Bloom behaves strange with camera stack

Unity Engine
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1

I am trying to add Bloom Effect to both scene and UI, so I set up 2 camera, one rendering Default Layer as base camera and one rendering UI Layer as overlay camera.
According to the manual, postprocess effects are applied to a camera stack instead of an individual overlay camera. So if I put the postprocess volume into UI Layer, the effect should be applied to the whole render result.
However, it is not. In the picture below, the blue sphere is in Default layer and the red sphere is in UI layer. it seems that bloom is only applied to UI layer.

7794606--984555--Bloom1.png
7794606--984555--Bloom1.png958×482 446 KB

If I set the volume to default layer, the blue sphere gets the bloom effect, but the red sphere does not. This is correct because the postprocess effect is applied after the base camera before the overlay camera.
7794606--984558--Bloom2.png
7794606--984558--Bloom2.png925×497 430 KB

To combine this two result, I tried setting the two camera to the same volume mask, or creating one volume for each camera. the result is shown below:
7794606--984561--Bloom3.png
7794606--984561--Bloom3.png996×502 439 KB
7794606--984564--Bloom4.png
7794606--984564--Bloom4.png898×502 408 KB

the scene became too bright, because the the bloom effect is applied twice on the whole scene, not individual layers. This is not the desired result, and apply bloom twice is too performance consuming.

So now I assume that the first setup is Actually the correct one. but the render result of the base camera is somehow clamped, so when the overlay camera renders, it lost the HDR information of the base camera, which results the bloom effect is very weak on the base layer. So is there any way to fix it?

2

You’re not the only one with this problem: Post Processing with multiple cameras is currently very problematic

The best way to fix it is if you have an enterprise support contract, go yell at Unity devs and tell them to prioritize fixing it. But for us plebs, we have to resort to various hacks. In your case you might be able to render both cameras to RTs without using stacking and without any PP, composite them yourself, then apply PP to the final result. I’m not sure how much performance that would cost; it probably would be reasonable on PC/console but pretty massive on tile renderers like mobile.

3

While, the blooming problem is solved by turning off the postprocess option in the base camera. However, this means I cannot apply other postprocess effects on the base layer.
From the Frame debugger, I can see that once I turn on the postprocess option of the base camera, An UberPostProcess shader is always rendered no matter I use any effect or not. And in the shader, Color grading is always enabled, which makes glowing colors less bright than the origin value. And that’s why the objects on base layer lose the bloom effect.
I just hope there is a way to disable the Color grading for the base camera, then everything is OK.