[Plib-users] Trouble making simple colored triangle

[Ben D. <be...@wa...>]

I'm in the process of porting my whole 3d project over to PLIB/SSG, and
mostly it's great, but i'm having a lot of trouble trying to build some
simple geometry with SSG.

As an example, consider a plain green triangle.  You might think that the
following code would work:

 sgVec4 color;
 sgVec3 pos;
 ssgSimpleState *state = new ssgSimpleState();
 sgSetVec4(color, 0.0f, 1.0f, 0.0f, 1.0f);
 state->setMaterial(GL_DIFFUSE, color);

 ssgVertexArray   *vertices = new ssgVertexArray(3);
 ssgNormalArray   *normals = new ssgNormalArray(3);

 ssgVtxTable *pVtx = new ssgVtxTable(GL_TRIANGLES, vertices,
  normals, NULL, NULL);

 for (int i = 0; i < 3; i++)
 {
  double a = i * PI2f / 3.0f;
  sgSetVec3(pos, cos(a), sin(a), 0);
  vertices->add(pos);
  sgSetVec3(pos, 0, 0, 1);
  normals->add(pos);
 }
 pVtx->setState(state);

But, this makes a triangle which is white, not green.  Stepping into the SSG
code, i discovered that in the function ssgVtxTable::draw_geometry(), there
is the line:
  if ( num_colours == 0 ) glColor4f   ( 1.0f, 1.0f, 1.0f, 1.0f ) ;


This forces all primitives to white if they don't have a color per vertex!
I worked around this problem by making and using a subclass of ssgVtxTable
with a draw_geometry that doesn't make this assumption.  The triangle was
still white, so i found the next problem.

In ssgSimpleState::apply(), it uses glMaterialfv to set the diffuse color.
However, GL is in "Color Material" mode by default, so the glMaterialfv
calls have no effect, making the shape default white.

I tried to solve this problem by adding a statement to the triangle code:
    state->disable(GL_COLOR_MATERIAL);

But, another problem (bug?) in ssgSimpleState::apply() prevents this from
working.  The Enable/Disable state code, required to disable "Color
Material", is called *after* the glMaterialfv code, but OGL requires the
glMaterialfv call to come after!

So, i tried to work around this by changing the order in ssgSimpleState.cxx.

That worked, so i finally got a green triangle.

Surely there is some way to do simple colored geometry without all the
hacking i had to do?  Please help, i would really love to be able to use
SSG.

Thanks,
Ben Discoe
http://vterrain.org/