UFO-Scripts/ptl *.ufo

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Particles are used for weapon and map effects. The particle engine is fully scriptable and already has some nice looking particles. To create new particles you should read throught the datatypes part.

They are small, short lived entities which are controlled by a script. UFO:AI uses particles to depict bullets, rockets, explosions, smoke, laser beams, and so on. Some effects are a combination of several particles – in fact a particle can spawn more particles.

Particle are local entities, like actors and aliens. Like other entities the particle has an origin, a think function, etc.

Units of Measurement


  • The Quake 2 engine measures time in milliseconds. For instance, the cl.time variable is defined as int time in the client_state_t struct, which means that its size varies depending on the size of an int on the local system.
    Specifically, if the local system defines integers as 16 bits, then cl.time has a maximum value of 32767, which means it overflows after about 33 seconds.
    The gnu-linux-386 architecture defines 32 bit integers, which means the cl.time value is good for about 24 days.
    We shall assume 32 bit integers and therefore a non-overflowing cl.time value.
  • The UFO:AI particle system measures time in seconds. Time values are stored in float type variables. Basically, the particle functions multiply the quake2 time by a scaling factor of 0.001.


Distance is measured in texels (See also Texel (graphics) at Wikipedia.).
A typical male actor is 56 texels tall and 24 texels wide. If the height of an average white man is about 180 cm then a texel is about 3.2 cm, or 5/4 inches.

A UFO:AI cell is 32x32x72 texels which is about 1x1x2.3 meters.

Particle Definitions

Every particle in the UFO:AI game is an instantiation of a particle definition. Particle definitions are declared by scripts in the base/ufos directory. A particle definition consists of three functions: a mandatory init function and optional think and run functions. Each function contains commands. A command may or may not have a reference (ie an argument). Here is an example of a particle definition (taken from base/ufos/weaponsfx.ufo).

// ==================
// flamethrower
// ==================

particle fireBall
    image  sfx/fireball
    blend  add
    style  facing
    tps    2.5
    push   pos "80 80"
    mul    *t
    add    pos "10 10"
    pop    *size
  think { tfade  out }

The Language

Particle References

A command may include a target of the operation. This is called the reference. The reference may take several forms: a constant, a stack, or a variable. If the reference is a constant and the command accepts more than one type of reference, then the constant will be preceded by a string specifying the type (float, pos, vector, or color). The octalthorpe (#) reference means that the command operates on the top element on the stack. Variables are more complicated ...

Particle Variables

UFO:AI defines twenty-two variables. Each variable has a specific type. Each variable corresponds to a property of the particle (ie a value in its local entity or particle type structure). Most commands can take a variable as an argument. When used as an argument, variables are prefixed with an asterisk “*”. For instance pop *v moves a value from the stack to variable v.


  • image: V_STRING path to model - relative from base/pics. This is mostly used for particles which do not have models, ie impact particles. Supported are all image types the engine supports (see Artwork).
  • model: V_STRING path to model - relative from base/models. Models are only rendered if the particle is an entity, which only happens for projectiles. Supported are all model types the engine supports (see Modelling).
  • skin: V_INT skin number that should be use for the model
  • blend: V_STRING The blending function for the particle image.
    • replace
    • blend
    • one
    • add
    • filter
    • invfilter
  • style: V_STRING Defines the basic type&shape of the particle.
    • facing The particle will always face the camera/viewer.
    • rotated Fixed rotation in the world (given by angles)
    • beam (Guess: The particle will be stretched from a first point (s) to a second one [defined by velocity (v)?])
    • line Will draw a line from a first point defined by s to a second one defined by velocity v
    • axis
    • circle Internal style - don't use this
  • thinkfade: V_STRING think fade
    • none Default - tps value does not change over time.
    • in (Guess: Increase the tps value linearly over time [starting with 0])
    • out Decrease the tps value linearly over time [starting with the original tps value and reducing to 0]
    • sin Alter the tps value over time in a sinus function [between the 0 and tps value]
    • saw (Guess: Alter the tps value over time linearly [between the 0 and tps value].)
    • blend
  • scroll_s: V_FLOAT Scrolls texture coordinates in S direction
  • scroll_t: V_FLOAT Scrolls texture coordinates in T direction
  • framefade: V_STRING frame fade
none, in, out, sin, saw, blend
  • size: V_POS typically size[0] is the length of the weapon in the direction in which it is fired.
Some particle represent beams from beams weapons – these beams are considered to travel infinitely fast ie they are instantaneous. For these instantaneous particles, the size is equal to the distance between the origin and the target.
  • scale: V_VECTOR scaling in x, y and z direction
  • color: V_COLOR color vector of RGBA
  • a: V_VECTOR acceleration vector
  • v: V_VECTOR velocity vector
  • s: V_VECTOR location vector
  • angles: V_VECTOR <pitch, yaw, roll>. Typically this is used for projectiles; it points from the muzzle coordinate towards the impact coordinate.
(see omega, below)
  • omega: V_VECTOR the rotation vector for the particle
(newAngles = oldAngles + frametime * omega)
  • life: V_FLOAT specifies how long a particle will be active (seconds)
  • rounds: V_INT specifies how many rounds a particle will be active
  • tps: V_FLOAT think per second - call the think function tps times each second, the first call at 1/tps seconds
  • lastthink: V_FLOAT
  • frame: V_INT see endframe
  • endframe: V_INT frame and endframe specifies the frame count for an animated particle
 frame 1
 endframe 10
 image +sfx/uexp_

This example will use the images sfx/uexp_01 - sfx/uexp_10 for the particles in the correct order to simulate something like an explosion.

  • fps: V_FLOAT how many frames per second (animate)
  • levelflags: V_INT display this particle only in specific levels
  • lastframe: V_FLOAT time (in seconds) when the think function was last executed (perhaps this can be used to delay or speed up particle actions).
  • dt: V_FLOAT time increment for rendering this particle (delta time)
  • t: V_FLOAT time that the particle has been active already
  • offset: V_VECTOR determines whether the particles image is 'shifted' along the long and lat of the particle. The x coord specifies shift perpendicular to the direction the particle moves and the y coord specifies shift parallel to the direction of motion.
  • light: V_INT specifies light intensity of dlight (dynamic light) (see also the color vector to define the light color)
  • physics: V_BOOL don't let the particles go through a wall
    • stick: V_BOOL this will make the particle stick to the brush that was hit
    • bounce: V_BOOL this will make the particle bounce away from the brush it hit
    • stayalive: V_BOOL only works in combination with physics - let the particles die 'naturally' when they hit something solid
  • autohide: V_BOOL autohide the particle if the current position is higher than the current selected worldlevel (useful for weather particles)

Stack functions and particle commands

In the init and run functions each command is written on a separate line, but that is not required. In fact, the whole script could be written without any line feeds or carriage returns. Line feeds/carriage returns are not required – but whitespace must appear around every token. Brace brackets, function names, commands and arguments are tokens. The fireBall particle only uses nine commands. In fact, there are nineteen particle commands. Commands have an argument data type in the same way that variables have a data type. Commands may be given a reference as an argument. The type of the argument (e.g. a reference) and the command must be the same. Here is a list of all the particle commands, with data types of their arguments and descriptions.

Marks the end of a function. Takes no arguments. This command is not usually seen in script code --– it is automatically added to the end of the function by the parser
  • push (any type)
Put reference on the stack. The reference may be of any type
  • pop (any type)
Pop value from the stack to the indicated variable
  • kpop (any type)
Pop value from the stack to the indicated variable, push the value back to the stack
Pop value from the stack, add it to reference value, push the sum to the stack
Pop value from the stack, subtract reference, push the result to the stack
Pop value from the stack, multiply it by reference, push the result to the stack
Pop value from the stack, divide it by reference, push the result to the stack
Calculate sine of reference, push result to the stack
Calculate cosine of reference, push result to the stack
Calculate tangent of reference, push result to the stack
Push a random value between 0 and the argument onto the stack. Cannot produce V_INT values to, e.g., spawn a random number of particles
Multiplies each entry of the given parameter from stack with a random value from 0 to 1
newvalue(s) = [parameter_1*(0...1), parameter_2*(0...1), etc...]
See also frand() in #q_shared.c and PC_RAND in cl_particle.c:CL_ParticleFunction
Push a random value between -argument and +argument onto the stack
Multiplies each entry of the given parameter from stack with a random value from -1 to 1
newvalue(s) = [parameter_1*(-1...1), parameter_2*(-1...1), etc...]
See also cfrand() in #q_shared.c and PC_CRAND in cl_particle.c:CL_ParticleFunction
Pop the last two values from the stack, use these values to create a pos type value, and push the pos value onto the stack. Takes no arguments
Pop the last three values from the stack, use these values to create a vector type value, and push this vector value onto the stack
Pop the last four values from the stack, use these values to create a color type value, and push this color value onto the stack
set inuse flag to false (deacticate the particle)
Instantiates a new particle. The argument must be a defined particle name. The spawned particle inherits the parent's s, v and a values
Instantiates one or more new particles. The argument must be a defined particle name. The number of particles to create is taken from the stack. The spawned particles inherit the parent's s, v and a values
child particles are always rendered relative to their parents. Combined with the use of offset (see above) this allows complex particles to be made that behave as single entities.

Basic Datatypes

  • If you are a coder you should already know float (V_FLOAT), integer (V_INT) and string (V_STRING) types.
  • A position type (V_POS) is a set of two positive integers.
  • A vector type (V_VECTOR) is a set of three floats.
  • A color type (V_COLOR) is a set of four floats, each float having a value between 0 and 1 inclusive (normalized red green blue and alpha channels).
  • style, fade and blend have specific allowed values (actually strings) defined in source/game/q_shared.c as shown below:
    • blend
    • replace
    • add
      glBlendFunc( GL_ONE, GL_ONE )
    • filter
      glBlendFunc( GL_ZERO, GL_SRC_COLOR )
    • invfilter
      glBlendFunc( GL_ZERO, GL_ONE_MINUS_SRC_COLOR )

Assignment Command

The assignment command is simply the name of the variable followed by the value. In the fireBall example image sfx/fireball assigns the string value sfx/fireball to the image variable. Note that strings to not require quotation marks. Also, remember that the data types of variable and its value must match.

Vector Components

It is possible to specify the individual components of a vector or color. This is done by adding a suffix to the argument. The suffix consists of a period “.” followed by the index of the desired component. For instance, in the particle laserPulse definition:

push float 2.2
pop *size.2

Moves the value 2.2 to size[1].

Projectile Particles

Weapons fire projectiles: beams, bullets, rockets, etc. When a weapons fires, the LE_AddProjectile function creates an entity, sets the s variable to the location of the muzzle of the gun and spawns particles representing the projectiles. The init function runs as soon as the particle spawns. The le->ptl element points to the particle, so projectiles are both entities and particles.

Next, the LE_AddProjectile function sets the particle's angle attribute to point toward the target. As far as I can tell, this does two things: it rotates the particle image so that the particle (eg a beam) is pointed the right way, and similarly for finite speed particle models. AFAIK the angles element has nothing to do with motion because that is accomplished using le->mins and/or the v and a variables.

Infinite speed projectiles

If the speed of a projectile is set to zero, then it is an infinite speed projectile: eg laser beam, bullet, SMG. In this case, LE_AddProjectile sets the length of the projectile (size[0]) to the distance between the muzzle and the target and sets the s variable to the midpoint between the muzzle and the target. If the weapons has impact effects, then LE_AddProjectile spawns the impact particles. Then LE_AddProjectile returns. LE_AddProjectile does not kill the particle – all infinite speed projectiles contain a kill command in the think function, so the particle will disappear when the particle entity performs its first think routine (don't forget to put such a kill command in your definition!). The lifetime of such a particle can be controlled in the init function by manipulating the tps and lastthink variables.

Finite speed projectiles

If the speed of a projectile is not zero, then it is a finite speed projectile: a rocket, napalm (from the flamethrower), or tachyon burst. These projectiles fly from target to impact. LE_AddProjectile does the following:

  • sets the s variable to the muzzle coordinate
  • spawns the particle (which also runs the particle's init function)
  • sets the particle's angles to point towards the impact coordinate
  • sets the le->maxs coordinate to the impact location
  • converts the speed to a velocity vector pointing in the direction of the target, and stores this vector in le->mins
  • offsets the particle's origin (le->origin) slightly towards the target, half the length (size[0]) away from the muzzle
  • sets the particle entity's endtime (le->endtime) to be equal to the time required for the projectile to travel to the impact coordinate
  • if the fire definition includes an impact, then set some references for the impact definition
  • set and execute the particle's think function

The LET_Projectile function implements motion for finite speed projectiles. Remember that the velocity of a projectile is stored in the le->mins vector. Each time the local entity does a think function LET_Projectile which moves the projectile allow the mins vector towards its target. For this reason, the particle definitions for finite speed projectiles never use the v or a variables. They also do not need the kill command.

Impact Particles

Impact particles are a little different from projectile particles – most importantly, impact particles are not entities. Since they are not entities, they do not receive an entity think function. Instead, once per frame the CL_Frame function calls the CL_ParticleRun function. For each active particle CL_ParticleRun:

  • advances the time
  • updates the location of the particle using the (s, v, a and dt particle variables)
  • executes the particles' run function
  • if it is the right time, executes the particle's think function
  • if it is the right time, shows a frame of the particle's animation
  • fades the particle (if fading is enabled)

If you want an impact particle to move then you must set the v or a variables in its particle definition. Don't forget to kill an impact particle with an explicit kill command, either in the think section or elsewhere!


  1. misc_particle
  2. Levelflags