It appears that whatever XYZ camera space value comes out, whilst the
z-value may well not be set, the XY values tally with a Z value of 1.
ie, the camera space vector is defined as an XY position on the Z=1 plane.
Can anyone confirm this?
I can only infer this from looking at the the location (using a locator)
whilst showing the view frustum.
Cheers,
Simon.
On Wednesday, June 1, 2016 at 9:01:44 AM UTC+1, simon smith wrote:
>
> Hi Guys,
>
> OK, so I'm trying to go from screen-space (ie, the final UV position from
> a cameras rendered image) back to a world space of the location selected on
> the image.
>
> If my understanding is correct, it is simply as follows ...
> 1) Get the screen space co-ordinate - center of the image is 0,0 and the
> extremes go from -1 to 1. The z-value is the depth
> 2) Use the cameras inverse projection matrix to turn this into camera
> space.
> 3) Use cameras world projection matrix to get a ray from the camera
> towards the scene (in world space).
> 4) Use this vector and the camera position to create a ray to then
> intersect the geometry.
> 5) Job done :)
>
> The problems arise in step (2).
> I have tried creating a 3-vector and a 4-vector to represent the screen
> space.
> The x/y is the screen coordinates, and I have put different values of Z in
> to see how that affects things, but it did not.
> (Does the z-value run from 0-1 from the near to far clipping planes?)
>
> I then tried a 4-vector, with 1 for the W-value but this made no
> difference either.
> As I increased the z-values I expected the resulting camera space Z value
> to scale accordingly.
>
> I feel that I'm missing something fundamental in how Maya applied or uses
> projection matricies.
> Should the W-value in the screen space always be 1? Changing the near
> clipping plane seems to change the transforms so that must be telling me
> something useful!
>
>
> This is the basis of the test code that I've been using:
> There is just a single default camera in the scene moved along along the
> z-axis.
> I did this as the resulting camera space vectors can be validated simply
> by inspection.
>
> I tried with test screenspace coordinates of:
> 0,0,0,1 -> expect to be at 0,0,0 in camera space
> 0,0,1,1 -> expect to be at 0,0,z in camera space - with z being non-zero
> (it was always still zero!)
>
> The x/y values that come out are always the same no matter what screen
> space z value I use, but how do I get a valid value for this?
> I need this z value to create a suitable test ray ....
>
> sl = om.MSelectionList()
> sl.add("cameraShape1")
>
> dpathCameraShape = om.MDagPath()
> sl.getDagPath(0, dpathCameraShape)
>
> dpathCameraShape.extendToShape()
> cam = om.MFnCamera(dpathCameraShape.node())
>
> matProj = floatMMatrixToMMatrix_(cam.projectionMatrix())
> matInvProj = matProj.inverse()
>
> matCam = dpathCameraShape.inclusiveMatrix()
> matInvCam = matCam.inverse()
>
> printMatrix_(matCam, "matCam")
> printMatrix_(matProj, "matProj")
>
> # Use test points at known locations in screenspace.
> point = om.MFloatPoint(0, 0, 0, 1)
> vect = om.MVector(point)
> result = vect * matInvProj
> print4Vec_(point, "Test Point In Screen Space:")
> print4Vec_(result, "Test Point In Camera Space:")
>
> If anyone could point me in the right direction that would be great!
> I just know it's going to be something bloody obvious, but cannnot see the
> wood for the trees :)
>
> Best Regards,
> Simon.
>
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