Projectin a 2D point into Worldspace, for makeing up worldspace ray together with actual viewing direction

[Christoph Hintermüller <hinzge@sbox.tugraz.at>]

Hi
I do have the folowing problem to solve:
1) leth the user draw a 2D polygon onto the screen
2) project each point of the polygon back to worldspace as near as 
possible to the eye
3) take reproceted points and the actual viewing direction to project 
the drawn polygon onto
    the underlying surface.
4) i have to save all necessary informations in order to be able to 
modifiy the selection with   further ones from other viewing directions 
and zooming sizes
5) i have tor check wether points from the acutal surfaceoutline lie 
inside tha new polygon    and decide onto the characteristics of the 
polygon wether to remove the point from the
    surface outline or not .
6) take the surfacepoints from the actual selection and check wether 
they ly inside the
    polygons beeing dravn before and again decide wether to add the 
poitn to the resultin
    outline or not
7) before adding/keeping/removing the points in 5) and 6) check wether 
the point can be seen
    from the actual point of view or not and if not do not touch it at 
all or ignore it.
for 1) i can use some tools allready done by others.
for three i consided kusing the SoRayPickaction with an Worldspace ray set.

But for 2) i don't know exactly what to use
I till now extract the camera from the XtExamineViewer, or better the 
cameras Viewing Volume
   the viewers Vewport Region But i don't know what i still need to make 
step 3 independent
   from the actual view and to be able to reapply 3 for 7) without 
bothering about the actual
   point and direction of view
for 3) i think of using SoRayPickAction in worldspace Ray mode
-  4) is oen by copying all the necessary informations
-  5) is so far doen by taking the saved viewing volume and using the 
world to screen
    projection function in order to get the projection point in 
normalized screen cooridnates
    and than the point is located in realtion to the saved original 
screen polygon.
-  6) is doen in the sam way as 5 only that instead of the allready 
acepted surfacepoints the
    newly determined surfacepoints are projected onto the screen the 
former polygons were drawn
-  7) use 3) for degtermining wether a point beeing located inside the 
the polygon can be seen
    form the actual point of view or not, if not the point is ignored 
(menas is either kept in
    surface outline or just left aside.

My problem is how to get a 2d screnn point into world space only 
dependent on the state
of the  viewer/camera   at thime when the corresponding poly gon was drawn
And which informations to save in order to reproduce the projections 
onto the screen and back at a time when the viwer/camera sate has 
definitly changed since saving.

I do need this in order to enable the user to draw exact outlines onto 
the underneath surface. This is used or better needed for example to 
mark a region on the surface inside
which an extrution having the drawn schape should take place.

the whole reprocection thing and viewer/cmaera independent stuff should 
be there for convenience reasons in order to allow the user to correct 
and refine the outline without
the need of complete redraw.

cu
Christoph Hintermüller
Technical University Graz Austria



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