().getRotate();
>       I thought the rotation should be like (0.7853982,0,0,1)。but the
> result is(0,0,0.382683,0,92388).
>       I've checked the source code of OSG, and I cann't get any
> inspriation from it.
>       Another case, I thought I should move the object to its center and
> do rotate then move it back (according to some book or paper). The code is
> below.
> osg::Vec3 Center = model1->getBound().center();
> model1->setMatrix(origin*osg::Matrix::translate(-Center)*
> osg::Matrix::rotate(osg::DegreesToRadians(45.0), 0, 0,
> 1)*osg::Matrix::translate(Center));
> The object is on the same position and rotation as the first case.
>        I try to get the rotation, I get more confused..
>        I thought I didn't change the position of the model, the tranlation
> I get
> osg::Vec3 translation = model1->getMatrix().getTrans();
> should be (0,0,0) but (-168.184,-141.218, 0) and the rotation is just like
> the first case.
>        Could you please help me to figure out why I got those results?
> Thank you!
> Cheers,
> Jiechang
> ------------------
> Read this topic online here:
> http://forum.openscenegraph.org/viewtopic.php?p=70883#70883
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