Move a physical ball with the touch

[jake1987.jj]

hi guy!

I have a physical ball and I would like to do this:

As the user moves his finger across the screen, the ball dies accordingly.

The ball must have the direction of touch and power (based on user stroke speed)

I know that to do this I have to use a physical impulse but what I do not know is the math to insert in the touch event.

Here is the video of a game that gives an idea of what I want to do.(0:12-0:25):

https://www.youtube.com/watch?time_continue=21&v=D2cItHrlUHU

can you give me a hand?

[roaminggamer]

Try a touch joint:

• https://docs.coronalabs.com/api/library/physics/newJoint.html
  • https://docs.coronalabs.com/guide/physics/physicsJoints/index.html
    • https://docs.coronalabs.com/guide/physics/physicsJoints/index.html#touch

[jake1987.jj]

Hi @roaminggamer

1.wow! Thank you so much for the quick response

2.My fault. I did not say that I would not use the joints. For two reasons:

• I think that with math I could manage some aspects
• As you can see in the video the ball stays still until the user finishes a stretch. if I used a joint and did:“tmpJoint:setTarget( event.x, event.y )” in moved phase then the ball would move immediately but I want it to move only when the user has finished. if instead I do it only in the final phase the ball may not have the thrust that…

[roaminggamer]

Actually, I just downloaded tigerball and a touch joint won’t help.

The essential mechanic steps are:

• touch begin:
  • collect t0 = system.getTimer() (Note: event has a time in it so you can use that too)
• touch end:
  • t1 - system.getTimer() (Note: event has a time in it so you can use that too)
  • dt = (t1 - t0)/1000
  • dx = event.x - event.xStart 
  • dy = event.y - event.yStart 
  • distance = math.sqrt( dx * dx + dy * dy )
  • factor = choose your own value
  • speed limit = choose your own value
  • speed = factor * distance/dt
  • if speed > speed limit, cap at speed limit
  • vec = normalize the vector <dx,dy>
  • scale the vector vec = <vec.x * speed, vec.y * speed>
  • apply a linear impulse to the ball center with magnitude <vec.x , vec.y>

[jake1987.jj]

Thanks really fantastic. Just what I wanted!

I did a test but there is something I do not understand.

local ball = display.newCircle( 160, 100, 20 ) ball:setFillColor( 1, 0, 0 ) physics.addBody(ball, "dynamic", {radius=ballRadius}) local t0 local t1 local factor = 1 local speedLimit = 10 function ball:touch( event ) if(event.phase=="began")then display.getCurrentStage( ):setFocus( self ) self.isFocus = true t0 = system.getTimer() elseif(self.isFocus)then if(event.phase=="moved")then elseif((event.phase=="ended")or(event.phase=="cancelled")) then display.getCurrentStage( ):setFocus( nil ) self.isFocus = nil t1 = system.getTimer() local dt = (t1 - t0)/1000 local dx = event.x - event.xStart local dy = event.y - event.yStart local distance = math.sqrt( dx \* dx + dy \* dy ) local speed = factor \* distance/dt if (speed \> speedLimit)then speed = speedLimit end local vecX, vecY = math.abs(dx), math.abs(dy) local impulseX, impulseY = vecX\*speed, vecY\*speed self:applyLinearImpulse( self.x, self.y, impulseX, impulseY ) end end end ball:addEventListener( "touch" )

if I for example:

1.the event begins

2.I execute the stroke

3.I wait a few seconds

4.release

the ball receives a huge impulse instead of a low one

Did I miss something?

[roaminggamer]

That sounds wrong.  If you wait to lift your finger, dt should be very large making the speed (impulse magnitude very small).

I’ll look over your code again in a moment.

Tip: Paste to notepad and replace tabs with three spaces before then copying and pasting the modified code here.  It makes the code more legible.

[roaminggamer]

• You didn’t normalize correctly.

• Your call to applyLinearImpulse is backwards, you have the forces in the wrong place: 

  object:applyLinearImpulse( xForce, yForce, bodyX, bodyY )

• You didn’t include the object mass.  (If you don’t do this, then later change the size of the ball you’ll need to re-balance your factor calculation.)

• I made some changes to the original recipe.

https://github.com/roaminggamer/RG_FreeStuff/raw/master/AskEd/2018/06/tigerBall.zip

local ballRadius = 20 local ball = display.newCircle( cx, bottom - 80, ballRadius ) ball.factor = 0.06 ball.limit = 50 ball:setFillColor( 1, 0, 0 ) physics.addBody( ball, "dynamic", { radius = ballRadius, bounce = 0 } ) ball.linearDamping = 1 function ball.enterFrame(self) display.remove(self.line) if( not self.lx ) then return end self.line = display.newLine( self.x, self.y, self.lx, self.ly ) self.line:setStrokeColor(1,1,0) self.line.strokeWidth = 4 end Runtime:addEventListener( "enterFrame", ball ) function ball.finalize( self ) Runtime:removeEventListener( "enterFrame", self ) end ball:addEventListener("finalize") local ground = display.newRect( cx, bottom - 50, fullw, 40) physics.addBody( ground, "static") function ball:touch( event ) local id = event.id if(event.phase=="began") then display.getCurrentStage( ):setFocus( self, id ) self.isFocus = true -- self.t0 = system.getTimer() -- elseif(self.isFocus) then if(event.phase=="moved") then self.lx = event.x self.ly = event.y elseif((event.phase=="ended")or(event.phase=="cancelled")) then display.getCurrentStage( ):setFocus( self, nil ) self.isFocus = nil -- local dt = (system.getTimer() - self.t0)/1000 -- local dx = event.x - event.xStart local dy = event.y - event.yStart print(dx,dy) local distance = math.sqrt( dx \* dx + dy \* dy ) -- This is how you normalize (not by using math.abs()): local vx = dx/distance local vy = dy/distance -- local magnitude = self.factor \* distance/dt --print( distance, dt, magnitude, self.t0, system.getTimer(), dx, dy ) magnitude = (magnitude \> self.limit) and self.limit or magnitude magnitude = magnitude \* self.mass -- self:applyLinearImpulse( vx \* magnitude, vy \* magnitude, self.x, self.y ) -- self.lx = nil self.ly = nil -- end end end ball:addEventListener( "touch" )

https://www.youtube.com/watch?v=7np_XgPwgRE&feature=youtu.be

[jake1987.jj]

Really fantastic now we are here!

And the question of mass would be my next question…

Really thanks again

[jake1987.jj]

Hi.

could I ask one last question?

with your project if I “tap” the ball, both (the ball and the ground) will disappear.

I understand that with those values a tap face splashed the ball (really short time and non-existent distance), but why does the background disappear?

I have already corrected the mistake by checking the positions in touch event only I do not understand this thing of the ground…

[roaminggamer]

A distance of 0 causes division by-zero.

Make this change:

 local dx = event.x - event.xStart local dy = event.y - event.yStart local distance = math.sqrt( dx \* dx + dy \* dy ) if( distance \> 0.01 ) then -- This is how you normalize (not by using math.abs()): local vx = dx/distance local vy = dy/distance -- local magnitude = self.factor \* distance/dt --print( distance, dt, magnitude, self.t0, system.getTimer(), dx, dy ) magnitude = (magnitude \> self.limit) and self.limit or magnitude magnitude = magnitude \* self.mass -- self:applyLinearImpulse( vx \* magnitude, vy \* magnitude, self.x, self.y ) end -- self.lx = nil self.ly = nil

[jake1987.jj]

Thanks again!

[jake1987.jj]

Still here to ask for advice.

Currently if I run a long stretch(more than a few seconds) and after release, the ball will have a small push.

I know this happens because I have sampled a lot of time.

While playing tiger ball it seems that only the last stretch is considered.

So to do this I am storing in a table all the positions it makes in the last 1-2 seconds then I delete all the previous ones.

This all around works. But I’d love to hear what you think and how you can solve it.

[roaminggamer]

Try a touch joint:

• https://docs.coronalabs.com/api/library/physics/newJoint.html
  • https://docs.coronalabs.com/guide/physics/physicsJoints/index.html
    • https://docs.coronalabs.com/guide/physics/physicsJoints/index.html#touch

[jake1987.jj]

Hi @roaminggamer

1.wow! Thank you so much for the quick response

2.My fault. I did not say that I would not use the joints. For two reasons:

• I think that with math I could manage some aspects
• As you can see in the video the ball stays still until the user finishes a stretch. if I used a joint and did:“tmpJoint:setTarget( event.x, event.y )” in moved phase then the ball would move immediately but I want it to move only when the user has finished. if instead I do it only in the final phase the ball may not have the thrust that…

[roaminggamer]

Actually, I just downloaded tigerball and a touch joint won’t help.

The essential mechanic steps are:

• touch begin:
  • collect t0 = system.getTimer() (Note: event has a time in it so you can use that too)
• touch end:
  • t1 - system.getTimer() (Note: event has a time in it so you can use that too)
  • dt = (t1 - t0)/1000
  • dx = event.x - event.xStart 
  • dy = event.y - event.yStart 
  • distance = math.sqrt( dx * dx + dy * dy )
  • factor = choose your own value
  • speed limit = choose your own value
  • speed = factor * distance/dt
  • if speed > speed limit, cap at speed limit
  • vec = normalize the vector <dx,dy>
  • scale the vector vec = <vec.x * speed, vec.y * speed>
  • apply a linear impulse to the ball center with magnitude <vec.x , vec.y>

[jake1987.jj]

Thanks really fantastic. Just what I wanted!

I did a test but there is something I do not understand.

local ball = display.newCircle( 160, 100, 20 ) ball:setFillColor( 1, 0, 0 ) physics.addBody(ball, "dynamic", {radius=ballRadius}) local t0 local t1 local factor = 1 local speedLimit = 10 function ball:touch( event ) if(event.phase=="began")then display.getCurrentStage( ):setFocus( self ) self.isFocus = true t0 = system.getTimer() elseif(self.isFocus)then if(event.phase=="moved")then elseif((event.phase=="ended")or(event.phase=="cancelled")) then display.getCurrentStage( ):setFocus( nil ) self.isFocus = nil t1 = system.getTimer() local dt = (t1 - t0)/1000 local dx = event.x - event.xStart local dy = event.y - event.yStart local distance = math.sqrt( dx \* dx + dy \* dy ) local speed = factor \* distance/dt if (speed \> speedLimit)then speed = speedLimit end local vecX, vecY = math.abs(dx), math.abs(dy) local impulseX, impulseY = vecX\*speed, vecY\*speed self:applyLinearImpulse( self.x, self.y, impulseX, impulseY ) end end end ball:addEventListener( "touch" )

if I for example:

1.the event begins

2.I execute the stroke

3.I wait a few seconds

4.release

the ball receives a huge impulse instead of a low one

Did I miss something?

[roaminggamer]

That sounds wrong.  If you wait to lift your finger, dt should be very large making the speed (impulse magnitude very small).

I’ll look over your code again in a moment.

Tip: Paste to notepad and replace tabs with three spaces before then copying and pasting the modified code here.  It makes the code more legible.

[roaminggamer]

• You didn’t normalize correctly.

• Your call to applyLinearImpulse is backwards, you have the forces in the wrong place: 

  object:applyLinearImpulse( xForce, yForce, bodyX, bodyY )

• You didn’t include the object mass.  (If you don’t do this, then later change the size of the ball you’ll need to re-balance your factor calculation.)

• I made some changes to the original recipe.

https://github.com/roaminggamer/RG_FreeStuff/raw/master/AskEd/2018/06/tigerBall.zip

local ballRadius = 20 local ball = display.newCircle( cx, bottom - 80, ballRadius ) ball.factor = 0.06 ball.limit = 50 ball:setFillColor( 1, 0, 0 ) physics.addBody( ball, "dynamic", { radius = ballRadius, bounce = 0 } ) ball.linearDamping = 1 function ball.enterFrame(self) display.remove(self.line) if( not self.lx ) then return end self.line = display.newLine( self.x, self.y, self.lx, self.ly ) self.line:setStrokeColor(1,1,0) self.line.strokeWidth = 4 end Runtime:addEventListener( "enterFrame", ball ) function ball.finalize( self ) Runtime:removeEventListener( "enterFrame", self ) end ball:addEventListener("finalize") local ground = display.newRect( cx, bottom - 50, fullw, 40) physics.addBody( ground, "static") function ball:touch( event ) local id = event.id if(event.phase=="began") then display.getCurrentStage( ):setFocus( self, id ) self.isFocus = true -- self.t0 = system.getTimer() -- elseif(self.isFocus) then if(event.phase=="moved") then self.lx = event.x self.ly = event.y elseif((event.phase=="ended")or(event.phase=="cancelled")) then display.getCurrentStage( ):setFocus( self, nil ) self.isFocus = nil -- local dt = (system.getTimer() - self.t0)/1000 -- local dx = event.x - event.xStart local dy = event.y - event.yStart print(dx,dy) local distance = math.sqrt( dx \* dx + dy \* dy ) -- This is how you normalize (not by using math.abs()): local vx = dx/distance local vy = dy/distance -- local magnitude = self.factor \* distance/dt --print( distance, dt, magnitude, self.t0, system.getTimer(), dx, dy ) magnitude = (magnitude \> self.limit) and self.limit or magnitude magnitude = magnitude \* self.mass -- self:applyLinearImpulse( vx \* magnitude, vy \* magnitude, self.x, self.y ) -- self.lx = nil self.ly = nil -- end end end ball:addEventListener( "touch" )

https://www.youtube.com/watch?v=7np_XgPwgRE&feature=youtu.be

[jake1987.jj]

Really fantastic now we are here!

And the question of mass would be my next question…

Really thanks again

[jake1987.jj]

Hi.

could I ask one last question?

with your project if I “tap” the ball, both (the ball and the ground) will disappear.

I understand that with those values a tap face splashed the ball (really short time and non-existent distance), but why does the background disappear?

I have already corrected the mistake by checking the positions in touch event only I do not understand this thing of the ground…