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    Error using VertexHelper

    DikkesnoekDikkesnoek Member
    edited October 2012 in General questions
    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
    Screen Shot 2012-10-03 at 17.57.48.png
    1033 x 706 - 257K
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    Comments

    • ar2rsawseenar2rsawseen Maintainer
      It seems that coordinates are clock wise, you need to use counter clockwise order.
      Just pass them in reverse order. ;)
      http://appcodingeasy.com/Gideros-Mobile
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    • DikkesnoekDikkesnoek Member
      edited October 2012
      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
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    • atilimatilim Maintainer
      edited October 2012
      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. :)
      http://en.wikipedia.org/wiki/Cytorus
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    • DikkesnoekDikkesnoek Member
      Thanks Atilim. I am using the game template. I think that I first
      need to modify some code to let it work.

      Regards,

      Marc
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    • DikkesnoekDikkesnoek Member
      edited October 2012
      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
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    • ar2rsawseenar2rsawseen Maintainer
      @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? :)
      http://appcodingeasy.com/Gideros-Mobile
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    • DikkesnoekDikkesnoek Member
      edited October 2012
      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



      Screen Shot 2012-10-04 at 09.20.32.png
      970 x 649 - 443K
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