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For some reason, the blocks are not dropping down due to gravity. Instead, once I hit play, they shoot off to the y-axis. I'm just trying to make the cubes drop straight down and land on the ground. I'm not sure if the box I have them in has anything to do with it, but I have a big box with the sides remaining to act as a backdrop. On frame 1 they are above where they need to drop and instantly on frame 2 they appear on the opposite of the wall as if it was pushed out. Then the frames after that it is shooting out and away like a shotgun

Here is a link to a GIF video of what it looks like. It was 1mb too large to upload on here.

---> https://ibb.co/nzddBqW <---

Frame 1 Frame 2

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  • $\begingroup$ The file is around 50mb, so I'm not able to provide you with a blend file. The service has a limit of a 30mb file upload. $\endgroup$ Commented Jul 19, 2021 at 22:59
  • $\begingroup$ If the backdrop is set as a rigid body but the normals are not pointing inward, it will spit out the active cubes. $\endgroup$
    – TheLabCat
    Commented Jul 20, 2021 at 3:33
  • $\begingroup$ Hi adlowe...there is ALWAYS a way to provide the blend file. If you don't want - i can accept that but of course our help is than more "guessing" than knowing. Here some ideas how you can do it: You could provide it via dropbox (nearly no limit) or any other cloud platform. You could simplify you blend file and delete the memory-expensive parts so that your file is under 30mb. $\endgroup$
    – Chris
    Commented Jul 20, 2021 at 5:02
  • $\begingroup$ @TheLabCat are the normals you're referring to, the red and blue faces. I had just found out about that part. $\endgroup$ Commented Jul 20, 2021 at 22:47

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The problem is that the small cubes are inside a large cube. Changing the normals of the outer cube's faces won't help anything (at least not in my version), Blender treats them as inside another rigid body object so they get rejected, i.e. thrown outwards (always in the direction of the wall which is closest to them).

To change this, your walls have to be separate objects, so that your small cubes are not inside any of these walls. Or at least they have to be separate, unconnected planes, and then you have to set Collisions > Shape > Mesh, because Convex Hull uses the bounding box of the objects so the small cubes will still be treated as inside the big cube.

The question is if it is necessary to have rigid bodies all around. You could easily get away in this scene with just a floor as rigid body, or maybe the floor and the one wall where the small cubes probably bounce against. All the other walls can be normal objects without influence on the simulation, I guess.

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  • $\begingroup$ Oh, so are you saying instead of adding a box and removing the faces I don't want, I should have extruded the edges on a plane to make the shape I need. I plan on beveling the edges where the wall and floor meet, maybe. I also didn't think about making 2 different objects and only having the floor as a rigid body. I just thought it's common to just do what I was trying to do and have one object. I assumed it would perform better with fewer objects in the scene or have a smaller file size. $\endgroup$ Commented Jul 20, 2021 at 22:52
  • $\begingroup$ @adlowe07 Don't know if you're familiar with the concept of convex hulls in geometry, to say it simple what it means in this case is that the collision shape is the smallest closed envelope containing all vertices/edges/faces of your box. Which means everything within this boundary is inside, even though by deleting faces it's open. If you put the small cubes right on the edge where the faces are gone and "open space" begins, they might be outside the convex hulls and simulated correctly. But the shape that's needed to enclose the remaining cube vertices is the border. $\endgroup$ Commented Jul 21, 2021 at 7:38
  • $\begingroup$ @adlowe07 Some people say they always use Mesh because it's more accurate, but usually Box works for cuboids and Sphere for spheres etc. Actually I once read somewhere, the basic shapes for basic meshes are simulated faster or better because they are simpler. Now, Convex Hull works well for more complex meshes - if there is nothing to simulate inbetween the mesh, like something inside a cube, or maybe inside a bottle... or chain links that are connected. For these things you need Mesh, because by calculating the complete mesh and not only the surrounding, it also works with "holes". $\endgroup$ Commented Jul 21, 2021 at 7:46

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