add consideration for 2d physics center_of_mass #29867

Raphael2048 · 2019-06-18T11:52:17Z

add consideration for 2d physics center_of_mass
This should close #12353

doc/classes/Physics2DDirectBodyState.xml

bojidar-bg · 2019-06-18T16:56:00Z

Doesn't calculating the center of mass from the shapes (instead of assuming it is at (0,0)) break compatibility?

Raphael2048 · 2019-06-19T06:18:44Z

It does, but in mostly cases, Rigidbody2D's center is at (0, 0).

capnm · 2019-06-21T09:42:25Z

There should be an explicit center of mass relative to the origin.
If set, this calculation shouldn't be done.

Raphael2048 · 2019-06-21T11:35:38Z

I'll try to add a method to do this after this commit is merged.
As this issue mentioned #7136

capnm · 2019-06-21T12:13:25Z

Could you perhaps include the first step and add a boolean option, e.g. 'relative to origin' by default true? That wouldn't break the compatibility.
I suspect that this is very error prone. Have you tested what happens when you change the scale or shape dynamically?

Raphael2048 · 2019-06-21T12:40:24Z

It's same as physic 3d's behavior. I think it's not needed to add it, because you will never want to shift your shape offset origin in fact. I have tested dynamically changing it.

capnm · 2019-06-21T14:15:37Z

Sorry, it's called origin in Blender, here it is the Rigidbody2D Transform. Until this PR, you could just use the Rigidbody2D transform to manually set the center of mass wherever you wanted, but you'll break this if you don't make it optional.
Personally, I think for non-trivial 2D objects, the homogeneous shape area is mostly useless here.

hubbyist · 2019-06-21T15:02:33Z

If options included "at origin", "at centroid", "at offset". When third is the default incompatibility may be avoided.

Xrayez · 2019-06-28T21:22:04Z

Well actually I've attempted to create a fix similar to this from 3D, fixed it to some extent, but the movement was weird, that's why I didn't make a PR in the first place, not sure what's different between this PR and:

https://github.com/Xrayez/godot/commit/207ea7d5be3c32df9055b341aeec8c278bc9543e

Seems like I didn't compute custom area, will need to test this then. 🙂

Xrayez · 2019-06-28T22:25:24Z

Tested, seems to work fine, not sure about the third case, perhaps the center of mass must be computed manually for polygon shapes?

If this is the case, having to override center of mass would be indeed useful.

center-of-mass-2d.zip

capnm · 2019-06-30T19:18:51Z

An another example where it went terribly wrong ;-)

Raphael2048 · 2019-07-01T04:50:42Z

I have updated my PR to take polygon into consideration. @Xrayez

servers/physics_2d/body_2d_sw.cpp

Xrayez · 2019-11-03T11:11:01Z

servers/physics_2d/shape_2d_sw.cpp

+real_t ConvexPolygonShape2DSW::get_area() const {
+
+	if (point_count < 3) return 0;
+
+	real_t signed_area = 0;
+	for (int i = 0; i < point_count; i++) {
+		Vector2 v1 = points[i].pos;
+		Vector2 v2 = points[(i + 1) % point_count].pos;
+		signed_area += v1.x * v2.y - v2.x * v1.y;
+	}
+	signed_area *= 0.5;
+	return ABS(signed_area);
+}
+
+Vector2 ConvexPolygonShape2DSW::get_shape_center_offset() const {
+
+	if (point_count < 3) return Vector2();
+
+	Vector2 centroid;
+	real_t signed_area = 0.0;
+	real_t a = 0.0;
+
+	for (int i = 0; i < point_count; i++) {
+		const Vector2 &v1 = points[i].pos;
+		const Vector2 &v2 = points[(i + 1) % point_count].pos;
+
+		a = v1.x * v2.y - v2.x * v1.y;
+		signed_area += a;
+		centroid += (v1 + v2) * a;
+	}
+
+	signed_area *= 0.5;
+	centroid /= (6.0 * signed_area);
+
+	return centroid;
+}
+


@akien-mga it would be better if #30222 and #30251 could be merged and used here directly as Geometry methods instead of "hardcoding" here, @raphael10241024 would need to rebase later once merged. 🙂

How should I convert points to a Vector?

Xrayez

Tested again with polygon center computations, fixes the third case on the GIF above.
I've also tested this on a project of mine with some heavy usage of polygons, works alright even with local (per project) center of mass workarounds, doesn't seem to break compat. The rigid bodies falling look even better subjectively. 🎉

Xrayez · 2019-11-03T11:19:33Z

The ability to change the center of mass is for another topic I guess, I'm not sure if changing the center offset is the proper way to simulate center of mass physics currently. I mean, if you do offset the shapes, you still get some weird simulation going on. If this does break compat, it might be the case for people already relying on the not-so-well-working-feature as a feature.

Xrayez · 2020-05-28T09:38:57Z

I wonder whether this has any chance to be merged at this point, because @reduz planned to rework this in 4.1 or so... (Waiting for Godot 😛).

I could work on salvaging this in case @raphael10241024 abandoned the PR.

aaronfranke · 2020-06-30T21:41:46Z

@raphael10241024 Is this still desired? If so, it needs to be rebased on the latest master branch.

If not, abandoned pull requests will be closed in the future as announced here.

aaronfranke · 2020-07-01T04:38:07Z

@raphael10241024 All of the CI builds failed because you are accessing a private variable.

servers/physics_2d/body_2d_sw.h: In member function 'virtual Vector2 PhysicsDirectBodyState2DSW::get_total_gravity() const':
servers/physics_2d/body_2d_sw.h:353:59: error: 'Vector2 Body2DSW::gravity' is private within this context
  353 |  virtual Vector2 get_total_gravity() const { return body->gravity; } // get gravity vector working on this body space/area
      |                                                           ^~~~~~~

aaronfranke · 2021-01-20T06:41:35Z

@raphael10241024 Is this still desired? If so, it needs to be rebased on the latest master branch.

If not, abandoned pull requests will be closed in the future as announced here.

YuriSizov · 2021-09-01T20:17:26Z

I'm going to close this as superseded by #49610. Thanks for the contribution nonetheless!

Raphael2048 requested a review from a team as a code owner June 18, 2019 11:52

Raphael2048 force-pushed the add_2d_center_of_mass branch 2 times, most recently from 1860c1e to 1bb74d7 Compare June 18, 2019 12:53

akien-mga added bug topic:physics labels Jun 18, 2019

akien-mga added this to the 3.2 milestone Jun 18, 2019

akien-mga reviewed Jun 18, 2019

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Raphael2048 force-pushed the add_2d_center_of_mass branch from ab2bc82 to 781bdeb Compare July 1, 2019 04:46

Xrayez suggested changes Jul 1, 2019

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akien-mga modified the milestones: 3.2, 4.0 Nov 26, 2019

aaronfranke added needs work topic:2d labels Jan 20, 2021

Raphael2048 requested review from aaronfranke, AndreaCatania, bruvzg, hpvb, JFonS, neikeq, reduz and vnen as code owners January 26, 2021 08:53

Raphael2048 force-pushed the add_2d_center_of_mass branch from d08608d to f9fbe86 Compare January 26, 2021 09:15

add considertion for 2d center of masss

3c8b6dd

Raphael2048 force-pushed the add_2d_center_of_mass branch from f9fbe86 to 3c8b6dd Compare January 26, 2021 09:20

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pouleyKetchoupp mentioned this pull request Jun 14, 2021

Proper support for custom mass properties in 2D/3D physics bodies #49610

Merged

YuriSizov closed this Sep 1, 2021

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add consideration for 2d physics center_of_mass #29867

add consideration for 2d physics center_of_mass #29867

Raphael2048 commented Jun 18, 2019

bojidar-bg commented Jun 18, 2019

Raphael2048 commented Jun 19, 2019

capnm commented Jun 21, 2019

Raphael2048 commented Jun 21, 2019

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add consideration for 2d physics center_of_mass #29867

add consideration for 2d physics center_of_mass #29867

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Raphael2048 commented Jun 18, 2019

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