Box2d star shape (and other shapes)

saeys · April 2018

Hi all,

I'm having an image of a star that i would like to behave just as the circles in the Collision Filtering example project, only it has the shape of a... well... star, not rolling circles. Maybe there is a way to make a box2d shape out of a sprite?

Can't make it a PolygonShape since max 8 vertices are allowed, nor a ChainShape since it must be a DYNAMIC_BODY.

hgy29 · April 2018

Do you need to check for collision the gap between the branches of your star ? if not then (assuming your star as less than 8 branches) you can use polygon shape (pentagon, hexagon, etc) for box2D. if there are more than 8 branches, then a circle collision shape would be a good approximation anyhow

saeys · April 2018

Well it's a five-pointed star, and my thought is to have several of those lying piled together. So in that case I would like to be able to check for collision between one star's branch and an other star's gap. Otherwise it may look strange in some cases.

So it isn't possible to check for collision at a sprite's boundaries or something like that?

Tested also to draw the PolygonShape vertices from edge to edge like a pentagram, but alas - Polygon should be convex and should have a CCW winding order, whatever that means.

saeys · April 2018

Found this: https://stackoverflow.com/questions/8543105/is-there-any-way-to-draw-the-concave-polygon-in-box2d-and-detect-the-collision-w

If this may be a solution, ie making five triangle shapes instead of the five pointed star and group the together in the body somehow, I'm not sure how to implement it in the code.

local function createStar(texture, x, y, filter)
	local body = world:createBody{type = b2.DYNAMIC_BODY, position = {x = x, y = y}}
 
	local shape = b2.PolygonShape.new()
	shape:set(29,0, 11,52, 56,20, 0,19, 46,52) -- polygon drawn CCW, but crosses itself creating an error
	body:createFixture{shape = shape, density = 1, restitution = 0.7, friction = 1.1, filter = filter}
 
	local sprite = Bitmap.new(Texture.new(texture, true))
	sprite:setAnchorPoint(0.5, 0.5)
	stage:addChild(sprite)
	actors[body] = sprite
end
 
local blueFilter = {categoryBits = 2, maskBits = 3}
 
createStar("star.png", 200, 0, blueFilter)

saeys · April 2018

Got it. Didn't expect it was possible to have several fixtures on the same body. So I created several fixtures in a row and changed the polygon shape before each fixture creation, into five triangles, forming a star:

	local shape = b2.PolygonShape.new()
	starFixtureProperties @ |shape = shape, density = 1, restitution = 0.5, friction = 1|
	shape:set(29,0, 39,16, 19,16)
	body:createFixture{starFixtureProperties}
	shape:set(56,20, 43,33, 39,16)
	body:createFixture{starFixtureProperties}
	shape:set(46,52, 28,44, 43,33)
	body:createFixture{starFixtureProperties}
	shape:set(11,52, 14,33, 28,44)
	body:createFixture{starFixtureProperties}
	shape:set(0,19, 19,16, 14,33)
	body:createFixture{starFixtureProperties}

May there be any problems with this solution that I haven't seen yet?

Also, the polygons were drawn CW (clockwise). If they were drawn CCW, it crashes with this slightly confusing error message:

Polygon should be convex and should have a CCW winding order.

pie · May 2018

@saeys have you tried PE?
https://www.codeandweb.com/physicseditor

saeys · May 2018

Looks interesting, thank you!

antix · May 2018

Maybe R.U.B.E (https://www.iforce2d.net/rube/)

totebo · May 2018

Interestingly I have seen that the latest Tiled supports polygons, so might be worth a look:

https://www.mapeditor.org

If you need something less tiley, I can second RUBE. I've used it since my first game and it's unbeatable as a Box2D editor.

Howdy, Stranger!

Categories

In this Discussion

Top Posters

Box2d star shape (and other shapes)

Comments

Howdy, Stranger!

Quick Links

Categories

In this Discussion

Top Posters

Box2d star shape (and other shapes)

Comments