Fix #80529: Smoke dissolve rate affected by time steps

Now results are similar under different time steps. Additionally it is
now working correctly with different time scale, where smoke lifetime
is proportional to time scale.

Pull Request: https://projects.blender.org/blender/blender/pulls/138347

extern/mantaflow/README.blender

@@ -9,3 +9,4 @@ Local modifications:
 * ./patches/precision-of-4d-vector.patch to increase precision of 4D vector normalization functions.
 * ./patches/liquid-mesh-performance.patch improve liquid mesh generation by puting calculation of inverse radius outside for loops.
 * ./patches/liquid-performance.patch improve liquid generation (without mesh) by precalculate sum of vectors and put it outside for loop.
+* ./patches/smoke-dissolve-rate.patch Dissolve smoke independently from number of timesteps in Frame.

extern/mantaflow/patches/smoke-dissolve-rate.patch

@@ -0,0 +1,38 @@
+commit 4a2c116053be7d445f061bee074796dc87529b3f
+Author: Bartosz Kosiorek <gang65@poczta.onet.pl>
+Date:   Fri May 2 19:59:49 2025 +0200
+
+    Physics: Dissolve smoke independently from number of timesteps in Frame
+    
+    Fixes: 80529
+
+diff --git a/extern/mantaflow/preprocessed/plugin/extforces.cpp b/extern/mantaflow/preprocessed/plugin/extforces.cpp
+index 88935fa7ae9..eaf3e7d39f4 100644
+--- a/extern/mantaflow/preprocessed/plugin/extforces.cpp
++++ b/extern/mantaflow/preprocessed/plugin/extforces.cpp
+@@ -1652,10 +1652,11 @@ void dissolveSmoke(const FlagGrid &flags,
+                    Grid<Real> *green = nullptr,
+                    Grid<Real> *blue = nullptr,
+                    int speed = 5,
+-                   bool logFalloff = true)
++                   bool logFalloff = true,
++                   const float dissolveScale = 1.0)
+ {
+-  float dydx = 1.0f / (float)speed;  // max density/speed = dydx
+-  float fac = 1.0f - dydx;
++  const float dydx = dissolveScale / (float)speed;  // max density (1.0) * scale / speed = dydx
++  const float fac = 1.0f - dydx;
+   KnDissolveSmoke(flags, density, heat, red, green, blue, speed, logFalloff, dydx, fac);
+ }
+ static PyObject *_W_11(PyObject *_self, PyObject *_linargs, PyObject *_kwds)
+@@ -1676,8 +1677,9 @@ static PyObject *_W_11(PyObject *_self, PyObject *_linargs, PyObject *_kwds)
+       Grid<Real> *blue = _args.getPtrOpt<Grid<Real>>("blue", 5, nullptr, &_lock);
+       int speed = _args.getOpt<int>("speed", 6, 5, &_lock);
+       bool logFalloff = _args.getOpt<bool>("logFalloff", 7, true, &_lock);
++      const float dissolveScale = _args.getOpt<float>("dissolveScale", 8, 1.0, &_lock);
+       _retval = getPyNone();
+-      dissolveSmoke(flags, density, heat, red, green, blue, speed, logFalloff);
++      dissolveSmoke(flags, density, heat, red, green, blue, speed, logFalloff, dissolveScale);
+       _args.check();
+     }
+     pbFinalizePlugin(parent, "dissolveSmoke", !noTiming);

extern/mantaflow/preprocessed/plugin/extforces.cpp

@@ -1652,10 +1652,11 @@ void dissolveSmoke(const FlagGrid &flags,
                    Grid<Real> *green = nullptr,
                    Grid<Real> *blue = nullptr,
                    int speed = 5,
-                   bool logFalloff = true)
+                   bool logFalloff = true,
+                   const float dissolveScale = 1.0)
 {
-  float dydx = 1.0f / (float)speed;  // max density/speed = dydx
-  float fac = 1.0f - dydx;
+  const float dydx = dissolveScale / (float)speed;  // max density (1.0) * scale / speed = dydx
+  const float fac = 1.0f - dydx;
   KnDissolveSmoke(flags, density, heat, red, green, blue, speed, logFalloff, dydx, fac);
 }
 static PyObject *_W_11(PyObject *_self, PyObject *_linargs, PyObject *_kwds)
@@ -1676,8 +1677,9 @@ static PyObject *_W_11(PyObject *_self, PyObject *_linargs, PyObject *_kwds)
       Grid<Real> *blue = _args.getPtrOpt<Grid<Real>>("blue", 5, nullptr, &_lock);
       int speed = _args.getOpt<int>("speed", 6, 5, &_lock);
       bool logFalloff = _args.getOpt<bool>("logFalloff", 7, true, &_lock);
+      const float dissolveScale = _args.getOpt<float>("dissolveScale", 8, 1.0, &_lock);
       _retval = getPyNone();
-      dissolveSmoke(flags, density, heat, red, green, blue, speed, logFalloff);
+      dissolveSmoke(flags, density, heat, red, green, blue, speed, logFalloff, dissolveScale);
       _args.check();
     }
     pbFinalizePlugin(parent, "dissolveSmoke", !noTiming);

intern/mantaflow/intern/strings/smoke_script.h

@@ -336,7 +336,8 @@ def smoke_step_$ID$():\n\
     \n\
     if using_dissolve_s$ID$:\n\
         mantaMsg('Dissolving smoke')\n\
-        dissolveSmoke(flags=flags_s$ID$, density=density_s$ID$, heat=heat_s$ID$, red=color_r_s$ID$, green=color_g_s$ID$, blue=color_b_s$ID$, speed=dissolveSpeed_s$ID$, logFalloff=using_logdissolve_s$ID$)\n\
+        dissolveSmoke(flags=flags_s$ID$, density=density_s$ID$, heat=heat_s$ID$, red=color_r_s$ID$, green=color_g_s$ID$, blue=color_b_s$ID$, \
+                      speed=dissolveSpeed_s$ID$, logFalloff=using_logdissolve_s$ID$, dissolveScale=s$ID$.timestep / frameLengthUnscaled_s$ID$)\n\
     \n\
     mantaMsg('Advecting density')\n\
     advectSemiLagrange(flags=flags_s$ID$, vel=vel_s$ID$, grid=density_s$ID$, order=2)\n\
@@ -469,7 +470,8 @@ def step_noise_$ID$():\n\
     \n\
     if using_dissolve_s$ID$:\n\
         mantaMsg('Dissolving noise')\n\
-        dissolveSmoke(flags=flags_sn$ID$, density=density_sn$ID$, heat=None, red=color_r_sn$ID$, green=color_g_sn$ID$, blue=color_b_sn$ID$, speed=dissolveSpeed_s$ID$, logFalloff=using_logdissolve_s$ID$)\n\
+        dissolveSmoke(flags=flags_sn$ID$, density=density_sn$ID$, heat=None, red=color_r_sn$ID$, green=color_g_sn$ID$, blue=color_b_sn$ID$, \
+                      speed=dissolveSpeed_s$ID$, logFalloff=using_logdissolve_s$ID$, dissolveScale=s$ID$.timestep / frameLengthUnscaled_s$ID$)\n\
     \n\
     mantaMsg('Advecting UVs and updating UV weight')\n\
     advectSemiLagrange(flags=flags_s$ID$, vel=vel_s$ID$, grid=uvGrid0_s$ID$, order=2)\n\