The Group input of the ExpressionME-L Node enables efficient parallel processing of particle data. When this input is created as the first input, all connected node data will be processed using multi-threading. If the Group input is not the first input, it will be processed in a single threaded, serial manner.
A variety of powerful functions are available for reading specific particle data. You can also choose to include or exclude subgroups in the evaluation process. When subgroups are included, all particles will be treated as if they were in a single combined pool.

Note, particle count is a variable and not a function. It is directly accessible through the given input name that has been created. See example below:

Accessing particle count in any given group:

// IO Block Start
   input_desc('bysize', port_mgroup); // create a internal Group called bysize
   input_desc('dust', port_group); // create a tP particle Group input called dust
// IO Block End

// accessing the dust group particles
for(var i = 0; i < dust_in_count; i += 1)
 {\\
    pid=group_in_id2pid(i);
    ..do something
 };

for(var i = 0; i < bysize_in_count; i += 1)
 {
    pid=group_in_id2pid(i);
    ..do something
 };

The group_in_id2pid function is used to convert a particle ID within a group to a real thinkingParticles (tP) particle ID. Particle Groups are like containers that can store a large number of particles. These particles can either be created within the group or moved into it. Each particle has its own internal ID that is used to address or affect the exact particle within thinkingParticles. This function is used to retrieve the system-wide ID of a particle within a group.

Example:
Every particle group can collect and hold particles in any way possible. Within each group, particles are arranged in a simple linear list ordered by creation, from 0 to the maximum number of particles in the group. To retrieve the unique internal particle ID, used by thinkingParticles internally, you can call this function. For example, to get the unique ID of particle 17, you would call realid=group_in_id2pid(17) and this function would return perhaps ID:376895 which is the particle known to tP.

Call:

group_in_id2pid(in_id)

Output:

Code Example: 

go through all particles in the group

var pid;
var ppos[3];

 for(var i = 0; i < group_in_count; i += 1)
 {
    pid=group_in_id2pid(i);
            
    ..as sample get the position from the pid
    get_pdata(pid, data_pos, ppos);
 };

 used to search for particles within a given radius of a specified world position in a group.

Call:

group_in_find(world_position, radius, max_found, optional skip_particleid)

This function has the following parameters:

Return:
The function returns a scalar value indicating the number of particles found.

Gathers any particle data from a set of found particles within a group. This function only makes sense, when group_in _find returned a result to iterate over.

Call:

 group_in_find_data(id, find_type, ret_value)

Return:
If the function returns 0, it means there was no data  and the value of ret_value will be unspecified. If the function returns 1, it means that access to the data was successful and ret_value has been set to the gathered data.

Usage:

To find the maximum of 5 particles at position (0,0,0) with a radius of 10 in the input group2, you can use the following code:

  var pos[3] = {0,0,0};
  var found = group2_in_find(pos, 10, 5);

if(found > 0)
{
    var pid2;
    var pvel2[3];
    var dir;    
   
    //iterate through the found data 
    for (var i := 0; i < found; i += 1)
    {
        pgroup2_in_find_data(i, find_pid, pid2);    //get the find pid\\
        pgroup2_in_find_data(i, find_dir,  dir);    //get the find direction
        get_pdata(pid2, data_vel , pvel2);          //get the velocity from the find pid        
        ......%%//%%do anything with the found data    
    };
};

Explanation:

• The group2_in_find function is called with the search position (0,0,0), the radius (10) and the maximum number of particles to find (5).
• If the function returns a value greater than 0, it means that at least one particle was found.
• The loop iterates through the found particles and calls the pgroup2_in_find_data function to get the particle ID of the found particles and gets the particles's direction.
• The get_pdata function is called to get the velocity of the particle with id pid2.

this function is used to create a new particle at a specified position. When the first input of the ME-L Expression node is a particle group, at least one particle needs to be present to execute this function.
Be careful not to create particles into the same group or you will end up with exponentionally creating particles of particles of particles …

Call:

group_in_create(pos, optional velocity)

Output:

This function intersects all particle shapes in a group.

Call:

group_in_isect(pos, dir, max_length, optional pskip, optional fskip)

Parameters:

Output:

This function gets the hit data of a group intersection. It is only valid after a group_in_isect result is not equal to 0.

Call:

group_in_isect_data(isect_data, vector_out[3])\\
group_in_isect_data(isect_data, scalar_out)\\
group_in_isect_data(isect_uvw, chan, uvw_out)

Parameters:

The isect_data object includes the following possible properties:

Output:

Example:

if(pgroup2_in_isect(wp, pvel*dt, 1.0))
{    
   pgroup2_in_isect_data(point_norm, dir);  
   pgroup2_in_isect_data(point_vel,  vel);
   dprod = dot(dir, pvel - vel);
   if(dprod < 0)
    {
        pvel = pvel - dir *  (dprod * bounce_in);
        set_pdata(pid, data_vel , pvel);
        pgroup2_in_isect_data(point_col,  color);
        set_pdata(pid, data_dchan+1, color);
    }   else
            dprod = 0;
};

Explanation:

This code example checks for intersections between a particle group and a ray, and modifies the velocity and color of the particles in the group based on the result of the intersection.

The code first calls the pgroup2_in_isect function, passing in the wp (world position) of the particle, the velocity of the particle multiplied by dt (delta time), and a maximum length of 1.0. If this function returns true, indicating that an intersection occurred, the code proceeds to retrieve data about the intersection using the pgroup2_in_isect_data function.

The pgroup2_in_isect_data function is called twice, the first time with the point_norm parameter and the dir variable as output, and the second time with the point_vel parameter and the vel variable as output. These calls retrieve the surface normal and velocity of the particle at the point of intersection.

The code then calculates the dot product of the surface normal and the difference between the particle's velocity and the vel variable. If this dot product is negative, the code modifies the particle's velocity by subtracting the surface normal multiplied by the dot product and the bounce_in variable. It then sets the modified velocity back to the particle using the set_pdata function, and sets the particle's color to the color variable using the set_pdata function as well. If the dot product is not negative, it is set to 0.

ME-L groups behave similarly to thinkingParticles' own particle groups, with the exception that they do not have a hierarchy. The same particle can be in several groups. Virtual ME-L groups are accessible throughout multiple ME-L nodes, either in one or many DynamicSets. Accessing virtual ME-L Groups between multiple nodes is done via a strict naming convention. By using the identical name in every ME-L Node, the same group properties will be shared as well.

As with any Node input, when the first input is an ME-L group, the data stored within will be processed in a multi-threaded parallel manner.

Adds a particle to the virtual ME-L group. Multiple calls with the same ID, will not add the same particle twice to this group.

Call:

mgroup_in_add(particle_id_in);

Return:
The function returns a scalar >=0 for a valid particle ID, -1 for no particle ID

Removes a particle from the virtual ME-L Group

Call:

mgroup_in_remove(particle_id_in);

Return:
The function returns a scalar >=0 for a valid particle ID, -1 for no particle ID

Checks if a particle is part of a virtual ME-L Group

Call:

mgroup_in_ingroup(particle_id_in);

Return:
The function returns a scalar >=0 for a valid particle ID, -1 for no particle ID

ME-L virtual Group Examples: Go manually through all particles in a ME-L Group 

for(var i = 0; i < mgroup_in_count; i+= 1)
        {
            pid = mgroup_in_id2pid(i);
            
            if(pid >= 0)
            {
                ..... do something
            }    
        }