# Uniform Grid

The Uniform Grid class Grid is a subclass of Spatial, and is defined in Grid.h

For simplicity, I put all implementations in Grid.h

The default grid uses a 16x16x16 cell subdivision.

## Insert Triangle

It calculates the bounding box of the triangle, and adds the triangle index to all cells covered.

<figure><img src="https://3464970502-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F3JUKGJZ67JX02QZdPhsy%2Fuploads%2FBBfuSthrCuxx9L85HWFk%2FuniformGrid.png?alt=media&#x26;token=362867de-153a-4016-86aa-341e4c91342d" alt="" width="261"><figcaption></figcaption></figure>

```cpp
    void Insert(int triIdx) override
    {
        Triangle t = getTriangle(triIdx);

        glm::vec3 triMin = glm::min(t.v0, glm::min(t.v1, t.v2));
        glm::vec3 triMax = glm::max(t.v0, glm::max(t.v1, t.v2));

        glm::ivec3 minCell = PosToCell(triMin);
        glm::ivec3 maxCell = PosToCell(triMax);

        for (int z = minCell.z; z <= maxCell.z; z++)
            for (int y = minCell.y; y <= maxCell.y; y++)
                for (int x = minCell.x; x <= maxCell.x; x++)
                {
                    int idx = x + dims.x * (y + dims.y * z);
                    cells[idx].push_back(triIdx);
                }
    }
```

## QueryAABB&#x20;

It just checks every cell covered by the input query box, retrieves its contents （intersections). Collision Detection can be based on this.

```cpp
    void QueryAABB(const AABB &box, std::vector<int> &out) const override
    {
        if (!AABBIntersects(box, bbox))
            return;

        glm::ivec3 minC = PosToCell(box.min);
        glm::ivec3 maxC = PosToCell(box.max);
        
        for (int z = minC.z; z <= maxC.z; z++)
            for (int y = minC.y; y <= maxC.y; y++)
                for (int x = minC.x; x <= maxC.x; x++)
                {
                    int idx = x + dims.x * (y + dims.y * z);
                    out.insert(out.end(), cells[idx].begin(), cells[idx].end());
                }
    }
```

## Raycast

This is ray intersection testing function. It is based on 3D-DDA

```cpp
   bool Raycast(const Ray &ray, HitInfo &outHit) override
    {
        float tHit;
        if (!RayAABB(ray.origin, ray.dir, bbox.min, bbox.max, tHit))
            return false;

        glm::vec3 p = ray.origin + ray.dir * std::max(0.0f, tHit);
        glm::ivec3 cell = PosToCell(p);

        glm::vec3 step = glm::sign(ray.dir);
        glm::vec3 tDelta = glm::abs(cellSize / ray.dir);
        glm::vec3 next;
        next.x = ((cell.x + (step.x > 0)) * cellSize.x + bbox.min.x - p.x) / ray.dir.x;
        next.y = ((cell.y + (step.y > 0)) * cellSize.y + bbox.min.y - p.y) / ray.dir.y;
        next.z = ((cell.z + (step.z > 0)) * cellSize.z + bbox.min.z - p.z) / ray.dir.z;

        float bestT = FLT_MAX;
        int bestIdx = -1;

        while (cell.x >= 0 && cell.y >= 0 && cell.z >= 0 &&
               cell.x < dims.x && cell.y < dims.y && cell.z < dims.z)
        {
            int idx = cell.x + dims.x * (cell.y + dims.y * cell.z);
            for (int triIdx : cells[idx]) {
                float t;
                Triangle tri = Spatial::getTriangle(triIdx);

                if (RayTriangle(ray, tri, t)) {
                    if (t < bestT) {
                        bestT = t;
                        bestIdx = triIdx;
                    }
                }
            }

            if (next.x < next.y) {
                if (next.x < next.z) {
                    cell.x += (int)step.x;
                    next.x += tDelta.x;
                } else {
                    cell.z += (int)step.z;
                    next.z += tDelta.z;
                }
            } else {
                if (next.y < next.z) {
                    cell.y += (int)step.y;
                    next.y += tDelta.y;
                } else {
                    cell.z += (int)step.z;
                    next.z += tDelta.z;
                }
            }
        }

        if (bestIdx >= 0) {
            outHit = {bestT, bestIdx};
            return true;
        }
        return false;
    }
```