rewrote the notes on the Creative patent after re-reading it

fixed my incomplete StencilOp changes (reverted back to the way it was before)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@5249 d7cf8633-e32d-0410-b094-e92efae38249

diff --git a/r_shadow.c b/r_shadow.c
index 613d447d9dcc04120a629bd736fa0d26ebfdfa0d..7a2d2d907d238d1d1e994b46fe883341314e6370 100644 (file)
--- a/r_shadow.c
+++ b/r_shadow.c
@@ -7,26 +7,25 @@ as far as the light's radius, if the light has a radius at all), capped at
 both front and back to avoid any problems (extrusion from dark faces also
 works but has a different set of problems)
 
-This is rendered using Carmack's Reverse technique, in which backfaces behind
-zbuffer (zfail) increment the stencil, and frontfaces behind zbuffer (zfail)
-decrement the stencil, the result is a stencil value of zero where shadows
-did not intersect the visible geometry, suitable as a stencil mask for
-rendering lighting everywhere but shadow.
-
-In our case to hopefully avoid the Creative Labs patent on Carmack's Reverse,
-we use a biased stencil clear of 128 (which also negates the need for the
-stencil wrap extension), we draw the frontfaces first and backfaces second
-(decrement, increment), and we redefine the DepthFunc to zpass when behind of
-surfaces and zfail when infront (this means zpass is decr/incr during volume
-rendering, not zfail).
+This is normally rendered using Carmack's Reverse technique, in which
+backfaces behind zbuffer (zfail) increment the stencil, and frontfaces behind
+zbuffer (zfail) decrement the stencil, the result is a stencil value of zero
+where shadows did not intersect the visible geometry, suitable as a stencil
+mask for rendering lighting everywhere but shadow.
+
+In our case to hopefully avoid the Creative Labs patent, we draw the backfaces
+as decrement and the frontfaces as increment, and we redefine the DepthFunc to
+GL_LESS (the patent uses GL_GEQUAL) which causes zfail when behind surfaces
+and zpass when infront (the patent draws where zpass with a GL_GEQUAL test),
+additionally we clear stencil to 128 to avoid the need for the unclamped
+incr/decr extension (not related to patent).
 
 Patent warning:
-This algorithm may be covered by Creative's patent (US Patent #6384822)
-on Carmack's Reverse paper (which I have not read), however that patent
-seems to be about drawing a stencil shadow from a model in an otherwise
-unshadowed scene, where as realtime lighting technology draws light where
-shadows do not lie, additionally the stencil clear, zfail/zpass rules and
-incr/decr order are different in this implementation.
+This algorithm may be covered by Creative's patent (US Patent #6384822),
+however that patent is quite specific about increment on backfaces and
+decrement on frontfaces where zpass with GL_GEQUAL depth test, which is
+opposite this implementation and partially opposite Carmack's Reverse paper
+(which uses GL_LESS, but increments on backfaces and decrements on frontfaces).
 
 
 
@@ -947,15 +946,15 @@ void R_Shadow_RenderVolume(int numvertices, int numtriangles, const float *verte
        GL_LockArrays(0, numvertices);
        if (r_shadowstage == R_SHADOWSTAGE_STENCIL)
        {
-               // increment stencil if backface is behind depthbuffer
+               // decrement stencil if backface is behind depthbuffer
                qglCullFace(GL_BACK); // quake is backwards, this culls front faces
-               qglStencilOp(GL_KEEP, GL_INCR, GL_KEEP);
+               qglStencilOp(GL_KEEP, GL_DECR, GL_KEEP);
                R_Mesh_Draw(0, numvertices, numtriangles, element3i);
                c_rt_shadowmeshes++;
                c_rt_shadowtris += numtriangles;
-               // decrement stencil if frontface is behind depthbuffer
+               // increment stencil if frontface is behind depthbuffer
                qglCullFace(GL_FRONT); // quake is backwards, this culls back faces
-               qglStencilOp(GL_KEEP, GL_DECR, GL_KEEP);
+               qglStencilOp(GL_KEEP, GL_INCR, GL_KEEP);
        }
        R_Mesh_Draw(0, numvertices, numtriangles, element3i);
        c_rt_shadowmeshes++;
@@ -1113,7 +1112,7 @@ void R_Shadow_Stage_StencilShadowVolumes(void)
        //}
        //else
        //      qglDisable(GL_POLYGON_OFFSET_FILL);
-       qglDepthFunc(GL_GEQUAL);
+       qglDepthFunc(GL_LESS);
        qglCullFace(GL_FRONT); // quake is backwards, this culls back faces
        qglEnable(GL_STENCIL_TEST);
        qglStencilFunc(GL_ALWAYS, 128, ~0);
@@ -1122,12 +1121,12 @@ void R_Shadow_Stage_StencilShadowVolumes(void)
                r_shadowstage = R_SHADOWSTAGE_STENCILTWOSIDE;
                qglDisable(GL_CULL_FACE);
                qglEnable(GL_STENCIL_TEST_TWO_SIDE_EXT);
-               qglActiveStencilFaceEXT(GL_BACK); // quake is backwards, this is front faces
-               qglStencilMask(~0);
-               qglStencilOp(GL_KEEP, GL_KEEP, GL_DECR);
                qglActiveStencilFaceEXT(GL_FRONT); // quake is backwards, this is back faces
                qglStencilMask(~0);
-               qglStencilOp(GL_KEEP, GL_KEEP, GL_INCR);
+               qglStencilOp(GL_KEEP, GL_DECR, GL_KEEP);
+               qglActiveStencilFaceEXT(GL_BACK); // quake is backwards, this is front faces
+               qglStencilMask(~0);
+               qglStencilOp(GL_KEEP, GL_INCR, GL_KEEP);
        }
        else
        {
@@ -2743,15 +2742,15 @@ void R_Shadow_DrawEntityShadow(entity_render_t *ent, rtlight_t *rtlight, int num
                                GL_LockArrays(0, mesh->numverts);
                                if (r_shadowstage == R_SHADOWSTAGE_STENCIL)
                                {
-                                       // increment stencil if backface is behind depthbuffer
+                                       // decrement stencil if backface is behind depthbuffer
                                        qglCullFace(GL_BACK); // quake is backwards, this culls front faces
-                                       qglStencilOp(GL_KEEP, GL_INCR, GL_KEEP);
+                                       qglStencilOp(GL_KEEP, GL_DECR, GL_KEEP);
                                        R_Mesh_Draw(0, mesh->numverts, mesh->numtriangles, mesh->element3i);
                                        c_rtcached_shadowmeshes++;
                                        c_rtcached_shadowtris += mesh->numtriangles;
-                                       // decrement stencil if frontface is behind depthbuffer
+                                       // increment stencil if frontface is behind depthbuffer
                                        qglCullFace(GL_FRONT); // quake is backwards, this culls back faces
-                                       qglStencilOp(GL_KEEP, GL_DECR, GL_KEEP);
+                                       qglStencilOp(GL_KEEP, GL_INCR, GL_KEEP);
                                }
                                R_Mesh_Draw(0, mesh->numverts, mesh->numtriangles, mesh->element3i);
                                c_rtcached_shadowmeshes++;