quote:Original post by Vlion
newxval = oldxval+accelxval;

quote:Original post by LilBudyWizer
No, you add the acceleration vector to the velocity vector. Acceleration has both a magnitude and direction. If you acceleration is a constant, a, then the velocity at time t is a*t+v0 where v0 is the velocity at time zero. The position at time t is 1/2*a*t^2+v0*t+p0 where p0 is the position at time zero. In 1D those are 1D vectors or in other words scalars. In 2D and 3D those are 2D and 3D vectors. The differance in velocity from t to t+dt is a*(t+dt)+v0-(a*t+v0) or a*t+a*dt+v0-a*t-v0 or a*dt. So if you add a*dt to the previous velocity you get the new velocity after dt more whatever time units you are using. If you have a fixed frame interval then you can use that interval length for your time basis so dt is 1.

One thing to note is that under accelerated motion your velocity is constantly changing. You move the average velocity times the time, not the ending velocity times the time. So it is 1/2*(v1+v2)*dt with constant acceleration. Using the equation above the differance in position from t to t+dt is 1/2*a*(t+dt)^2+v0*(t+dt)+p0-(1/2*a*t^2+v0*t+po) or a*t*dt+1/2*a*dt^2+v0*dt or dt*(1/2*a*dt+(a*t+v0)), but a*t+v0 is the velocity at the start of the frame, v1, and a*dt is the change in velocity, dv, during the frame. So the change in position is dt*(1/2*dv+v1). If you call the velocity at the end of the frame v2 then that can be rewriten as dt*(1/2*(v2-v1)+v1) which can be rewriten as 1/2*(v1+v2)*dt.

You apply your max velocity by simply adding an if after calculating the velocity at the end of the frame. If it is greater than the maximum velocity then you set it to the maximum velocity. To be completely accurate you would have to calculate when within the frame you hit the maximum velocity, calculate the displacement up to that time and then the additional displacement to the end of the frame. It isn''t an accurate model to start with and even if it was the tiny differance in position over such a short interval isn''t worth worrying about. The maximum error is 1/2*dv*dt which most likely is not even a pixel so your user is not going to notice.

      //        This ought to give you the basic idea.//        The keydown(...) function is of course not real.//        Also, remember that all angles are in radians.#include <math.h>struct vector{     float x, y;};struct point{     float x, y;};struct spaceship{     vector velocity;     point coord;     float angle;}    int main(){     spaceship player; memset(&player, 0, sizeof(spaceship));     for(bool exit = false; !exit; ) //Simple game loop     {            if(keydown(LEFT))                  player.angle -= ROTATION_SPEED;            if(keydown(RIGHT))                  player.angle += ROTATION_SPEED;            if(keydown(UP))            {                  player.velocity.x += sinf(angle) * THRUSTER_FORCE;                  player.velocity.y += cosf(angle) * THRUSTER_FORCE;            }            if(keydown(DOWN))            {                  player.velocity.x -= sinf(angle) * THRUSTER_FORCE;                  player.velocity.y -= cosf(angle) * THRUSTER_FORCE;            }            player.coord.x += player.velocity.x;            player.coord.y += player.velocity.y;     }     return 0;}      

  	m_MaxVelocity = 2.0f;	m_Acceleration = 0.02f;	m_XVelocity += (cosf(m_Angle * 3.14159f / 180 ) * m_Acceleration);	m_YVelocity += (sinf(m_Angle * 3.14159f / 180 ) * m_Acceleration);	if( m_XVelocity > 0 )	{		if( m_XVelocity > (cosf(m_Angle * 3.14159f / 180) * m_MaxVelocity) )			m_XVelocity = (cosf(m_Angle * 3.14159f / 180) * m_MaxVelocity);	}	else	{		if( m_XVelocity < (cosf(m_Angle * 3.14159f / 180) * m_MaxVelocity) )			m_XVelocity = (cosf(m_Angle * 3.14159f / 180) * m_MaxVelocity);	}	if( m_YVelocity > 0 )	{		if( m_YVelocity > (sinf(m_Angle * 3.14159f / 180) * m_MaxVelocity) )			m_YVelocity = (sinf(m_Angle * 3.14159f / 180) * m_MaxVelocity);	}	else	{		if( m_YVelocity < (sinf(m_Angle * 3.14159f / 180) * m_MaxVelocity) )			m_YVelocity = (sinf(m_Angle * 3.14159f / 180) * m_MaxVelocity);	}