Hello, all. I’m trying to implement stencil shadows in a project I’m working on, using ARB vertex shaders, and I’ve run into some trouble. I’m following this article on Gamasutra, and it does not seem to be working.

Before rendering the shadow volume (both front and back faces), I enable my vertex program, which is as follows:

!!ARBvp1.0

ATTRIB vPos = vertex.position;

OUTPUT oPos = result.position;PARAM lPos = program.env[0];

TEMP temp;

ADD temp.xyz, vPos, -lPos;

MAD temp, vPos.w, lPos, temp;

DP4 oPos.x, state.matrix.mvp.row[0], temp;

DP4 oPos.y, state.matrix.mvp.row[1], temp;

DP4 oPos.z, state.matrix.mvp.row[2], temp;

DP4 oPos.w, state.matrix.mvp.row[3], temp;END

My understanding is that this program will extrude vertices with a zero W component, and do nothing to vertices with a one W component.

When rendering the shadow volume, I do:

glVertex4f (verts[1][0], verts[1][1], verts[1][2], 1);

glVertex4f (verts[0][0], verts[0][1], verts[0][2], 1);glVertex4f (verts[0][0], verts[0][1], verts[0][2], 0);

glVertex4f (verts[1][0], verts[1][1], verts[1][2], 0);

Where the verts array holds the coordinates of the edge that is being extruded.

Before drawing these shadow volumes, I also do:

glProgramEnvParameter4fARB (GL_VERTEX_PROGRAM_ARB, 0, lightvec[0], lightvec[1], lightvec[2], 1.0f);

Where lightvec holds the location of the light I’m using to create the shadow volume.

Nothing actually shows up when using the vertex shader.

When I used the following code to calculate the extrusion of the points by hand, it worked. (I already did glVertex3f for the first two, unextruded points)

Vector d1, d2;

d1 = verts[0] - lightvec;

d2 = verts[1] - lightvec;d1.Normalize ();

d2.Normalize ();d1 = d1 * 10000;

d2 = d2 * 10000;glVertex3fv (d1);

glVertex3fv (d2);

Any assistance here would be very much appreciated!