Unity Skybox shaders don't work in HDR

I'm hoping someone can help me figure out how to fix the built-in Unity skybox shaders to work when HDR is enabled. Currently it seems the mobile skybox shader works great, but the other three non-mobile skybox shaders and anything based off of them does not work. Here's the current problem, which shows up most obviously when using an alpha-blended shader (like a cloud) in front of a dark sky:

Regular Skybox Shader, No HDR (correct)


Mobile Skybox Shader, HDR enabled (also correct)


Regular Skybox Shader, HDR enabled (definitely not correct)



I'm guessing the mobile one works because it just does a simple "combine texture" and that's it, while the non-mobile ones do this:

Code (csharp):

1.  
2. fixed4 skybox_frag (v2f i, sampler2D smp)
3. {
4.     fixed4 tex = tex2D (smp, i.texcoord);
5.     fixed4 col;
6.     col.rgb = tex.rgb + _Tint.rgb - unity_ColorSpaceGrey;
7.     col.a = tex.a * _Tint.a;
8.     return col;
9. }
10.  

Something about that unity_ColorSpaceGrey seems suspicious but I don't really understand what's going on. Anyone have any insight?

 

Following up on this, I changed that suspicious colorspacegrey line to this:

Code (csharp):

1.  
2. col.rgb = tex.rgb * _Tint.rgb;
3.  

And it seems to work great. Can someone explain to me why the original shader doesn't use a simple multiply for the tint, and what ColorSpaceGrey is? I'm guessing that maybe in non-HDR the col.rgb gets clamped to 0,0,0, but in HDR it goes negative?

 

I think I answered my own question, as this also fixes the problem:

Code (csharp):

1.  
2. col.rgb = clamp(tex.rgb + _Tint.rgb - unity_ColorSpaceGrey, 0, 100);
3.  

So in HDR, colors can go negative, which means any shader that relies on subtracting without remembering to clamp to zero is going to fail. Am I right that this is a bug? I can't imagine that a dark sky is "supposed to" become a negative color in HDR; it looks like they just forgot to clamp.

 

Having this issue too now, saving the files as a .jpeg instead of HDR helped in our case we aren't using HDR for a mobile VR project