Alpha blending with Cg shaders

OpenGL OpenGL: Basic Coding
1

I am trying to use alpha blending in my Cg shaders. This is my code in the application where I first render a plane (behind) and then render a sphere (in front of plane that should be transparent):


	glDisable(GL_LIGHTING);
	glEnable(GL_BLEND);
	glBlendFunc(GL_ONE,GL_ZERO);

	// Test alpha blending.
	render_plane(-3, 0,2, 0, 0,0, 1, 10);

        // Render transparent sphere.
	glPushMatrix();
		glScalef(2.0, 2.0, 2.0);
		glTranslatef(0.0, 0.0, 1.5);
		render_sphere();
	glPopMatrix();

	//--- Disable GL blending
	glDisable(GL_BLEND);
	glEnable(GL_LIGHTING);

I would like the sphere to be transparent so I can see the plane thats behind it.

In the fragment program for the plane I set the

OUT.color.a = 1.0

and in the fragment program for the sphere I set the

OUT.color.a = 0.5.

But the sphere is not transparent, what am I doing wrong?

2

You blend func is wrong.
Try one of these instead :

glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA); // classic transparency
glBlendFunc( GL_ONE, GL_ONE_MINUS_SRC_ALPHA ); // pre-multiplied alpha