Wheel joint

[horacebury]

Trying to implement an axel joint and because the wheel joint does not provide the same limits and motor that the piston joint does I’m doing this with a pivot and a piston.

The two objects to be joined are connected via a middle object (the axel) and, unlike other joint types, this does not stop the two objects from colliding. This means that they need a collision filter or they will jitter all over the place.

Finally, because the two objects to be joined are joined via a third (the axel) the composite joint is now squishy.

Can anyone improve on this?

require("physics") physics.start() physics.setGravity( 0, 10 ) physics.setDrawMode("hybrid") local a = display.newRect( 400, 400, 800, 50 ) a.rotation = -35 local b = display.newRect( 600, 350, 50, 300 ) b.fill = {.5,.5,.5} physics.addBody(a,"static",{isSensor=false,density=.1, filter={ groupIndex=-1 }}) physics.addBody(b,"dynamic",{isSensor=false,density=.1, filter={ groupIndex=-1 }}) local ax, ay = a:localToContent( -400, 0 ) local bx, by = a:localToContent( 400, 0 ) local cx, cy = b:localToContent( 0, -150 ) local dx, dy = b:localToContent( 0, 150 ) local d = display.newGroup() d.x, d.y = 600, 260 physics.addBody( d, "dynamic", { radius=40, isSensor=true } ) local j = physics.newJoint( "piston", a, d, 600, 260, bx-a.x, by-a.y ) j.isLimitEnabled = true j:setLimits( -600, 200 ) local p = physics.newJoint( "pivot", d, b, 600, 260 )

[davebollinger]

just off the top of my head, haven’t run the code, but looks like your mass ratios might be wonky.  (a common cause of “squishy” joints)  try setting density of a and b to 1.0, and set d to 10.0 – just as experimental values to start from.  that ought to stiffen up the joints, but might be too extreme, maybe causing unwanted side-effects elsewhere, so experiment.

>> unlike other joint types, this does not stop the two objects from colliding

the non-collision of jointed bodies doesn’t “cascade” or “chain”, it’s only between the two explicit bodies involved.  so a doesn’t collide with d, nor does b collide with b, because they’re jointed.  but a will still collide with b because they’re not directly jointed.

>> require(“physics”)

>> physics.start()

argh!!  that SHOULDN’T work – it ONLY works because sloppy physics lib leaks a global (a personal pet peeve)

[davebollinger]

added:  OH!.. just saw it:  make d a proper circle, not an empty display group – THAT’S probably what’s causing wonky mass ratios!!

[horacebury]

• non-collision of jointed bodies doesn’t “cascade” or “chain”, it’s only between the two explicit bodies involved.

I don’t believe this to be the case - I’ve created multi-piece ropes or cables which have the upside of no piece colliding with any other piece. These are typically built with a series of pivot joints between, essentially, individual rectangles. That’s simple to build, so I won’t provide code, unless requested.

• argh!!  that SHOULDN’T work

Totally. Unfortunately, I’ve made it a habit!

• a doesn’t collide with d, nor does b collide with b,

Not sure what you mean, but if you’re saying that with 3 bodies jointed thusly: 1—2---3 then 1 and 3 would collide; As above, I have seen this break and it leads me to believe that there is a certain amount of cascading or inheritance.

• make d a proper circle

I’m convinced (from experience) that (other than with outline shapes) all physics bodies can be constructed on completely empty display groups; An image or shape is not required.

Thank you for your advice about the body densities. I think that was the trick. This works for me - note the continued use of the group, not circle:

local physics = require("physics") physics.start() physics.setGravity( 0, 10 ) physics.setDrawMode("hybrid") local a = display.newRect( 400, 400, 800, 50 ) a.rotation = -35 local b = display.newRect( 600, 350, 50, 300 ) b.fill = {.5,.5,.5} physics.addBody(a,"static",{isSensor=false,density=.1, filter={ groupIndex=-1 }}) physics.addBody(b,"dynamic",{isSensor=false,density=.1, filter={ groupIndex=-1 }}) local ax, ay = a:localToContent( -400, 0 ) local bx, by = a:localToContent( 400, 0 ) local cx, cy = b:localToContent( 0, -150 ) local dx, dy = b:localToContent( 0, 150 ) local d = display.newGroup() d.x, d.y = 600, 260 physics.addBody( d, "dynamic", { density=2, radius=40, isSensor=true } ) local j = physics.newJoint( "piston", a, d, 600, 260, bx-a.x, by-a.y ) j.isLimitEnabled = true j:setLimits( -600, 200 ) local p = physics.newJoint( "pivot", d, b, 600, 260 )

Thanks for your help 

[horacebury]

My apologies. Having just written the cable code from scratch, I see you’re right; The various pieces do in fact collide with each other unless given a collision filter…

local function touch(e) if (e.phase == "began") then e.target.hasFocus = true display.currentStage:setFocus(e.target) e.target.touchjoint = physics.newJoint( "touch", e.target, e.x, e.y ) if (e.target.touch) then e.target:touch(e) end return true elseif (e.target.hasFocus) then e.target.touchjoint:setTarget( e.x, e.y ) if (e.phase == "moved") then else e.target.hasFocus = nil display.currentStage:setFocus(nil) e.target.touchjoint:removeSelf() e.target.touchjoint = nil end if (e.target.touch) then e.target:touch(e) end return true end return false end local function testCable() local physics = require("physics") physics.start() physics.setGravity( 0, 0 ) physics.setDrawMode("hybrid") local group = display.newGroup() for i=1, 10 do local rect = display.newRoundedRect( group, 100+i\*50, 100, 50, 20, 10 ) physics.addBody( rect, "dynamic", { density=1 } ) if (i \> 1) then physics.newJoint( "pivot", group[i-1], group[i], 100+i\*50-25, 100 ) end rect:addEventListener( "touch", touch ) end end testCable()

[davebollinger]

just off the top of my head, haven’t run the code, but looks like your mass ratios might be wonky.  (a common cause of “squishy” joints)  try setting density of a and b to 1.0, and set d to 10.0 – just as experimental values to start from.  that ought to stiffen up the joints, but might be too extreme, maybe causing unwanted side-effects elsewhere, so experiment.

>> unlike other joint types, this does not stop the two objects from colliding

the non-collision of jointed bodies doesn’t “cascade” or “chain”, it’s only between the two explicit bodies involved.  so a doesn’t collide with d, nor does b collide with b, because they’re jointed.  but a will still collide with b because they’re not directly jointed.

>> require(“physics”)

>> physics.start()

argh!!  that SHOULDN’T work – it ONLY works because sloppy physics lib leaks a global (a personal pet peeve)

[davebollinger]

added:  OH!.. just saw it:  make d a proper circle, not an empty display group – THAT’S probably what’s causing wonky mass ratios!!

[horacebury]

• non-collision of jointed bodies doesn’t “cascade” or “chain”, it’s only between the two explicit bodies involved.

I don’t believe this to be the case - I’ve created multi-piece ropes or cables which have the upside of no piece colliding with any other piece. These are typically built with a series of pivot joints between, essentially, individual rectangles. That’s simple to build, so I won’t provide code, unless requested.

• argh!!  that SHOULDN’T work

Totally. Unfortunately, I’ve made it a habit!

• a doesn’t collide with d, nor does b collide with b,

Not sure what you mean, but if you’re saying that with 3 bodies jointed thusly: 1—2---3 then 1 and 3 would collide; As above, I have seen this break and it leads me to believe that there is a certain amount of cascading or inheritance.

• make d a proper circle

I’m convinced (from experience) that (other than with outline shapes) all physics bodies can be constructed on completely empty display groups; An image or shape is not required.

Thank you for your advice about the body densities. I think that was the trick. This works for me - note the continued use of the group, not circle:

local physics = require("physics") physics.start() physics.setGravity( 0, 10 ) physics.setDrawMode("hybrid") local a = display.newRect( 400, 400, 800, 50 ) a.rotation = -35 local b = display.newRect( 600, 350, 50, 300 ) b.fill = {.5,.5,.5} physics.addBody(a,"static",{isSensor=false,density=.1, filter={ groupIndex=-1 }}) physics.addBody(b,"dynamic",{isSensor=false,density=.1, filter={ groupIndex=-1 }}) local ax, ay = a:localToContent( -400, 0 ) local bx, by = a:localToContent( 400, 0 ) local cx, cy = b:localToContent( 0, -150 ) local dx, dy = b:localToContent( 0, 150 ) local d = display.newGroup() d.x, d.y = 600, 260 physics.addBody( d, "dynamic", { density=2, radius=40, isSensor=true } ) local j = physics.newJoint( "piston", a, d, 600, 260, bx-a.x, by-a.y ) j.isLimitEnabled = true j:setLimits( -600, 200 ) local p = physics.newJoint( "pivot", d, b, 600, 260 )

Thanks for your help 

[horacebury]

My apologies. Having just written the cable code from scratch, I see you’re right; The various pieces do in fact collide with each other unless given a collision filter…

local function touch(e) if (e.phase == "began") then e.target.hasFocus = true display.currentStage:setFocus(e.target) e.target.touchjoint = physics.newJoint( "touch", e.target, e.x, e.y ) if (e.target.touch) then e.target:touch(e) end return true elseif (e.target.hasFocus) then e.target.touchjoint:setTarget( e.x, e.y ) if (e.phase == "moved") then else e.target.hasFocus = nil display.currentStage:setFocus(nil) e.target.touchjoint:removeSelf() e.target.touchjoint = nil end if (e.target.touch) then e.target:touch(e) end return true end return false end local function testCable() local physics = require("physics") physics.start() physics.setGravity( 0, 0 ) physics.setDrawMode("hybrid") local group = display.newGroup() for i=1, 10 do local rect = display.newRoundedRect( group, 100+i\*50, 100, 50, 20, 10 ) physics.addBody( rect, "dynamic", { density=1 } ) if (i \> 1) then physics.newJoint( "pivot", group[i-1], group[i], 100+i\*50-25, 100 ) end rect:addEventListener( "touch", touch ) end end testCable()