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Difference between revisions of "SPG:Running"

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Notes:
 
Notes:
  
Research applies to all four of the Genesis/Mega Drive games, and Sonic CD.
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Research applies to all four of the [[Sega Mega Drive|Genesis/Mega Drive]] games, and [[Sonic CD]].
  
The following only applies when Sonic is on flat, dry land with no special power-ups.  Curves, water physics, Supersonic, and Super Fast Shoes will be covered in separate guides.
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The following only applies when [[Sonic]] is on flat, dry land with no special power-ups.  Curves, water physics, [[Super Sonic]], and [[Speed Shoes]] will be covered in separate guides.
  
 
===Accleration===
 
===Accleration===
  
Sonic accelerates at a speed of 0.046875 (''acc'').  When you first press to the right from a standstill, his X speed increases by ''acc'' every step.  When you first press to the left from a standstill, his X speed decreases by ''acc'' every step.
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Sonic accelerates at a speed of 0.046875 (''acc'').  When you first press {{right}} from a standstill, his X speed increases by ''acc'' every step.  When you first press {{left}} from a standstill, his X speed decreases by ''acc'' every step.
  
 
===Deceleration===
 
===Deceleration===
  
If Sonic is already moving when you press left or right, rather than at a standstill, the computer checks whether you are pressing the joypad in the direction he's already moving.  If so, ''acc'' is added to his X speed as normal.  However, if you are pressing in the opposite direction than he's already moving, the deceleration constant (''dec''), a value of 0.5, is added instead.  Thus Sonic can turn around quickly.  If no distinction is made between ''acc'' and ''dec'', Sonic takes too long to overcome his current velocity, frustrating the player.  A good engine must not make such a day one mistake.
+
If Sonic is already moving when you press {{left}} or {{right}}, rather than at a standstill, the computer checks whether you are pressing the joypad in the direction he's already moving.  If so, ''acc'' is added to his X speed as normal.  However, if you are pressing in the opposite direction than he's already moving, the deceleration constant (''dec''), a value of 0.5, is added instead.  Thus Sonic can turn around quickly.  If no distinction is made between ''acc'' and ''dec'', Sonic takes too long to overcome his current velocity, frustrating the player.  A good engine must not make such a day one mistake.
 +
 
 +
One might think that if X speed happened to equal 0.1, and you pressed {{left}}, ''dec'' would be subtracted, resulting in an X speed value of -0.4.  Oddly, this is not the case in any of the Mega Drive games.  Instead, at any time an addition or subtraction of ''dec'' results in X speed changing sign, X speed is set to ''dec''.  For example, in the instance above, X speed would become -0.5.  The bizarre result of this is that you can press {{right}} for one step, and then press {{left}} (or vice versa), and start running faster than if you had just pressed {{left}} alone!  Now, the resulting speed is still lower than one pixel per step, so it isn't very noticeable, but nonetheless it is true.  You may not want to bother emulating this anomaly.
  
One might think that if X speed happened to equal 0.1, and you pressed left, ''dec'' would be subtracted, resulting in an X speed value of -0.4.  Oddly, this is not the case in any of the Mega Drive games.  Instead, at any time an addition or subtraction of ''dec'' results in X speed changing sign, X speed is set to ''dec''.  For example, in the instance above, X speed would become -0.5.  The bizarre result of this is that you can press to the right for one step, and then press to the left (or vice versa), and start running faster than if you had just pressed left alone!  Now, the resulting speed is still lower than one pixel per step, so it isn't very noticeable, but nonetheless it is true.  You may not want to bother emulating this anomaly.
 
 
 
===Friction===
 
===Friction===
  
If you are not pressing left or right, friction (''frc'') kicks in.  The value of ''frc'' is 0.046875 - the same, incidentally, as ''acc''.  In any step in which the game recieves no horizontal joypad input, ''frc'' times the sign of X speed is subtracted from X speed, unless absolute X speed is less than ''frc'', in which case X speed is simply set to zero.
+
If you are not pressing {{left}} or {{right}}, friction (''frc'') kicks in.  The value of ''frc'' is 0.046875 - the same, incidentally, as ''acc''.  In any step in which the game recieves no horizontal joypad input, ''frc'' times the sign of X speed is subtracted from X speed, unless absolute X speed is less than ''frc'', in which case X speed is simply set to zero.
  
 
   {
 
   {
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   }
 
   }
  
Sonic CD actually uses a fix like this because Sonic can perform the "Super Peel Out" (or "Dash", in Japan, which is what I'll call it here), which launches him forward at a speed of 12 pixels per step.  Sonic can't accelerate to this speed under normal conditions, but if he reaches it from a Dash, he can continue to run at such a speed without slowing down, as long as you continue to press in the direction of his motion.  If you should release the button, friction will take over.  If you press again, friction will cease, and X speed will remain constant, but it will not rebuild to 12 without another Dash.
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[[Sonic CD]] actually uses a fix like this because Sonic can perform the "[[Super Peel Out]]" (or "Dash", in Japan, which is what I'll call it here), which launches him forward at a speed of 12 pixels per step.  Sonic can't accelerate to this speed under normal conditions, but if he reaches it from a Dash, he can continue to run at such a speed without slowing down, as long as you continue to press in the direction of his motion.  If you should release the button, friction will take over.  If you press again, friction will cease, and X speed will remain constant, but it will not rebuild to 12 without another Dash.
  
 
However, the programmers of Sonic CD neglected to apply the fix while Sonic is in the air, so if Sonic were to Dash off of a cliff, while you held in the direction of his motion, X speed would cut to 6 as he leaves the ground, regardless of how much higher it was at the time.  Again, that's just not right.
 
However, the programmers of Sonic CD neglected to apply the fix while Sonic is in the air, so if Sonic were to Dash off of a cliff, while you held in the direction of his motion, X speed would cut to 6 as he leaves the ground, regardless of how much higher it was at the time.  Again, that's just not right.
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===Other Characters===
 
===Other Characters===
  
All the characters - Sonic, Knuckles, and Tails - have the same acceleration, deceleration, top speed, running and braking values.  They handle identically, with no difference at all besides their special moves and their sprites (and the annoying fact that Knuckles jumps lower than the other two).
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All the characters - [[Sonic]], [[Tails]], and [[Knuckles]] - have the same acceleration, deceleration, top speed, running and braking values.  They handle identically, with no difference at all besides their special moves and their sprites (and the annoying fact that Knuckles jumps lower than the other two).
  
 
[[Category:Sonic Physics Guide]]
 
[[Category:Sonic Physics Guide]]

Revision as of 15:53, 13 July 2009

Notes:

Research applies to all four of the Genesis/Mega Drive games, and Sonic CD.

The following only applies when Sonic is on flat, dry land with no special power-ups. Curves, water physics, Super Sonic, and Speed Shoes will be covered in separate guides.

Accleration

Sonic accelerates at a speed of 0.046875 (acc). When you first press Right from a standstill, his X speed increases by acc every step. When you first press Left from a standstill, his X speed decreases by acc every step.

Deceleration

If Sonic is already moving when you press Left or Right, rather than at a standstill, the computer checks whether you are pressing the joypad in the direction he's already moving. If so, acc is added to his X speed as normal. However, if you are pressing in the opposite direction than he's already moving, the deceleration constant (dec), a value of 0.5, is added instead. Thus Sonic can turn around quickly. If no distinction is made between acc and dec, Sonic takes too long to overcome his current velocity, frustrating the player. A good engine must not make such a day one mistake.

One might think that if X speed happened to equal 0.1, and you pressed Left, dec would be subtracted, resulting in an X speed value of -0.4. Oddly, this is not the case in any of the Mega Drive games. Instead, at any time an addition or subtraction of dec results in X speed changing sign, X speed is set to dec. For example, in the instance above, X speed would become -0.5. The bizarre result of this is that you can press Right for one step, and then press Left (or vice versa), and start running faster than if you had just pressed Left alone! Now, the resulting speed is still lower than one pixel per step, so it isn't very noticeable, but nonetheless it is true. You may not want to bother emulating this anomaly.

Friction

If you are not pressing Left or Right, friction (frc) kicks in. The value of frc is 0.046875 - the same, incidentally, as acc. In any step in which the game recieves no horizontal joypad input, frc times the sign of X speed is subtracted from X speed, unless absolute X speed is less than frc, in which case X speed is simply set to zero.

 {
 ; friction, only if player isn't pressing left or right
 X speed = X speed - minimum(absolute(X speed),frc) * sign(X speed)
 }

Top Speed

Sonic can only accelerate so long. At some point, he reaches top speed and can no longer move any faster under his own power. This maximum speed (max) is 6 pixels per step. So, after acc is added to X speed, the computer checks to see if X speed exceeds max. If it does, it's set to max.

This means, of course, that if Sonic is somehow running at a higher speed than he can possibly achieve on his own (perhaps by way of having been impelled by a spring), if you press in the direction he's moving, the computer will add acc, notice that X speed exceeds max, and set X speed to max. Thus it is possible to curtail your forward momentum by pressing in the very direction of your motion. That's just not right! This can be solved in your engine by checking to see if X speed is less than max before adding acc. Only if X speed is less than max does the computer add acc to it and check if X speed exceeds max. Problem solved.

 {
 if the player is pressing left
   {
   if X speed > 0
     {
     X speed -= dec
     }
   else
   if X speed > -max
     {
     X speed = X speed - acc
     if X speed < -max then X speed = -max
     }
   }
 else
 if the player is pressing right
   {
   if X speed < 0
     {
     X speed += dec
     }
   else
   if X speed < max
     {
     X speed = X speed + acc
     if X speed > max then X speed = max
     }
   }
 }

Sonic CD actually uses a fix like this because Sonic can perform the "Super Peel Out" (or "Dash", in Japan, which is what I'll call it here), which launches him forward at a speed of 12 pixels per step. Sonic can't accelerate to this speed under normal conditions, but if he reaches it from a Dash, he can continue to run at such a speed without slowing down, as long as you continue to press in the direction of his motion. If you should release the button, friction will take over. If you press again, friction will cease, and X speed will remain constant, but it will not rebuild to 12 without another Dash.

However, the programmers of Sonic CD neglected to apply the fix while Sonic is in the air, so if Sonic were to Dash off of a cliff, while you held in the direction of his motion, X speed would cut to 6 as he leaves the ground, regardless of how much higher it was at the time. Again, that's just not right.

Running Animation

If you include Sonic CD in the mix, Sonic has 3 different running animations. He is depicted as standing still only if X speed is exactly zero. If he has any X speed whatsoever, he enters his walking animation. Once his X speed equals (or exceeds) 6, he enters his running animation, with the whirling feet. Once his X speed equals (or exceeds) 10, he enters his Dashing animation, with the figure-eight feet. Of course, the Dashing animation is only in Sonic CD.

Braking Animation

Sonic enters his braking animation when you turn around only if his absolute X speed is equal to or more than 4.5. In Sonic 1 and Sonic CD, he then stays in the braking animation until X speed reaches zero or changes sign. In the other 3 games, Sonic returns to his walking animation after the braking animation finishes displaying all of its frames.

Other Characters

All the characters - Sonic, Tails, and Knuckles - have the same acceleration, deceleration, top speed, running and braking values. They handle identically, with no difference at all besides their special moves and their sprites (and the annoying fact that Knuckles jumps lower than the other two).