SPG:Rebound

Sonic Physics Guide
Basics Positions And Speeds Angles Variables Calculations (Technical) Pixel and Subpixel Hex Angles Characters Sonic Tails Knuckles
Collision
Solid Tiles (Terrain Part 1) Sensors Solid Terrain (Terrain Part 2) Layers Loops Slope Collision (Slopes Part 1) The Player's Sensors While Grounded 360 Degree Collision While Airborne Hitboxes The Player's Hitbox Trigger Areas Solid Objects Solid Objects Sloped Objects Jump Through Platforms Pushable Blocks Item Monitor
Physics
Slope Physics (Slopes Part 2) Moving Along Slopes 360 Degree Movement Falling and Slipping Down Slopes Landing On The Ground Air State Running Jumping Rolling Getting Hit Rebound Underwater Air Physics Super Speeds
Gameplay
Main Game Loop Game Objects Game Enemies Badniks Bosses Ring Loss Special Abilities Spindash Super Peel Out Drop Dash Insta-Shield Flying Gliding Climbing Elemental Shields Flame Shield Bubble Shield Lightning Shield Player 2 Player 2 CPU Respawning Special Stages Rotating Maze
Presentation
Camera Animations Animation System Variable Speed Animation Timings Normal Animation Timings Animation Rules
Special
Overlay Scripts Sonic 1 Overlay Sonic 2 Overlay

Notes:

• The research applies to all four of the Sega Mega Drive games and Sonic CD.

Badniks

When the Player destroys a Badnik, and they are in the air, the game checks to see if they should rebound or not.

If the Player's Y Position is greater than that of the Badnik, or their Y Speed is negative, they will not rebound. Instead, sign(Y Speed) will be subtracted from Y Speed (this is not limited to 0. If Y Speed were 0.25 (64 subpixels), it would not be set to 0, but to -0.75 (-192 subpixels)). This slight reduction of vertical speed helps give the impression that the Badnik had some weight to it.

On the other hand, if the Player's Y Position is less than that of the Badnik and their Y Speed is positive, they will rebound. Their Y Speed will simply be multiplied by -1, reversing it fully.

Note: X Speed is totally unaffected by destroying a Badnik, both in the air and when rolling on the ground.

Bounce Height

But can't you control the height of the bounce by holding or not holding the jump button? Well yes, but that is actually part of the main jumping code.

If the Player has jumped (and only if they has jumped) into the Badnik, the variable jump height code is still in effect. As mentioned in the Variable Jump Height - during a jump, at any point if the player isn't holding jump and their Y Speed is less than -4, it will be set to -4. For this reason, if you are not holding jump when you collide with the enemy, and if their new Y Speed is less than -4, it will be set to -4 because the jump code does that already. Effectively, allowing you to control the rebound height.

This means that if you weren't jumping, and instead land on a Badnik by rolling, you cannot control your bounce height at all, as the jump height code is not in effect.

Note:

• This bounce control scenario applies to all other similar rebounds, such as from Item Monitors.

Monitors

Note: For more details about collision with Item Monitors, see Solid Objects and Game Objects.

Sonic 1 Method

(used in Sonic 1, Sonic 2, Sonic CD, and Sonic 3)

When the Player, spinning, collides with a monitor while in the air, the game checks whether their Y Speed is positive or not. If it is, they will rebound off of the Item Monitor in the same way as they do Badniks - only without bothering to compare their Y Position to that of the Monitor. The Player will rebound upward every time, no matter what.

However, if their Y Speed is negative upon collision with the Monitor, it then compares the Player's Y Position with that of the Monitor. If the Player's Y Position is less, then they are moved outside of the Monitor to the side, as if it were solid, and it is not destroyed. If the Player's Y Position is greater, then the Player bumps into the bottom of the Monitor, bouncing it upward a little bit (for example, if the monitor is in a tree this knocks it down.)

Sonic & Knuckles Method

(used in Sonic & Knuckles and Sonic 3 & Knuckles)

In Sonic & Knuckles, the Item Monitor collision behaviour has been revised. Monitors are not considered solid when the Player hits them while their Y Speed is negative, and they can't bounce them up by hitting them from beneath. This means the Flame Shield Monitor in the tree in Angel Island Act 2 can't be knocked down when Sonic 3 is locked on to Sonic & Knuckles, even though it can be in Sonic 3 alone.

If their Y Speed is positive when they hits the Monitor, they will rebound normally as they do in the other games. But if Y Speed is negative, their Y Speed will be multiplied by -1. This means if they jumps into a Monitor from underneath, they'll be bounced downward. Also, if they hit a Monitor in its corner as they jump over it, they'll be bounced back toward the ground.

Note: In all the games, X Speed is totally unaffected by destroying a Monitor, both in the air and when rolling on the ground.

Bosses

When the Player hits a boss while in the air, their X Speed and Y Speed are both divided by 2 and reversed (so, multiplied by -0.5). Though, if on the ground, their Ground Speed is not affected at all. In your game, you may wish to fix that and have Ground Speed reverse and divide by 2.

If they are rebounded by a boss upward by this, then the standard jump circumstances still apply: releasing the Jump button while Y Speed is less than -4 will set Y Speed to -4, air drag is calculated, and so on.

Notes

• Gravity is added to Y Speed before the engine checks for and/or performs the rebounding routine.