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Jiechang Guo Newbie
Joined: 22 Sep 2016 Posts: 6

Posted: Fri May 05, 2017 11:54 am Post subject: 【NEWBIE】Question about osg::MatrixTransform::getMatrix().getTrans() and getRotate() 


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 

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Wojtek User
Joined: 02 Nov 2011 Posts: 78

Posted: Fri May 05, 2017 1:22 pm Post subject: 【NEWBIE】Question about osg::MatrixTransform::getMatrix().getTrans() and getRotate() 


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
20170505 13:54 GMT+02:00 Jiechang Guo < ( Only registered users can see emails on this board! Get registred or enter the forums! 
)>:
Quote:

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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Jiechang Guo Newbie
Joined: 22 Sep 2016 Posts: 6

Posted: Sat May 06, 2017 6:51 am Post subject: Re: 【NEWBIE】Question about osg::MatrixTransform::getMatrix().getTrans() and getRotate() 


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<osg::MatrixTransform> 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 

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Wojtek User
Joined: 02 Nov 2011 Posts: 78

Posted: Sat May 06, 2017 11:03 am Post subject: 【NEWBIE】Question about osg::MatrixTransform::getMatrix().getTrans() and getRotate() 


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
20170506 8:51 GMT+02:00 Jiechang Guo < ( Only registered users can see emails on this board! Get registred or enter the forums! 
)>:
Quote:

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<osg::MatrixTransform> 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
_______________________________________________
osgusers mailing list
(Only registered users can see emails on this board! Get registred or enter the forums!  )
http://lists.openscenegraph.org/listinfo.cgi/osgusersopenscenegraph.org


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