Re: [PyOpenGL-Users] get Translation inside matrix stack

[Gary H. <gh...@is...>]

Prashant Saxena wrote:
> I am sorry but I didn't get any success with your code. I tried this:
>
> glPushMatrix()
> glTranslate(5,0,0)
> glTranslate(10,0,0)
> glTranslate(5,0,0)
> # To get total increment of x axis(which is 5+10+5=20)
> modelview = glGetFloatv( GL_MODELVIEW_MATRIX )
> print modelview
> glPopMatrix()

My mistake.   The glGetDouble/glGetFloat call is the right thing here, 
but I was confusing my Python-to-OpenGL libraries.  Your syntax is 
correct for PyOpenGL.  (Mine was for a library called Pyglet.)

>
> but I am getting this:
> [[1.0, 0.0, 0.0, 0.0], [0.0, 1.0, 0.0, 0.0], [0.0, 0.0, 1.0, 0.0], 
> [0.0, 0.0, 0.0, 1.0]]
> Which does,'t solve my problem.

There are several matrix stacks maintained by OpenGL.  Which one are you 
using when you issue the glPushMatrix-glTranslate-glPopMatrix calls?   
Probably not the GL_MODELVIEW_MATRIX since that's still the identity 
when you get it back.   You need to choose your matrix first:
    glMatrixMode(GL_MODELVIEW)  # Specify which matrix
    then do your matrix ops
    ... = glGetFloatv( GL_MODELVIEW_MATRIX )  # Retrieve the matrix back

Gary Herron


>
> ----- Original Message ----
> From: Prashant Saxena <ani...@ya...>
> To: Gary Herron <gh...@is...>
> Sent: Tuesday, July 22, 2008 9:59:38 PM
> Subject: Re: [PyOpenGL-Users] get Translation inside matrix stack
>
> I am sorry but I didn't get any success with your code. I tried this:
>
> glPushMatrix()
> glTranslate(5,0,0)
> glTranslate(10,0,0)
> glTranslate(5,0,0)
> # To get total increment of x axis(which is 5+10+5=20)
> modelview = glGetFloatv( GL_MODELVIEW_MATRIX )
> print modelview
> glPopMatrix()
>
> but I am getting this:
> [[1.0, 0.0, 0.0, 0.0], [0.0, 1.0, 0.0, 0.0], [0.0, 0.0, 1.0, 0.0], 
> [0.0, 0.0, 0.0, 1.0]]
> Which does,'t solve my problem.
>
>
> ----- Original Message ----
> From: Gary Herron <gh...@is...>
> Cc: pyo...@li...
> Sent: Tuesday, July 22, 2008 8:48:24 PM
> Subject: Re: [PyOpenGL-Users] get Translation inside matrix stack
>
> Shantytown Saxena wrote:
> >
> > glPushMatrix()
> > glTranslate(5,0,0)
> > glTranslate(10,0,0)
> > glTranslate(5,0,0)
> > # How do I get here total increment of x axis(which is 5+10+5=20)
> > before I Pop up.
> > glPopMatrix()
>
> All the transformations are recorded by OpenGL in a 4x4 matrix.
> You can recover that matrix like this:
>
>     buff = (ctypes.c_double*16)()
>     pyglet.gl <http://pyglet.gl.gl>.glGetDoublev(stack, buff)
>
> If you started with the identity and did no other transformations you'll
> find the 20 in that 4x4 array.
>
> In either the [0,3] or [3,0] element.  (I've forgotten which way Opengl
> stores the array now.)
>
> Gary Herron
>
>
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