Exporting png from custom render texture in HDRP results in darker image

Hello, fellow gamedevs!
I am trying to render icons in hdrp and save them to png. I am creating a custom render texture and clicking export in the context menu. It seems to save the file, however the color seems off - it's way darker than the original image. Refer to the screenshots below.
After some googling and research I've found out that it's related to hdrp rendering in linear color space and the color format of the exported image. I could find suggestions to turn on check srgb mode on the render texture, however it doesn't seem like I have that option (screenshot below). There's an srgb option int color format dropdown, however I can't seem to select it - it goes back to UNORM options.(screen below)

Render texture in the editor and how it's supposed to look:



Exported png looks darker:


No srgb checkbox in the inspector:


The srgb option in color format dropdown that I can't select:

 

Just tried this myself, first with a custom render texture and then with a render texture. I'm getting the same issue. For reference I'm using 2020.1.3f1 on windows.

Have you tried initializing a new CRT at runtime to see if it allows it then?

When I first saw the darker image the first thing that came to my mind was that you may have been blitting to the render texture prior to all post-processing completing. Using the frame debugger I've noticed the render target appearing dark until after the FinalPost. I believe you are correct in assuming that the color space is the culprit here, but just in case that turns out to be a dead end I thought I'd mention this possibility.

 

I managed to get around this by saving it as a 32bit exr then applying the sRGB filter in photoshop, hope this is fixed soon though.

 

This is still an issue. I can't get our project to export normal images for the life of me. Doesn't matter if it's regular render textures or custom ones.

 

I can confirm, this is still a problem! I'm working with 2021.3.0f1 (LTS) atm.
This has litterally wasted an entire day diagnosing until I realized it doesnt happen on the Built-in render-pipeline, only in URP and HDRP.
Have not been able to find any solultion and editing externally is not an option. I'm working on an editor tool, to prevent the need for exactly that.

*Indeed also happens in Built-in render-pipeline when ColorSpace is set to Linear. As well as, works as expected when using URP and ColorSpace is set to Gamma.

If there is a fix by now that works for all render-pipelines, could anyone please share. I would really appreciate it.

 

I have found a solution that worked well for me. It's kinda slow, but in my case, that wasn't an issue:

• Copy the render texture to a texture2d
• Use GetPixels on this texture and store the pixels in an array
• Loop over each item of the array (they are colors)
• Use the gamma or linear property of the item (I forgot which is the right one) and assign it back to the array
• Use SetPixels to store the array in the texture again
• Export EncodeToPNG (or what you want) to export the texture to a file

This will give you the correct image. I spent more than two days trying to figure this out until I accidentally stumbled over the gamma/linear properties of the Color class, which solve the problem. Shame that the encode function doesn't have a switch for this or does it correctly automatically. But this works.

 

Hmm... that would really go against what I'm trying to achieve with my tool. I'm computing textures with compute-shader, for speed. If I'd then have to loop over them on the CPU side that would defeat the purpose.

For clarification, the output I get back is already being passed to a Texture2D and adjusted for linear ColorSpace. All looks fine, until I try to save that Texture2D. It's litterally just the

Code (CSharp):

1. File.WriteAllBytes(path, _preview.EncodeToPNG());

bit that seems to mess things up.



As you can see, the computed output is fine.

 

Sorry. That's the best I've got.

 

I seem to have found a word-around...
It appears that by default, even when using linear color-space, textures are imported with gamma color-space. (if it were linear, they'd have to be imported with sRGB unchecked, apparently.)

When I prepare my texture, I set it up to be in gamma (I think final parameter defaults to false, but just for illustration)



I've added a simple bool in my compute shader, so I know if I'm working in linear color-space or not.


And in the shader, I run every result through one final step that adjusts for gamma


Final result seems to be correct


Source of inspiration: https://medium.com/@abdulla.aldandarawy/unity-always-be-linear-1a30db4765db

PS: It's not 100% perfect, but pretty close.

 

Hm yeah I can see a little bit of difference, but not much. Isn't there a shader function for the conversion to get it exact?

 

Great call!

Changed it to:

Code (CSharp):

1. float4 gamma_correction(float4 source)
2. {
3.     if (is_linear)
4.        return float4(LinearToGammaSpace(source), LinearToGammaSpaceExact(source.a));
5.  
6.     return source;
7. }
8.  

Now the output is identical looking

 

Excellent!