Projectile ballistic coefficients (drag factors)

Add a field to the projectile datablock for the ballistic coefficient...

In engine\game\projectile.h immediately behind the declaration

bool isBallistic;

add

F32 ballisticCoefficient;

Provide a suitable default value in the constructor...

In engine\game\projectile.cc in the ProjectileData::ProjectileData()
constructor just behind the statement

isBallistic = false;

add

ballisticCoefficient = 1.0; // completely frictionless

Make the field console accessible...

In engine\game\projectile.cc at the end of the ProjectileData::initPersistFields()
function add

addNamedFieldV(ballisticCoefficient, TypeF32, ProjectileData, new FRangeValidator(0, 1));

Add client/server data exchange support for it...

In engine\game\projectile.cc in the ProjectileData::packData(BitStream* stream)
function just behind the statement

stream->write(gravityMod);

add

stream->write(ballisticCoefficient);

In engine\game\projectile.cc in the ProjectileData::unpackData(BitStream* stream)
function just behind the statement

stream->read(&gravityMod);

add

stream->read(&ballisticCoefficient);

Now for the physics simulation changes...

In engine\game\projectile.cc in the Projectile::processTick(const Move* move)
function change the block of code

// Otherwise, we have to do some simulation work.
   oldPosition = mCurrPosition;

   if( mDataBlock->isBallistic ) 
      mCurrVelocity.z -= 9.81f * mDataBlock->gravityMod * (F32(TickMs) / 1000.0f);
   
   newPosition = oldPosition + mCurrVelocity * (F32(TickMs) / 1000.0f);

   if (bool(mSourceObject))
      mSourceObject->disableCollision();

to read as follows

// Otherwise, we have to do some simulation work.
   oldPosition = mCurrPosition;

   F32 dT = F32(TickMs) * 0.001f; // 1/1000.0 division avoidance
   if( mDataBlock->isBallistic ) {
   // modified to account for drag coefficient or form factor
      F32		dV = (1-mDataBlock->ballisticCoefficient) * dT;
      Point3F	d(mCurrVelocity.x * dV,mCurrVelocity.y * dV, 9.81f * mDataBlock->gravityMod * dT);
      mCurrVelocity -= d;
	}
	newPosition = oldPosition + mCurrVelocity * dT;

   if (bool(mSourceObject))
      mSourceObject->disableCollision();

Do a Build of the engine.

Now for some example script changes to put the ballistic coefficient into service...

In example\fps\server\scripts\rifle.cs in datablock ProjectileData(RifleProjectile)
add

ballisticCoefficient = 0.95; // 0 - horrid drag, 1 - no drag

And for comparison purposes...

In example\starter.fps\server\scripts\crossbow.cs in datablock ProjectileData(CrossbowProjectile)
add

ballisticCoefficient = 0.85; // 0 - horrid drag, 1 - no drag

Once this change is incorporated, if you set a projectiles ballisticCoefficient to any value less than 1.0 its velocity drops off in a non linear manner during flight. If set to 1.0 ballisticCoefficient has no effect. If set to 0.0, the projectile will drop to your feet (infinite drag, essentially). Good values may be derived empirically. Note that the field name is a bit of a misnomer since this is not a real ballistic coefficient (BC) (derived by measurements, based on a standard atmosphere, gravity, etc. and relative to a an established standard projectile), but the name conveys the effect acheived through its use. Real rifle bullets BCs typically range from about 0.15 to 0.55 so if you want to tie your BCs to real numbers, adjust by adding a constant of 0.40 to the real BC to get a value that
will work well in the simulation.