Weightless Physics object

[bartbes]

That is very much a bug, feel free to report it.

[ghostwriter]

Reported, Issue #167

[tentus]

The solution by ghostwriter indeed works, and is now used in multiple places in Kurosuke! Thanks man!

[kikito]

I've done some research on this.

Zero mass is static for Box2D. But can't you just set a very low mass instead? After all, even light has mass (albeit practically 0).

Actually, Box2D has been able to move objects with 0 mass for some time:

b2_kinematicBody

A kinematic body moves under simulation according to its velocity. Kinematic bodies do not respond to forces. They can be moved manually by the user, but normally a kinematic body is moved by setting its velocity. A kinematic body behaves as if it has infinite mass, however, Box2D stores zero for the mass and the inverse mass. Kinematic bodies do not collide with other static or kinematic bodies.

It's just that the version bundled with LÖVE is from before kinematic bodies were included. Which is a shame, because kinematic bodies seem exactly what tentus needs for his platforms.

There's an issue to update box2d to 2.1.x, but it's on hold.

By looking at the box2d source code currently used in LÖVE, it seems that there are only two kinds of bodies: "static" and "dynamic". A body is labelled as "static" if both the inverses of its mass and inertia are 0.

So I tried creating an object with mass=0 and inertia=not-0, and managed to move the object a bit, but it only moved when it hit other objects, and not in the direction that I wanted. Press any key and while the two bodies collide to see the effect:

Code: Select all

function love.load()
   world = love.physics.newWorld(0, 0, 800, 600)
   world:setGravity(0, 10)

   body1 = love.physics.newBody(world, 200, 200, 0, 1)
   shape1 = love.physics.newRectangleShape(body1, -20, -20, 40, 20)
   
   body2 = love.physics.newBody(world, 200, 50)
   shape2 = love.physics.newRectangleShape(body2, -10, -10, 20, 20)
   body2:setMassFromShapes()

end

function love.draw()
   love.graphics.polygon("fill", shape1:getPoints())
   love.graphics.polygon("line", shape2:getPoints())
   if pressed then
      love.graphics.print("Pressed", 8, 8)
   end
   if body1:isStatic() then
     love.graphics.print("Static", 8,20)
   end
end

function love.update(dt)
   if pressed then body1:setLinearVelocity(10, 0) end
   world:update(dt)
end

function love.keypressed(key, unicode)
   pressed = true
end

I doubt this could be used for making platforms, though.

[Taehl]

As an alternate solution: What if you made a large, invisible box that nothing except the platform collides with. Then you just slide your platform around on top of it. Since the player can't touch or see it, they'd never know it was there.

[tentus]

As an alternate solution: What if you made a large, invisible box that nothing except the platform collides with. Then you just slide your platform around on top of it. Since the player can't touch or see it, they'd never know it was there.

Because that locks you to a single horizontal axis. Platforms can move up and down, or at angles. If I was shooting for just left-to-right, I'd just use body:setY() every frame, using the starting Y position.

Actually, how would you do a box that only collides with platforms? Toggle its sensorness on every collision?

I also have other floating objects, such as birds (well, microbes that float, but lets call them birds). Platforms were the simplest example of a weightless object.

@kikito:
You make me sad, knowing that the perfect solution is just out of reach. Ah well, maybe in a future version of Love.

[Robin]

Actually, how would you do a box that only collides with platforms? Toggle its sensorness on every collision?

No, use shape:setMask().

[Taehl]

Ah, I thought it was horizontal platforms that were causing you problems. Oh well.

[tentus]

Actually, how would you do a box that only collides with platforms? Toggle its sensorness on every collision?

No, use shape:setMask().

Ah! I have not used masks yet, I should acquaint myself. Let's see... you would use setCategory(x) on the invisible box, and then setMask(x) on the the platform, right? Given that both xes are the same number of course... and there are at most 16 possible categories, correct?

I love getting answers I hadn't even considered, it keeps my brain active.