Error using VertexHelper

Dikkesnoek

Hello my friends,

I am trying to create a PolygonShape like this:

local landingGround = levelSelf.world:createBody({}) landingGround.name = "landingGround" local polygonShape = b2.PolygonShape.new() polygonShape:set(40.0, -163.0, 186.5, -95.0, 478.5, -82.0, 478.5, -202.0, 38.5, -163.5)   landingGround:createFixture({shape = polygonShape, density = 0})

I will get an error: "Polygon should be convex and should have a CCW winding order". I've copied
the values directly from VertexHelper (see the attached screenshot). As you see, the Polygon is
convex. Any ideas?

Regards,

Marc

ar2rsawseen

It seems that coordinates are clock wise, you need to use counter clockwise order.
Just pass them in reverse order.

Dikkesnoek

Thanks. Weird that VertexHelper will generate the data in CW order.
When I reverse the vertices:

polygonShape:set(38,-163, 478,-202, 478,-82, 186,-95, 40,-163)

I will get the same error.

Marc

atilim

Remove one of the (40.0, -163.0) or (38.5, -163.5). They are very near to each other and make the polygon concave.

Edit: If removing one of these vertices doesn't solve the problem, then pass them in reverse order.

Dikkesnoek

Thanks Atilim. I am using the game template. I think that I first
need to modify some code to let it work.

Regards,

Marc

Dikkesnoek

Is there any additional information how to use the Polygonshape with
more vertices? I've got it work but not with the coordinates generated
by VertexHelper. I have to invert the Y values and rearrange the sequence
to get it work. Also which position values do you give a Polygonshape's
body?

Marc

ar2rsawseen

@Dikkesnoek

all the coordinates for PolygonShape are relative, it means you can, for example, select one of the coordinate as 0, and provide other accordingly to the distance from this 0 coordinate. Then By setting position of the body that wears this PolygonShape, you can move it and position where you need it. It's similar using Shape object in Gideros.

Can you post some code or/and screenshot how it looks for you now?

Dikkesnoek

Here it is. It's a test. As you can see in the first screenshot above I want to
create a shape on top of a part of the rock. I test on a 960x640px resolution.
When I open the picture in VertexHelper (I don't know if there is an other
to to do this) and draw the shape I will get the following coordinates:

38,-163,
478,-202,
477,-82,
186,-95

As I remember from Cocos2D and Box2D, that vertices will be specified from
the center of the picture. (0, 0) will be at (480, 320). To get it work in Gideros
you will need to invert the Y-coordinates and reorder it to meet the CCW rule:

478,202,
38,163,
186,95,
477,82

This is not really clear to me. Also how to position the shape. This is what I
did:

local body = levelSelf.world:createBody{type = b2.STATIC_BODY} body:setPosition(480, 320) shape = b2.PolygonShape.new() shape:set(478,202, 38,163, 186,95, 477,82) body:createFixture{shape = shape, density = 1, restitution = 0.2, friction = 1}

See the result in the attached screenshot.

I think that the use of Physics is still quiet complicated to a lot of people. The
manuals are still not ready on this point. Is it an idea (I wan't to help if possible)
to create a graphical example of a screen and show what a world, body, fixture,
shape, etc. is. This could make it very clear.


Regards,

Marc