# The Skyquad

A skyquad is to a quad what a skybox is to a box. The skyquad allows us to render a sky by sending just a single quad down the rendering pipeline. The skyquad is useful not only in OpenGL, but also in ray tracing, where we have rays passing through an image plane. I believe it is useful to go over the ray equation in a ray tracer first, so let's start there.

## The Ray Equation

In a ray tracer, we have a camera and a window into the world - the image to be rendered - and the goal is to determine the ray passing through each of the pixels of the image. This is depicted in the following figure:

The problem of determining a ray through a given pixel can be summarised as follows, using the same notation as in the figure:

**Input:**

- $\vec{u}$, $\vec{r}$, and $\vec{f}$ - the camera's up, right, and forward vectors, respectively.
- $\vec{o}$ - the camera's position.
- fovy - the camera's vertical field of view angle.
- $(w,h)$ - the width and height of the image, respectively.
- $(x,y)$ - the coordinates of the target point in the image, ranging from $(0,0)$ to $(w,h)$.

**Output:**

- The ray $r$ originating at $\vec{o}$ and passing through $(x,y)$.

In addition, we will use the following conventions:

- $(0,0)$ corresponds to the bottom-left corner of the image.
- $(w,h)$ corresponds to the top-right corner of the image.

Note that $(x,y)$ need not correspond with the integer coordinates of a pixel; it can be any real point on the image plane.

### Determining the distance to the image plane

The first step is determining the distance $d$ from the camera to the image plane, as shown in the image below:

From the image, we can see that:

$$tan(\frac{fovy}{2}) = \frac{h/2}{d}$$

Solving for $d$ yields:

\begin{align} tan(\frac{fovy}{2}) &= \frac{h/2}{d} \\ & \Leftrightarrow \\\ d &= \frac{h/2}{tan(\frac{fovy}{2})} \\ &= \frac{h}{2}\;cot(\frac{fovy}{2}) \end{align}

### Deriving the ray equation

Next, from the first figure, we can observe that the center of the image is determined by:

$$\vec{c} = \vec{o} + d\vec{f}$$

To reach out the target point $(x,y)$ from $\vec{c}$, we offset $\vec{c}$ by a delta. We push $\vec{c}$ $d_x$ units along the camera's right vector $\vec{r}$ and $d_y$ units along the camera's up vector $\vec{u}$:

$$[x,y] = \vec{c} + d_x\vec{r} + d_y\vec{u}$$

Deriving $d_x$ and $d_y$ is straightforward. When $x=0$, $d_x=-w/2$, and when $x=w$, $d_x=w/2$. The case for $d_y$ is similar. This results in:

$$d_x = x - \frac{w}{2}$$

$$d_y = y - \frac{h}{2}$$

The vector from the camera position $\vec{o}$ to the point $(x,y)$ on the image plane is then given by:

\begin{align} \vec{v} &= [x,y] − \vec{o} \\ &= \vec{c} + d_x\vec{r} + d_y\vec{u} - \vec{o} \\ &= \vec{o} + d\vec{f} + d_x\vec{r} + d_y\vec{u} - \vec{o} \\ &= d\vec{f} + d_x\vec{r} + d_y\vec{u} \\ &= d\vec{f} + (x - \frac{w}{2})\vec{r} + (y - \frac{h}{2})\vec{u} \end{align}

Finally, the ray equation is $r = \vec{o} + \lambda\vec{t}$, where $\vec{t}$ is conveniently defined as $\vec{v}$ normalised: $\vec{t} = \frac{\vec{v}}{||\vec{v}||}$.

## The Skyquad in OpenGL

To draw the skyquad in OpenGL, we render a quad directly in NDC space [1], with $z=1$ to send it to the background. For each pixel in the quad, we compute the ray originating from the camera and passing through that pixel. We then use the ray's direction to sample a cubemap texture holding the skybox data. The fetched texel determines the colour value of the pixel.

To simplify our calculations, we compute the ray direction in NDC space and then apply the viewport's aspect ratio $r$ and the camera's rotation matrix $R$ to transform it into world space. NDC space has many interesting properties that make the calculations easier:

- $\vec{u} = (0,1,0)$, $\vec{r} = (1,0,0)$, and $\vec{f} = (0,0,-1)$, assuming a right-handed coordinate system.
- $\vec{o} = \vec{0}$
- $d$ = 1 (we set $z=1$ for the quad that we render)
- $(w,h) = (1,1)$

With the above properties in mind, the distance to the image plane $d$ becomes:

\begin{align} d &= \frac{h}{2}\;cot(\frac{fovy}{2}) \\ &= \frac{1}{2}\;cot(\frac{fovy}{2}) \end{align}

The ray direction vector in NDC space boils down to:

\begin{align} \vec{v}' &= d\vec{f} + x - \frac{w}{2}\vec{r} + (y - \frac{h}{2})\vec{u} \\ &= d[0,0,-1] + x - \frac{1}{2}[1,0,0] + (y - \frac{h}{2})[0,1,0] \\ &= [x - \frac{1}{2}, y - \frac{1}{2}, -d] \end{align}

Next, apply the viewport's aspect ratio $r$ to properly scale the ray:

$$v = v' * [r,1]$$

And finally, normalise the vector and apply the camera's rotation matrix $R$ to obtain the direction in world space:

$$\vec{t} = R \frac{\vec{v}}{||\vec{v}||}$$

To render the skyquad, we send a 2D quad with coordinates ranging from $(-1,-1)$ to $(1,1)$ down the pipeline with the shader program that follows. Note that we need not compute a ray per pixel; instead, we compute a ray for every vertex of the quad and let the pipeline interpolate the rays:

**Vertex shader**

```
uniform mat3 Rotation;
uniform float fovy;
uniform float aspect;
layout (location = 0) in vec2 Position;
out vec3 Ray;
vec3 skyRay (vec2 Texcoord)
{
float d = 0.5 / tan(fovy/2.0);
return normalize(vec3((Texcoord.x - 0.5) * aspect, Texcoord.y - 0.5, -d));
}
void main ()
{
Ray = Rotation * skyRay(Position*0.5 + 0.5); // map from [-1,1] to [0,1]
gl_Position = vec4(Position, 0.0, 1.0);
}
```

**Fragment shader**

```
uniform samplerCube tex;
in vec3 Ray;
layout (location = 0) out vec4 Colour;
void main ()
{
vec3 R = normalize(Ray);
Colour = vec4(pow(texture(tex, R).rgb, vec3(1.0/2.2)), 1.0);
}
```

That was a lot of maths for such a simple shader! But the final solution is simple, and works for both ray tracing and OpenGL.

And as always, the mandatory screenshot:

[1] Technically it is clip space, but since we set $w=1$ in the shader then the perspective division leaves the coordinates unchanged.