Fix T91733, T92486: Cycles wrong shadow catcher with volumes

Changes:
* After hitting a shadow catcher, re-initialize the volume stack taking
  into account shadow catcher ray visibility. This ensures that volume objects
  are included in the stack only if they are shadow catchers.
* If there is a volume to be shaded in front of the shadow catcher, the split
  is now performed in the shade_volume kernel after volume shading is done.
* Previously the background pass behind a shadow catcher was done as part of
  the regular path, now it is done as part of the shadow catcher path.

For a shadow catcher path with volumes and visible background, operations are
done in this order now:

* intersect_closest
* shade_volume
* shadow catcher split
* intersect_volume_stack
* shade_background
* shade_surface

The world volume is currently assumed to be CG, that is it does not exist in
the footage. We may consider adding an option to control this, or change the
default. With a volume object this control is already possible.

This includes refactoring to centralize the logic for next kernel scheduling
in intersect_closest.h.

Differential Revision: https://developer.blender.org/D13093

1 files changed, 4 insertions, 16 deletions

diff --git a/intern/cycles/kernel/integrator/shade_background.h b/intern/cycles/kernel/integrator/shade_background.h
index 71a590749bd..24482e85b05 100644
--- a/intern/cycles/kernel/integrator/shade_background.h
+++ b/intern/cycles/kernel/integrator/shade_background.h
@@ -192,23 +192,11 @@ ccl_device void integrator_shade_background(KernelGlobals kg,
 
 #ifdef __SHADOW_CATCHER__
   if (INTEGRATOR_STATE(state, path, flag) & PATH_RAY_SHADOW_CATCHER_BACKGROUND) {
+    /* Special case for shadow catcher where we want to fill the background pass
+     * behind the shadow catcher but also continue tracing the path. */
     INTEGRATOR_STATE_WRITE(state, path, flag) &= ~PATH_RAY_SHADOW_CATCHER_BACKGROUND;
-
-    const int isect_prim = INTEGRATOR_STATE(state, isect, prim);
-    const int isect_type = INTEGRATOR_STATE(state, isect, type);
-    const int shader = intersection_get_shader_from_isect_prim(kg, isect_prim, isect_type);
-    const int shader_flags = kernel_tex_fetch(__shaders, shader).flags;
-
-    if (shader_flags & SD_HAS_RAYTRACE) {
-      INTEGRATOR_PATH_NEXT_SORTED(DEVICE_KERNEL_INTEGRATOR_SHADE_BACKGROUND,
-                                  DEVICE_KERNEL_INTEGRATOR_SHADE_SURFACE_RAYTRACE,
-                                  shader);
-    }
-    else {
-      INTEGRATOR_PATH_NEXT_SORTED(DEVICE_KERNEL_INTEGRATOR_SHADE_BACKGROUND,
-                                  DEVICE_KERNEL_INTEGRATOR_SHADE_SURFACE,
-                                  shader);
-    }
+    integrator_intersect_next_kernel_after_shadow_catcher_background<
+        DEVICE_KERNEL_INTEGRATOR_SHADE_BACKGROUND>(kg, state);
     return;
   }
 #endif