From Sonic Retro
|Sonic Physics Guide|
- The research applies to all four of the Sega Mega Drive games and Sonic CD.
- Variables and constants for Sonic and other characters such as X Position and acc will be referenced frequently, they can be found in Basics.
- An object's actual Width Radius and Height Radius variables are separate to an object's hitbox width radius and height radius.
- More detailed information about how hitboxes and trigger areas work, as well as the Player's hitbox, can be found at SPG:Hitboxes.
- 1 Introduction
- 2 Interaction
- 3 Badniks
- 3.1 Motobugs
- 3.2 Choppers
- 3.3 Buzz Bombers
- 3.4 Crabmeats
- 3.5 Caterkillers
- 4 Bosses
Much like other objects, Badniks behave and are constructed in many different ways, some simple and some rather more intricate. This page will cover just a few examples of specific Badniks and Bosses ranging from simple to complex to help you understand how they are put together.
Both Badniks and Bosses typically have at least 1 main attackable hitbox.
Motobugs move at a speed of 1. Once they touch a wall, they wait for 1 second before starting off in the other direction.
Motobugs have a Width Radius of 8 and a Height Radius of 14, resulting in a 17 x 29 rectangle.
They have a hitbox with a width radius of 20 and a height radius of 16, resulting in a 41 x 33 rectangle.
They check for the floor with 1 downwards facing sensor in at it's X Position and Y Position + Height Radius. They will ignore the floor if the distance found is less than -8 or greater than 12. This will trigger a turn.
Motobugs don't check for walls and will only turn when the floor below them runs out. Motobugs don't check for floor while turning.
Choppers have a gravity of 0.09375 and bounce with a speed of -7 at the Y Position where they spawned.
Choppers have a hitbox with a width radius of 12 and a height radius of 16, resulting in a 25 x 33 rectangle.
Buzz Bombers move at a speed of 4 (or -4).
Buzz Bombers have a hitbox with a width radius of 24 and a height radius of 12, resulting in a 49 x 25 rectangle.
Buzz Bomber Attack
If the Player's X Position ends up within a 96px range radius in either direction of the Buzz Bomber's X Position, the Buzz Bomber will prepare to shoot. It will pause for 30 frames before actually shooting (one of those frames being the last frame it had moved). After this, the 3 shooting animation subimages last for 14, 8, and 9 frames respectively. The animation makes it appear the projectile is spawning, but it does not spawn yet. During any action the wings will animate a sub image every 2 frames.
Once this animation ends, the real projectile object spawns.
29 frames later, the Buzz Bomber will begin flying again.
Buzz Bomber Projectile
The projectile moves with an X Speed of 2 or -2, and a Y Speed of 2.
Crabmeats move at a speed of 0.5 (or -0.5) while walking. They will walk for up to 127 frames. When they do turn they simply multiply their speed by -1.
Crabmeats have a Width Radius of 8 and a Height Radius of 16, resulting in a 17 x 33 rectangle.
They have a hitbox with a width radius of 16 and a height radius of 16, resulting in a 33 x 33 rectangle.
They check for the floor with 1 downwards facing sensor which moves depending on the direction it is walking. It also fluctuates position every frame. Every other frame the sensor is positioned at X Position and Y Position + Height Radius, and on the other frames when walking right the sensor is at it's X Position + 16 and Y Position + Height Radius, and when moving left the sensor is at it's X Position - 16 and Y Position + Height Radius. This means it will never step too far off a cliff before turning, but is also checking under it. You could simply use 2 sensors, but this saves processing each frame.
When the sensor is out to the side, they will ignore the floor if the distance found is less than -8 or greater than 12. When no floor is found by that sensor, they will stop (to perform their shooting/turning routine). The middle sensor purely just realigns the Crabmeat to the floor and no distance limits apply.
When walking, a Crabmeat will wait for 128 frames before stopping. Once stopped, it will wait 90 frames before firing projectiles. Upon firing, it will wait an additional 60 frames before beginning to walk again in the opposite direction.
Crabmeats also use natural turning (when finding a ledge) as an opportunity to attack, stopping and performing the same routine.
Crabmeat's projectiles spawn at it's X Position - 16 and X Position + 16, both at it's Y Position.
When shot, Crabmeat's projectiles are given a Y Speed of -4, and an X Speed of either positive or negative 1. They have a gravity of 0.21875.
Caterkillers are comprised of 4 segments, the head, and 3 body segments. The Head Segment is the only vulnerable part, while the body segments have hitboxes which damage the player (these also trigger the scattering upon being touched, but their main job is to hurt Sonic and they cannot be destroyed like a Badnik normally can). They head will also trigger scattering if not destroyed when touched.
The head segment has a Width Radius of 8 and a Height Radius of 7, resulting in a 17 x 15 rectangle. The hitboxes of all segments are just a little bit bigger, with a width radius of 8 and a height radius of 8, resulting in a 17 x 17 rectangle.
Only the head segment has a sensor for detecting the floor at it's X Position and Y Position + Height Radius. It is only active when the head moves a pixel.
The way they move is rather complex compared to other enemies. Firstly, let's go over timings.
Caterkillers scrunch up then flatten/stretch out, on a loop. They spend 16 frames moving, then 8 frames staying still. So this would be scrunching up for 16 frames, then not moving for 8, then stretching out for 16, then not moving for 8, and repeat.
Firstly this will go over how they work while on open ground, then what they do to track properly on slopes, then cover what they do when meeting a wall or a ledge.
For easier reference, we will number the segments like so:
When starting to scrunch (and if the Cater killer isn't turning at all), each body part should already be spaced 12px apart. So if the Head Segment's X position was 50, the next body segment would have an X position of 62, or 38 if it is facing right, and so on. It's mouth is also set to open.
The Head Segment won't proceed (its X speed is 0), instead this is where the back end catches up with the front end.
Segment 1 will proceed with the head's X speed plus -0.25 (0.25 when the segment is moving right). This results in a movement of 4 pixels within the 16 frames.
Segment 2 will move with Segment 1's X speed plus -0.25 (0.25 when the segment is moving right). This results in a movement of 8 pixels within the 16 frames.
Segment 3 will move with Segment 2's X speed plus -0.25 (0.25 when the segment is moving right). This results in a movement of 12 pixels within the 16 frames.
Both the Head Segment and Segment 2 will animate upwards 7 pixels within the 16 frames. The other segments remain flat to the floor. Animate being the keyword here, the actual position and hitbox do not rise at all.
When starting to stretch out (and if the Caterkiller isn't turning at all), each body part should already be spaced 8px apart. So if the Head Segment's X position was 50, the next body segment would have an X position of 58, or 42 if it is facing right, and so on. It's mouth is also set to closed.
Stretching out is basically the opposite, where the head moves forward and the back end stays still.
The Head Segment will proceed with an X speed of -0.75 (0.75 when moving right). This results in a movement of 12 pixels within the 16 frames.
Segment 1 will proceed with the head's speed plus 0.25 (-0.25 when the segment is moving right). This results in a movement of 8 pixels within the 16 frames.
Segment 2 will proceed with Segment 1's X speed plus 0.25 (-0.25 when the segment is moving right). This results in a movement of 4 pixels within the 16 frames.
Segment 3 doesn't proceed, it has Segment 2's X speed plus 0.25 (-0.25 when the segment is moving right). This results in 0 X speed.
This time, both the Head Segment and Segment 2 will animate downwards 7 pixels within the 16 frames, back to being flat to the floor.
As a segment rises, it does so in a particular way.
Across the 16 frames, this is how many pixels the raised segments will be from the ground while scrunching up.
0, 0, 0, 0, 1, 1, 2, 3, 4, 4, 5, 6, 6, 7, 7, 7
This will be reversed as they move back down while stretching out.
In your framework this could be baked into frames of animation or as an extra y offset subtracted from the draw position.
The Head Segment sticks to the floor using a downwards sensor much like Sonic does as it moves. They will turn the floor if the distance found is less than -8 or greater than 12.
Well, each body part needs to also stick to slopes as they move. This is done by having the Head Segment record and remember all of it's Y offsets in a small array (only on the movement frames) and having the next body part read from this as they move. This saves the game having to read data from tiles for each body segment. This array should be around 16 entries long.
Y Offset Arrays
Note: We will assume that position 0 of an array is the oldest value, while new values are added to the end.
After colliding with the floor, X Speed is added to it's X Position, the Head Segment will check each frame whether it has moved a pixel - this being if it's floored X position after moving does not equal it's floored X position before moving.
If movement has occurred, the current Y offset will be added to it's Y offset array.
The other segments will use this array to reposition themselves. Using Segment 1 as an example, it has an index which it will read from the array, this should be at around 12 back from the most recent value. This is because the segments are 12 pixels away from each other when fully stretched.
When Segment 1 moves a pixel (which occurs at different frames to the Head Segment), it will trim the oldest value from the Head Segment's array (effectively moving 1 Y offset array entry into the future). Then, it will read a value from the Head Segment's array at the index position. If this value is valid, the Y position of the segment will be adjusted according to the read value. After this happens, whatever the value is, Segment 1 adds this value to it's own array for Segment 2 to use in the exact same way.
Segment 2 and Segment 3 work in the exact same way, except Segment 2 will use Segment 1's array, and Segment 3 will use Segment 2's array (also, Segment 3 doesn't need to record any values to it's own array, being the last segment).
The result of this is while the arrays are all added to at the rate of that segment's motion, it's read from at the rate of the next segment's motion. This ensures that each segment will meet the same Y offset value when arriving at a particular X position.
Note: There's a possibility of this de-syncing if the timing of the states aren't perfectly in sync or speeds of the segments aren't perfectly balanced.
Ledges And Walls
When the Caterkiller has to turn, it doesn't suddenly stop or even turn all at once. Each body segment independently changes direction.
On the frames that the Head Segment meets a ledge or touches a wall, instead of recording a Y offset from the ground, a value of 128 is stored in the array. This will signal the next segment to turn around also, when they read it. Segments will not align themselves to the ground if nothing is found below or they have encountered a turning signal. The segment may spend 2 or more frames off the side of a ledge by 1 pixel however since the array only updates upon a pixel of movement, a turn signal isn't recorded multiple times.
So, one by one, the segments will change direction as they encounter this signal in the arrays, and each reach the point of turn.
On the movement frame a segment (including the head) turns, the fractional part of it's X position needs to be flipped. This is to ensure the segments are always perfectly pixel aligned after the 16 frame movement has passed. So, if the Head Segment has been moving left, and is at an X position of 50.75 when it turns, the 0.75 portion of the position is flipped (1-0.75 = 0.25) resulting in an X position of 50.25 afterwards. This happens for any segment and will help keep the segment at the same position within a pixel relative to it's direction, so nothing de-syncs.
- 1 pixel seems to be subtracted from a segment's X position when it turns to face left.
- The segment will not stop at any point during a turn, and will continue to move in whichever direction it's now facing as if nothing happened.
When you touch any part of the Caterkiller (unless in doing so you destroy it's head), it will break apart and each segment will bounce away.
Segment Scatter Speed
Upon scattering, each segment first is given its own X speed. Here, starting at the Head Segment and ending with Segment 3.
2, 1.5, -1.5, -2
This speed is negated if the segment is facing to the right. On this frame, their Y speed is set to -4
These will fall with a gravity of (0.21875), and upon contact with the floor their Y speed is set to -4 each time as a bounce.
Bosses work very similarly to Badniks collision-wise. The key difference is hitting them doesn't immediately destroy it, the boss will visually flash to show damage for 32 frames. While flashing, the hitbox is not active. This is to prevent spamming attacks and allow Sonic to rebound from the boss.
In Sonic 1, a flying Eggmobile will typically bob up and down (using sine a wave) with a range of 8 pixels once every 128 frames.
Once destroyed, it explodes for 180 frames, before dropping, raising a bit, and then flying away with an X Speed of 4 and a Y Speed of -0.5.
Green Hill Zone Boss
The Green Hill Zone boss is comprised of 2 main elements, the Eggmobile and the Boulder.
The Eggmobile has a attackable hitbox with a width radius of 24 and a height radius of 24, resulting in a 49 x 49 rectangle.
The Boulder has a hurt hitbox with a width radius of 20 and a height radius of 20, resulting in a 41 x 41 rectangle.
Upon entering and settling into the starting position at the centre X of the screen, the Boulder will spawn at the Eggmobile's X and Y Position and begin to lower 1px per frame until it is 128 pixels below the Eggmobile.
After this, the Eggmobile waits 24 frames before doing it's initial movement.
The Boulder will start to swing towards the right, with one full back and forth swing every 256 frames.
The Eggmobile will start moving to the left with an X Speed of -0.25. This is because the movement starts at the centre the very first time and not at one of the sides, and because the chain is starting at the bottom and has to swing up.
This lasts for 128 frames, and the Boulder has swung up and to the left, and back down again, and is directly under the Eggmobile. The Eggmobile will then stop and rotate to the right. This when the main movement starts.
- The boulder continues to swing the same speed throughout.
Because the Eggmobile pauses when the Boulder is directly below, it waits 64 frames for the Boulder to continue and swing up behind. When the boulder is at it's peak behind the Eggmobile, it will move at an X Speed of 1 or -1 depending on the current direction.
Movement also lasts for 64 frames, and the boulder has swung down and is directly below the Eggmobile again. The Eggmobile turns around and pauses, and the loop starts again, flipped.