Coordinate system

Anonymous 9 years ago 0
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lhs wrote at 2012-01-20 11:34:


Though this is a great piece of work, I just can't figure out why is z axis pointing up. That just looks so wrong to me, am I missing something?

objo wrote at 2012-01-20 19:03:

Yes, Z is the default up direction in this library, but both HelixViewport3D and CameraController also supports Y as up direction.

I have mostly used this library for engneering/math projects, that's why I chose this convention. (I know it is common in computer graphcis to use Y up, and the default UpDirection of Media3D.ProjectionCamera is 0,1,0)

lhs wrote at 2012-01-20 19:20:

Thanks for reply.

I just thought that it was WPF related, so Y should be up...

What is the easiest way to switch the logic, without rotating models or going through every line of code?

objo wrote at 2012-01-20 19:23:

sorry, I forgot:

set ModelUpDirection="0,1,0" on your HelixViewport3D.

See the UpdirectionDemo example.

lhs wrote at 2012-01-20 19:46:

Tried that, but I get right view instead of front view. Still need to rotate models, which will mess up my transforms (long story...).

Never mind, I thought that you could save me some time, but I guess that I will have to do it myself.

Thanks anyway. Keep up the good work.

objo wrote at 2012-01-26 19:50:

sorry, I don't see the problem here. Do you need a left handed system? Let us know if there is a bug in the handling of the up direction!

przem321 wrote at 2012-01-28 00:42:

I guess the problem is that the view-cube is a little bit messed up. Usually, in graphics libraries like OpenGL and WPF, which both use a RHS (right-handed coordinate system) the Z-axis points towards the camera, the Y-axis to the sky and the X-axis to the right.  In a LHS system (like Direct3D) the Z-axis points away from the camera. In the Helix-Viewport generally everything works well except the view-cube which shows an R instead of F, B instead R, and so on, and the default camera. But one can customize the cube and the camera anyway. ...

Well, this is not a bug and also a minor issue. Personally, I would also prefer the have the front direction looking along the negative Z-axis as the very default setting, just because this is standard, not only in WPF, but also OpenGL... well I know, there is no real standard, Maya handles it differently, Blender and 3D Studio too, in truth this topic is a mess... just for this reason I stick to the coordinate-system of the libraries, and for WPF it is like explained. 

Anyway, lhs, you don't have to play around with your model transforms (if the model is in RHS), you just have to get used to the view-cube and, I recommend it, change the default camera position to something like 


            view.Camera.Position = new Point3D(4, 2, 16);
            view.Camera.LookDirection = new Vector3D(-4, -2, -16);
            view.Camera.UpDirection = new Vector3D(0, 1, 0);


Btw, we have already elaborated about the default camera here: http://helixtoolkit.codeplex.com/discussions/286186


PS: Maybe one could add a property to set the 'front-direction' just analogous to the 'up-direction' property? This shouldn't cause problems since the third axis would be derived automatically according to the CS?

objo wrote at 2012-01-29 09:15:

Great post! How would you implement support for a front-direction property? Apply a ScaleTransform3D(1,1,-1) to the model? Also note that there is a ViewCubeFrontText property in HelixViewport3D that can be used if you need to change the text on the faces of the view cube. 

przem321 wrote at 2012-01-31 04:23:

Yes, I think changing the text of the view-cube is the simplest way to achieve the goal. I wouldn't work with any additional transforms, the front-direction property should just imply how to set up the view-cube, i.e. setting the model-up and front-direction should imply how to setup the view cube naming, etc. This is basically not a really important issue, just cosmetics. 

RobPerkins wrote at 2012-07-05 09:22:

I've been reading and using Helix for awhile now and I'm very pleased so far. 

One thing I do need is a way to transform the coordinate system to an RHS. The display requirements I have are for X increasing to the right, Y increasing "into" the screen, and Z increasing up. It's said to be the coordinate system used by draftsmen. The one I see "by default" is a left-hand system with Y increasing towards the user. 

Is it possible to set properties for world coordinates in a Z-up RHS?  Possible to subclass HelixViewport3D for that? What approach would you suggest? Unless I'm seeing this wrong? 

objo wrote at 2012-07-07 00:48:

Isn't the ProjectionCamera only supporting LHS? I have not studied the code yet, may be wrong...

If a MatrixCamera is needed to support RHS the camera controller may need some changes.. 

I found simple explanations to LHS and RHS projection matrices in this presentation: http://users.ece.gatech.edu/lanterma/mpg10/mpglecture04f10_3dto2dproj.pdf

Direct3D has LH and RH versions of all the projection functions, e.g. http://msdn.microsoft.com/en-us/library/windows/desktop/bb204936(v=vs.85).aspx

RobPerkins wrote at 2012-07-07 01:02:

Let's see..

Helix appears to rotate the WPF default 90 degrees CCW about X, so that Z is up. I need that much, plus a -Y transformation. I can compute that at load and apply a reverse transformation during hit testing, that part is no problem.

I just need the coordinate system axes graphic to also invert the Y arrow, and I'm good to go.

przem321 wrote at 2012-07-26 15:41:

The link you have posted is somewhat misleading. WPF and OpenGL both use the RHS!!! This applies to the HelixToolkit too.

objo wrote at 2012-08-09 01:02:

Was getting confused here. Agree that WPF use RHS coordinate system for the cameras!

I see there is a need to create an example showing the different configurations...

See http://helixtoolkit.codeplex.com/workitem/9962

RobPerkins wrote at 2012-08-09 02:42:

I agree! I'm confused myself and was about to ask, with the "default" axis arrows, which color is x, which is y, which z...

I'm not the one to write that demo, though, in the confused state I'm in...

RobPerkins wrote at 2012-10-02 04:52:

RobPerkins wrote:

I agree! I'm confused myself and was about to ask, with the "default" axis arrows, which color is x, which is y, which z...

I'm not the one to write that demo, though, in the confused state I'm in...

Not confused any longer. I'd modified the axis arrows and caused my own pain. It's RHS with no problems at all once I cleared that up.