Retrieve Object Position for Projection Shadow Calculation - c++

Question

I am working on a class project and I need to implement shadows into our project, using OpenGL-esque coding. Our specific project is a spider running around a house. I need to make shadows appear on the floor below the spider (don't care about shadows for anything else).
The main question I need answered is this: When calculating shadows, How do I retrieve a point on the spider? In our notes, it says we have 3 givens in order to begin our initial calculations: 1) Plane (we know A,B,C,D) 2) Light source at (Lx,Ly,Lz) 3) Point q in space: Shadow caster
I do not understand how to retrieve "q". We load the spider mesh using "Mesh" and "SuperMesh" classes, and pass the mesh onto the vertex shader, but I do not understand how to reference points on the spider while on the CPU side.
I'm hesitant to post all of my code, mainly because this project uses a special library "llgl" that's based on OpenGL that our teacher made explicity for this class (makes debugging super easy, right?), and I'm not entirely sure what all would be needed in order to have my question properly answered. I'll include the Mesh and SuperMesh classes, as well as the declarations of loading the spider and passing it to the vertex shader as a matrix (which is more relevant). If you need more code, please request it. Hopefully it's more of a "theory" response than a syntax response, so I don't frustrate anyone trying to understand our code, haha.
Thank you for your time.
Main: (literally all of the excess code is cut out for simplicity)
...
SuperMesh* spider = new SuperMesh("spider/SmoothSpider.spec.mesh");
mat4 spiderR = rotationY(M_PI); // Rotation matrix for spider
vec3 spiderT(0,-2.5,-8.0); // Translation for spider
...
... (in draw)
//prog1 simply defines the txt files for the vertex and fragment shader
prog1->setUniform("worldMatrix", // sets world matrix for spider
spiderR * translation(vec4(spiderT.x,
spiderT.y,
spiderT.z,
0.0)));
spider->draw(prog1);
...
Mesh: (Hopefully not needed)
#ifndef MESH_H
#define MESH_H
#include "llgl.h"
#include <string>
#include <vector>
using namespace std;
using namespace llgl;
class Mesh{
public:
string fname;
bool initialized;
vector<uint8_t> vdata;
vector<uint8_t> idata;
vec4 color;
vec4 scolor;
int isize;
GLenum ienum;
unsigned vbuff,ibuff;
uint32_t ni;
TextureX* diffusetex;
TextureX* ramp;//used for cell shading, this is the "gradient"
Mesh(string fname);
void setup();
void draw(Program& prog);
void draw(Program* prog){ draw(*prog); }
};
#endif
SuperMesh:
#pragma once
#include "Mesh.h"
#include <iostream>
#include <vector>
#include <string>
#include <fstream>
using namespace std;
struct SuperMesh
{
vector<Mesh*> m;
string fname;
bool loaded;
SuperMesh(string specfile)
{
fname = specfile;
loaded = false;
}
void load()
{
if(loaded)
return;
ifstream in(fname.c_str());
if(!in.good())
throw runtime_error("Cannot load file " + fname);
string pfx;
unsigned slashidx;
slashidx = fname.find("/");
if(slashidx == string::npos)
slashidx = fname.find("\\");
if(slashidx != string::npos)
pfx = fname.substr(0, slashidx) + "/";
while(1)
{
string s;
getline(in, s);
if(in.fail())
break;
if(s.find("binarymesh ") == 0)
{
string fname = s.substr(11);
m.push_back(new Mesh(pfx + fname));
m.back()->setup();
}
}
loaded = true;
}
void draw(Program* p)
{
if(!loaded)
load();
for(unsigned i = 0; i < m.size(); ++i)
{
m[i]->draw(p);
}
}
};