Name:			GLQuake stencil shadows - the z-fail method
Author:			Rich Whitehouse
E-mail:			thefatal@telefragged.com
Homepage:		http://www.telefragged.com/thefatal/
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This is a follow-up to my stencil shadows tutorial, in which I will explain
how to use the "z-fail" stencil shadow method. Basically, this method reverses
the face culling of the shadow volume, and increments/decrements the stencil
buffer on each pass if the depth test fails, as opposed to passes. The main
advantage of using this method is that we no longer get unfortunate depth
issues when the view point gets within a shadow volume. The disadvantage is
that it is slower and requires us to do more rendering because with this
method we must put caps on our shadow volumes, because we aren't relying
on depth testing. In this case, however, adding volume caps is going to be
relatively trivial.

First open up gl_rmain.c and go to the GL_ShadowPass function that you have
already added from the last tutorial. Below this check:

			if (GL_TriFacingLight(tri, verts, lightPosition))

After the for loop and whatnot, we want to add this:


			//draw the front cap for the shadow volume
			glBegin(GL_TRIANGLES);
			for ( j = 0; j < 3; j++ )
			{
				v1 = &verts[tri->vertindex[j]].v[0];
				glVertex3f(v1[0], v1[1], v1[2]);
			}
			glEnd();

			//Now the back cap. We draw it backwards to assure it is drawn
			//facing outward from the shadow volume.
			glBegin(GL_TRIANGLES);
			for (j = 0; j < 3; j++ )
			{
				v1 = &verts[tri->vertindex[2-j]].v[0];
				v3[0] = ( v1[0]-lightPosition[0] )*PROJECTION_DISTANCE;
				v3[1] = ( v1[1]-lightPosition[1] )*PROJECTION_DISTANCE;
				v3[2] = ( v1[2]-lightPosition[2] )*PROJECTION_DISTANCE;

				glVertex3f( v1[0]+v3[0], v1[1]+v3[1], v1[2]+v3[2] );
			}
			glEnd();

As I said, it is rather trivial. This is all we had to do for our caps. Now
go down to the GL_DrawAliasStencilShadow function right below the one you're
in. We want to change the contents to this:

{
	glCullFace(GL_FRONT);
	glStencilOp( GL_KEEP, GL_INCR, GL_KEEP );
	GL_ShadowPass( paliashdr, posenum, lightPosition );

	glCullFace(GL_BACK);
	glStencilOp( GL_KEEP, GL_DECR, GL_KEEP );
	GL_ShadowPass( paliashdr, posenum, lightPosition );
}

As I described above, this is reversing the face culling from the standard
method and changing the increment/decerement to when the depth test fails,
and simply keeping the value otherwise. Next go down to the function
GL_StencilShadowing. After this chunk of code:

	while (g_numStencilEnts > 0)
	{ //go through the list and do all the shadows
		g_numStencilEnts--;
		GL_StencilShadowModel(g_stencilEnts[g_numStencilEnts]);
	}

We want to add:

	glCullFace(GL_FRONT);
	glDepthFunc(GL_LEQUAL);

And above that same chunk of code (the while (g_numStencil.. etc. line) we
want to add this:

	glDepthFunc(GL_LESS);

Since GLQuake has the cvar gl_ztrick enabled by default, we want to
disable that now as well, because it conflicts with our depth settings.
Just open gl_vidnt.c (assuming you intend to make a Windows build) and
find this line:

cvar_t	gl_ztrick = {"gl_ztrick","1"};

Change that "1" to "0" and you're done. Now start the game, set r_shadows
to 2, and notice that when your view enters a shadow volume everything
looks as it should.

-Rich