This is my implementation of Alan Zucconi’s tutorial on Volumetric Rendering.

This is a vertex and fragment shader.
An important observation I made is the order of declaring functions is important. A function must be declared before it can be called by another function.
I expanded on the tutorial by declaring Properties, making the center of the sphere and its size available from Unity’s inspector. This way I could play with values without updating the shader each time.
Properties
{
_Center ("Center", Vector) = (0, 0, 0, 0)
_Radius ("Radius", Float) = 0.5
}
In the vertex function I get the world position of the vertex for use in the fragment function.
v2f vert (appdata_base input)
{
v2f output;
output.vertex = UnityObjectToClipPos(input.vertex);
output.wPos = mul(unity_ObjectToWorld, input.vertex).xyz;
return output;
}
The fragment function is where most of the work in this shader happens.
fixed4 frag (v2f input) : SV_Target
{
float3 worldPos = input.wPos;
float3 viewDir = normalize(input.wPos - _WorldSpaceCameraPos);
fixed4 color;
if (raymarchHit(worldPos, viewDir))
{
color = fixed4(1,0,0,1);
}
else
{
color = fixed4(1,1,1,1);
}
return color;
}
Full listing:
Shader "Custom/VolumetricSphere"
{
Properties
{
_Center ("Center", Vector) = (0, 0, 0, 0)
_Radius ("Radius", Float) = 0.5
}
SubShader
{
Tags { "RenderType"="Opaque" }
Pass
{
CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#include "UnityCG.cginc"
float3 _Center;
float _Radius;
#define STEPS 64
#define STEP_SIZE 0.01
struct v2f
{
float4 vertex : SV_POSITION;
float3 wPos : TEXCOORD1;
};
bool sphereHit(float3 pos)
{
return distance(pos, _Center) < _Radius;
}
bool raymarchHit(float3 worldPos, float3 viewDir)
{
for (int i = 0; i < STEPS; i++)
{
if (sphereHit(worldPos))
{
return true;
}
worldPos += viewDir * STEP_SIZE;
}
return false;
}
v2f vert (appdata_base input)
{
v2f output;
output.vertex = UnityObjectToClipPos(input.vertex);
output.wPos = mul(unity_ObjectToWorld, input.vertex).xyz;
return output;
}
fixed4 frag (v2f input) : SV_Target
{
float3 worldPos = input.wPos;
float3 viewDir = normalize(input.wPos - _WorldSpaceCameraPos);
fixed4 color;
if (raymarchHit(worldPos, viewDir))
{
color = fixed4(1,0,0,1);
}
else
{
color = fixed4(1,1,1,1);
}
return color;
}
ENDCG
}
}
}