in vec2 vertPosition;
out vec2 uv;

uniform vec3 headPos;
uniform int batchSize;

struct particle_t {
  vec4 positionAndSize;
  vec4 velocity;
  vec4 padding2; // for LÖVR's limited ShaderBlock type :(
  vec4 padding3; // for LÖVR's limited ShaderBlock type :(
};
layout(std430) buffer particleData {
  float reloadFlag;
  particle_t particles[];
};

vec4 position(mat4 projection, mat4 transform, vec4 vertex) {
  int index = gl_VertexID / 4 + gl_InstanceID * batchSize;
  particle_t particle = particles[index];
  vec3 pos = particle.positionAndSize.xyz;
  float size = particle.positionAndSize.w;
  uv = vertPosition * 0.5 + 0.5;

  vec3 forward = normalize(pos - headPos);
  vec3 right = normalize(cross(forward, vec3(0, 1, 0)));
  vec3 up = cross(right, forward);
  vec3 offset = mat2x3(up, right) * (vertPosition * size);

  return projection * transform * vec4(pos + offset, 1.0);
}