Atmosphere seems really bright when HDR is on. Check to make sure we're not accidentally doubly applying tonemapping for the atmosphere when HDR is on. Maybe verify if this issue was present with ACES tonemapping or not.
HDR off
HDR on
Notice that the atmosphere does get "brighter" when HDR is on. I notice it most as the blue part shrinks to almost nothing.
Another weird part of this is that if you crank the brightness way up you can see the total height of the atmosphere including the fully transparent pixels. That might be a separate bug but it also reveals that with HDR off those pixels are semi-transparent but with HDR on they become fully opaque.
This might hint at the actual issue here being that when HDR is on the transparency or light accumulation is not being calculated correctly
This is based off an observation from @ggetz while we were working on https://github.com/CesiumGS/cesium/issues/12126
HDR off
HDR on
Notice that the atmosphere does get "brighter" when HDR is on. I notice it most as the blue part shrinks to almost nothing.
Another weird part of this is that if you crank the brightness way up you can see the total height of the atmosphere including the fully transparent pixels. That might be a separate bug but it also reveals that with HDR off those pixels are semi-transparent but with HDR on they become fully opaque. This might hint at the actual issue here being that when HDR is on the transparency or light accumulation is not being calculated correctly
HDR off + brightness
HDR on + brightness
Reproduce