Cycles: change volume step size controls, auto adjust based on voxel size

By default it will now set the step size to the voxel size for smoke and
volume objects, and 1/10th the bounding box for procedural volume shaders.

New settings are:
* Scene render/preview step rate: to globally adjust detail and performance
* Material step rate: multiplied with auto detected per-object step size
* World step size: distance to steo for world shader

Differential Revision: https://developer.blender.org/D1777

23 files changed, 212 insertions, 66 deletions

diff --git a/intern/cycles/blender/addon/properties.py b/intern/cycles/blender/addon/properties.py
index cd72da128aa..c91e210bbd8 100644
--- a/intern/cycles/blender/addon/properties.py
+++ b/intern/cycles/blender/addon/properties.py
@@ -444,13 +444,20 @@ class CyclesRenderSettings(bpy.types.PropertyGroup):
         default=8,
     )
 
-    volume_step_size: FloatProperty(
-        name="Step Size",
-        description="Distance between volume shader samples when rendering the volume "
-        "(lower values give more accurate and detailed results, but also increased render time)",
-        default=0.1,
-        min=0.0000001, max=100000.0, soft_min=0.01, soft_max=1.0, precision=4,
-        unit='LENGTH'
+    volume_step_rate: FloatProperty(
+        name="Step Rate",
+        description="Globally adjust detail for volume rendering, on top of automatically estimated step size. "
+                    "Higher values reduce render time, lower values render with more detail",
+        default=1.0,
+        min=0.01, max=100.0, soft_min=0.1, soft_max=10.0, precision=2
+    )
+
+    volume_preview_step_rate: FloatProperty(
+        name="Step Rate",
+        description="Globally adjust detail for volume rendering, on top of automatically estimated step size. "
+                    "Higher values reduce render time, lower values render with more detail",
+        default=1.0,
+        min=0.01, max=100.0, soft_min=0.1, soft_max=10.0, precision=2
     )
 
     volume_max_steps: IntProperty(
@@ -934,6 +941,14 @@ class CyclesMaterialSettings(bpy.types.PropertyGroup):
         default='LINEAR',
     )
 
+    volume_step_rate: FloatProperty(
+        name="Step Rate",
+        description="Scale the distance between volume shader samples when rendering the volume "
+                    "(lower values give more accurate and detailed results, but also increased render time)",
+        default=1.0,
+        min=0.001, max=1000.0, soft_min=0.1, soft_max=10.0, precision=4
+    )
+
     displacement_method: EnumProperty(
         name="Displacement Method",
         description="Method to use for the displacement",
@@ -1044,6 +1059,13 @@ class CyclesWorldSettings(bpy.types.PropertyGroup):
         items=enum_volume_interpolation,
         default='LINEAR',
     )
+    volume_step_size: FloatProperty(
+        name="Step Size",
+        description="Distance between volume shader samples when rendering the volume "
+                    "(lower values give more accurate and detailed results, but also increased render time)",
+        default=1.0,
+        min=0.0000001, max=100000.0, soft_min=0.1, soft_max=100.0, precision=4
+    )
 
     @classmethod
     def register(cls):
diff --git a/intern/cycles/blender/addon/ui.py b/intern/cycles/blender/addon/ui.py
index e545d436c85..5f994faa435 100644
--- a/intern/cycles/blender/addon/ui.py
+++ b/intern/cycles/blender/addon/ui.py
@@ -373,7 +373,7 @@ class CYCLES_RENDER_PT_subdivision(CyclesButtonsPanel, Panel):
         col = layout.column()
         sub = col.column(align=True)
         sub.prop(cscene, "dicing_rate", text="Dicing Rate Render")
-        sub.prop(cscene, "preview_dicing_rate", text="Preview")
+        sub.prop(cscene, "preview_dicing_rate", text="Viewport")
 
         col.separator()
 
@@ -428,9 +428,11 @@ class CYCLES_RENDER_PT_volumes(CyclesButtonsPanel, Panel):
         scene = context.scene
         cscene = scene.cycles
 
-        col = layout.column()
-        col.prop(cscene, "volume_step_size", text="Step Size")
-        col.prop(cscene, "volume_max_steps", text="Max Steps")
+        col = layout.column(align=True)
+        col.prop(cscene, "volume_step_rate", text="Step Rate Render")
+        col.prop(cscene, "volume_preview_step_rate", text="Viewport")
+
+        layout.prop(cscene, "volume_max_steps", text="Max Steps")
 
 
 class CYCLES_RENDER_PT_light_paths(CyclesButtonsPanel, Panel):
@@ -1696,6 +1698,9 @@ class CYCLES_WORLD_PT_settings_volume(CyclesButtonsPanel, Panel):
         sub.prop(cworld, "volume_sampling", text="Sampling")
         col.prop(cworld, "volume_interpolation", text="Interpolation")
         col.prop(cworld, "homogeneous_volume", text="Homogeneous")
+        sub = col.column()
+        sub.active = not cworld.homogeneous_volume
+        sub.prop(cworld, "volume_step_size")
 
 
 class CYCLES_MATERIAL_PT_preview(CyclesButtonsPanel, Panel):
@@ -1827,6 +1832,9 @@ class CYCLES_MATERIAL_PT_settings_volume(CyclesButtonsPanel, Panel):
         sub.prop(cmat, "volume_sampling", text="Sampling")
         col.prop(cmat, "volume_interpolation", text="Interpolation")
         col.prop(cmat, "homogeneous_volume", text="Homogeneous")
+        sub = col.column()
+        sub.active = not cmat.homogeneous_volume
+        sub.prop(cmat, "volume_step_rate")
 
     def draw(self, context):
         self.draw_shared(self, context, context.material)
diff --git a/intern/cycles/blender/blender_shader.cpp b/intern/cycles/blender/blender_shader.cpp
index dc226805664..59c1539d207 100644
--- a/intern/cycles/blender/blender_shader.cpp
+++ b/intern/cycles/blender/blender_shader.cpp
@@ -1260,6 +1260,7 @@ void BlenderSync::sync_materials(BL::Depsgraph &b_depsgraph, bool update_all)
       shader->heterogeneous_volume = !get_boolean(cmat, "homogeneous_volume");
       shader->volume_sampling_method = get_volume_sampling(cmat);
       shader->volume_interpolation_method = get_volume_interpolation(cmat);
+      shader->volume_step_rate = get_float(cmat, "volume_step_rate");
       shader->displacement_method = get_displacement_method(cmat);
 
       shader->set_graph(graph);
@@ -1324,6 +1325,7 @@ void BlenderSync::sync_world(BL::Depsgraph &b_depsgraph, BL::SpaceView3D &b_v3d,
       shader->heterogeneous_volume = !get_boolean(cworld, "homogeneous_volume");
       shader->volume_sampling_method = get_volume_sampling(cworld);
       shader->volume_interpolation_method = get_volume_interpolation(cworld);
+      shader->volume_step_rate = get_float(cworld, "volume_step_size");
     }
     else if (new_viewport_parameters.use_scene_world && b_world) {
       BackgroundNode *background = new BackgroundNode();
diff --git a/intern/cycles/blender/blender_sync.cpp b/intern/cycles/blender/blender_sync.cpp
index 3d932825ae7..9f7797efb97 100644
--- a/intern/cycles/blender/blender_sync.cpp
+++ b/intern/cycles/blender/blender_sync.cpp
@@ -262,7 +262,8 @@ void BlenderSync::sync_integrator()
   integrator->transparent_max_bounce = get_int(cscene, "transparent_max_bounces");
 
   integrator->volume_max_steps = get_int(cscene, "volume_max_steps");
-  integrator->volume_step_size = get_float(cscene, "volume_step_size");
+  integrator->volume_step_rate = (preview) ? get_float(cscene, "volume_preview_step_rate") :
+                                             get_float(cscene, "volume_step_rate");
 
   integrator->caustics_reflective = get_boolean(cscene, "caustics_reflective");
   integrator->caustics_refractive = get_boolean(cscene, "caustics_refractive");
diff --git a/intern/cycles/kernel/geom/geom_object.h b/intern/cycles/kernel/geom/geom_object.h
index ac2ddf575ca..c6d02ed9702 100644
--- a/intern/cycles/kernel/geom/geom_object.h
+++ b/intern/cycles/kernel/geom/geom_object.h
@@ -320,6 +320,17 @@ ccl_device_inline uint object_patch_map_offset(KernelGlobals *kg, int object)
   return kernel_tex_fetch(__objects, object).patch_map_offset;
 }
 
+/* Volume step size */
+
+ccl_device_inline float object_volume_step_size(KernelGlobals *kg, int object)
+{
+  if (object == OBJECT_NONE) {
+    return kernel_data.background.volume_step_size;
+  }
+
+  return kernel_tex_fetch(__object_volume_step, object);
+}
+
 /* Pass ID for shader */
 
 ccl_device int shader_pass_id(KernelGlobals *kg, const ShaderData *sd)
diff --git a/intern/cycles/kernel/kernel_path.h b/intern/cycles/kernel/kernel_path.h
index 74377f2c069..db35303e3f1 100644
--- a/intern/cycles/kernel/kernel_path.h
+++ b/intern/cycles/kernel/kernel_path.h
@@ -171,19 +171,19 @@ ccl_device_forceinline VolumeIntegrateResult kernel_path_volume(KernelGlobals *k
   Ray volume_ray = *ray;
   volume_ray.t = (hit) ? isect->t : FLT_MAX;
 
-  bool heterogeneous = volume_stack_is_heterogeneous(kg, state->volume_stack);
+  float step_size = volume_stack_step_size(kg, state->volume_stack);
 
 #    ifdef __VOLUME_DECOUPLED__
   int sampling_method = volume_stack_sampling_method(kg, state->volume_stack);
   bool direct = (state->flag & PATH_RAY_CAMERA) != 0;
-  bool decoupled = kernel_volume_use_decoupled(kg, heterogeneous, direct, sampling_method);
+  bool decoupled = kernel_volume_use_decoupled(kg, step_size, direct, sampling_method);
 
   if (decoupled) {
     /* cache steps along volume for repeated sampling */
     VolumeSegment volume_segment;
 
     shader_setup_from_volume(kg, sd, &volume_ray);
-    kernel_volume_decoupled_record(kg, state, &volume_ray, sd, &volume_segment, heterogeneous);
+    kernel_volume_decoupled_record(kg, state, &volume_ray, sd, &volume_segment, step_size);
 
     volume_segment.sampling_method = sampling_method;
 
@@ -229,7 +229,7 @@ ccl_device_forceinline VolumeIntegrateResult kernel_path_volume(KernelGlobals *k
   {
     /* integrate along volume segment with distance sampling */
     VolumeIntegrateResult result = kernel_volume_integrate(
-        kg, state, sd, &volume_ray, L, throughput, heterogeneous);
+        kg, state, sd, &volume_ray, L, throughput, step_size);
 
 #    ifdef __VOLUME_SCATTER__
     if (result == VOLUME_PATH_SCATTERED) {
diff --git a/intern/cycles/kernel/kernel_path_branched.h b/intern/cycles/kernel/kernel_path_branched.h
index 0d5781fe3d1..337c4fb1d10 100644
--- a/intern/cycles/kernel/kernel_path_branched.h
+++ b/intern/cycles/kernel/kernel_path_branched.h
@@ -91,7 +91,7 @@ ccl_device_forceinline void kernel_branched_path_volume(KernelGlobals *kg,
   Ray volume_ray = *ray;
   volume_ray.t = (hit) ? isect->t : FLT_MAX;
 
-  bool heterogeneous = volume_stack_is_heterogeneous(kg, state->volume_stack);
+  float step_size = volume_stack_step_size(kg, state->volume_stack);
 
 #      ifdef __VOLUME_DECOUPLED__
   /* decoupled ray marching only supported on CPU */
@@ -100,7 +100,7 @@ ccl_device_forceinline void kernel_branched_path_volume(KernelGlobals *kg,
     VolumeSegment volume_segment;
 
     shader_setup_from_volume(kg, sd, &volume_ray);
-    kernel_volume_decoupled_record(kg, state, &volume_ray, sd, &volume_segment, heterogeneous);
+    kernel_volume_decoupled_record(kg, state, &volume_ray, sd, &volume_segment, step_size);
 
     /* direct light sampling */
     if (volume_segment.closure_flag & SD_SCATTER) {
@@ -171,7 +171,7 @@ ccl_device_forceinline void kernel_branched_path_volume(KernelGlobals *kg,
       path_state_branch(&ps, j, num_samples);
 
       VolumeIntegrateResult result = kernel_volume_integrate(
-          kg, &ps, sd, &volume_ray, L, &tp, heterogeneous);
+          kg, &ps, sd, &volume_ray, L, &tp, step_size);
 
 #      ifdef __VOLUME_SCATTER__
       if (result == VOLUME_PATH_SCATTERED) {
diff --git a/intern/cycles/kernel/kernel_textures.h b/intern/cycles/kernel/kernel_textures.h
index 1cae34348c9..c8e01677d09 100644
--- a/intern/cycles/kernel/kernel_textures.h
+++ b/intern/cycles/kernel/kernel_textures.h
@@ -35,6 +35,7 @@ KERNEL_TEX(KernelObject, __objects)
 KERNEL_TEX(Transform, __object_motion_pass)
 KERNEL_TEX(DecomposedTransform, __object_motion)
 KERNEL_TEX(uint, __object_flag)
+KERNEL_TEX(float, __object_volume_step)
 
 /* cameras */
 KERNEL_TEX(DecomposedTransform, __camera_motion)
diff --git a/intern/cycles/kernel/kernel_types.h b/intern/cycles/kernel/kernel_types.h
index c5be93e2cda..6a0e74efc91 100644
--- a/intern/cycles/kernel/kernel_types.h
+++ b/intern/cycles/kernel/kernel_types.h
@@ -887,13 +887,13 @@ enum ShaderDataFlag {
   SD_HAS_DISPLACEMENT = (1 << 26),
   /* Has constant emission (value stored in __shaders) */
   SD_HAS_CONSTANT_EMISSION = (1 << 27),
-  /* Needs to access attributes */
-  SD_NEED_ATTRIBUTES = (1 << 28),
+  /* Needs to access attributes for volume rendering */
+  SD_NEED_VOLUME_ATTRIBUTES = (1 << 28),
 
   SD_SHADER_FLAGS = (SD_USE_MIS | SD_HAS_TRANSPARENT_SHADOW | SD_HAS_VOLUME | SD_HAS_ONLY_VOLUME |
                      SD_HETEROGENEOUS_VOLUME | SD_HAS_BSSRDF_BUMP | SD_VOLUME_EQUIANGULAR |
                      SD_VOLUME_MIS | SD_VOLUME_CUBIC | SD_HAS_BUMP | SD_HAS_DISPLACEMENT |
-                     SD_HAS_CONSTANT_EMISSION | SD_NEED_ATTRIBUTES)
+                     SD_HAS_CONSTANT_EMISSION | SD_NEED_VOLUME_ATTRIBUTES)
 };
 
 /* Object flags. */
@@ -1275,6 +1275,7 @@ typedef struct KernelBackground {
   /* only shader index */
   int surface_shader;
   int volume_shader;
+  float volume_step_size;
   int transparent;
   float transparent_roughness_squared_threshold;
 
@@ -1282,7 +1283,6 @@ typedef struct KernelBackground {
   float ao_factor;
   float ao_distance;
   float ao_bounces_factor;
-  float ao_pad;
 } KernelBackground;
 static_assert_align(KernelBackground, 16);
 
@@ -1355,7 +1355,7 @@ typedef struct KernelIntegrator {
   /* volume render */
   int use_volumes;
   int volume_max_steps;
-  float volume_step_size;
+  float volume_step_rate;
   int volume_samples;
 
   int start_sample;
diff --git a/intern/cycles/kernel/kernel_volume.h b/intern/cycles/kernel/kernel_volume.h
index f443bb88463..035fcdbc8e1 100644
--- a/intern/cycles/kernel/kernel_volume.h
+++ b/intern/cycles/kernel/kernel_volume.h
@@ -101,15 +101,19 @@ ccl_device float kernel_volume_channel_get(float3 value, int channel)
 
 #ifdef __VOLUME__
 
-ccl_device bool volume_stack_is_heterogeneous(KernelGlobals *kg, ccl_addr_space VolumeStack *stack)
+ccl_device float volume_stack_step_size(KernelGlobals *kg, ccl_addr_space VolumeStack *stack)
 {
+  float step_size = FLT_MAX;
+
   for (int i = 0; stack[i].shader != SHADER_NONE; i++) {
     int shader_flag = kernel_tex_fetch(__shaders, (stack[i].shader & SHADER_MASK)).flags;
 
+    bool heterogeneous = false;
+
     if (shader_flag & SD_HETEROGENEOUS_VOLUME) {
-      return true;
+      heterogeneous = true;
     }
-    else if (shader_flag & SD_NEED_ATTRIBUTES) {
+    else if (shader_flag & SD_NEED_VOLUME_ATTRIBUTES) {
       /* We want to render world or objects without any volume grids
        * as homogeneous, but can only verify this at run-time since other
        * heterogeneous volume objects may be using the same shader. */
@@ -117,13 +121,19 @@ ccl_device bool volume_stack_is_heterogeneous(KernelGlobals *kg, ccl_addr_space
       if (object != OBJECT_NONE) {
         int object_flag = kernel_tex_fetch(__object_flag, object);
         if (object_flag & SD_OBJECT_HAS_VOLUME_ATTRIBUTES) {
-          return true;
+          heterogeneous = true;
         }
       }
     }
+
+    if (heterogeneous) {
+      float object_step_size = object_volume_step_size(kg, stack[i].object);
+      object_step_size *= kernel_data.integrator.volume_step_rate;
+      step_size = fminf(object_step_size, step_size);
+    }
   }
 
-  return false;
+  return step_size;
 }
 
 ccl_device int volume_stack_sampling_method(KernelGlobals *kg, VolumeStack *stack)
@@ -158,12 +168,13 @@ ccl_device int volume_stack_sampling_method(KernelGlobals *kg, VolumeStack *stac
 
 ccl_device_inline void kernel_volume_step_init(KernelGlobals *kg,
                                                ccl_addr_space PathState *state,
+                                               const float object_step_size,
                                                float t,
                                                float *step_size,
                                                float *step_offset)
 {
   const int max_steps = kernel_data.integrator.volume_max_steps;
-  float step = min(kernel_data.integrator.volume_step_size, t);
+  float step = min(object_step_size, t);
 
   /* compute exact steps in advance for malloc */
   if (t > max_steps * step) {
@@ -199,7 +210,8 @@ ccl_device void kernel_volume_shadow_heterogeneous(KernelGlobals *kg,
                                                    ccl_addr_space PathState *state,
                                                    Ray *ray,
                                                    ShaderData *sd,
-                                                   float3 *throughput)
+                                                   float3 *throughput,
+                                                   const float object_step_size)
 {
   float3 tp = *throughput;
   const float tp_eps = 1e-6f; /* todo: this is likely not the right value */
@@ -207,7 +219,7 @@ ccl_device void kernel_volume_shadow_heterogeneous(KernelGlobals *kg,
   /* prepare for stepping */
   int max_steps = kernel_data.integrator.volume_max_steps;
   float step_offset, step_size;
-  kernel_volume_step_init(kg, state, ray->t, &step_size, &step_offset);
+  kernel_volume_step_init(kg, state, object_step_size, ray->t, &step_size, &step_offset);
 
   /* compute extinction at the start */
   float t = 0.0f;
@@ -264,8 +276,9 @@ ccl_device_noinline void kernel_volume_shadow(KernelGlobals *kg,
 {
   shader_setup_from_volume(kg, shadow_sd, ray);
 
-  if (volume_stack_is_heterogeneous(kg, state->volume_stack))
-    kernel_volume_shadow_heterogeneous(kg, state, ray, shadow_sd, throughput);
+  float step_size = volume_stack_step_size(kg, state->volume_stack);
+  if (step_size != FLT_MAX)
+    kernel_volume_shadow_heterogeneous(kg, state, ray, shadow_sd, throughput, step_size);
   else
     kernel_volume_shadow_homogeneous(kg, state, ray, shadow_sd, throughput);
 }
@@ -533,7 +546,8 @@ kernel_volume_integrate_heterogeneous_distance(KernelGlobals *kg,
                                                Ray *ray,
                                                ShaderData *sd,
                                                PathRadiance *L,
-                                               ccl_addr_space float3 *throughput)
+                                               ccl_addr_space float3 *throughput,
+                                               const float object_step_size)
 {
   float3 tp = *throughput;
   const float tp_eps = 1e-6f; /* todo: this is likely not the right value */
@@ -541,7 +555,7 @@ kernel_volume_integrate_heterogeneous_distance(KernelGlobals *kg,
   /* prepare for stepping */
   int max_steps = kernel_data.integrator.volume_max_steps;
   float step_offset, step_size;
-  kernel_volume_step_init(kg, state, ray->t, &step_size, &step_offset);
+  kernel_volume_step_init(kg, state, object_step_size, ray->t, &step_size, &step_offset);
 
   /* compute coefficients at the start */
   float t = 0.0f;
@@ -679,12 +693,13 @@ kernel_volume_integrate(KernelGlobals *kg,
                         Ray *ray,
                         PathRadiance *L,
                         ccl_addr_space float3 *throughput,
-                        bool heterogeneous)
+                        float step_size)
 {
   shader_setup_from_volume(kg, sd, ray);
 
-  if (heterogeneous)
-    return kernel_volume_integrate_heterogeneous_distance(kg, state, ray, sd, L, throughput);
+  if (step_size != FLT_MAX)
+    return kernel_volume_integrate_heterogeneous_distance(
+        kg, state, ray, sd, L, throughput, step_size);
   else
     return kernel_volume_integrate_homogeneous(kg, state, ray, sd, L, throughput, true);
 }
@@ -735,7 +750,7 @@ ccl_device void kernel_volume_decoupled_record(KernelGlobals *kg,
                                                Ray *ray,
                                                ShaderData *sd,
                                                VolumeSegment *segment,
-                                               bool heterogeneous)
+                                               const float object_step_size)
 {
   const float tp_eps = 1e-6f; /* todo: this is likely not the right value */
 
@@ -743,9 +758,9 @@ ccl_device void kernel_volume_decoupled_record(KernelGlobals *kg,
   int max_steps;
   float step_size, step_offset;
 
-  if (heterogeneous) {
+  if (object_step_size != FLT_MAX) {
     max_steps = kernel_data.integrator.volume_max_steps;
-    kernel_volume_step_init(kg, state, ray->t, &step_size, &step_offset);
+    kernel_volume_step_init(kg, state, object_step_size, ray->t, &step_size, &step_offset);
 
 #      ifdef __KERNEL_CPU__
     /* NOTE: For the branched path tracing it's possible to have direct
diff --git a/intern/cycles/kernel/split/kernel_do_volume.h b/intern/cycles/kernel/split/kernel_do_volume.h
index 45b839db05f..b24699ec39c 100644
--- a/intern/cycles/kernel/split/kernel_do_volume.h
+++ b/intern/cycles/kernel/split/kernel_do_volume.h
@@ -44,7 +44,7 @@ ccl_device_noinline bool kernel_split_branched_path_volume_indirect_light_iter(K
                      branched_state->isect.t :
                      FLT_MAX;
 
-  bool heterogeneous = volume_stack_is_heterogeneous(kg, branched_state->path_state.volume_stack);
+  float step_size = volume_stack_step_size(kg, branched_state->path_state.volume_stack);
 
   for (int j = branched_state->next_sample; j < num_samples; j++) {
     ccl_global PathState *ps = &kernel_split_state.path_state[ray_index];
@@ -61,7 +61,7 @@ ccl_device_noinline bool kernel_split_branched_path_volume_indirect_light_iter(K
 
     /* integrate along volume segment with distance sampling */
     VolumeIntegrateResult result = kernel_volume_integrate(
-        kg, ps, sd, &volume_ray, L, tp, heterogeneous);
+        kg, ps, sd, &volume_ray, L, tp, step_size);
 
 #  ifdef __VOLUME_SCATTER__
     if (result == VOLUME_PATH_SCATTERED) {
@@ -164,12 +164,12 @@ ccl_device void kernel_do_volume(KernelGlobals *kg)
       if (!kernel_data.integrator.branched ||
           IS_FLAG(ray_state, ray_index, RAY_BRANCHED_INDIRECT)) {
 #  endif /* __BRANCHED_PATH__ */
-        bool heterogeneous = volume_stack_is_heterogeneous(kg, state->volume_stack);
+        float step_size = volume_stack_step_size(kg, state->volume_stack);
 
         {
           /* integrate along volume segment with distance sampling */
           VolumeIntegrateResult result = kernel_volume_integrate(
-              kg, state, sd, &volume_ray, L, throughput, heterogeneous);
+              kg, state, sd, &volume_ray, L, throughput, step_size);
 
 #  ifdef __VOLUME_SCATTER__
           if (result == VOLUME_PATH_SCATTERED) {
diff --git a/intern/cycles/render/background.cpp b/intern/cycles/render/background.cpp
index 6553ca735e4..5a61ead2c54 100644
--- a/intern/cycles/render/background.cpp
+++ b/intern/cycles/render/background.cpp
@@ -43,6 +43,8 @@ NODE_DEFINE(Background)
   SOCKET_BOOLEAN(transparent_glass, "Transparent Glass", false);
   SOCKET_FLOAT(transparent_roughness_threshold, "Transparent Roughness Threshold", 0.0f);
 
+  SOCKET_FLOAT(volume_step_size, "Volume Step Size", 0.1f);
+
   SOCKET_NODE(shader, "Shader", &Shader::node_type);
 
   return type;
@@ -91,6 +93,8 @@ void Background::device_update(Device *device, DeviceScene *dscene, Scene *scene
   else
     kbackground->volume_shader = SHADER_NONE;
 
+  kbackground->volume_step_size = volume_step_size * scene->integrator->volume_step_rate;
+
   /* No background node, make world shader invisible to all rays, to skip evaluation in kernel. */
   if (bg_shader->graph->nodes.size() <= 1) {
     kbackground->surface_shader |= SHADER_EXCLUDE_ANY;
diff --git a/intern/cycles/render/background.h b/intern/cycles/render/background.h
index fb27430f9a3..c2ca1f75179 100644
--- a/intern/cycles/render/background.h
+++ b/intern/cycles/render/background.h
@@ -45,6 +45,8 @@ class Background : public Node {
   bool transparent_glass;
   float transparent_roughness_threshold;
 
+  float volume_step_size;
+
   bool need_update;
 
   Background();
diff --git a/intern/cycles/render/integrator.cpp b/intern/cycles/render/integrator.cpp
index ee1aa5988bf..d856f7a300f 100644
--- a/intern/cycles/render/integrator.cpp
+++ b/intern/cycles/render/integrator.cpp
@@ -50,7 +50,7 @@ NODE_DEFINE(Integrator)
   SOCKET_INT(ao_bounces, "AO Bounces", 0);
 
   SOCKET_INT(volume_max_steps, "Volume Max Steps", 1024);
-  SOCKET_FLOAT(volume_step_size, "Volume Step Size", 0.1f);
+  SOCKET_FLOAT(volume_step_rate, "Volume Step Rate", 1.0f);
 
   SOCKET_BOOLEAN(caustics_reflective, "Reflective Caustics", true);
   SOCKET_BOOLEAN(caustics_refractive, "Refractive Caustics", true);
@@ -143,7 +143,7 @@ void Integrator::device_update(Device *device, DeviceScene *dscene, Scene *scene
   }
 
   kintegrator->volume_max_steps = volume_max_steps;
-  kintegrator->volume_step_size = volume_step_size;
+  kintegrator->volume_step_rate = volume_step_rate;
 
   kintegrator->caustics_reflective = caustics_reflective;
   kintegrator->caustics_refractive = caustics_refractive;
diff --git a/intern/cycles/render/integrator.h b/intern/cycles/render/integrator.h
index 9930e907aea..9804caebe6e 100644
--- a/intern/cycles/render/integrator.h
+++ b/intern/cycles/render/integrator.h
@@ -45,7 +45,7 @@ class Integrator : public Node {
   int ao_bounces;
 
   int volume_max_steps;
-  float volume_step_size;
+  float volume_step_rate;
 
   bool caustics_reflective;
   bool caustics_refractive;
diff --git a/intern/cycles/render/object.cpp b/intern/cycles/render/object.cpp
index 4987b6089d7..c84007823d2 100644
--- a/intern/cycles/render/object.cpp
+++ b/intern/cycles/render/object.cpp
@@ -17,6 +17,7 @@
 #include "render/camera.h"
 #include "device/device.h"
 #include "render/hair.h"
+#include "render/integrator.h"
 #include "render/light.h"
 #include "render/mesh.h"
 #include "render/curves.h"
@@ -65,6 +66,7 @@ struct UpdateObjectTransformState {
   KernelObject *objects;
   Transform *object_motion_pass;
   DecomposedTransform *object_motion;
+  float *object_volume_step;
 
   /* Flags which will be synchronized to Integrator. */
   bool have_motion;
@@ -266,6 +268,65 @@ uint Object::visibility_for_tracing() const
   return trace_visibility;
 }
 
+float Object::compute_volume_step_size() const
+{
+  if (!geometry->has_volume) {
+    return FLT_MAX;
+  }
+
+  /* Compute step rate from shaders. */
+  float step_rate = FLT_MAX;
+
+  foreach (Shader *shader, geometry->used_shaders) {
+    if (shader->has_volume) {
+      if ((shader->heterogeneous_volume && shader->has_volume_spatial_varying) ||
+          (shader->has_volume_attribute_dependency)) {
+        step_rate = fminf(shader->volume_step_rate, step_rate);
+      }
+    }
+  }
+
+  if (step_rate == FLT_MAX) {
+    return FLT_MAX;
+  }
+
+  /* Compute step size from voxel grids. */
+  float step_size = FLT_MAX;
+
+  foreach (Attribute &attr, geometry->attributes.attributes) {
+    if (attr.element == ATTR_ELEMENT_VOXEL) {
+      ImageHandle &handle = attr.data_voxel();
+      const ImageMetaData &metadata = handle.metadata();
+      if (metadata.width == 0 || metadata.height == 0 || metadata.depth == 0) {
+        continue;
+      }
+
+      /* Step size is transformed from voxel to world space. */
+      Transform voxel_tfm = tfm;
+      if (metadata.use_transform_3d) {
+        voxel_tfm = tfm * transform_inverse(metadata.transform_3d);
+      }
+
+      float3 size = make_float3(
+          1.0f / metadata.width, 1.0f / metadata.height, 1.0f / metadata.depth);
+      float voxel_step_size = min3(fabs(transform_direction(&voxel_tfm, size)));
+
+      if (voxel_step_size > 0.0f) {
+        step_size = fminf(voxel_step_size, step_size);
+      }
+    }
+  }
+
+  if (step_size == FLT_MAX) {
+    /* Fall back to 1/10th of bounds for procedural volumes. */
+    step_size = 0.1f * average(bounds.size());
+  }
+
+  step_size *= step_rate;
+
+  return step_size;
+}
+
 int Object::get_device_index() const
 {
   return index;
@@ -451,6 +512,7 @@ void ObjectManager::device_update_object_transform(UpdateObjectTransformState *s
     flag |= SD_OBJECT_HOLDOUT_MASK;
   }
   state->object_flag[ob->index] = flag;
+  state->object_volume_step[ob->index] = FLT_MAX;
 
   /* Have curves. */
   if (geom->type == Geometry::HAIR) {
@@ -504,6 +566,7 @@ void ObjectManager::device_update_transforms(DeviceScene *dscene, Scene *scene,
 
   state.objects = dscene->objects.alloc(scene->objects.size());
   state.object_flag = dscene->object_flag.alloc(scene->objects.size());
+  state.object_volume_step = dscene->object_volume_step.alloc(scene->objects.size());
   state.object_motion = NULL;
   state.object_motion_pass = NULL;
 
@@ -624,6 +687,7 @@ void ObjectManager::device_update_flags(
 
   /* Object info flag. */
   uint *object_flag = dscene->object_flag.data();
+  float *object_volume_step = dscene->object_volume_step.data();
 
   /* Object volume intersection. */
   vector<Object *> volume_objects;
@@ -634,6 +698,10 @@ void ObjectManager::device_update_flags(
         volume_objects.push_back(object);
       }
       has_volume_objects = true;
+      object_volume_step[object->index] = object->compute_volume_step_size();
+    }
+    else {
+      object_volume_step[object->index] = FLT_MAX;
     }
   }
 
@@ -651,6 +719,7 @@ void ObjectManager::device_update_flags(
     else {
       object_flag[object->index] &= ~(SD_OBJECT_HAS_VOLUME | SD_OBJECT_HAS_VOLUME_ATTRIBUTES);
     }
+
     if (object->is_shadow_catcher) {
       object_flag[object->index] |= SD_OBJECT_SHADOW_CATCHER;
     }
@@ -679,6 +748,7 @@ void ObjectManager::device_update_flags(
 
   /* Copy object flag. */
   dscene->object_flag.copy_to_device();
+  dscene->object_volume_step.copy_to_device();
 }
 
 void ObjectManager::device_update_mesh_offsets(Device *, DeviceScene *dscene, Scene *scene)
@@ -725,6 +795,7 @@ void ObjectManager::device_free(Device *, DeviceScene *dscene)
   dscene->object_motion_pass.free();
   dscene->object_motion.free();
   dscene->object_flag.free();
+  dscene->object_volume_step.free();
 }
 
 void ObjectManager::apply_static_transforms(DeviceScene *dscene, Scene *scene, Progress &progress)
diff --git a/intern/cycles/render/object.h b/intern/cycles/render/object.h
index 7bd3edf769b..2c2870cd0f2 100644
--- a/intern/cycles/render/object.h
+++ b/intern/cycles/render/object.h
@@ -97,6 +97,9 @@ class Object : public Node {
   /* Returns the index that is used in the kernel for this object. */
   int get_device_index() const;
 
+  /* Compute step size from attributes, shaders, transforms. */
+  float compute_volume_step_size() const;
+
  protected:
   /* Specifies the position of the object in scene->objects and
    * in the device vectors. Gets set in device_update. */
diff --git a/intern/cycles/render/osl.cpp b/intern/cycles/render/osl.cpp
index d17d7270cd5..74eb61e2dff 100644
--- a/intern/cycles/render/osl.cpp
+++ b/intern/cycles/render/osl.cpp
@@ -760,16 +760,14 @@ void OSLCompiler::add(ShaderNode *node, const char *name, bool isfilepath)
   else if (current_type == SHADER_TYPE_VOLUME) {
     if (node->has_spatial_varying())
       current_shader->has_volume_spatial_varying = true;
+    if (node->has_attribute_dependency())
+      current_shader->has_volume_attribute_dependency = true;
   }
 
   if (node->has_object_dependency()) {
     current_shader->has_object_dependency = true;
   }
 
-  if (node->has_attribute_dependency()) {
-    current_shader->has_attribute_dependency = true;
-  }
-
   if (node->has_integrator_dependency()) {
     current_shader->has_integrator_dependency = true;
   }
@@ -1143,8 +1141,8 @@ void OSLCompiler::compile(OSLGlobals *og, Shader *shader)
     shader->has_displacement = false;
     shader->has_surface_spatial_varying = false;
     shader->has_volume_spatial_varying = false;
+    shader->has_volume_attribute_dependency = false;
     shader->has_object_dependency = false;
-    shader->has_attribute_dependency = false;
     shader->has_integrator_dependency = false;
 
     /* generate surface shader */
diff --git a/intern/cycles/render/scene.cpp b/intern/cycles/render/scene.cpp
index 76f62fd6690..77c66779c20 100644
--- a/intern/cycles/render/scene.cpp
+++ b/intern/cycles/render/scene.cpp
@@ -63,6 +63,7 @@ DeviceScene::DeviceScene(Device *device)
       object_motion_pass(device, "__object_motion_pass", MEM_GLOBAL),
       object_motion(device, "__object_motion", MEM_GLOBAL),
       object_flag(device, "__object_flag", MEM_GLOBAL),
+      object_volume_step(device, "__object_volume_step", MEM_GLOBAL),
       camera_motion(device, "__camera_motion", MEM_GLOBAL),
       attributes_map(device, "__attributes_map", MEM_GLOBAL),
       attributes_float(device, "__attributes_float", MEM_GLOBAL),
diff --git a/intern/cycles/render/scene.h b/intern/cycles/render/scene.h
index d2570138b53..6b10a901d7b 100644
--- a/intern/cycles/render/scene.h
+++ b/intern/cycles/render/scene.h
@@ -91,6 +91,7 @@ class DeviceScene {
   device_vector<Transform> object_motion_pass;
   device_vector<DecomposedTransform> object_motion;
   device_vector<uint> object_flag;
+  device_vector<float> object_volume_step;
 
   /* cameras */
   device_vector<DecomposedTransform> camera_motion;
diff --git a/intern/cycles/render/shader.cpp b/intern/cycles/render/shader.cpp
index cc6eb2e5e7f..747fc58f81a 100644
--- a/intern/cycles/render/shader.cpp
+++ b/intern/cycles/render/shader.cpp
@@ -178,6 +178,8 @@ NODE_DEFINE(Shader)
               volume_interpolation_method_enum,
               VOLUME_INTERPOLATION_LINEAR);
 
+  SOCKET_FLOAT(volume_step_rate, "Volume Step Rate", 1.0f);
+
   static NodeEnum displacement_method_enum;
   displacement_method_enum.insert("bump", DISPLACE_BUMP);
   displacement_method_enum.insert("true", DISPLACE_TRUE);
@@ -203,10 +205,11 @@ Shader::Shader() : Node(node_type)
   has_bssrdf_bump = false;
   has_surface_spatial_varying = false;
   has_volume_spatial_varying = false;
+  has_volume_attribute_dependency = false;
   has_object_dependency = false;
-  has_attribute_dependency = false;
   has_integrator_dependency = false;
   has_volume_connected = false;
+  prev_volume_step_rate = 0.0f;
 
   displacement_method = DISPLACE_BUMP;
 
@@ -353,9 +356,10 @@ void Shader::tag_update(Scene *scene)
     scene->geometry_manager->need_update = true;
   }
 
-  if (has_volume != prev_has_volume) {
+  if (has_volume != prev_has_volume || volume_step_rate != prev_volume_step_rate) {
     scene->geometry_manager->need_flags_update = true;
     scene->object_manager->need_flags_update = true;
+    prev_volume_step_rate = volume_step_rate;
   }
 }
 
@@ -533,10 +537,12 @@ void ShaderManager::device_update_common(Device *device,
     /* in this case we can assume transparent surface */
     if (shader->has_volume_connected && !shader->has_surface)
       flag |= SD_HAS_ONLY_VOLUME;
-    if (shader->heterogeneous_volume && shader->has_volume_spatial_varying)
-      flag |= SD_HETEROGENEOUS_VOLUME;
-    if (shader->has_attribute_dependency)
-      flag |= SD_NEED_ATTRIBUTES;
+    if (shader->has_volume) {
+      if (shader->heterogeneous_volume && shader->has_volume_spatial_varying)
+        flag |= SD_HETEROGENEOUS_VOLUME;
+    }
+    if (shader->has_volume_attribute_dependency)
+      flag |= SD_NEED_VOLUME_ATTRIBUTES;
     if (shader->has_bssrdf_bump)
       flag |= SD_HAS_BSSRDF_BUMP;
     if (device->info.has_volume_decoupled) {
diff --git a/intern/cycles/render/shader.h b/intern/cycles/render/shader.h
index 5a5b42de994..1509150228f 100644
--- a/intern/cycles/render/shader.h
+++ b/intern/cycles/render/shader.h
@@ -92,6 +92,8 @@ class Shader : public Node {
   bool heterogeneous_volume;
   VolumeSampling volume_sampling_method;
   int volume_interpolation_method;
+  float volume_step_rate;
+  float prev_volume_step_rate;
 
   /* synchronization */
   bool need_update;
@@ -118,8 +120,8 @@ class Shader : public Node {
   bool has_bssrdf_bump;
   bool has_surface_spatial_varying;
   bool has_volume_spatial_varying;
+  bool has_volume_attribute_dependency;
   bool has_object_dependency;
-  bool has_attribute_dependency;
   bool has_integrator_dependency;
 
   /* displacement */
diff --git a/intern/cycles/render/svm.cpp b/intern/cycles/render/svm.cpp
index 2946614a3b7..c4218f7df1e 100644
--- a/intern/cycles/render/svm.cpp
+++ b/intern/cycles/render/svm.cpp
@@ -444,16 +444,14 @@ void SVMCompiler::generate_node(ShaderNode *node, ShaderNodeSet &done)
   else if (current_type == SHADER_TYPE_VOLUME) {
     if (node->has_spatial_varying())
       current_shader->has_volume_spatial_varying = true;
+    if (node->has_attribute_dependency())
+      current_shader->has_volume_attribute_dependency = true;
   }
 
   if (node->has_object_dependency()) {
     current_shader->has_object_dependency = true;
   }
 
-  if (node->has_attribute_dependency()) {
-    current_shader->has_attribute_dependency = true;
-  }
-
   if (node->has_integrator_dependency()) {
     current_shader->has_integrator_dependency = true;
   }
@@ -864,8 +862,8 @@ void SVMCompiler::compile(Shader *shader, array<int4> &svm_nodes, int index, Sum
   shader->has_displacement = false;
   shader->has_surface_spatial_varying = false;
   shader->has_volume_spatial_varying = false;
+  shader->has_volume_attribute_dependency = false;
   shader->has_object_dependency = false;
-  shader->has_attribute_dependency = false;
   shader->has_integrator_dependency = false;
 
   /* generate bump shader */