iPad and iPhone show "An error occurred while rendering. Rendering has
stopped." on load. Cesium's AtmosphereStageVS calls
czm_computeGroundAtmosphereScattering with its `out` parameters bound
directly to varyings:
czm_computeGroundAtmosphereScattering(
v_positionWC, lightDirection,
v_atmosphereRayleighColor, v_atmosphereMieColor,
v_atmosphereOpacity);
ANGLE's Metal backend wraps each `out` argument in an ANGLE_out(...)
helper taking `thread T &`, but vertex-output members live in
`__metal_generic` and cannot bind to `thread`, so the program fails to
LINK and Cesium tears down the render loop.
This is the only shader in Cesium that binds varyings to `out` params --
SkyAtmosphere (czm_computeScattering) and the ground-atmosphere fragment
path both route through locals, so they are unaffected and stay on.
ModelSceneGraph.configurePipeline attaches the stage under a single
condition, `fog.enabled && fog.renderable`, so clearing `fog.renderable`
keeps the broken program from ever being built.
`renderable` rather than `enabled` is deliberate: Fog.update returns
early when `enabled` is false and forfeits the density that drives 3D
Tiles screen-space-error scaling, and visualSettings saves/restores
`fog.enabled` around IR boost, which would resurrect the broken shader
mid-session.
Detection is a WebGL2 link probe of the same out-param/varying pattern,
so a future driver fix silently restores the effect with no code change,
OR'd with iOS/iPadOS platform detection as a backstop for when no probe
context can be created. iPadOS desktop mode is identified by the
Macintosh UA token plus maxTouchPoints > 1, since real Macs report zero
and navigator.platform is deprecated.
Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01HoRD3yHVgfTHeikTmsUoHf
