Smoke simulator questions (free bake, negative frame start, density/temp diff)

I’m trying to make Quick Smoke work on a BI scene in Blender 2.73 and am confused by how the Physics tab operates for smoke simulations. I have a few questions:

  1. What are the “rules” regarding baking/free baking smoke? If I have Free Bake on, sometimes the simulation works if I go frame-by-frame starting at frame 1. Most of the time, Free Bake doesn’t work and I have to press the Bake button to bake the simulation.

  2. Is possible to start a smoke simulation on a negative frame? When I set my simulation from frame -14 to frame 200 and bake it, it will show the correct state of the smoke on frame 0 but the simulation will stay frozen beyond that.

  3. In a Domain/Flow simulation, I see that there’s a “Density” and “Temp Diff” control for both the Domain and the Flow. Conceptually, I think I understand what the parameter should do for the flow object (the density of the smoke and how fast should it rise), but what do these parameters mean for the domain object?


  1. “Free bake” means “get rid of the current baked cache”. It is not a baking mode, it’s just an operator to “get rid of the current baked cache”. If you don’t bake a simulation, then the simulation data will be written in memory. Playing an animation (by pressing alt+A) or stepping frame-by-frame will work because the data for the previous frame would have been written in memory and is therefore known, so we’re able to generate the data for the current frame from it. The fact that you have to start stepping frame-by-frame at frame 1 is because this is the frame that initializes the simulation, we’ll be able to generate all the values for frame 2 and so on… Hopefully I’m clear enough here.

  2. Technically yes. But this feature is buggy as hell, I just tried and I’m not sure if I should report a bug or not. Might need to dig into the know todos/limitations list. Though it seems to only happens if we set “Allow negative frames” in user prefs.

  3. It is my understanding that the domain would basically represent the medium in which the smoke evolves (e.g.: water, air…). I need to dig into that subject a little more.

Thanks for the info. That’s very helpful.

I guess what’s weird is that when I go frame-to-frame without baking the simulation, sometimes the sim works, but a lot of the time it doesn’t. I pretty much can’t rely on the sim working without baking it first.