Gamedev:CollisionVolumes

What is a collision volume?

The collision volume is a geometric shape around (almost) all Spring units. If a projectile collides with this shape, the engine registers it as a hit. Other names are hitbox, hitsphere etc.

How to see them ingame

Press alt + b and look at the black wireframe shapes around units. If you can not see them, the collision volume might be blocked from view by the 3D model or be very offset like underground or in the sky. Red shapes indicate the unit was hit.

What else are they used for?

• line-of-fire calculations
• mouse-click detection when selecting a single unit

How to edit their shape

What are they measured in?

in elmos.

In Upspring

in the 3D view, click on the blue "3D" text and a menu opens.
Click "draw model radius + height"
At the right side, go to the "Model" tab and edit the values.
With Upspring, you can only define sphere shaped collision volumes.

editing_hitbox_in_upspring.jpg

in the UnitDef

There are numerous tags that controll shape, size, position of the collision volume. You have to restart spring.exe for the changes to take effect, there is no way to reload unitdefs.

collisionVolumeOffsets = [[0 0 0]],

position of the collision volume, in this example center of the unit. The value is relative to the unit center set in Upspring.

collisionVolumeType = [[box]],

defines the shape. possible are: box, ellipsoid and cylinders: CylX, CylY, CylZ

collisionVolumeScales = =  [[40 20 55]]

dimensions along X Y Z axes.

boxxy the hitbox editor

This creation allows you move hitboxes by pressing buttons. This way you can get the needed numbers and copy them to the unitdef file.

forum thread

Multiple collision volumes

A unit can have multiple collision volumes. For example a tank could use a box for the chasis and a cylinder for the turret.

Each piece can have its own collision volume.

usePieceCollisionVolumes = true,

will cause each piece on the model to have an autogenerated hitvolume; unwanted hitvolumes can then be removed with Lua.

Can they be changed during gameplay?

Yes. One example would be "pop up" turrets that get a smaller hitbox when sunken into the ground etc. See Spring.SetUnitCollisionVolumeData() and Spring.SetUnitPieceCollisionVolumeData() at Lua_SyncedCtrl. There seems to be no noticeable cpu usage when dynamically adjusting collision volumes. Example: Dynamic Collision Volume gadget and it's configuration file.

CPU Usage of non-sphere volumes

For the engine, collision with a sphere are fastest to calculate. Cylinders and boxes require more math and so are slower. How much does it really matter? No idea.

Unit radius & height VS collision volume

Even when a unit defines a collision volume that is not a sphere, the radius and height values set in Upspring still have a number of purposes. To name a few examples: height is used to decide whether a ship can move over an underwater structure, radius is used in collision handling for aircraft.

Which spot on an enemy do units target

The model center set in Upspring

Common problems (and how to solve)

weapons miss the unit

Weapons like Dgun and torpedos might pass through under the unit if the hitvolume does not extend a bit into the ground/underwater.

Intersecting hitboxes

Two near units might refuse to shoot if their turrets intersect with each other's hitboxes.

Fast projectiles passing through collision volumes

The problem: fast projectiles sometimes fly through small hit volumes without causing damage: The solution: collisionVolumeTest = 1, in the unitDef, switches to continuous rather than discrete collision tests and hits will then register correctly.

related weapon tags

targetBorder

Units will target the nearest / farthest edge (targetBorder=1 / targetBorder=-1) of a hit volume instead of its center (targetBorder=0) along their line of fire. Works on all volume types.

Aircraft

The hitspheres of airplanes seem to be scaled down -50% by the engine.

This is removed in 0.83, see here

Engine code

related engine parts are: 1 ... 2 ... ...