# Problem with steering behavior

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Hello! I am new and just beginning to learn about steering behavior. I need such behavior for the game I am making. Can you please look into my code and try to see what I am missing or at least misunderstood in terms of steering behavior?

My problem is that my sprite doesn't steer. So here is my code:


void PathFinder::moveToDestination( float e )
{

if( !_isPathFound ) return;

float tilesize = static_cast< float >( tileset->getTilesize() );
float speed    = e * movableObjects->getVelocity();// NOTE : This should be getSpeed()

const float MAX_FORCE = 2.5;
const float mass      = 80.0f;
const float tolerance = 1.0f;

sf::Vector2f desired_velocity;
sf::Vector2f steer;
sf::Vector2f velocity;

spritePosition.x = movableObjects->getSpritePositionX();
spritePosition.y = movableObjects->getSpritePositionY();

current_velocity = normalize( targetPosition - spritePosition   );
desired_velocity = normalize( targetPosition - current_velocity ) * speed;

steer            = desired_velocity - current_velocity;
steer            = truncate< float >( steer , MAX_FORCE );
steer           /= mass;

velocity         = truncate< float >( current_velocity + steer , speed );

float movex = spritePosition.x + velocity.x;
float movey = spritePosition.y + velocity.y;

cout << " current velocity : " << current_velocity.x << " x " << current_velocity.y << endl;
cout << " desired velocity : " << desired_velocity.x << " x " << desired_velocity.y << endl;
cout << " steer            : " << steer.x << " x " << steer.y << endl;
cout << " velocity         : " << velocity.x << " x " << velocity.y << endl;
cout << endl;

if( abs( targetPosition.x - spritePosition.x ) <= tolerance
&& abs( targetPosition.y - spritePosition.y ) <= tolerance )
{
_isPathFound = false;
}

movableObjects->setSpritePosition( movex , movey );
}



Here is my code for VectorMath file which contains a bunch of templated functions: Maybe I got something wrong in here

#ifndef VECTORMATH_HPP
#define VECTORMATH_HPP

#include<cmath>

template< typename T >
T length( const sf::Vector2< T > &v )
{
return sqrt( ( v.x * v.x ) + ( v.y * v.y ) );
}

template< typename T  >
sf::Vector2< T > normalize( const sf::Vector2< T > &v )
{
return v / length( v );
}

template< typename T >
sf::Vector2< T > truncate( const sf::Vector2< T > &v , float nmax )
{
float i = nmax / length( v );

i = ( i < 1.0f )? 1.0f : i;

sf::Vector2< T > n = v;
n *= i;

return n;
}

#endif // VECTORMATH_HPP



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How about adding some comments to your code to make it easier to understand what you are trying to do at each stage.

I had a quick glance at your code and this looks fishy to me:

current_velocity = normalize( targetPosition - spritePosition );
desired_velocity = normalize( targetPosition - current_velocity ) * speed;

(targetPosition - current_velocity) ... what units are you using here?  A position minus a velocity doesn't make sense.

Have a look at some good steering tutorials here:

http://gamedev.tutsplus.com/tutorials/implementation/understanding-steering-behaviors-path-following/

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When I do steering, I use a polar coordinate kind of thing. Instead of velocity vectors, things have a facing direction and speed. They have a max angle change of like 5-10 degrees per frame, and add or subtract that to their facing direction to turn toward the target point.

Speed can be instantaneous start/stop, or have an acceleration rate, depending on what kind of movement you're going for. If accelerating, then you have to "look ahead" and start slowing down before you get to the target, or you'll end up orbiting it.

For path following, it can make for smoother movement if you have an imaginary rabbit (like in dog racing) that moves along the path (using linear interpolation between path nodes), always keeping a set distance ahead of the character, and the character steers toward the rabbit instead of a path node.

Steering can make for very nice AI behavior, especially for being relatively easy to program. You can make a system where characters have various behaviors, such as following a path, maintaining a position relative to another character (e.g. airplanes flying in formation), avoiding walls or other obstacles, etc.. For each behavior, calculate the ideal angle change and speed change, and a weight based on how urgent it is (e.g. obstacle avoidance weight goes up by inverse squared distance from the obstacle). Sum up all the ideal changes, multiplied by their weights, and then clamp to the actual maximum angle change per frame and acceleration rate.

Works really well with the angle system I'm always preaching about, where you use a 16 bit number to represent a full circle. 65536 "degrees" per rotation, and it automatically wraps to zero if you go over. And you can reinterpret it as signed 16 bit for a -180 to +180 angle, which is extremely handy when calculating whether to turn left or right, and by how far.

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How about adding some comments to your code to make it easier to understand what you are trying to do at each stage.

I had a quick glance at your code and this looks fishy to me:

current_velocity = normalize( targetPosition - spritePosition );
desired_velocity = normalize( targetPosition - current_velocity ) * speed;

(targetPosition - current_velocity) ... what units are you using here?  A position minus a velocity doesn't make sense.

Also in this code fragment and nearby:

speed looks more or less like a variable timestep (it shouldn't be called "speed" and it is redundant with poorly-named parameter e).

Obviously, current_velocity isn't the current velocity of anything. You must store the object's velocity, not only its position.

Presumably, steer would like to be an acceleration, but it fluctuates ambiguously and contradictorily in the no man's land between length, velocity, force, impulse and acceleration.

You should really use physically correct formulas and variables: random naming and inconsistent variable types are a terrible form of obfuscation of which you are the first victim.

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Hello everyone! Thanks for your kind replies! I have finally made it! Thank you so much for your suggestions!

here is the final code I have made so far:

    if( !_isPathFound ) return;

float tilesize = static_cast< float >( tileset->getTilesize() );

const float MAX_SPEED    = e * movableObjects->getVelocity();
const float MAX_VELOCITY = 3.0f;
const float MAX_FORCE    = 10.4;
const float mass         = 50.0f;
const float tolerance    = 1.0f;

sf::Vector2f desired_velocity;
sf::Vector2f steer;
sf::Vector2f velocity;

spritePosition.x = movableObjects->getSpritePositionX();
spritePosition.y = movableObjects->getSpritePositionY();

desired_velocity = normalize( targetPosition - spritePosition ) * MAX_VELOCITY;
steer.x = desired_velocity.x - MAX_SPEED;
steer.y = desired_velocity.y - MAX_SPEED;

steer = truncate( steer , MAX_FORCE );
steer /=  mass;

velocity = truncate( velocity + steer , MAX_SPEED );

float movex = spritePosition.x + velocity.x;
float movey = spritePosition.y + velocity.y;

if( abs( targetPosition.x - spritePosition.x ) <= tolerance
&& abs( targetPosition.y - spritePosition.y ) <= tolerance )
{
_isPathFound = false;
}

movableObjects->setSpritePosition( movex , movey );


This one doesn't handle the arrival yet. It just get to the point where I check if it is near to the tolerated value then stop rendering or stop moving at that point. I might try adding arrival behavior soon on this one.

I still manage to figure out how can I orient the sprite on its velocity  to the target as it move.
But for now at least it works.

Thank you very much for your help and time!

Edited by neonwarge04

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