xyz2equirect: Cartesian to Equirectangular Conversion

Spherical coordinate projection

The xyz2equirect function converts 3D Cartesian coordinates on the unit sphere to equirectangular UV coordinates. This is the inverse operation of equirect2xyz, mapping 3D directions back to panoramic image coordinates:

$$\theta = \text{atan2}(z, x) + \pi$$ $$\phi = \text{acos}(-y)$$ $$\begin{pmatrix} u \\ v \end{pmatrix} = \begin{pmatrix} \frac{\theta}{2\pi} \\ \frac{\phi}{\pi} \end{pmatrix}$$

Where $\theta$ represents azimuth and $\phi$ represents elevation angle from the north pole.

Direction Mapping

3D Direction	UV Output	Description
$(1, 0, 0)$	$(0.5, 0.5)$	East (right)
$(-1, 0, 0)$	$(0, 0.5)$	West (left)
$(0, 1, 0)$	$(*, 0)$	North pole (top)
$(0, -1, 0)$	$(*, 1)$	South pole (bottom)
$(0, 0, 1)$	$(0.25, 0.5)$	Forward (front)

ライブエディター

const fragment = () => {
      const center = vec2(0.5)
      const radius = length(uv.sub(center))
      const dir = normalize(vec3(uv.sub(center), sqrt(max(0, float(0.25).sub(radius.mul(radius))))))
      const equiUV = xyz2equirect(dir)
      const pattern = sin(equiUV.x.mul(20)).mul(sin(equiUV.y.mul(15)))
      const color = vec3(pattern.mul(0.5).add(0.5))
      return vec4(color, 1)
}

結果

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