Fluid: Diffusion settings now optional

For optimization purposes these settings need to be enabled explicitly from now on.

intern/mantaflow/intern/MANTA_main.cpp

@@ -981,6 +981,8 @@ std::string MANTA::getRealValue(const std::string &varName, FluidModifierData *m
     ss << (mmd->domain->flags & FLUID_DOMAIN_USE_FRACTIONS ? "True" : "False");
   else if (varName == "DELETE_IN_OBSTACLE")
     ss << (mmd->domain->flags & FLUID_DOMAIN_DELETE_IN_OBSTACLE ? "True" : "False");
+  else if (varName == "USING_DIFFUSION")
+    ss << (mmd->domain->flags & FLUID_DOMAIN_USE_DIFFUSION ? "True" : "False");
   else
     std::cout << "ERROR: Unknown option: " << varName << std::endl;
   return ss.str();

intern/mantaflow/intern/strings/fluid_script.h

@@ -112,6 +112,7 @@ using_invel_s$ID$        = $USING_INVEL$\n\
 using_outflow_s$ID$      = $USING_OUTFLOW$\n\
 using_sndparts_s$ID$     = $USING_SNDPARTS$\n\
 using_speedvectors_s$ID$ = $USING_SPEEDVECTORS$\n\
+using_diffusion_s$ID$    = $USING_DIFFUSION$\n\
 \n\
 # Fluid time params\n\
 timeTotal_s$ID$    = $TIME_TOTAL$\n\

intern/mantaflow/intern/strings/liquid_script.h

@@ -266,14 +266,14 @@ def liquid_step_$ID$():\n\
     if using_obstacle_s$ID$:\n\
         mantaMsg('Extrapolating object velocity')\n\
         # ensure velocities inside of obs object, slightly add obvels outside of obs object\n\
-        extrapolateVec3Simple(vel=obvelC_s$ID$, phi=phiObsIn_s$ID$, distance=int(res_s$ID$/2), inside=True)\n\
+        extrapolateVec3Simple(vel=obvelC_s$ID$, phi=phiObsIn_s$ID$, distance=6, inside=True)\n\
         extrapolateVec3Simple(vel=obvelC_s$ID$, phi=phiObsIn_s$ID$, distance=3, inside=False)\n\
         resampleVec3ToMac(source=obvelC_s$ID$, target=obvel_s$ID$)\n\
     \n\
     extrapolateMACSimple(flags=flags_s$ID$, vel=vel_s$ID$, distance=2, intoObs=True if using_fractions_s$ID$ else False)\n\
     \n\
     # vel diffusion / viscosity!\n\
-    if viscosity_s$ID$ > 0.:\n\
+    if using_diffusion_s$ID$:\n\
         mantaMsg('Viscosity')\n\
         # diffusion param for solve = const * dt / dx^2\n\
         alphaV = viscosity_s$ID$ * s$ID$.timestep * float(res_s$ID$*res_s$ID$)\n\

release/scripts/startup/bl_ui/properties_physics_fluid.py

@@ -957,6 +957,14 @@ class PHYSICS_PT_diffusion(PhysicButtonsPanel, Panel):
 
         return (context.engine in cls.COMPAT_ENGINES)
 
+    def draw_header(self, context):
+        md = context.fluid.domain_settings
+        domain = context.fluid.domain_settings
+        is_baking_any = domain.is_cache_baking_any
+        has_baked_any = domain.has_cache_baked_any
+        self.layout.enabled = not is_baking_any and not has_baked_any
+        self.layout.prop(md, "use_diffusion", text="")
+
     def draw_header_preset(self, _context):
         FLUID_PT_presets.draw_panel_header(self.layout)
 

source/blender/makesdna/DNA_fluid_types.h

@@ -42,6 +42,7 @@ enum {
   FLUID_DOMAIN_EXPORT_MANTA_SCRIPT = (1 << 12), /* Export mantaflow script during bake. */
   FLUID_DOMAIN_USE_FRACTIONS = (1 << 13),       /* Use second order obstacles. */
   FLUID_DOMAIN_DELETE_IN_OBSTACLE = (1 << 14),  /* Delete fluid inside obstacles. */
+  FLUID_DOMAIN_USE_DIFFUSION = (1 << 15), /* Use diffusion (e.g. viscosity, surface tension). */
 };
 
 /* Border collisions. */

source/blender/makesrna/intern/rna_fluid.c

@@ -1563,6 +1563,13 @@ static void rna_def_fluid_domain_settings(BlenderRNA *brna)
 
   /*  diffusion options */
 
+  prop = RNA_def_property(srna, "use_diffusion", PROP_BOOLEAN, PROP_NONE);
+  RNA_def_property_boolean_sdna(prop, NULL, "flags", FLUID_DOMAIN_USE_DIFFUSION);
+  RNA_def_property_ui_text(
+      prop, "Use Diffusion", "Enable fluid diffusion settings (e.g. viscosity, surface tension)");
+  RNA_def_property_clear_flag(prop, PROP_ANIMATABLE);
+  RNA_def_property_update(prop, NC_OBJECT | ND_MODIFIER, "rna_Fluid_reset");
+
   prop = RNA_def_property(srna, "surface_tension", PROP_FLOAT, PROP_NONE);
   RNA_def_property_range(prop, 0.0, 100.0);
   RNA_def_property_ui_text(