i want to have a spaceship, that moves left when u press left, up when up is pressed, etc., but i want to have a maxium speed limit. right now i use x_pos += x_vel y_pos += y_vel x_vel += x_accel y_vel += y_accel if (left_key_pressed) { if (x_vel > -10.0f) {x_vel += time_passed / 10.0f} if (x_vel < -10.0f) {x_vel = 10.0f} } and same thing for other keys. but that makes the speed limit too sudden. i want your acceleration to *slowly* slow down to 0 as your speed becomes close to 10.0f. also, it has to work with negative and positive initial speeds. thank you for any help, shurcool.

i''m really sorry for the double-post >, please reply here, and delete the other post.

You essentially want a function with a negative slope, right?

y = m *x + b

m is negative (the slope), y is your new velocity, x is your original velocity, b is the starting velocity for this attenuation function. This will reduce your velocity steadily from b to zero - and beyond. It's your responsibility to check if y is zero and stop then.

Of course, that's total junk. A better solution is to use a simple parabolic of the form

y = a *x 2 + b

and confine the range of your x . This would ensure that your value ramped up smoothly to a maxium value b when x was zero, and then ramped down. a is just a constant multiplier. To restrict x to positive values (from 0 to c ), use

y = a *(x - c )2 + b

You want to position your b such that it was the speed at which your acceleration started to decrease. You could also use a sine function to achieve the effect. If you wanted a rapid initial acceleration, slowing to a plateau and then gently decreasing to zero, you'd need to aggregate functions (add them together). A few sine and cosines of carefully selected complements, summed up, would give you the desired effect. You could also bias (add a constant or variable to) the function for values of x less than a certain threshold.

Hope I've helped to not only provide a solution but also provoke thought on better ones.

**Edited for consistent typeface

Edited by - Oluseyi on July 3, 2001 7:35:20 PM

Alternatively, use a logarithmic scale for your velocity. This gives a realistic looking acceleration and deceleration. Here's some (pseudo) code.

(In 1 dimension (easy to extrapolate to N dimensions))

Variables:

iIndex: a number that represents the sum of all left key strokes (which count for -1) and all right key strokes (which count for +1).

rVelocity: the 1-D speed of the object on a logarithmic scale

pseudo-code:

  int   iIndexfloat rVelocityif (KEY_PRESSED){   if (key==LEFT_KEY)   {      decrement iIndex    }   if (key==RIGHT_KEY)   {      increment iIndex   }   rVelocity = V_MAX * (1 - exp(-iIndex*iIndex))}   

where exp(x) is a function that returns the exponential e x.

Good luck,

Tim

Edited by - Timkin on July 3, 2001 12:09:55 AM

thanks, guys, you''ve really helped a lot! i have changed my needs a little bit, because now i will not only have arrow keys moving the ship in 4 directions (4 more if you press 2 keys at once), but it will be able to rotate, so that you face different directions. i will figure out how to do that, since now i know how to figure out the friction. and all i needed was: friction = speed * speed * friction_constant (0.1 in my case). thanks a lot,
shurcool.

hey,
i am using time_passed in it, and that constant acceleration is dependant on keys pressed. i also made sure that when up and right arrow keys are pressed, user will not go faster than if he only pressed the up key, by making the x and y accelration variables equal to sin(45) (in degrees) times 0.001 (that''s what y_accel is equal to when up key is pressed). i have also tried the method grhodes_at_work suggested (the last one, "quick-and-dirty take"), but the air resistance way seems to work better and more realistic. i will post the source code of the physics part here, and you can download the exe at http://www16.brinkster.com/shurcool/ (but please note that i haven''t worked on graphics part yet, just the physics! so please don''t scream at my 7 lines and strigs of text! ). here''s the physics part source code, i think it works fine for my needs:

if (player[player_num].vel_x > 0.0f)
{
player[player_num].vel_x -= player[player_num].vel_x * player[player_num].vel_x / 20.0f;
}
if (player[player_num].vel_x < 0.0f)
{
player[player_num].vel_x += player[player_num].vel_x * player[player_num].vel_x / 20.0f;
}
if (player[player_num].vel_y > 0.0f)
{
player[player_num].vel_y -= player[player_num].vel_y * player[player_num].vel_y / 20.0f;
}
if (player[player_num].vel_y < 0.0f)
{
player[player_num].vel_y += player[player_num].vel_y * player[player_num].vel_y / 20.0f;
}
player[player_num].rot += mouse_move_x;
if (player[player_num].rot > 360.0f) {player[player_num].rot -= 360.0f;}
if (player[player_num].rot < 0.0f) {player[player_num].rot += 360.0f;}
if (suction)
{
player[player_num].vel_x += (0 - player[player_num].x_pos) / 12500.0f; // Suction Of The Vortex
player[player_num].vel_y += (0 - player[player_num].y_pos) / 12500.0f; // Suction Of The Vortex
}
if (PosValf((0 - player[player_num].x_pos) * (0 - player[player_num].x_pos) + (0 - player[player_num].y_pos) * (0 - player[player_num].y_pos)) < 25.0f)
{
// Die
print_inside = true;
}
player[player_num].x_pos += player[player_num].vel_x;
player[player_num].y_pos += player[player_num].vel_y;



if ( key_pressed[''S''] && s_wu )
{
suction = !suction;
s_wu = false;
}
if ( !key_pressed[''S''] )
{
s_wu = true;
}

if ( (!key_pressed[VK_LEFT] || !key_pressed[VK_RIGHT]) && (!key_pressed[VK_UP] || !key_pressed[VK_DOWN]) )
{
if ( key_pressed[VK_UP] )
{
if ( key_pressed[VK_LEFT] )
{
player[player_num].vel_x -= (float)sin(player[player_num].rot / 57.2957795130823208767981548141052f - 0.785398163397448309615660845819876f) * 0.00157079632679489661923132169163975f;
player[player_num].vel_y -= (float)cos(player[player_num].rot / 57.2957795130823208767981548141052f - 0.785398163397448309615660845819876f) * 0.00157079632679489661923132169163975f;
}
else if ( key_pressed[VK_RIGHT] )
{
player[player_num].vel_x -= (float)sin(player[player_num].rot / 57.2957795130823208767981548141052f + 0.785398163397448309615660845819876f) * 0.00157079632679489661923132169163975f;
player[player_num].vel_y -= (float)cos(player[player_num].rot / 57.2957795130823208767981548141052f + 0.785398163397448309615660845819876f) * 0.00157079632679489661923132169163975f;
}
else
{
player[player_num].vel_x -= (float)sin(player[player_num].rot / 57.2957795130823208767981548141052f) * 0.002f;
player[player_num].vel_y -= (float)cos(player[player_num].rot / 57.2957795130823208767981548141052f) * 0.002f;
}
}
else if ( key_pressed[VK_DOWN] )
{
if ( key_pressed[VK_RIGHT] )
{
player[player_num].vel_x += (float)sin(player[player_num].rot / 57.2957795130823208767981548141052f - 0.785398163397448309615660845819876f) * 0.00157079632679489661923132169163975f;
player[player_num].vel_y += (float)cos(player[player_num].rot / 57.2957795130823208767981548141052f - 0.785398163397448309615660845819876f) * 0.00157079632679489661923132169163975f;
}
else if ( key_pressed[VK_LEFT] )
{
player[player_num].vel_x += (float)sin(player[player_num].rot / 57.2957795130823208767981548141052f + 0.785398163397448309615660845819876f) * 0.00157079632679489661923132169163975f;
player[player_num].vel_y += (float)cos(player[player_num].rot / 57.2957795130823208767981548141052f + 0.785398163397448309615660845819876f) * 0.00157079632679489661923132169163975f;
}
else
{
player[player_num].vel_x += (float)sin(player[player_num].rot / 57.2957795130823208767981548141052f) * 0.002f;
player[player_num].vel_y += (float)cos(player[player_num].rot / 57.2957795130823208767981548141052f) * 0.002f;
}
}
else if ( key_pressed[VK_RIGHT] )
{
player[player_num].vel_x -= (float)cos(player[player_num].rot / 57.2957795130823208767981548141052f) * 0.002f;
player[player_num].vel_y += (float)sin(player[player_num].rot / 57.2957795130823208767981548141052f) * 0.002f;
}
else if ( key_pressed[VK_LEFT] )
{
player[player_num].vel_x += (float)cos(player[player_num].rot / 57.2957795130823208767981548141052f) * 0.002f;
player[player_num].vel_y -= (float)sin(player[player_num].rot / 57.2957795130823208767981548141052f) * 0.002f;
}
}