From Sonic Retro


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.


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

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	
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


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 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.

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.


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.

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.

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.

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.

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:

if (the player is pressing left)
    if (gsp > 0)
        gsp -= dec;
    else if (gsp > -top)
        gsp -= acc;
else if (the player is pressing right)
    if (gsp < 0)
        gsp += dec;
    else if (gsp < top)
        gsp += acc;
else 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).