Proper scene set up for, not meters

Support Basics & Interface
1

The default scene set up, at least in 2.8+ is meters, right?

First, I noticed most tutorials I’ve scene for things like physics don’t change anything…but then they model things Not to scale. This bugs the heck out of me because that can cause Lots of issues. First, your simulation looks cool, but if you want it to look real, then it’s wrong.

Unit scale is one that interests me because the manual explicitly says “This only influences the values displayed in the user interface and not how things behave internally. For example, physics simulations don’t take the unit scale into account.”

But when I did a small simulation in mm that failed miserably, I changed the unit scale and it failed miserably Differently. But wouldn’t that suggest that it shouldn’t have effected it at all?

So I’m curious what the proper way to set up scenes would be for different units, and scales. I live in the US, so I have to use inches Often, but realistically I’m working in mm. I switch to inches just to enter a bunch of inch information, but generally I’ll switch back to mm, cm, or meters, depending on the scale I’m working with. House? Meters. Car, cm, computer chair-keyring, mm. things the size of computer chair could go either way, but the small details like screws, mm is convenient to me.

An explanation of how to actually set things up for different units and scene sizes, especially when related to physical things like simulations, phyiscal lights and materials, etc.
And it Does make a difference, is there an add-on or something that helps you out switching back and forth? Sure, I could just save scenes with the settings applied in each and just start there. But sometimes you need to switch after the fact.

2

So far as I know, the mapping of these real-world units of measure to “BU’s = Blender Units” is arbitrary. I fully expect that the multipliers for “feet” vs. “meters” are correct.

The extremely-important lesson is simply: “pay attention to scale!” Grab that 100’ tape measure and actually measure things, and make sure that everything is “to scale” in relation to everything else.

When I’m roughing-out a scene, initially the objects are just simple geometric shapes. But, they are the right size. And, they are placed on a set which is also “the right size.” “This room is 11 x 14 feet with an eight-foot ceiling. That window is 2 x 4 feet and is 36 inches above the floor.” You really can’t just “eyeball it” and have it come out right.

3

So what I don’t understand is it Does make a BIG difference. I just tried to throw down a cloth on a tray (similar to a tablecloth) and figured, why not do a quick simulation to get started. It was going wonky, and nothing was working out right.
I was set to inches, dropping a 48x48 plane onto a 30x24 tray.
When I changed the unit scale to be .0254 it changed my decimal point, but ALSO changed the size of the objects in my scene. I went into edit mode and scaled them up by the inverse which I don’t remember at the moment but I did the math at the time. Something like 3.39 anyway.
I then deleted the collision, and added it again to sort of “reset” it. And did the same to the cloth.
And wouldn’t you know it, my simulation time sped up by at Least a full factor, and all the crazy bits were fixed.

So setting scale units before you actually get going Is important. I’m pretty surprised I haven’t seen Anyone mention it yet in all the tutorials I’ve seen in the maybe year I’ve been learning blender.

So I guess that Definitely leads me to the question…has anyone made an add-on to scale All the scene units if for some reason you get someone else’s file, or you just plain forgot until halfway through? It’d have to scale values for modifiers as well, so could potentially be quite time consuming depending on the scene.

4

I think that what really matters is the relative scale of the objects. “A Blender Unit (BU) can be anything you want it to be,” but if you want the scene to really look right, it is critically important that the proportions are consistent with reality. If you don’t, “something will feel ‘off,’” even if the audience can’t quite put their finger on just what it is.

If you watch an actual “movie shoot,” you’ll notice that there is always someone on set with a very long tape measure, and someone else who’s writing down every number that she calls out. Nowadays I’m told that some digital cameras come equipped with laser rangefinders that automatically capture the distance to various objects in the scene, recording the measurements automatically and digitally. (Such rangefinders are now readily available in hardware stores at a moderate cost.)

— Edit: This reply does not consider simulations.

5

Absolute scale does matter in simulations and shaders. A tiny fluid emitter will dribble out drops of water that stick together. And some shaders, like glass, will look much different depending on scale.

That said there are some times when you need to make things larger to work better, rigid body sims have some stuff hard coded in them that makes smaller scales fail sometimes.

6

only that a blender unit is 1 meter. Your unit size is relative to other things, so that only makes sense if you change all of those other things, which is where unit scale comes in.

Gravity is a constant. It’s set to 9.8m/s^2, which happens to be 9.8 blender units.
Light values with light falloff is also scale dependent.
And DOF? It always cracks me up when someone on youtube does something then is confused why they have to use a value of 8 this time, but last time it looked good at 2, then they say something like, “DOF is just fickle like that sometimes.” But no, they just don’t understand that scale matters.

But yeah, if you adjust ALL of the values yourself, then scale is relative. But it’s a lot less work to just get it right. I’m looking to find out the best way to get it right if you want to set it to something that isn’t meters.

7

In my basic tests though, that Is effected by scene scale. So I’m wondering if those things are fixed.
In some google sleuthing I found a fix for photographic lights respecting scene scale, So wondered if that’s all it took.
But the big annoyance is, if you didn’t set the unit scale First, changing it scales down all your units (which is revealing at why your simulations probably failed). So I was wondering if anyone made an add-on to scale all those number inverse to the scale you used.

Shoot, you go from inches/mm and scale up to meters, do your simulation, bake, then go back to the proper scale to ensure lighting, materials, fog, etc are correct.

8

If for any reason – including those stated – you want “BU = Meter,” then you should measure and lay-out your scene that way. The bounding-boxes of the objects should be of the “correct” size, as should be the set itself, and their placement on the “ground” should reflect actual distances. Only when you do this will the scene look “right,” no matter where the cameras may be placed. It’s tedious but worth it. I snap labeled empties to the center of each corresponding object to mark where I originally placed it – like is sometimes done on a real set with “gaffer tape.”