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If I'm using blender game engine, how do blender units work in relation to gravity, acceleration, or weight? Is there a rough physical world equivalent?

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    $\begingroup$ I think it's usually 1 BU = 1 meter with regards to physics simulations. $\endgroup$ – El'endia Starman May 24 '13 at 3:27
  • $\begingroup$ What do you mean by rough physical world equivalent. Units are used in the bge the same way they are used in the 3d view. $\endgroup$ – iKlsR May 24 '13 at 3:31
  • $\begingroup$ in relation to gravity, acceleration, or weight. $\endgroup$ – RobotHumans May 24 '13 at 3:34
  • $\begingroup$ i think you need to keep an eye on the size of units when you go to 3D print especially, if not you will be trying to make something that does not fit the printer. a lot of difference whether you use metric or british when scaling things this subject needs more explaining $\endgroup$ – user698 Jun 16 '13 at 11:49
  • $\begingroup$ @jmm I appreciate that you read multiple questions from me probably (the 3d printing and the blender units/real world thing). They are separate though. With 3d printing unit conversion would be arbitrary and would depend on the specificity required on the model. This question was more concerned with physics simulations and how they interact with gravity. I can add this to my question if you think it would be helpful. $\endgroup$ – RobotHumans Jun 16 '13 at 11:52
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Blender units

The Blender units system is a decimal unit system that can be whatever you want it to be. By default, it's implicitly considered meters. If you go to the scene tab in the properties panel, then to the gravity section, you can see that the rate of acceleration due to gravity is (0.000, 0.000, -9.810). 9.810 is the rate of acceleration in meters per second squared. So as you can see, it's implicitly considered meters. Now, if you change this value to -.0981 and every rigidbody's mass to 1100 of what it was, you would now be working in centimeters and grams (or the cgs system).

Metric units

First off, let me start by defining a list of metric measurements that Blender uses.

Length

  • (km) kilometer [1000 meters]
  • (m) meter
  • (cm) centimeter [1100 meter]
  • (mm) millimeters [11000 meter]
  • (µm) micrometers [1100000000 meter]

Mass

  • (ton) metric ton [1000 kilograms]
  • (kg) kilogram
  • (g) gram [11000 kilogram]

If you go to the units section in the scene tab, you can see that there are three different options ("None, Metric, Imperial"). "None" is Blender units. If you select metric units, Blender will explicitly use metric units. Check back in the physics panel, and you'll now see (0m/s2, 0m/s2, -9.81m/s2).

For another example, select a mesh and hit n in the scene view to bring up the properties panel, go to the transform section and then dimensions. If you change one of the x, y, or z dimensions to a large number, you will see that it now uses the kilometers instead of meters. And if you change it to a ridiculously small number, like .000002, it will use micrometers.

Imperial units

Imperial units are the worst to work with. Also known as the British units. This is what backward America uses. Let me again define the units that Blender utilizes.

Length

  • (mi) mile [5280 feet]
  • (ft) foot [also symbolized with a single quote: ']
  • (in) inch [112 foot] [also symbolized with double quote: "]
  • thou or mil [112000 foot]

Mass (technically weight)

  • ton [2000 pounds]
  • (lb) pound
  • (oz) ounce [116 pound]

Separate units

If you are using metric or imperial units, you can use the 'separate units' checkbox which makes Blender separate out a unit of measurement into two separate units of measurement of the same system.

For example, without 'separate units' toggled, a field might display something like 2.538m. But with 'separate units' toggled, it would display 2m 53.8cm

Scripting

One thing you should note is that when working with scripts, and you use a function like bpy.ops.mesh.primitive_cube_add(location=(1.0, 1.5, 2.0)), which creates and positions a cube, the values are in Blender units even if you are working in metric or imperial. So this function, when using the imperial system, would place a cube at (3.281', 4.921', 6.562') or
(3' 3.37", 4' 11.1", 6' 6.74") with separate units.

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    $\begingroup$ Can you add a credible link to the source of this information? $\endgroup$ – iKlsR May 24 '13 at 4:12
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    $\begingroup$ That would be me. I fired up Blender and just learned all of this. Sorry. $\endgroup$ – Daniel Pendergast May 24 '13 at 4:15
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    $\begingroup$ When you're using the weight units in the Imperial system, does Blender convert them into mass using the setting for gravity, or does it just assume standard Earth gravity for this calculation? $\endgroup$ – Gwen May 24 '13 at 4:15
  • $\begingroup$ Aaand my answer is blown out of the water. +1! $\endgroup$ – El'endia Starman May 24 '13 at 4:16
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    $\begingroup$ Note that units are really useful only for sims. If you're creating a solar system, you'd maybe say 1BU = 1000km. Blender uses internally floats, so you have only limited data range and precision to work with. Likewise, if you're working on microscopic scale, you could assume 1BU = 1mm for the same reason. $\endgroup$ – jesterKing May 24 '13 at 7:08
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By default, 1 BU = 1 meter. Proof: the strength of gravity.

enter image description here

Earth's gravity is about 9.81 m/s2 (or 32 ft/s2, if you prefer) downwards.

Now, if you aren't using any physics simulations, 1 BU can be anything you like, and if you're using physics simulations, then you can tweak the strength of gravity anyway to match with the scale you've chosen.

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  • $\begingroup$ But, in my experience, it has always seemed as though Blender's gravity has been too light. I always end up setting it to 20 or 30. This could be because I build all of my models too big though. $\endgroup$ – Gwen May 24 '13 at 4:13
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    $\begingroup$ @Gwenn: Probably because you're building all your models too big. For instance, the default cube is two meters long, wide, and high, which is a BIG cube. $\endgroup$ – El'endia Starman May 24 '13 at 4:16
  • $\begingroup$ @Compro01. You need to assume that the time unit is second. It could also have been animation frames for example. $\endgroup$ – user877329 May 8 '14 at 6:39
  • $\begingroup$ @user877329 - Not my answer. I just fixed up some formatting on it. $\endgroup$ – Compro01 May 8 '14 at 8:53

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