Jump to content
  • 0
Sign in to follow this  
animats

"Plane physics"

Question

OK, today's question as I try to push the limits of low-poly. Plane physics.

So I want to make a rectangular prim with one rectangular hole in it, as a floor with a hole for a stairway. Preferably at 1 LI.

So I make a cube in Blender, stretch it out, and use a Boolean to punch a square hole in it. Fine.

I bring that into SL, set it to Prim mode, and it works fine as long as it's 0.5m or more thick. Known bug/feature. But I don't want it that thick.

So I try a separate physics model, one plane with a hole in it. Prim physics mode. This works. Almost. Objects fall through the hole, but the physics plane is centered in Z in the middle of the visible object, even though it's on top of the object in Blender. Avatars and objects sink halfway through the floor.

The problem is that to export a physics model from Blender, you have to use "Export Selected" mode, and that turns out coordinates relative to the selection set. So the viewable model and physics model don't line up. No matter what Z height I use for the physics plane, it always ends up in-world centered at the Z height of the visible model. The physics engine did the right thing; this is Blender vs. the SL/Firestorm uploader. Uploading with

OK. So, I try two planes in the physics model, one on top, and one on the bottom, so physics model and visible model have the same bounds. Prim mode again. What do I get? A physics model that connects the two planes as bounds of a solid. The hole works only if the two planes are at least 0.5m or greater apart.

vanezorderproblem_012.thumb.png.797656da43cf716565709aacae4776a3.png

Left, two separate planes 0.5m apart. Right, 0.49m apart. On the right, the hole is impenetrable. Beq Janus's fix to Firestorm shows this correctly.

Yes, there are workarounds, like using multiple boxes that don't quite touch to build up the plane, but they all increase the land impact. The stuff above is 1 LI.

The frustrating thing is that it's a Blender and uploader limitation, not a physics engine problem. A plane with a hole in it has the right physics. It just won't be in quite the right place. You can use this trick with a thin wall, with a physics plane embedded in the middle of the wall. Floors, not so much, because your feet sink in.

For what it's worth, I put in a Blender bug report asking for layer information in .dae files. If the Blender crowd does that, the SL uploader could in future recognize layers, and we could get rid of having to export separate physics files. Perhaps someday.

 

 

Share this post


Link to post
Share on other sites

1 answer to this question

Recommended Posts

  • 0
4 hours ago, animats said:

OK, today's question as I try to push the limits of low-poly. Plane physics.

So I want to make a rectangular prim with one rectangular hole in it, as a floor with a hole for a stairway. Preferably at 1 LI.

So I make a cube in Blender, stretch it out, and use a Boolean to punch a square hole in it. Fine.

I bring that into SL, set it to Prim mode, and it works fine as long as it's 0.5m or more thick. Known bug/feature. But I don't want it that thick.

So I try a separate physics model, one plane with a hole in it. Prim physics mode. This works. Almost. Objects fall through the hole, but the physics plane is centered in Z in the middle of the visible object, even though it's on top of the object in Blender. Avatars and objects sink halfway through the floor.

The problem is that to export a physics model from Blender, you have to use "Export Selected" mode, and that turns out coordinates relative to the selection set. So the viewable model and physics model don't line up. No matter what Z height I use for the physics plane, it always ends up in-world centered at the Z height of the visible model. The physics engine did the right thing; this is Blender vs. the SL/Firestorm uploader. Uploading with

OK. So, I try two planes in the physics model, one on top, and one on the bottom, so physics model and visible model have the same bounds. Prim mode again. What do I get? A physics model that connects the two planes as bounds of a solid. The hole works only if the two planes are at least 0.5m or greater apart.

vanezorderproblem_012.thumb.png.797656da43cf716565709aacae4776a3.png

Left, two separate planes 0.5m apart. Right, 0.49m apart. On the right, the hole is impenetrable. Beq Janus's fix to Firestorm shows this correctly.

Yes, there are workarounds, like using multiple boxes that don't quite touch to build up the plane, but they all increase the land impact. The stuff above is 1 LI.

The frustrating thing is that it's a Blender and uploader limitation, not a physics engine problem. A plane with a hole in it has the right physics. It just won't be in quite the right place. You can use this trick with a thin wall, with a physics plane embedded in the middle of the wall. Floors, not so much, because your feet sink in.

For what it's worth, I put in a Blender bug report asking for layer information in .dae files. If the Blender crowd does that, the SL uploader could in future recognize layers, and we could get rid of having to export separate physics files. Perhaps someday.

 

 

I got a bit lost since I am just waking up --- BUT it has been known for at least five years that planes physics won't work on walls less than .5 so the obvious thing to do would be to use cube physics. Actually I almost always use cube physics. Also if you didn't, you need to apply rotation, location and scale to BOTH the main model and the physics model before uploading so that they line up. 

And if the picture on the left in blue is your planes physics model (and not the main model) it is WAY too complex. Physics models should be as simple as you can get --- so a series of quadrilateral pieces.  In planes physics they can overlap. Planes physics also HATES tiny pieces so you wouldn't have anything in the "inside four corner" area at all, just top and bottom.

Sorry if I misunderstood your post; I did reread :D.   And also note that the Firestorm view of physics is sometimes wrong (this may have been corrected in the latest FS release but with a few people reporting major physics issues I didn't install that update). So you need to test. Also, it COULD be the newest FS version issue if you have that installed.  

Going off to work on a roof so no testing from me today :D.   Good luck. 

 

 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×