Cycles Volume Render: support for rendering of homogeneous volume with absorption.

This is the simplest possible volume rendering case, constant density inside
the volume and no scattering or emission. My plan is to tweak, verify and commit
more volume rendering effects one by one, doing it all at once makes it
difficult to verify correctness and track down bugs.

Documentation is here:
http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Materials/Volume

Currently this hooks into path tracing in 3 ways, which should get us pretty
far until we add more advanced light sampling. These 3 hooks are repeated in
the path tracing, branched path tracing and transparent shadow code:

* Determine active volume shader at start of the path
* Change active volume shader on transmission through a surface
* Light attenuation over line segments between camera, surfaces and background

This is work by "storm", Stuart Broadfoot, Thomas Dinges and myself.

1 files changed, 5 insertions, 6 deletions

diff --git a/intern/cycles/render/scene.cpp b/intern/cycles/render/scene.cpp
index 52e355df382..da790b6fcc9 100644
--- a/intern/cycles/render/scene.cpp
+++ b/intern/cycles/render/scene.cpp
@@ -133,7 +133,6 @@ void Scene::device_update(Device *device_, Progress& progress)
 	/* The order of updates is important, because there's dependencies between
 	 * the different managers, using data computed by previous managers.
 	 *
-	 * - Background generates shader graph compiled by shader manager.
 	 * - Image manager uploads images used by shaders.
 	 * - Camera may be used for adapative subdivison.
 	 * - Displacement shader must have all shader data available.
@@ -142,11 +141,6 @@ void Scene::device_update(Device *device_, Progress& progress)
 	
 	image_manager->set_pack_images(device->info.pack_images);
 
-	progress.set_status("Updating Background");
-	background->device_update(device, &dscene, this);
-
-	if(progress.get_cancel()) return;
-
 	progress.set_status("Updating Shaders");
 	shader_manager->device_update(device, &dscene, this, progress);
 
@@ -157,6 +151,11 @@ void Scene::device_update(Device *device_, Progress& progress)
 
 	if(progress.get_cancel()) return;
 
+	progress.set_status("Updating Background");
+	background->device_update(device, &dscene, this);
+
+	if(progress.get_cancel()) return;
+
 	progress.set_status("Updating Camera");
 	camera->device_update(device, &dscene, this);