Figure. SDF for a sphere [ag-0017]
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Context

Example. SDF of a sphere [ag-000L]

The SDF of a sphere with radius $r$ centered at the origin $\boldsymbol {o}$ is \[ f(\boldsymbol {p})=\lVert \boldsymbol {p}\rVert -r \] as depicted below:

Figure. SDF for a sphere [ag-0017]
✍️source

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The SDF above for a sphere translates to the following GLSL:

float sdSphere(vec3 p, float r)
{
  return length(p) - r;
}