Question

Path following - How I do I stick to the path but increase speed and thus velocity of a moving object?

Asked by: Dr_Spankenstein

If I have an object that is following a path using the following line:

object.Position += direction * speed * (float)gameTime.ElapsedGameTime.TotalSeconds;

I know that the direction can be found by subtracting the object's position from the next node position and I can therefore move an object from one node to the next until the final destination node is reached.  

The next node will currently be found when the object's position is within a certain proximity of the current destination node.

However, if I increase the speed at which the object is travelling in a given direction then I will not be able to detect if the object is within the proximity limit as it has "overshot" this limit within a single frame.

How do I get an object to strictly follow a path (I.e. have a very small proximity limit) even when increasing the speed of the object causes it fly past a node undetected?

Thank you.


However,

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Asked On
2009-09-29 at 08:01:06ID24770315
Tags

Graph

,

AI

,

C#

,

XNA

,

Nodes

Topics

Physics & Artificial Intelligence in Game Programming

,

3D Game Programming

,

Microsoft Visual C#.Net

Participating Experts
1
Points
500
Comments
7

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Answers

 

by: h2g2guyPosted on 2009-09-29 at 13:42:49ID: 25453246

What about this:

Before moving your object, find the displacement between the object and the next node as a Vector2 (or 3, depending on the game).  Then, immediately after moving the object, find the displacement again.  Normalize both vectors.  If they point in opposite directions, then you know that you have passed through the node.

Keep in mind:  if you use the code I typed in below, rounding errors might make the 'if' statement always return false.  If this happens, let me know.

Vector2 beforeDirection, afterDirection;
 
//find direction as you normally do
 
beforeDirection = direction;
beforeDirection.Normalize();
 
object.Position += direction * speed * 
     (float)gameTime.ElapsedGameTime.TotalSeconds;
 
//if you are not sure if you've hit the node, find the direction
//again the same way you normally do.  store in afterDirection.
 
afterDirection.Normalize();
 
if(-afterDirection == beforeDirection)
{
     //you know that you've overshot the node
}

                                              
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by: Dr_SpankensteinPosted on 2009-09-29 at 15:19:56ID: 25453984

Thank you!  I will try this out after some much needed sleep.

Just a thought before bed.  Would the best method/algorithm in this situation be:

- Find out how much I have overshot the desination node
- Move the object back to the destination node.
- Find the direction to move in for the next node
- Move in the new direction with the amount the object overshot the last node
- If the object overshoots the next node repeat all of the above, if not carry on moving normally.

Or can you suggest a better method than this?

Thanks for your help.

 

by: h2g2guyPosted on 2009-09-29 at 16:37:47ID: 25454471

That sounds to me like the best possible way.  (At least, I can't think of anything better!)

 

by: Dr_SpankensteinPosted on 2009-09-30 at 07:41:09ID: 25459391

I've been working on moving the object from one node to another but have come unstuck when trying to work out how I should add the remaining distance (object_Distance_Remaining) when "overshooting" a node.

position_Final += ?????

Any help would be most welcome.

The code is not very optimized and heavily commented but it is included below to show you what I have so far:

            Vector3 position_Final = Vector3.Zero;
 
            // If the object is at the current destination node
            if (AI.AtNode(debug_Polygon[0].Position, nodes[node_Index]))
            {
                // Increase the node index
                node_Index++;
            }
 
            // Find next node
            Vector3 node = nodes[node_Index];
 
            Vector3 position_Start = debug_Polygon[0].Position;
 
            // Find the direction to the next node based on the object's position
            Vector3 node_Direction_To = node - position_Start;
            
            // Distance must be calculated from NON normalized direction
            float node_Distance_To = node_Direction_To.LengthSquared();
 
            // Now normalize the direction to the next node (for smooth movement)
            node_Direction_To.Normalize();
 
            // Set final position to current object position
            position_Final = position_Start;
 
            // Calculate the final position based on direction to the next node and object's speed
            position_Final += node_Direction_To * speed * (float)gameTime.ElapsedGameTime.TotalSeconds;
 
            // Calculate the distance travelled this frame
            float object_Distance_Travelled = (position_Final - position_Start).LengthSquared();
 
            // If the distance from the object's original position to its new position is greater than the distance to the next node
            // [I.e. The object has overshot the node by a certain distance]
            if (object_Distance_Travelled > node_Distance_To)
            {
                float object_Distance_Remaining = object_Distance_Travelled - node_Distance_To;
                Console.WriteLine("Overshot node by " + object_Distance_Remaining);
 
                // Set position to current node
                position_Start = nodes[node_Index];
 
                // Find the next node (ONLY IF NOT FINAL NODE)
                node_Index++;
                node = nodes[node_Index];
 
                // Find the direction to the next node based on the object's position
                node_Direction_To = node - position_Start;
 
                // Distance must be calculated from NON normalized direction
                node_Distance_To = node_Direction_To.LengthSquared();
 
                // Now normalize the direction to the next node (for smooth movement)
                node_Direction_To.Normalize();
 
                // Set final position to current object position
                position_Final = position_Start;
 
                // FIND NEW DISTANCE BASED ON: object_Distance_Remaining
                position_Final += ?????
            }

                                              
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by: h2g2guyPosted on 2009-09-30 at 13:22:33ID: 25463067

Wouldn't it be the following?  I'm not sure if I understand your question...

position_Final += object_Distance_Remaining * node_Direction_To

                                              
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by: Dr_SpankensteinPosted on 2009-09-30 at 13:27:03ID: 25463127

I think that's it yes!  I guess this would work correctly only if "node_Direction_To" was normalized?

To reiterate the question:

I would like to move in the direction of the next node by the remaining length.

 

by: h2g2guyPosted on 2009-09-30 at 14:06:20ID: 25463652

You are correct in your assumption, and fortunately I did understand the question (ha, ha)!

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