Question How to correctly sample HDR cubemap?

Hi everyone,

Unity default skybox shader samples a cubemap like this

Code (CSharp):

1. half4 tex = texCUBE (_Tex, i.texcoord);
2. half3 c = DecodeHDR (tex, _Tex_HDR);

What does DecodeHDR do? When should I use it? Does unity populate _##_HDR for all textures in materials or the ones I bind using SetGlobalTexture?

 

It decodes HDR (High Dynamic Range) textures that have been stored in RGBM or dLDR format. HDR a texture with values ">1.0". What is RGBM and dLDR?
https://docs.unity3d.com/Manual/Lightmaps-TechnicalInformation.html

And this is the code for that function.

data

is the texture color value,

decodeInstructions

is the

*_HDR

for that texture.

Code (CSharp):

1. // Decodes HDR textures
2. // handles dLDR, RGBM formats
3. inline half3 DecodeHDR (half4 data, half4 decodeInstructions)
4. {
5.     // Take into account texture alpha if decodeInstructions.w is true(the alpha value affects the RGB channels)
6.     half alpha = decodeInstructions.w * (data.a - 1.0) + 1.0;
7.  
8.     // If Linear mode is not supported we can skip exponent part
9.     #if defined(UNITY_COLORSPACE_GAMMA)
10.         return (decodeInstructions.x * alpha) * data.rgb;
11.     #else
12.     #   if defined(UNITY_USE_NATIVE_HDR)
13.             return decodeInstructions.x * data.rgb; // Multiplier for future HDRI relative to absolute conversion.
14.     #   else
15.             return (decodeInstructions.x * pow(alpha, decodeInstructions.y)) * data.rgb;
16.     #   endif
17.     #endif
18. }

Whenever you might use a pre-computed HDR texture, either one generated by Unity (i.e.: lightmaps, reflection probes), or imported from an external source (.hdr or .exr image for custom reflection probes, or sky boxes), but are using non-HDR texture formats. That can be either due to the platform not supporting them (ie: mobile), or you manually selecting a non-HDR texture format on that texture's import settings.

Yes. If there's an

*_HDR

for a texture that's being used by that shader, Unity will fill it out with the required data. For non-HDR or native-HDR images it'll be values that will have no affect on the texture (

float4(1.0, 1.0, 1.0, 0.0)

), and on HDR images it'll be the values to decode it from the current format for the current color space.


Note: HDR Render Textures don't need to be decoded because they're already guaranteed to be in a native-HDR format. Even on mobile.

 

Thank you very much @bgolus! Though it's the first time you answer my question directly, your posts on this forum are absolutely invaluable source of information and helped me immensely.

 

I have tried DecodeHDR in compute shader. But it seems that Unity does not fill the

*_HDR

variable for me in compute shader. All the decoded data were 0, because of the default value 0 in

*_HDR

variable without Unity's feeding. if I set

*_HDR

as half4(1,1,1,1), the values extracted from HDR map were truncated.(max to 1)

May I ask is there any other method to get decoded HDR value from map in compute shader?

 

I am using unity shadergraph and I am trying to write a custom node that will decode an HDR texture I am passing in, but no matter what I do I can't find the syntax or methods to make it work. Can you point me in the right direction?

Code (CSharp):

1. inline float4 SampleHDRTexture2D(UnityTexture2D _tex, UnitySamplerState ss, float2 uv)
2. {
3.     //somehow decode
4.     return color;
5. }

Is it even possible to extract HDR values from SAMPLE_TEXTURE2D?