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

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

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

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