pyopengl-users

[PyOpenGL-Users] Coloring Nurb Surface

[Michka P. <mic...@gm...>]

Hello

I have managed to display a nurb surface. Now I want it to be colored in
function of the height (with a color gradient). The documentation doesn't
help me a lot, and the tutorials found on the internet are mostly to old or
not adapted to my case. (Since most of them are written in C, not adapted to
pyOpenGL or not using Nurbs)

So here is my code which displays the surface (the controlpoints are random,
only here to test if it works) :

    mat_diffuse = ( 0.7, 0.4, 0.1, 1.0 )
    mat_specular = ( 1.0, 1.0, 1.0, 1.0 )
    mat_shininess = 100.0

    glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular)
    glMaterialfv(GL_FRONT, GL_SHININESS, mat_shininess)
    glEnable(GL_LIGHTING)
    glEnable(GL_LIGHT0)
    glEnable(GL_DEPTH_TEST)
    glEnable(GL_AUTO_NORMAL)
    glEnable(GL_NORMALIZE)

    nurb = gluNewNurbsRenderer()
    gluNurbsProperty(nurb, GLU_SAMPLING_TOLERANCE, 20.0) # Space between
polygons
    gluNurbsProperty(nurb, GLU_DISPLAY_MODE, GLU_FILL)

    ctrlpts = []
    grid_size = 15

    for u in range(grid_size):
        ctrlpts.append([])
        for v in range(grid_size):
            ctrlpts[u].append([u, v, 0.0])

    ctrlpts[3][3][2] = 1.0
    ctrlpts[4][4][2] = 1.0
    ctrlpts[3][4][2] = 1.0
    ctrlpts[4][3][2] = 1.0
    ctrlpts[2][3][2] = 1.0
    ctrlpts[4][2][2] = 1.0
    degree=3
    knot_num = len(ctrlpts) + degree
    knots =  [ float(i)/(knot_num-1) for i in range( knot_num ) ]

    # Render
    glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_diffuse)
    gluBeginSurface(nurb)
    gluNurbsSurface(nurb, knots, knots, ctrlpts, GL_MAP2_VERTEX_3)
    gluEndSurface(nurb)



Now, I have read about GL_MAP2_COLOR_4, but I don't know how and where to
use it ? Some say I should tessellate my nurb surface, use a callback
function and try to change the color of the vertexes. But How ?

Perhaps somebody has a tutorial or some good hints for me ?

Thank you

Michka Popoff

Re: [PyOpenGL-Users] Coloring Nurb Surface

[Dirk R. <dir...@gm...>]

	Hi Michka,

On 02/23/2011 08:34 AM, Michka Popoff wrote:
>
> Perhaps somebody has a tutorial or some good hints for me ?

generally the most generic and efficient way for that is to use glTexGen to turn 
your height into a texture coordinate and use a 1D texture for the colors to 
interpolate between. This allows you to fully specify the color that are 
interpolated and also enables you to do non-linear mappings (e.g. have a bright 
band at a specific height). Give it a try!

Hope it helps

	Dirk

Re: [PyOpenGL-Users] Coloring Nurb Surface

[Ian M. <geo...@gm...>]

I'd use a vertex shader.

Re: [PyOpenGL-Users] Coloring Nurb Surface

[Michka P. <mic...@gm...>]

Thanks for answering

Dirk :
generally the most generic and efficient way for that is to use glTexGen to
turn
your height into a texture coordinate and use a 1D texture for the colors to
interpolate between. This allows you to fully specify the color that are
interpolated and also enables you to do non-linear mappings (e.g. have a
bright
band at a specific height). Give it a try!

Ian Mallett :
I'd use a vertex shader.


The two ideas seem okay for me, but I think I am stuck : I have no influence
on the nurb displayed, because I don't know how to modify it before it is
displayed. (or after).

I tried a few things :

1) I used gluNurbsProperty(nurb, GLU_NURBS_MODE, GLU_NURBS_TESSELLATOR) and
gluNurbsCallback(nurb,GLU_NURBS_COLOR,color). I was able to print the colors
of every point in the callback function. But I can not modify them. So why
does this function exist ? And as the nurb is now tesselated, how can I
display it ?

2) I used gluNurbsSurface(nurb, knots, knots, ctrlpts, GL_MAP2_COLOR_4). In
the callback documentation for gluNurbsCallback() I have found :

This callback is effective only when the user provides  a  color map
(*GL**_**MAP1**_**COLOR**_**4* or *GL**_**MAP2**_**COLOR**_**4*).
``color'' contains four components: R,G,B,A

So where can I provide my own color map ? Documentation never says anything
about this.

My ideas :

1) I could subdivide my big nurb surface in a lot of little nurb patches,
and put a colored texture on it. Like the typical molehill.py tutorial you
can find everywhere. I think it is quite the same as Dirk's idea ?

2) I could rewrite everything with my own polygons, where I can define the
colors myself. Will not be as smooth as it is now, but should work.

Michka Popoff

Re: [PyOpenGL-Users] Coloring Nurb Surface

[Nicolas R. <Nic...@in...>]

On Feb 24, 2011, at 9:55 AM, Michka Popoff wrote:

> Thanks for answering
> 
> Dirk :
> generally the most generic and efficient way for that is to use glTexGen to turn
> your height into a texture coordinate and use a 1D texture for the colors to
> interpolate between. This allows you to fully specify the color that are
> interpolated and also enables you to do non-linear mappings (e.g. have a bright
> band at a specific height). Give it a try!
> 
> Ian Mallett :
> I'd use a vertex shader.
> 

Have a look at glumpy that implement those two ideas (1D texture + shader):
http://code.google.com/p/glumpy/


Nicolas