My question is pretty simple yet I cannot find an answer anywhere in documentation or on stack.
I am working on a game in Corona SDK and I have created a physics body that will be thrown across the screen. The physics body is an image. The object is meant to be a projectile and I would like to use the natural physics inherent to Corona to govern how the object reacts to being thrown across the screen. In order to do this it would be ideal for the object's center of mass to be at the nose/point of the object, somewhat like a badminton shuttle.
The question is: is there a way to set a custom center of mass of a physics body in Corona?
I have figured out how to do this, so I am posting my solution here so that others may find it.
Corona does not have a method or property that can be used to change the center of mass for a particular object.
The best way to change the center of mass of an object is to create another object with a heavier density than the original, position Object B at the x,y of the desired position for the center of mass, then make object B's density greater than Object A. Then using a weld joint, join the two objects and thereafter apply any linear forces to the center of mass of Object B. Object B will then drag Object A along as it is rigidly attached via the weld joint.
Related
I have seen numerous AR application behaving like this: there is a dot in the middle of the screen and we can position that dot on some objects and some content is displayed (I attacked an image if i was not clear enough). My question is how is this kind of behaviour obtained in Unity, my guess is that from that point you cast a ray, but I don't think that AR placed objects, from an ADF for example, can be found with the hit from that ray. The dot selecting objects placed on AR
I have made it work, with the aid of the Google Tango's Area Learning demo scene. I've placed some objects in the area and I have started to send a Raycast from the middle of the camera with "ViewportPointToRay" method. When that Ray would collide with a GameObject you can implement whatever functionality you need.
I am currently working on a 2D endless runner, written in Swift3 and using Spritekit.
Short question:
Is there a way to only check for collisions on the right side of my character's rectangular physics body?
More info:
The platform on which the character runs is made of puzzle pieces and the user builds upon it as the game progresses. The character progresses left to right, in respect to the background (which goes right to left).
I want the character to automatically jump when he collides with a piece on his right side. However, any pieces that the player puts to the right of him (same Y value as the character) is of the same class as the pieces underneath him.
So the same code that checks for collision between the character and pieces to his right, and make him jump, will also make him jump as long as the game detects collision between the character and the pieces under him.
I have not been able to find another problem like mine, since usually others' characters are colliding with objects of different classes from their ground class.
Thanks!
P.S. I have tried to make my character a SKSpriteNode with two physics bodies, but I could not find any helpful documentation. If it helps any, my character also performs a looping running animation--though I can't imagine that would harm anything.
You could achieve that by detecting collisions with your rectangle and then deciding whether the collision was with the side of your interest or not. Here is a discussion about how to do that. Good luck!
Have you tried adding a non visible sub node (e.g. feetNode) to your character's sprite node and giving that sub node the physics body (class) for floor contact ?
Depending on the rest of your logic, it may allow you to use a different physics class for your character have more flexibility in collision detection.
In fact, you could probably use that approach with several sub nodes in your character's sprite node and have multiple collision behaviours for the character depending on what hits it.
Once you obtain the contact point of the 2 bodies that are colliding, determine which body is the one that is colliding by checking the categorymasks and then check its CGPoints x position. This x position can be compared to the other body's x position to know exactly which side it is colliding from.
if Body A's x position > Body B's x position, Body A is on the right and if not, its on the left.
As simple as that.
Hope this helps!
Can you tell me how to do this, and if not help me solve this problem? Is it possible to set the new anchor points outside out of the object? I am trying to get an object to rotate around another object. They are physics objects, so I cannot do a transparency trick. Any ideas? Thanks!
You could put the two objects in a display group and set the group's anchor point to be at the center of your main object. The code at http://coronalabs.com/blog/2013/10/15/tutorial-anchor-points-in-graphics-2-0/ should be sufficient. See if that will do the trick for you. Note though that you will have to apply a negative rotation to the main object because you don't want it turning too. If several objects are "in orbit" around main object at different speeds then this trick won't work.
I'm pretty new to Box2D and cocos2d. I'm trying to do something which I thought would be pretty simple, but it is turning out to be more difficult than I had expected and I cannot find a working answer anywhere.
Basically, I want to move a b2body and rotate it to a certain point with animation.
I can get it to the correct position and rotation with:
targetBody->SetTransform(b2Vec2(10.0f,1.0f),10);
But I have no idea how to animate it there over time. I tried using cocos2d animation on the sprite used for the body, but that doesn't do anything. Any ideas?
There are a couple of ways you could do this.
You could use SetTransform every time step to update the position of the body gradually over time, in effect performing the animation yourself. The drawback with this method is that the body is 'teleporting' to the new position rather than moving, so it has no momentum, which means you can get odd results if it hits anything along the way. Still, if you know it will not hit anything or does not need to react to a collision this would be ok.
The other way is to SetLinearVelocity and SetAngularVelocity to give the body proper movement. This will keep the results of a collision realistic, and you don't need to keep updating anything every timestep. On the other hand, you will need to detect when the body has arrived at the desired position and then set the velocities back to zero, otherwise it will just keep moving. For dynamic bodies you will also need to counter gravity somehow, either by setting the gravity scale of the body to zero, or by applying an upwards force to balance gravity, or by changing the body type to kinematic during the move.
In general, you use Box2D to simulate the physical behavior of objects in relation to each other. The rules of mechanics implemented by Box2D dictate how your cocos2d CCSprites move if you continuously update the translation and rotation of your sprites according to their corresponding Box2d b2Body. You will have some kind of repeatedly invoked tick: method in which you step your Box2d world along in time, and in which you update your sprite positions according to simulation results of Box2d.
This pattern corresponds to b2Bodys of type b2_dynamicBody. Physical laws dictate the motion of the body in this case, and your sprites will follow these simulation results. This is why setting a conflicting position of a sprite by means of a CCAction or even directly will be undone almost instantaneously with the next tick:.
Solution 1: kinematic body type
However, there do exist other modes for a b2Body, and one of these is b2_kinematicBody in which the translation is no longer governed by the world but by the velocities or angular speeds you dictate through setters. So it would be one solution to work with body type b2_kinematicBody as in:
b2BodyDef bdef;
bdef.type = b2_kinematicBody;
With this you would manipulate the velocity and angular speed of a b2Body explicitly. In this case, Box2d is still responsible for moving bodies around, but according the velocities you dictate, and without any force effects of collision or gravity applied to this particular body. Also with this strategy, your CCSprites will follow b2Bodys. Setting conflicting positions for your sprites directly or by applying a CCAction would not make sense for the same reason as described above.
Solution 2: decouple sprites from Box2d
An alternative way to animating sprites would be to fully decouple those sprites from Box2d. In this case, you would simply not have any b2Body that governs the position of the sprite you are going to animate. Now, in this case, only you will dictate the position and rotation of your CCSprite, i.e. directly either through its position property or by applying a CCAction.
Recently I have a new project that uses Box2D as physics engine. And I am having some trouble with the body's position.
I want to draw images on the sprite, so I just use body.GetPosition as image's position. But I found that body.GetPosition returns the same value as body.GetWorldCenter, and I just wondering that I did something wrong.
I use box2d flash 2.1a in my project. And I currently subtract this position with half-width and half-height to walk around. Also, can you tell me if there is some way to solve my problem or draw the image in the correct positon in Box2D.
Thank you.
update
so sorry to post before read manual.
after RTM I got that body has two points,first is origin point and the second is center of mass ,so a regular shape's world center is always the same as it's origin point. sorry to post is.
Have you set the image in the userdata for the body? Your getting the body's position but you need to have a function that runs each world tick that can set the images position to the new location. Box2d is just numbers defining a simulation. Those numbers are constantly updating so the image needs to have position set each time there is a game tick. Hope this helps