Hello,
I’m a little confused about how the threshold parameter of the volume bloom parameter. What is the correct value that i put so that only materials using HDR will react with the volume?
Thanks
Threshold is really a tweak value not where you properly control it. To properly control it relies on your emissive value in shaders and of course how strong your lights are in the scene and so on. Basically, it’s a soft cut off for final tweaking, so there isn’t a correct value.
I recommend building an experiment scene with boxes of varying emission, lights etc to get a proper handle on your look.
In the old days, pre HDR buffers and linear engines, threshold was simply a measure of how bright the pixel had to be, and with non HDR ranges of before it was much more effective in deciding what would bloom and what would not.
It still is a threshold value but now the buffer has a lot more information and is directly controlled in your shaders and so on.
What would be the correct way to make that it applies only to materials to using hdr then? Only trial and error?
Which pipeline and so on? If it’s HDRP then the template scene will likely have decent defaults. I’m not sure about the URP scene.
You control it using the emissive output in the master node on shadergraph, or in emission slot.
I’m using URP in a 2d project.
In that case just still - control it via the material.
I’m sorry i’m not sure what you mean? Here i have a material that uses a color node with HDR to boost the output. It works well, however some bloom also applies on standard sprite lit shader even tough i set the threshold at two.
Usually boosting the colour value in base color will do it, or if you use a lit variant you can control it with masks. See How to make a sprite glow? for example.
I don’t work in 2D any more but the principles are the same. The bigger the number in the HDR buffer, the more bloom has to work with, particularly if that value is over 1, although that also brings you into exposure territory and other lit shenanigans you can probably just avoid.
But how do i make it NOT react with bloom? 
The mask on the 2D lit shader allows you to make it not react, or react.