[Solved] Physics Homework meets KAG

Hey again guys, sorry for clogging up the help section as of late. (It needed a bit of action anyway though.)
I've spent a little time on a small side project; belt conveyors. Something I've fantasized about for quite a while. The problem is, and I've run into it a bit before in regards to controlling zombie movement, CBlob.AddForce. I can't seem to wrap my head around what this function actually does. The documentation does mention it's not actually a force, which is frustratingly enough true, but what exactly are we sending to this function? Velocity? Acceleration?
What I want to do, is that the belt conveyor blocks push whatever blob touches it above, but assuming that blob is not moving on it's own as if it's a player-controlled entity for example, the velocity should be fixed. Whatever I do, lower weight blobs ends up with a much higher velocity. How would I go about doing that? What do I pass on to CBlob.AddForce?
Snippet from onTick handler:

Code:

//Create an array of blob object references
CBlob@[] nearbyBlobs;

//Create a handle for a blob object
CBlob@ nearbyBlob;

//Check if any blobs are within radius
if(this.getMap().getBlobsInRadius(this.getPosition(), this.getRadius(), @nearbyBlobs)) {

  //Determine current mode
  bool isFacingLeft = this.isFacingLeft();
 
  //Iterate through blob objects
  for(u8 i = 0; i<nearbyBlobs.length; i++) {
 
    //Keep a reference to this blob object
    @nearbyBlob = nearbyBlobs[i];
   
    //Create a vector representing the relative displacement
    Vec2f relativeDisplacement = nearbyBlob.getPosition() - this.getPosition();
   
    //Check if blob is not this blob, not tagged as conveyor and above
    if(nearbyBlob !is this && !nearbyBlob.hasTag("isConveyor") && relativeDisplacement.y < 0.5f) {
   
      //Create a movement vector
      Vec2f movement(Transports::ConveyorVariables::SLOW_VELOCITY * nearbyBlob.getMass(), 0.0f);
     
      //Check if conveyor is facing left
      if(isFacingLeft) {
     
        //Set horizontal movement in left direction
        movement.x *= -1.0f;
     
      }
     
      //Add a force to the left
      nearbyBlob.AddForce(movement);
     
    }
   
   
  }
 
}

 

I think I was going about this the wrong way.

It is acceleration (a) I need, and that's what's passed to AddForce, like you say (F=m*a). Strictly speaking, I'd rather have seen this method called accelerate, and without the confusing description, but anyway.

What I've been doing wrong, of course, is to make this call in the onTick handler, leading to crazy high acceleration. I have to do this in the onCollision handler, and when a blob collides with the belt conveyor, I think I'll have to keep track of the other blob to make sure that I don't accelerate it further as long as it didn't leave the track (other than to compensate for friction, perhaps). To do this I'll have to set a "touching belt conveyor"-tag on the blobs when they collide with the belt conveyor. Then I'll have to store blob IDs in the onEndCollision handler when they leave (but not by movement on the y-axis), and in onCollision check if the ID of the current colliding blob recently left a neighboring belt conveyor. A little tricky!

Edit:Actually, I might not have to store the IDs after all, if I just remove the touching flag in the onEndCollision check. Any suggestion as to how to check if the blob is moving upwards? MovementVars.keys? Somehow, .getVelocity() often returns a y-component that isn't 0, and the player character is visually jittering a bit. (code available at: https://github.com/ANybakk/kag-tran...res/Conveyor/BeltConveyor/BeltConveyorBlob.as )

 

I made some too, but I ended up using 'blob.setVelocity'. It's not that realistic for objects that collide the conveyor with high speed, but it's good for now.

 

That looks pretty good, Osmal. You made it work with mechanisms too.

Did you experience any kind of jitter/wobbeling? How did you actually end up setting velocity, in onCollision or onTick? What about character blobs, how does it work when you press buttons?

Btw, I'm curious; how do people do video capture in gif format? Is this something the engine supports, or some 3rd party software?

 

Stuff move on it smooth enough I think. I set it in onCollision and in onEndCollision
blob.setVelocity(Vec2f((this.isFacingLeft() ? -4.0f : 4.0f), 0.0f));
It currently doesnt move stuff that is placed on it when its not powered, and then powered - it needs the collision.

I use ScreenToGif program, its okay I suppose

 

Okay, so the description is right. You add a force, but only for that tick, meaning that the change is temporary, but the effect is not. At least that's how I interpret what you're saying. If that is the "interface" available for adding force, it's just not going to work. I guess I expected the blob to keep track of whatever forces was working on it. Stuff on belt conveyors move at a constant speed because the friction keeps them there (otherwise they would flip around), When they first hit the belt, they are accelerated, and when they leave, they will decelerate. But anyway, if I can't say that a force/acceleration is going to be applied until a certain point in time, I guess I'll have to set velocity to mimic the behavior. The question then is how I'm going to manipulate a blob's velocity when that blob could be doing all kinds of things at the time, for instance being controlled by a player or if it's a rolling boulder. I'm not sure atm.

 

Thanks for more excellent input, Geti. I think that is the right way to go.

I'm still having trouble with player characters bumping into things on the track, causing vertical movement and thus losing their "touching" flag, and makes it harder to see if it's working properly. You mentioned some "dummy" tile changes, Geti, are you referring to stuff done in LoaderUtilities.as, such as map.AddTileFlag(index, Tile::SOLID | Tile::COLLISION); ? I tried that, but I'm still experiencing this issue. My default sprite frame has some transparent pixels on the edges, because when there's only one tile, it should visually appear to have rounded edges. Does the game engine use these sprite pixels for collision checking, even though the animation or frame is changed at some point?

A snippet from the configuration file:

Code:

$shape_factory                            = box2d_shape
@$shape_scripts                           =
f32 shape_mass                            = 200.0
f32 shape_radius                          = 0.0
f32 shape_friction                        = 0.1
f32 shape_elasticity                      = 0.0
f32 shape_buoyancy                        = 0.0
f32 shape_drag                            = 0.2
bool shape_collides                       = yes
bool shape_ladder                         = no
bool shape_platform                       = no
@f32 verticesXY                           = 0.0;  0.0;
                                            8.0;  0.0;
                                            8.0;  8.0;
                                            0.0;  8.0;
u8 block_support                          = 0
bool block_background                     = no
bool block_lightpasses                    = no
bool block_snaptogrid                     = yes

Edit: This actually happens on vanilla platform blocks too when a player walks/runs over them. You can see it by the red dots being drawn in debug mode, that occasionally, there's a slight deviance on the y-axis. Does not happen on regular tiles (wood, stone etc)

 

Adding tiles in the background seems like a hack, considering blob shapes can be set to collide and otherwise act as "tiles" in the configuration file. I'm a bit curious about why collisions for stationary blobs are any different from tiles. I'd call that a pretty big flaw. That it's clearly visible even with vanilla blob blocks like the platform, doesn't really make it borderline either. Skinney's components are basically all using this hack. I don't like it.

Okay, I think it's safe to tag this topic as solved now. It's not working quite perfect, but it's pretty darn close now. I think I'll have to add friction into the calculations at some point, so that the belt isn't totally slippery when disabled, for instance. And materials don't seem to move anymore. Anyway. here's the general recipe:

• onCollision: Set a "touching" flag on the other blob so that you know it's touching the belt. Also store the ID of the belt conveyor segment.
  
• onEndCollision: Remove the "touching" flag if the other blob's velocity on the y-axis indicates it's leaving the belt (<0.0f).
  
• onTick: Iterate over all "touching" blobs within range (one tile above) that is touching this segment. Determine the difference in desired velocity with the actual velocity of the blob, and with it calculate what acceleration, and force, to apply (keeping in mind the tick rate). and update velocity.
  
• onSetStatic: Set a tile type with proper collision, and bring the sprite in front of it
  
• onDie: Remove tile
  
• .cfg: Disable shape collision

(Latest commit, for those interested: https://github.com/ANybakk/kag-transports/tree/692a41c27746ec44b2c26e11b4fdc702fdf9c241)

Update: Changed since the solution ended up quite differently later on. The touching-flag approach ended up just being a problem when it came to objects such as logs, which sometimes has vertical movement. Also not messing with forces anymore.

 

One extra question:
I quickly found out I have to take touching blobs' friction into account, but something's not right. The question is, the value set for friction (and drag) in a blob's configuration file, is that per second, per frame or per some fixed rate?

Code:

//Determine what acceleration to apply (a = v / t)
Vec2f acceleration = (targetVelocity - currentVelocity) / (Transports::ConveyorVariables::TIME_FOR_TARGET_VELOCITY * getTicksASecond());

//Determine blob's friction
f32 friction = nearbyBlob.getShape().getFriction() / getTicksASecond();

//Add force impulse (F = m * a)
nearbyBlob.AddForce((acceleration + Vec2f(friction, 0.0f)) * nearbyBlob.getMass());

 

Had to rewrite this post a couple of times.. sorry about that!

So the friction value is a force (in Newton)?

The problem is high-friction things such as materials. For them to reach the next belt conveyor segment before they reach a velocity of 0, I have to compensate for this friction. I added friction as an additional force (marginally larger), but I end up with different velocities again; materials go flying. If I divide that force with getTicksASecond(), the force is not strong enough to move it. So, I'm a bit puzzled (back to my question).

Code:

//Determine what acceleration to apply (a = v / t)
Vec2f acceleration = (targetVelocity - currentVelocity) / (Transports::ConveyorVariables::TIME_FOR_TARGET_VELOCITY * getTicksASecond());

//Obtain friction value
f32 friction = nearbyBlob.getShape().getFriction();

//Check if conveyor is facing left
if(isFacingLeft) {

  //Set left direction
  friction *= -1.0f;

}

//Add force impulses (F = m * a + static friction)
nearbyBlob.AddForce(acceleration * nearbyBlob.getMass() + Vec2f(friction + 0.1f, 0.0f));

Edit: By only applying the force when velocity reaches 0, the materials don't fly, but their velocity isn't steady, and deviates slightly from other things. Hmm...

Code:

//Determine what acceleration to apply (a = v / t)
Vec2f acceleration = (targetVelocity - currentVelocity) / (Transports::ConveyorVariables::TIME_FOR_TARGET_VELOCITY * getTicksASecond());

//Set a friction compensation force by determining the difference in friction
f32 friction = nearbyBlob.getShape().getFriction() - this.getShape().getFriction() + 0.1f;

//Check if conveyor is facing left
if(isFacingLeft) {

  //Set left direction
  friction *= -1.0f;

}

//Check if velocity isn't 0
if(currentVelocity.x != 0.0f) {

  //Set friction to 0
  friction = 0.0f;

}

//Add force impulses (F = m * a + static friction)
nearbyBlob.AddForce(acceleration * nearbyBlob.getMass() + Vec2f(friction, 0.0f));

 

Clearly, this is more complicated than I thought. I honestly don't remember this from my school days, but then physics wasn't exactly my favorite subject either :o)

I tried doing as you say, Verrazano, but I'm still not seeing the result I expect. The materials are travelling really slowly (or super fast without dividing by tick rate). The belt conveyor ("this") has a friction of 0.3, while mat_wood is 0.8 and builder is 0.07. I'm not sure what else can be at work here.

Code:

//Determine what acceleration to apply (a = v / t)
Vec2f acceleration = (targetVelocity - currentVelocity) / (Transports::ConveyorVariables::TIME_FOR_TARGET_VELOCITY * getTicksASecond());

//Determine what force to apply (F = m * a)
Vec2f force = acceleration * nearbyBlob.getMass();

//Determine what friction countering force to apply
Vec2f frictionForce = Vec2f(0.0f, 0.0f);

//Check if velocity is 0
if(currentVelocity.x == 0.0f) {

  //Determine the friction between the blob and conveyor segment
  f32 frictionFactor = Maths::Sqrt(nearbyBlob.getShape().getFriction() * this.getShape().getFriction());

  //Determine the force of friction that is necessary to surpass to initiate movement (Fs = Fn * us, Fn = m * g * cos(a))
  frictionForce.x = (nearbyBlob.getMass() * sv_gravity * Maths::Cos(0.0f) * frictionFactor) / getTicksASecond() + 0.1f;

  //Check if conveyor is facing left
  if(isFacingLeft) {

    //Set left direction
    frictionForce.x *= -1.0f;

  }

}

//Add force impulses
nearbyBlob.AddForce(force + frictionForce);