Thrust in Box2D

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Davidobot
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Thrust in Box2D

Post by Davidobot »

So, I have a square that I want to apply force to so that the force applied is always perpendicular to the base of the square. How would one accomplish this? See the diagram:
Image
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vrld
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Re: Thrust in Box2D

Post by vrld »

Rotate the force vector according to the rotation of the shape: http://en.wikipedia.org/wiki/Rotation_m ... dimensions. In code:

Code: Select all

function rotate(rot, x,y)
    local s,c = math.sin(rot), math.cos(rot)
    return c*x - s*y, s*x + c*y
end
[shameless-plug]Or use hump.vector that includes this function.[/shameless-plug]
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chezrom
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Re: Thrust in Box2D

Post by chezrom »

Sorry vrld for your hump.vector library, but Box2D has this functions ...

You can use body:getWorldVector(local_x,local_y) => world_x,world_y to transform a vector is the body/local coordinate to the world coordinate.
The local vector is simple to determine because your square shape is always aligned with the axes of the local coordinates.

Also use body:getWorldPoint(local_x,local_y) to determine where you applicate your force (by default it is applied to the center of mass, that is different of the origin of the local coordinates).
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Davidobot
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Re: Thrust in Box2D

Post by Davidobot »

How would one use this with Body:applyForce?
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chezrom
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Re: Thrust in Box2D

Post by chezrom »

For example :

Code: Select all

body:applyForce(body:getWorldVector(local_fx,local_fy))
The force is applied to the centre of mass, as show in your drawing.
You can have, if your thrust is directed down so the reaction force is up local_fx = 0 and local_fy= - gravity * factor
You must call applyForce at each update when the thrust is here.
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Davidobot
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Re: Thrust in Box2D

Post by Davidobot »

chezrom wrote:For example :

Code: Select all

body:applyForce(body:getWorldVector(local_fx,local_fy))
The force is applied to the centre of mass, as show in your drawing.
You can have, if your thrust is directed down so the reaction force is up local_fx = 0 and local_fy= - gravity * factor
You must call applyForce at each update when the thrust is here.
Thank you! Works flawlessly.
PM me on here or elsewhere if you'd like to discuss porting your game to Nintendo Switch via mazette!
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