Constant PARTICLE_COUNT_GLSL 

pub const PARTICLE_COUNT_GLSL: &str = r#"
#version 430 core

layout(local_size_x = 256) in;

struct Particle { vec4 position; vec4 velocity; vec4 color; float size; float mass; uint flags; uint attractor; };

layout(std430, binding = 0) readonly buffer ParticleBuffer { Particle particles[]; };
layout(std430, binding = 1) buffer IndirectBuffer {
    uint vertex_count;
    uint instance_count;
    uint first_vertex;
    uint base_instance;
};

uniform uint num_particles;

shared uint local_count;

void main() {
    if (gl_LocalInvocationID.x == 0) local_count = 0;
    barrier();

    uint idx = gl_GlobalInvocationID.x;
    if (idx < num_particles && (particles[idx].flags & 1u) != 0u) {
        atomicAdd(local_count, 1u);
    }
    barrier();

    if (gl_LocalInvocationID.x == 0) {
        atomicAdd(instance_count, local_count);
    }

    if (idx == 0) vertex_count = 4u; // Billboard quad = 4 verts
}
"#;

Indirect draw generation — count alive particles and build draw command.