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Update particle builtins description in Spatial shader #11879

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13 changes: 10 additions & 3 deletions tutorials/shaders/shader_reference/spatial_shader.rst
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Expand Up @@ -234,9 +234,16 @@ However, the value passed to the fragment shader still comes from ``VERTEX``.
For instancing, the ``INSTANCE_CUSTOM`` variable contains the instance custom data. When using particles, this information
is usually:

* **x**: Rotation angle in radians.
* **y**: Phase during lifetime (``0.0`` to ``1.0``).
* **z**: Animation frame.
* **x**: Current rotation angle in radians.
* **y**: Time the particle has been active in seconds.
* **z**: Current animation frame.
* **w**: Particle lifetime in seconds.
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@QbieShay QbieShay Apr 5, 2026

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This is incorrect. I don't know how to easily phrase it, but it contains the randomness of the lifetime.

So for example a certain particle may have 0.9 in CUSTOM.w, meaning the particle lives for 90% of the specified lifetime. So when this particle dies, CUSTOM.y will contain 0.9. Dividing Y by W gives the correct result.

I count on you all to figure out a phrasing for this, i have no idea how to make it intuitive.


To get the lifetime phase (ranging from ``0.0`` to ``1.0``), use ``CUSTOM.y / CUSTOM.w``.

Additionally, ``USERDATA1`` is defined with the following value in the built-in :ref:`class_ParticleProcessMaterial`:

* **w**: Accumulated angle from angular velocity in radians, as applied by the angular velocity texture.

This allows you to easily adjust the shader to a particle system using default particle material. When writing a custom particle
shader, this value can be used as desired.
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