Atmosphere: per-frame constants once + reuse computed haze #816
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sky_line/Slipstream#816
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Per pixel only rel = P - cam (direction, length) and the normal's scalar ambient factor vary; sunlit, the amblit base, haze weights and attenuation constants are per frame but recomputed per pixel (exp / pow / sRGB conversions + ~10 storage loads). Compute them once on the CPU (setAtmosphere). Also: water calls the scatter twice on the same ray and the deferred fade band three times: pass the computed additive / attenuation through. Haze + atmospherics are ~17% of lighting cycles. Lossless up to rounding.
First measure whether the cost is ALU or load latency (replace the scatter body with constants, keep the loads).
Context: measured on a GTX 1080 (Pascal) at 4K with the test-control channel and the shader clock (RenderVKShaderClockLighting) / pipeline stats (RenderVKPipelineStats). The frame is GPU-bound but not bandwidth-bound (memory controller ~36% busy on the 1080, ~14% on a 5090): it is latency / occupancy bound, so trading a little memory traffic for fewer registers and fewer dependent loads should help every GPU generation.
Fixed in Slipstream 1.3.0 (stable): https://git.slipstreamviewer.org/sky_line/Slipstream/releases/tag/v1.3.0
If you still see this on 1.3.0, reply here and we'll reopen it.