cocos2d for iPhone

A fast, easy to use, free, and community supported 2D game engine

1. 

   alvinlin123
   Member

   I have used cocos2d before, but am very new to cocos3d. I am trying to make an app that will rotate an object as user drag around the screen.

   For proof of concept, I am using the POD model comes with the example, namely the introducintPOD_float.pod. I can display the model no problem, but when I try to rotate the model along the X-axis by setting the "rotate" to (45.0, 0.0, 0.0), the model rotates around it's base. Is there any way to change the rotate behaviour so that the model will rotate around its center along the X-Axis?

   I mean in cocos3d you can set the anchor to (0.5, 0.5) so that a spriet rotates around it's center instead of corner. Is there a similar way to do it in cocos3d?

   Thanks in advance!

   Posted 11 months ago #
2. 

   Bill Hollings
   Moderator

   @alvinlin123

   In the 3D world, models are positioned and rotated based on the origin of your model's vertices (ie- (0,0,0) in your model's local coordinate system).

   This is defined by how you lay the model out in your 3D editor.

   For example, the CC3DemoMashUp demo app contains many different models. The vertices of the beachball are arranged so that (0,0,0) is in the center of the ball (which makes sense for a ball). The vertices of the teapot are arranged so that (0,0,0) is at the center of the bottom of the teapot (I guess so that it can easily be made to "sit" on a tabletop or something). So these two models will rotate differently.

   In cocos3d, to define a different rotation or location origin, create a CC3Node and add your model node to it as a child node. Set the location of the child node to an offset that will move the point on the child that you want to rotate around to the local origin of the parent CC3Node.

   Taking the teapot as an example, if the center bottom of the teapot is (0,0,0) and the center top of the teapot has a vertex (0, 1, 0), then if you set the location of the teapot as (0, -0.5, 0), and add it to another CC3Node, the center of the teapot will now be at the origin of the parent CC3Node.

   If you will be applying scaling, do this in the native scale of the child (teapot), and then perform any scaling you need to do using the parent node. Don't scale the teapot, scale the parent CC3Node, so that the teapot always stays the same relative to the parent.

   Now, rotate the parent node, which will rotate around its own origin, and will carry the teapot with it as it rotates. Since the teapot is positioned so that its geometrical center is at the origin of the parent node, the teapot will rotate around its geometrical center now.

   You can use the same technique for rotating any object, including lights and the camera. This technique is also used for making orbiting cameras, by using a parent CC3Node as a "boom" and locating the camera somewhere on the boom and then rotating the boom. The parent boom node will rotate around its origin, and carry the camera, which is located away from the boom's origin around with it as it rotates. In the CC3DemoMashUp, this is how the rainbow teapot rotates around the textured teapot. For fun, you can try replacing the rainbow teapot with the camera and see what happens.

   ...Bill

   Posted 11 months ago #
3. 

   alvinlin123
   Member

   Thanks for the lighting fast reply Bill!

   Posted 11 months ago #
4. 

   backboy_15
   Member

   Hi Bill Hollings,
   I am very very sorry that i cant understand the calculation.what i did is

   CC3Node *groundNode=[[[CC3Node alloc] initWithName:@"ground"] autorelease];
   	CC3MeshNode* earth = (CC3MeshNode*)[self getNodeNamed: @"Sphere"];
   	[earth setLocation:cc3v(0,0,0)];
   	[groundNode addChild:earth];
   	[groundNode setLocation:cc3v(0,0,296)];
   	[self addChild:groundNode];
   	CCActionInterval* partialRot = [CC3RotateBy actionWithDuration: .60 rotateBy: cc3v(30, 0.0, 0.0)];
   	[earth runAction: [CCRepeatForever actionWithAction: partialRot]];

   I want my earth to be positioned in cc3v(0,0,296) and rorate around its own x axis... but :((

   Posted 8 months ago #
5. 

   Bill Hollings
   Moderator

   @backboy_15

   If the mesh of your earth is centered at its own origin (0,0,0), and most sphere's are, then you can just set its location property to (0, 0, 296). You can either add it to the ground or to the world.

   This thread concerns the case where your model is NOT centered at its own origin (0,0,0). You would then add another CC3Node, add the earth to it with an offset to move its center to the (0,0,0) location, and add the new CC3Node to the world.

   The other option when the model is not centered on its own origin is to adjust the mesh itself so that it is. You can use the movePivotToCenterOfGeometry method to do that.

   If you don't need to offset the center of your model, then you can ignore this thread.

   And are you sure you want to rotate around the X-axis? Remember that the default is to have the Y-axis as the up/down axis.

   ...Bill

   Posted 8 months ago #

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