Understanding Scene Unit Scale

     Hello everyone this is my first question here. I am modeling things that are quite small, things measured in inches and millimeters. Because of this I have found videos that explain that I should change the Unit Scale and then change the grid size also.

     I am having a hard time understanding what exactly is happening when I only scale the Unit Scale up or down from 1.0. I see the grid change and everything, but it gets so wide when I change it to say 0.001 for metric that you can't see it anymore. Why then do I need to scale the grid back down under the overlay properties?

     I understand that under the hood there is a conversion between Blender Units and the Unit System and the units I decide on, since it makes it easier behind the scenes code wise and precision wise. I guess I just don't understand what exactly the unit scale 1.0 represents and what the grid spaces in the world represent by defaults, even.

To summarize...

     1.) What does this setting affect, like does it affect lighting/materials etc.

     2.) I can see that it also changes the view clipping, should I be setting that

     manually?

Sorry if this sounds like rambling, I can expand further if I need to.

  • Martin Bergwerf replied

    With a Scene Unit Scale set to 1.0, the Default Cube is 2m by 2m by 2m. The Clip Start is 0.01m. The Clip End is 1000m.

    When the Unit Scale is changed to 0.001, the Default Cube is 2mm by 2mm by 2mm. The Clip Start is 0.01mm. The Clip End is 1000mm.

    The reason to change the Unit Scale are for accuracy (there are only a certain number of digits behind the decimal point, that can be represented by a fixed number of Bytes) and it also is easier for us, when modeling very small or very large items.

    The Grid is there for convenience. So changing that to the same Scale as the Unit Scale makes it easier to model.

    Changing the Unit Scale does not affect Shading, Lighting, Physics or so, but a smaller or larger model does change the behavior of Light, Shading and Physics.

    I hope that helps.

  • Omar Domenech replied

    So modeling to real world scale is super important for your scene. If you're modeling a chair and you have made it the size of a building you'll have trouble down the road if say for example you'll want to link your assets to another scene and things will vary in sizes too much and you'll have to manually adjust everything and by eye you wont judge it right. Also the power of the lights will be affected, you'll have to crank up the power a lot since you are trying to light a chair that is as tall as a building.

    So if you want to model a grain of rice, now you have something very small and you'll ideally make it in real world scale, so now you have to zoom way way in and navigate around the grain of rice you are modeling and things like viewport navigation become a bit cumbersome at that scale. So that is why you adjust the scale of your scene, so that so zoomed in to your grain of rice, things behave as if it was normal size in the viewport while really being the actual size of the grain of rice. 

  • Brennan Buchanan(slimmy83) replied

    Thanks guys greatly appreciate it. I took a look again today fresh after sleeping and I seem to understand it. I the only thing I still am a little fuzzy on if anything is that when you set it to imperial is each minor square, the smallest square in the grid you get when zooming in 1ft^2 and then the same for metric just its 1m^2? That would make sense because then you just need to resize the grid from 1:1 with the scale of the scene to down to what you need for snapping to the units you scaled the scene down to.

  • Dwayne Savage(dillenbata3) replied

    Scene unit scale only affects what is displayed in the UI. It doesn't affect the internal data. Basically it multiples the data by the scale and then uses that as the representation of the scale. Think of a model car. They give a scale ratio like 8 to 1 or 8:1. Which just means it's 1/8(0.125) of actual size. In blenders case the 1 is the data and the 8 is the scale units. The grid uses the defined length.(meters by default). So when you change the unit scale you make it smaller/larger than the meter so the scale is adjusted to keep showing a meter. So you have to adjust it to represent the new displayed unit by adjusting it's scale. The clip start and clip end should automatically be adjusted (be multipled by the unit scale). Hope that helps.

  • Dwayne Savage(dillenbata3) replied

    If you're going to be working in inches or millimeters I recommend doing one of the following:


    1. Set length to different unit instead of using unit scale.


    2. Use advanced entry. For number fields this is easy just use unit markers. For feet you can use ft or ' for inches you can use in or " for yard use yd side note: blender will automatically convert them to meters if you're using metric. Metric unit marks will be converted if you're using imperial. For millimeters mm for centimeters cm and meters m you can also use math and combine like this: 2m 3cm + 2' 3"


    Now for hotkeys. When you press G you can then press = to switch to advanced mode then you can enter just like a number field including math. If you do this a lot then F4->preferences->input->keyboard->check default to advanced mode. This way you don't have to keep hitting = to switch to advanced mode.