BACKGROUND
Using kivy with openGL and GLU style functions, I have gotten modelview and projection matrices working. The model matrix is not used, so the modelview matrix is just really a transform based upon the location of the camera.
# cx,cy,cz are the location of the camera. cz is always positive
modelview = Matrix().look_at(cx,cy,cz, # eye position coords
cx,cy,0.0, # looking at this pt.
0.0,1.0,0.0) # a vector pointing up
The projection matrix is also straightforward and working as desired.
aspect_ratio = float(Window.width)/Window.height
projection.perspective(90.0, aspect_ratio, 1.0, 400.0)
The look_at and perspective functions come from kivy's Matrix class which is emulating/encapsulating some GLU functions of the same (or similar) name.
Projection seems to be working just right, but...
PROBLEM
A point on the screen can be thought of as defining a ray from the eye of the camera off to infinity. I'd like to know the world x,y coordinates of where that ray intersects the plane defined by z=0.
Here is (one of) my (many) attempt(s) to get this to work.
x,y # <--- mouse coords (I've tried range of [0..1], or [0..screenwidth])
z=0 # I have also tried z=cz
# create inverse proj. matrix with near set to the distance
# the camera is from z=0
proj = Matrix().identity()
proj.perspective(90.0, aspect_ratio, cz, cz+1.0)
proj_i = proj.inverse()
# create inverse modelview matrix
modelview_i = modelview_mat.inverse()
x,y,z = proj_i.transform_point(x,y,z)
x,y,z = modelview_i.transform_point(x,y,z)
This doesn't work. Specifically, when I have the camera steadily moving away (cz is increasing slowly). The x,y of mouse clicks do not change as the camera moves.
What am I doing wrong?
I know there are many similar questions floating about, but I haven't been able to find the information I am looking for.
