I have made another webgl fragment shader on shadertoy: Tokyo. This shader shows an abstract, procedural, view of Tokyo by night in the rain. In this post I will write up some notes about how this shader is made. (By the way, I have never been to Tokyo).

## Ray-marching Distance field

Similar to a lot of the 3d-shaders at shadertoy, the core of this shader is a 3d scene, rendered by ray-marching a distance field.

A distance field is a (signed) function which gives (a lower bound of) the distance to the closest surface at any point in space. To create the distance function of the 3d scene of Tokyo, I combined the distance functions of some basic objects (like boxes, spheres, planes and cylinders). The separate distance functions, and how to combine these functions, can be found in this article of Iñigo Quilez:

http://www.iquilezles.org/www/articles/distfunctions/distfunctions.htm

To generate an image of the scene, the shader initialises a (3D) ray for each pixel of the image. With the constructed ray the scene is marched. At each step the length of the ray is incremented by the minimal distance given by the distance field function until a surface is hit.

More information about rendering images using distance fields can be found at the (excellent) post by Iñigo Quilez:

http://www.iquilezles.org/www/articles/raymarchingdf/raymarchingdf.htm

A reference shader (also by Iñigo Quilez) can be found at shadertoy:

https://www.shadertoy.com/view/Xds3zN

## Color and fog

Because I wanted the shader to run at a smooth framerate, the maximum number of steps of incrementing each ray (for each pixel) is limited to 128 steps. Therefore, the scene is only rendered to a maximum distance, as you can see in the image above. To get a larger view distance, I added an extra intersection check with the ground plane for rays which didn’t hit a surface using the raymarching algorithm.

The scene color is based on the x-coordinate of the intersection point, resulting in a black street, some white stripes and an uniform grey color for the sidewalk and buildings. A very dark lighting (it is night after all) and some extra blueish fog is added to the scene, to hide the short viewing distance.

## Lights

To get the dark, rainy, ‘film noir’, mood I was aiming for, I needed some very bright lights with halos. The lights and halos are rendered using the same distance field as the field I used to ray march the scene to get the closest intersection point.

To get the information needed for rendering the lights and halos, some boxes and spheres which are used to construct the distance field are marked as lights. While stepping trough the distance field, the distance function not only returns the minimal distance to a surface, but also keeps track of the minimum of the minimal distance to the closest light (for each pixel). Finally, this minimal distance is used to render the halos around the lights shown.

## Reflections

To get the rainy look, reflection is added by ray marching the distance field after the first intersection along the reflection vector. This reflection vector is calculated by the normal (disturbed by some noise, and random ’tile-offsets’ for the tiles of the sidewalk) and the incoming view vector.

The colour of the reflected ray is calculated using the same functions as the color of the primary ray. Both colours are added resulting in the final colour of the scene.

Finally rain is rendered at top of the scene by animating some stretched noise.

You can find (the full source of) the fragment shader on Shadertoy: https://www.shadertoy.com/view/Xtf3zn.

I have also rendered a movie of this shader. You can find this movie on YouTube.