Karting without a hitch

Physics in SuperTuxKart are a careful mix between realistic elements and gameplay elements to smooth the experience and avoid many of the ways the kart could get completely out of control. Kart racers are all about doing crazy jumps and flying around corners without having to worry if the vehicle will suddenly flip and tumble.

I began contributing to the game during 1.0’s development and I remember well the work to fix a physics issue present in 0.9.3 that sometimes led to uncontrollable karts floating on downward slopes.

When STK 1.0 came out, it already had many years of fine-tuning behind it, delivering a satisfying experience in most situations.

In this blog post, I’m going to present many changes that have been done for SuperTuxKart Evolution to further enhance how the kart behaves: a major redesign of collisions between karts, aligning the obtention of collectibles with visible contact, changes to jumps, as well as subtler improvements.

Kart-on-kart collisions

Collisions between karts are perhaps one of the gameplay’s least satisfying elements in SuperTuxKart 1.5. It happens more by accident than deliberately, because the player initiating the collision will very often lose a lot of speed. Realistic, but not very fun.

In 1.5, collisions are only sought to pass off a bomb.

The philosophy behind the improvements brought by STK Evolution has been to make accidental collisions less disruptive while also creating more opportunities for deliberate collisions: pushing a rival off the track or into a road hazard is now a legitimate tactic.

To achieve this, we use several tricks:

• An impulse pushing both karts away from each other is generated whenever a kart collides with the side of another kart. It is not instantaneous, rather it applies a pushing force for a short period of time, with a fade-in and a fade-out period.
• We look at several factors to determine the strength of the impulse for each kart: relative speed, relative weight, which kart is pushing the other, and at what angle.
• We prevent the kart from being rotated by the collision or the impulse: collisions cause lateral motion, not imprevisible changes of direction. Steering continues working normally while the impulse is applied.

As a result of all these changes and a lot of careful tuning, the effect of collisions is more predictable and manageable for players.

Accidental collisions are usually harmless, there is more incentive to deliberately try to push a rival off the track, and factors such as kart weight, collision angle and relative speed matter much more than they used to.

All this together transform collisions from an annoyance into a fun part of the game.

Note that all the changes discussed above apply to collisions where the side of a kart is touched. This includes some collisions that may look at first glance to be more from the back, because the kart hitting from behind is substantially on the left or the right.

Collisions with the front of a kart touching the back of another, which are only a small portion of all collisions, are deliberately left unchanged, with the kart behind transferring speed to the kart in front.

To match the gameplay improvements with some eye-candy, we also plan on adding new kart animations that would be triggered when karts hit an obstacle or each other.

These changes also fix an issue present in SuperTuxKart 1.5 and earlier versions that frequent participants of online multiplayer encounter sooner or later : the collision deadlock.

When the kart in front got pushed from the side in the back, it would create a rotational force: the back of the kart would move away from the pushing kart, while the front would move towards it.

The arrows show the rotational forces the pushed kart is subject to without the impulse separating the karts.

To try to keep going in its intended direction, the kart being pushed would start turning in the opposite direction. It prevents veering off-track, but it means that now both karts are pushing against each other.

At this stage, neither kart is able to steer freely, nor to disengage safely : both karts locked together would usually soon crash. This situation can never arise in SuperTuxKart Evolution.

Touching items on the track

In SuperTuxKart 1.5, hitboxes are used for collisions with karts, walls and other physical objects, but they don’t matter when it comes to collecting nitro bottles, bananas, gums laying on the ground, etc.

In 1.5, the gum is not triggered despite clear contact with the kart.

In 1.5, if this kart gets any closer to the nitro can, it will collect it, despite a clear gap between the kart and the nitro.

With SuperTuxKart Evolution, this changes. Whereas the only thing that counted previously was the distance between the kart’s center and the item’s center, kart and item size are made relevant again.

As a consequence:

• There is close alignment between visual touch and item collection. There is no need to internalize the collection distance to pick optimal trajectories.
• A kart having a smaller hitbox is not always an advantage. It helps to avoid dangerous items and for cornering, but more precision and focus is required to pick up beneficial items.
• It offers another way to differentiate between item types. Small nitro bottle are harder to catch than big nitro cans, STK Evolution introduces small gums that are easier to avoid than regular gums, and more possibilities are open.

STK Evolution also fixes a related issue, curious yet frustrating: karts flying slightly above a gum laying on the ground would trigger it and slow down drastically. Now, at least one wheel needs to be touching the ground for the kart to stick to the gum.

In STK 1.5, this kart would have triggered the gum it is jumping above.

Jumps, tilted karts and more

It's great to see that you are interested in SuperTuxKart!

This Itch devlog is a copy of a post published on our official blog. I wanted to experiment with longer form updates here.

It's a bit of a fight to copy things over, so if you would like to read the four remaining sections that talk about changes to jumps, about making karts tilted on the side quickly controllable again, and collisions with the terrain with some neat graphs and pictures, head over to our blog.

A kart tilted on its side. In 1.5, guaranteed crash. In Evolution, not so.

If you would like to support our work and help bring about SuperTuxKart Evolution, you can donate here through Itch by downloading the latest release of STK using the button below and picking your price for the game. 

Donations play a major role to keep the project sustainable. A donation package with two tracks in early access is also available for donations of 5$ or more.