[HOpenGL] (not?) using preservingMatrix

[jamin1001]

Right, I buy that.  But I don't see how the C line

glGetFloatv (GL_MODELVIEW_MATRIX, your_matrix_result) ;

can be done, since it writes a result into your_matrix_result, so I would
need something like:

preservingMatrix $ do
    loadIdentity
    -- do whatever you want with your your matrix
    rs <- get ((matrix $ Just $ Modelview 0)::StateVar(GLmatrix GLdouble))
-- this does not compile, but I need rs
    return ()

I want the matrix so that I can then transform a vector.  

Speaking of which, are there any ready-made ways to transform a vector with
a matrix, or do I need to do that by hand (eg., see the post
http://groups.google.com/group/comp.graphics.api.opengl/browse_thread/thread/831ee85b6f97eb15/2de3e6d9c6a1a9e5)

 
Thanks again,

Jamin



Sebastian Sylvan wrote:
> 
> On 1/5/07, jamin1001 <jamin1001 at yahoo.com> wrote:
>>
>> In C OpenGL, you can do something like this to use matrices for
>> computation
>> without affecting context:
>>
>> glMatrixMode (GL_MODELVIEW) ;
>> glPushMatrix () ;
>> glLoadMatrix (your_matrix) ;
>> // do whatever you want with your your matrix
>> // for exampl glMultMatrix (your_matrix2) ;
>> ...
>> // at the end of your calculations use
>> glGetFloatv (GL_MODELVIEW_MATRIX, your_matrix_result) ;
>> glPopMatrix () ; // nothing has changed !!!
>>
>> I'm not exactly sure how this is done with HOpenGL. i.e., how can I get
>> the
>> result of a matrix transformation not effecting the MODELVIEW, etc.
>> matrices?  Could someone give an example?
>>
>>
>> Thanks,
>>
>> Jamin
> 
> glPushMatrix() ; foo ; glPopMatrix();
> 
> Is written like so:
> 
> preservingMatrix foo
> 
> Often you would write things like
> do foo
>     preservingMatrix $
>         do bar
>             baz
>     foo2
> 
> Here bar and baz may modify the matrices, but as soon as you leave the
> scope of the action passed to preservingMatrix, the matrix stack is
> popped (glPopMatrix()).
> 
> The Haskell way is better, since there is no way of ever forgetting to
> pop the matrix - preservingMatrix is like a user defined control
> structure. That's what first class actions buys you.
> 
> -- 
> Sebastian Sylvan
> +46(0)736-818655
> UIN: 44640862
> _______________________________________________
> HOpenGL mailing list
> HOpenGL at haskell.org
> http://www.haskell.org/mailman/listinfo/hopengl
> 
> 

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