Difference between revisions of "SPG:Running"

Revision as of 22:23, 17 August 2019

Notes:

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

The following describes ground speed, the forces that impact it and the effects of player input. It only applies when Sonic is on dry land with no special power-ups. Curves, water physics, Super Sonic, and Speed Shoes will be covered in separate guides.

Variables

The following variables/constants will be referenced frequently in this section.

//Variables
xsp: the speed in which sonic is moving horizontally
ysp: the speed in which sonic is moving vertically
gsp: the speed in which sonic is moving on the ground	
//Constants
acc: 0.046875
dec: 0.5
frc: 0.046875 (same as acc)
top: 6

Ground Speed Conversion

gsp is used to calculate xsp and ysp when landed on normal ground. This is further described in The Three Speed Variables.

xsp = gsp*cos(angle);
ysp = gsp*-sin(angle);

Movement Rules

Note: In the original games, holding and at the same time will run both direction's code in one step, if you want in your engine, you can disable + input (Emulators like Fusion will just cancel the right input if left is also held).

Acceleration

If you're holding , Sonic's gsp decreases by acc every step. If you're holding (Also applies if you're holding , which you may not want in your engine), his gsp increases by acc every step.

Deceleration

If Sonic is already moving when you press or , rather than at a standstill, the computer checks whether you are holding the direction he's already moving. If so, acc is added to his gsp as normal. However if you are pressing in the opposite direction than he's already moving, the deceleration constant (dec) 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 gsp happened to equal 0.1, and you pressed , dec would be subtracted, resulting in an gsp value of -0.4. Oddly, this is not the case in any of the original games. Instead, at any time an addition or subtraction of dec results in gsp changing sign, gsp is set to 0.5. For example, in the instance above, gsp would become -0.5. The bizarre result of this is that you can press for one step, and then press (or vice versa), and start running faster than if you had just pressed alone! Now, the resulting speed is still lower than one pixel per step, so it isn't very noticeable, but nonetheless it is true.

Friction

If you are not pressing or , friction (frc) kicks in. In any step in which the game recieves no horizontal input, frc is subtracted from gsp (depending on the sign of gsp), where if it then passes over 0, it's set back to 0.

Top Speed

Sonic can only accelerate up to a certain point. At some point, he reaches top speed and can no longer move any faster under his own power. So, after acc is added to gsp, if gsp exceeds top it's set to top.

In Sonic 1, if Sonic is already running at a higher speed than he can possibly achieve on his own (such as having been impelled by a spring), if you press in the direction he's moving, the computer will add acc to gsp, notice that gsp exceeds top, and set gsp to top. Thus it is possible to curtail your forward momentum by pressing in the very direction of your motion. This can be solved in your engine (and was fixed in Sonic 2 and beyond) by checking to see if gsp is less than top before adding acc. Only if gsp is already less than top will it check if gsp exceeds top.

Sonic CD and Sonic 2 actually has a fix like this, where the top check won't happen if gsp is already above top, because in Sonic CD can perform the "Super Peel Out", which launches him above his typical top, and in Sonic 2 because it's supposed to be a much faster game. The issue is, in both games, it doesn't fix this error when Sonic is in mid-air, causing him to usually lose his speed when running off of a ledge.

Here's some code logic that can accurately emulate movement and friction:

if (the player is pressing left)
{
    if (gsp > 0)
    {
        gsp -= dec;
        if (gsp <= 0)
            gsp = -0.5;
    }
    else if (gsp > -top)
    {
        gsp -= acc;
        if (gsp <= -top)
            gsp = -top;
    }
}

if (the player is pressing right)
{
    if (gsp < 0)
    {
        gsp += dec;
        if (gsp >= 0)
            gsp = 0.5;
    }
    else if (gsp < top)
    {
        gsp += acc;
        if (gsp >= top)
            gsp = top;
    }
}

if (the player is not pressing left or right)
    gsp -= minimum(absolute(gsp), frc) * sign(gsp);

Animation Rules

Running Animation

If you include Sonic CD in the mix, Sonic has 3 different running animations. He is depicted as standing still only if gsp is exactly zero. If he has any gsp whatsoever, he enters his walking animation, the frame advences in relation to his ground speed.

Once his gsp equals (or exceeds) 6, he enters his running animation, with the whirling feet. Once his gsp 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 gsp is equal to or more than 4.5. In Sonic 1 and Sonic CD, he then stays in the braking animation until gsp 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.

Character Specific

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).

@@ Line 26: / Line 26: @@
 ==Movement Rules==
+Note: In the original games, holding {{left}} and {{right}} at the same time will run both direction's code in one step, if you want in your engine, you can disable {{left}} + {{right}} input (Emulators like Fusion will just cancel the right input if left is also held).
 ===Acceleration===
-When you hold {{right}} from a standstill, his ''gsp'' increases by ''acc'' every step.  When you hold {{left}} from a standstill, his ''gsp'' decreases by ''acc'' every step.
+If you're holding {{left}}, Sonic's ''gsp'' decreases by ''acc'' every step. If you're holding {{right}} (Also applies if you're holding {{left}}, which you may not want in your engine), his ''gsp'' increases 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 ''gsp'' as normal.  However if you are pressing in the opposite direction than he's already moving, the deceleration constant (''dec'') 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 holding the direction he's already moving. If so, ''acc'' is added to his ''gsp'' as normal.  However if you are pressing in the opposite direction than he's already moving, the deceleration constant (''dec'') 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 ''gsp'' happened to equal 0.1, and you pressed {{left}}, ''dec'' would be subtracted, resulting in an ''gsp'' 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 ''gsp'' changing sign, ''gsp'' is set to ''dec''.  For example, in the instance above, ''gsp'' would become -0.5.  The bizarre result of this is that you can press {{left}} for one step, and then press {{right}} (or vice versa), and start running faster than if you had just pressed {{right}} 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 ''gsp'' happened to equal 0.1, and you pressed {{left}}, ''dec'' would be subtracted, resulting in an ''gsp'' value of -0.4. Oddly, this is not the case in any of the original games. Instead, at any time an addition or subtraction of ''dec'' results in ''gsp'' changing sign, ''gsp'' is set to 0.5. For example, in the instance above, ''gsp'' would become -0.5. The bizarre result of this is that you can press {{left}} for one step, and then press {{right}} (or vice versa), and start running faster than if you had just pressed {{right}} alone! Now, the resulting speed is still lower than one pixel per step, so it isn't very noticeable, but nonetheless it is true.
 ===Friction===
-If you are not pressing {{left}} or {{right}}, friction (''frc'') kicks in.  In any step in which the game recieves no horizontal joypad input, ''frc'' times the sign of ''gsp'' is subtracted from ''gsp'', unless absolute ''gsp'' is less than ''frc'', in which case ''gsp'' is simply set to zero.
+If you are not pressing {{left}} or {{right}}, friction (''frc'') kicks in.  In any step in which the game recieves no horizontal input, ''frc'' is subtracted from ''gsp'' (depending on the sign of ''gsp''), where if it then passes over 0, it's set back to 0.
 ===Top Speed===
-Sonic can only accelerate up to a certain point.  At some point, he reaches top speed and can no longer move any faster under his own power. So, after ''acc'' is added to ''gsp'', the computer checks to see if ''gsp'' exceeds ''top''.  If it does, it's set to ''top''.
+Sonic can only accelerate up to a certain point.  At some point, he reaches top speed and can no longer move any faster under his own power. So, after ''acc'' is added to ''gsp'', if ''gsp'' exceeds ''top'' it's set to ''top''.
-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 ''gsp'' exceeds ''top'', and set ''gsp'' to ''top''.  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 ''gsp'' is less than ''top'' before adding ''acc''.  Only if ''gsp'' is less than ''top'' does the computer add ''acc'' to it and check if ''gsp'' exceeds ''top''.  Problem solved.
+In Sonic 1, if Sonic is already running at a higher speed than he can possibly achieve on his own (such as having been impelled by a spring), if you press in the direction he's moving, the computer will add ''acc'' to ''gsp'', notice that ''gsp'' exceeds ''top'', and set ''gsp'' to ''top''.  Thus it is possible to curtail your forward momentum by pressing in the very direction of your motion.  This can be solved in your engine (and was fixed in Sonic 2 and beyond) by checking to see if ''gsp'' is less than ''top'' before adding ''acc''. Only if ''gsp'' is already less than ''top'' will it check if ''gsp'' exceeds ''top''.
-[[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 ''gsp'' will remain constant, but it will not rebuild to 12 without another Dash.
+[[Sonic CD]] and [[Sonic 2]] actually has a fix like this, where the ''top'' check won't happen if ''gsp'' is already above ''top'', because in Sonic CD can perform the "[[Super Peel Out]]", which launches him above his typical ''top'', and in Sonic 2 because it's supposed to be a much faster game. The issue is, in both games, it doesn't fix this error when Sonic is in mid-air, causing him to usually lose his speed when running off of a ledge.
-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, ''gsp'' 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.
 Here's some code logic that can accurately emulate movement and friction:
@@ Line 58: / Line 57: @@
      {
          gsp -= dec;
+        if (gsp <= 0)
+            gsp = -0.5;
      }
      else if (gsp > -top)
      {
          gsp -= acc;
+        if (gsp <= -top)
+            gsp = -top;
      }
 }
-else if (the player is pressing right)
+if (the player is pressing right)
 {
      if (gsp < 0)
      {
          gsp += dec;
+        if (gsp >= 0)
+            gsp = 0.5;
      }
      else if (gsp < top)
      {
          gsp += acc;
+        if (gsp >= top)
+            gsp = top;
      }
 }
-else gsp -= minimum(absolute(gsp ), frc)*sign(gsp);
+if (the player is not pressing left or right)
+    gsp -= minimum(absolute(gsp), frc) * sign(gsp);
 </nowiki>