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

[Wojciech Lewandowski]

Hi Jiechang,

I am not sure I am able to pinpoint your problem. I see some weak spots but
I am not sure if those are the true causes of your problem. And don't want
to give wrong clues. Can you write short repro program which demos your
problem ? I may then fix it and send you back.

To learn you may try to separate rotations and translations by using two
matrix transforms above loaded model.

MatrixTransformTranslate->MatrixTransformRotate->Object.

Apply only rotations to MatrixTransformRotate
and translations to MatrixTransformTranslate.

Cheers,
Wojtek



2017-05-06 8:51 GMT+02:00 Jiechang Guo :

> Hi Wojtek,
> First, Thank you very much for your detailed reply.
> 1. It's my mistake to say rotation around Y axis, I always think the z
> axis is actually the y axis.
> 2. The origin variable is
> osg::Matrix origin = model1->getMatrix();
> I update this variable everytime when I translate or rotate the model. And
> multiply it with my transform matrix so that I can get the correct result
> after changing the position or orientation  the model many times. Please
> Correct  Me if I'm not correct.
> 3. OMG..I tried what you told me to. I just... I think I undestand what's
> going on in side the constructor. No wonder I got that results and some
> previous work about trackball rotate I did  is wrong. Thank you.
> 4. I've done some experiments about the order of the origin matrix. I get
> the same result either I multiply it at first or at last...
> The code is below:
> osg::ref_ptr model1 = new osg::MatrixTransform;
> model1->addChild(osgDB::readNodeFile("E:\\objdata\\FEMUR.obj", a));
> osg::Matrix origin = model1->getMatrix();
> model1->setMatrix(origin*osg::Matrix::translate(100, 0, 0));
> osg::Vec3 Center = model1->getBound().center();
> origin = model1->getMatrix();
> osg::Quat quat(osg::PI_4, osg::Z_AXIS);
> model1->setMatrix(origin*osg::Matrix::translate(-Center)*
> osg::Matrix::rotate(quat)*osg::Matrix::translate(Center)*osg::Matrix::translate(100,
> 0, 0));
>
> The reason that I want to get the Trans() and Rotate() is that I'm
> doing a task: Compute the deviation of the origin model and target model.
> These two model are the same and when the origin model is being manipulted
> to the position of target model(which is a mesh model) I have to compute
> whether they are  overlaped and skip to another task.
> Actually, I've already implemented this function, but I was confused by:
> when I do only rotate task, the trans I get from getMatrix().getTrans() is
> changing. I even don't know why it works when I only compute the trans
> deviation. The code is below.
> model1Translation = m1.model->getMatrix().getTrans();
> model1Quat = m1.model->getMatrix().getRotate();
> model2Translation = m2.model->getMatrix().getTrans();
> model2Quat = m2.model->getMatrix().getRotate();
> osg::Vec3 positionbias = model2Translation - model1Translation;
> osg::Quat rotationbias = model2Quat - model1Quat;
> if (abs(positionbias.x()) <= 2 && abs(positionbias.y()) <= 2 &&
> abs(positionbias.z()) <= 2)
> {
> //if (abs(rotationbias.x())<=0.1&&)
> //{
> hm->pressNext();
> //}
> }
>
> Cheers,
>   Jiechang
>
> --
> Read this topic online here:
> http://forum.openscenegraph.org/viewtopic.php?p=70887#70887
>
>
>
>
>
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Re: [osg-users] 【NEWBIE】Question about osg::MatrixTransform::getMatrix().getTrans() and getRotate()

[Wojciech Lewandowski]

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
directly.
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 -->
model1->setMatrix(origin*osg::Matrix::translate(-Center)*
osg::Matrix::rotate(osg::DegreesToRadians(45.0), 0, 0,
1)*osg::Matrix::translate(Center));
I suppose you rather want to make it like this --> model1->setMatrix(osg::
Matrix::translate(-Center)*osg::Matrix::rotate(osg::DegreesToRadians(45.0),
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 :

> 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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