help i am trying to implement LOD in my game engine

[Abdelilah-Majid]

hi guys;
so i have this 3D game engine under development and i am building it with c and the raylib library
i has implemented Frustum_culling and data oriented design and field of view and i am planing to implement a collision detection optimization techneek and multi threading in the future and also any optimization idea that comes to my mind;
and i am now working on lod but i am stuck

the meshopt_simplify doesnt reduce the indices that much if i didn't set the mesh indices to the lod array and the meshopt_simplifySloppy produce a very bad looking mesh

and if i set the mesh indices to the lod array it cant upscale to the original array(i know this is a stupid thing to say but again the first method doesn't reduce the indices that much if i didn't do this)

here is my code: (sorry for the weird format )

void Renderer3D_LOD_gen(Renderer3D *renderer3D) {
for (unsigned long long int i = 0; i < renderer3D->number_of_existing_modules; i++) {
if (renderer3D->Modules[i].exist) {
if (renderer3D->Modules[i].visible_by_engine) {
if (renderer3D->Modules[i].visible) {

      unsigned int index_count = renderer3D->Modules[i].model.meshes[0].triangleCount * 3;
      float threshold = 0.2f;
      size_t target_index_count = (size_t)(index_count * threshold);
      float target_error = Renderer3D_LOD_get_normalized_float_distance_from_camera(renderer3D, renderer3D->Modules[i].pos)
        * meshopt_simplifyScale(renderer3D->Modules[i].model.meshes[0].vertices, renderer3D->Modules[i].model.meshes[0].vertexCount, sizeof(float));

      unsigned int *lod = malloc(sizeof(unsigned int) * index_count);
      float lod_error = 0.f;
      unsigned int *indices = malloc(sizeof(unsigned int) * index_count);

      for (unsigned int j = 0; j < index_count; j++) {
        indices[j] = (unsigned int)renderer3D->Modules[i].model.meshes[0].indices[j];
      }

      size_t lod_size = meshopt_simplify(lod, indices, index_count, renderer3D->Modules[i].model.meshes[0].vertices,
                                               renderer3D->Modules[i].model.meshes[0].vertexCount, sizeof(float), target_index_count, target_error, &lod_error);

      unsigned short *old_indices = renderer3D->Modules[i].model.meshes[0].indices;
      renderer3D->Modules[i].model.meshes[0].triangleCount = lod_size / 3;
      old_indices = realloc(old_indices, sizeof(unsigned short) * lod_size);
      for (unsigned int j = 0; j < lod_size; j++) {
        old_indices[j] = (unsigned short)lod[j];
      }

      printf("index_size= %d\nvertex_count= %d\nlod_size=%zu\ntarget_error= %f\n", index_count, renderer3D->Modules[i].model.meshes[0].vertexCount,
             lod_size, target_error);
    }
  }
} else {
  break;

}
}
}

and also this:

float Renderer3D_LOD_get_normalized_float_distance_from_camera(Renderer3D *renderer3D, Vector3 target_obj) {

float distance = Vector3Distance(target_obj, renderer3D->View_calling_optimizer->camera_3D->position);
float result = Normalize(distance, 0, renderer3D->View_calling_optimizer->view_depth);

return result;
}

can anyone help me get this working (snippets of code will help so much)
thank you all;