[HOpenGL] Projection and Modelview Matrices

[Sven Panne]

Guilherme Lopes wrote:
>     I want to do View Frustum Culling using Octrees, and everything is 
> working except for one small detail: I cannot find a way to access the 
> Projection Matrix, nor the Modelview matrix, which are taken for granted 
> in the C/C++ versions of the library.
>     Am I missing something? I can of course calculate the clipping 
> planes using lookat information and the arguments to the frustum 
> function, but cannot believe there isn't a more elegant solution... ;-)

Sorry for replying so slowly, but a HW/SW upgrade kept me busy the last weeks.
If you are using the OpenGL package coming with GHC 6.2(.2), you can access
the current matrix via, well, "currentMatrix":   :-)

    currentMatrix :: (Matrix m, MatrixComponent c) => StateVar (m c)

i.e.

    currentMatrix $= blah
    foo <- get currentMatrix

Perhaps you have to set the matrix mode via "matrixMode" before/after doing
that. This is not very elegant, so "currentMatrix" has been deprecated starting
with the OpenGL package shipping with the soon-to-be-released GHC 6.4. The new
"official" and more general way is:

    matrix :: (Matrix m, MatrixComponent c) => Maybe MatrixMode -> StateVar (m c)

where "matrix Nothing" is the same as "currentMatrix". In your example you can use:

    pm  <- get (matrix Projection)
    mvm <- get (matrix (Modelview 0))

I really have to continue writing documentation... *sigh*

Hope that helps,
    S.