# Chapter 20: Advanced Lighting

As I read this book, I summarize what I think is wrong. If you think my comments are wrong then please let me know. We can dicuss more and update your opinion.

## Getting started

There is a runtime error in because Shaders.metal files isn't included in the bundle. But we have the default library.

In Renderer.swift, in initializeMetal(), replace this:

```
guard let path = Bundle.main.path(forResource: "Shaders", ofType: "metal") else { fatalError() }
let input = try String(contentsOfFile: path, encoding: String.Encoding.utf8)
let library = try device.makeLibrary(source: input, options: nil)
```

With:

```
let library = device.makeDefaultLibrary()!
```

## Drawing a checkerboard pattern

This book explains the different between truncated division and floored division but it's not cinch to understand. Below figure will help you understand easily.

![](./difference-divisions.jpeg)

## Refraction

There is a runtime error in because Shaders.metal files isn't included in the bundle. But we have the default library.

In Renderer.swift, in initializeMetal(), replace this:

```
guard let path = Bundle.main.path(forResource: "Shaders", ofType: "metal") else { fatalError() }
let input = try String(contentsOfFile: path, encoding: String.Encoding.utf8)
let library = try device.makeLibrary(source: input, options: nil)
```

With:

```
let library = device.makeDefaultLibrary()!
```

## Raytraced water

There is a runtime error in because Shaders.metal files isn't included in the bundle. But we have the default library.

In Renderer.swift, in initializeMetal(), remove this:

```
let path = Bundle.main.path(forResource: "Shaders", ofType: "metal")
```

Replace this:

```
let input = try String(contentsOfFile: path!, encoding: String.Encoding.utf8)
let library = try device!.makeLibrary(source: input, options: nil)
```

With:

```
let library = device.makeDefaultLibrary()!
```

## 3. Clipping planes

This book uses vector_float4 to define a clip plane without any explanation. The plane consists of the normal of plane and the distance from the origin. From a below figure, we can know the plane can be represented by `ax + by + cz + d = 0`. You can get more detail at [here](https://brilliant.org/wiki/3d-coordinate-geometry-equation-of-a-plane)

![](./plane.png)

Thus float4 is the best type to store the plane. First xyz is the normal and w is distance.

In Renderer.swift, you have added this line in draw(in:) below // Water render:

```
var clipPlane = float4(0, 1, 0, 0.1)
```

Now you can understand what it is. This plane is pointing towards the positive y axis and located at 0.1 on the y axis.

Maybe you can't understand why this book uses dot product for the clip distance. Let's recap the dot product to understand this.

![](./dot-product.png)

One of the feature of the dot product is projection. From doing dot product between a vertex position and a clip plane, you can know this vertex position is before that plane or after that plane. If a position is located before the plane then the result of dot product is negative. In other case, the result is positive. Now you can easily understand this:

```
vertex_out.clip_distance[0] = dot(uniforms.modelMatrix * vertex_in.position, uniforms.clipPlane);
```