How to transcode an HDR supported texture format

Unity Engine
1

Hi there.

I am attempting to decode a lightmap (RGBD PNG) into a RGB9e5Float texture or a RGBAHalf texture.

I am able to load my own data into an RGBA32 texture pretty easily with the Color32 struct…but I cannot find a corresponding struct or sample code to load data into a RGB9e5Float or RGBAHalf texture.

I feel like I am missing something trivial and the unity docs aren’t making things clearer for me.

Any help would be greatly appreciated

2

All HDR images simply use values larger than 1.0, but the way this is encoded is probably case-by-case, depending on the exact encoding format. I don’t know the exact specifications, but Color32 for sure cannot grasp HDR values because it’s limited to 8-bit values

Check this out for use with normal floating-point Colors ColorUsageAttribute

3

Hi orionsyndrome

Thanks for replying

I am aware that Color32 wouldn’t work for me, but it is the only struct that is available to me. I’d expect to find a RGB9e5Float struct and a RGBAHalf struct.

So I have an array of bytes that are transcoded in RGBA32 i.e I am able to call:
texture.LoadRawTextureData((IntPtr)data,(int)length); on a fresh RGBA32 texture.

The alpha value of each pixel is going to be used a divisor to achieve larger HDR values. The logic looks something like this:

float3 DecodeRGBD(float4 rgbd)
{
    return rgbd.rgb * ((MaxRange / 255.0) / rgbd.a);
}

(Assuming MaxRange is 65025)

I do not want to do this calculation at the shader level - this would be inefficient. I want to use pack this transformed data into an HDR compatible (ideally compressed) texture format. I am having trouble achieving this in unity.

4

Do you mean on the C# side or HLSL? Because there is also Color, which is the struct you actually want. Color32 is strictly a 32-bit integer (4 bytes), while Color is 4x 32-bit floating points (16 bytes). You can’t possibly pack HDR information with Color32, and Unity likely expects you to avoid it, at least that’s my (educated) guess. Now I wish I had some solid experience with HDR from the engineering side of the things, but alas, I haven’t done anything with it, if we don’t count gawking at the end-product of ToneMapper.

5

If I wasn’t clear, you simply don’t have enough information entropy to encode HDR with Color32, because it’s used to encode 24-bit True Color + alpha, that’s just 8 bits per channel or 256 shades. You can divide all day long, but the losses are immense (with max range of 2^9 you have only 128 values to encode LDR; with max range of 2^10 only 64 etc; you are progressively losing information such that for a max range of 2^16 your entire LDR fits into just 1 value, black or white). Maybe you can make HDR Nintendo Gameboy with it, if you allow me to indulge myself with a bad joke.

6

C# side.

Ah thanks for that information. I will try that struct with a RGBAFloat texture format.

I think my problem comes down to low level data manipulation in C#.

Haha I too wish that gawking at tonemapper results gave me HDR powers!

1 Like
7

Don’t forget that attribute as well. I think it matters.

I.e. (this was the actual example I found)

[ColorUsage(false, true)] public Color hdrColorWithoutAlpha = Color.white;
8

Thanks orionsyndrome

I am aware of this attribute have used it. However this problem I am dealing with doesn’t involve public input - it’s purely data.

9

I know what you mean, but frankly, haven’t used it, maybe it grants additional powers in other contexts? It’s hard to tell.