[vtkusers] Problem with alpha blending

[Francisco Jesús Martínez Serrano]

Dear vtk users,

I'm trying to create an application to visualize the results of
galaxy-formation simulations
using particles. The technique that I want to use is the technique
that was used to make
these videos:

http://www.aip.de/People/AKhalatyan/COSMOLOGY/STEREO/MW_ANDROMEDA/MW.htm

It is basically a billboarding using a texture for each particle that
reproduces the density field of each particle.

The billboarding implementation that I have so far consists of adding
all the particles as
input of a vtkGlyph3D object and a vtkPlaneSource as source. The
texture (consisting of a
white square R=G=B=255 with varying alpha value for transparency) is
then added to the
actor of the vtkPolyDataMapper of the glyph. I have set up a callback
that sets the normal
vector of each plane in the glyph directed towards the camera:

void OrientNormal( vtkObject* vtkNotUsed(object),unsigned long
eventId,void* sr,void* vtkNotUsed(calldata))
{
	CallbackData* cd = reinterpret_cast<CallbackData*>(sr);
	vtkCamera* cam = cd->cam;
	vtkPolyData* pdata = cd->pdata;
	vtkPointData *PD = pdata->GetPointData();
	vtkGlyph3D* glyph = cd->glyph;

	double *rz;
	double rzm[3],rym[3],rxm[3];
	double *pos;
	double *cpos = cam->GetPosition();
	// Vector data for directions
	vtkFloatArray *FA = vtkFloatArray::New();
	FA->SetName("directions");
	FA->SetNumberOfComponents(3);
	uint n=pdata->GetNumberOfPoints();
	for (uint i=0; i<n; i++)
	{
		if ( cam->GetParallelProjection() != 0 )
		{
			rz = cam->GetDirectionOfProjection();
			rzm[0] = -rz[0];
			rzm[1] = -rz[1];
			rzm[2] = -rz[2];
		}
		else
		{
			pos=pdata->GetPoint(i);
			rzm[0] = -cpos[0] + pos[0];
			rzm[1] = -cpos[1] + pos[1];
			rzm[2] = -cpos[2] + pos[2];
//			vtkMath::Normalize(rzm);
		}
		FA->InsertNextTuple(rzm);
	}
	// Add the data arrays to the point data
	PD->SetNormals(FA);
	FA->Delete();
}


The problem comes with some relative postions of particles which don't
work as supposed.
An example can be seen at:

http://franjesus.googlepages.com/alphaproblem.png

Here the green particle is on the front (you can tell by the size, all
particles have the same
radius) and is partially covering the yellow and cyan particles. For
the yellow particle the
desired effect is obtained and there's a blended transition zone. With
the cyan particle, the
problem happens and the green particle totally covers it until the
alpha channel goes to zero
(ie. like a threshold).

Is there any option of the renderer that I have to set so get it
working correctly?
Might it be a problem with the OpenGL implementation? (nvidia linux 9755)