Saving image from image editor resets Image Texture color space #74610

System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 441.87

Blender Version
Broken: version: 2.82 (sub 7), branch: master, commit date: 2020-02-12 16:20, hash: 77d23b0bd7
Worked: (optional)

Short description of error
The bug originally reported here https://developer.blender.org/T74284 which has been irresponsibly ignored has even more dangerous side effects. When using Blender's image editor to edit and save images, each time image is saved, Image Texture node color space is reset.

Exact steps for others to reproduce the error

Create a new image datablock
Edit it using Blender's image editor
Save it as an external image file (JPG)
Load this externally saved image file as an image texture
Set image texture color space to anything else than sRGB (default)
Resave the image from the Image Editor

Result: Color Space of the loaded texture gets reset to sRGB

Expected: Color Space never changes once set up by user.

This issue make it impossible to reliably use any non sRGB image data authored by Blender in Blender.

Added subscriber: @Rawalanche

Added subscriber: @rjg

This comment was removed by @rjg

In #74610#888414, @rjg wrote:
I removed my last comment, because it was incorrect. The image data-block references the file. Therefore the selected color space should match the data stored in the image.

How can you decide color space of the data just based on the image container format? For example PNG image can contain diffuse color map or normal map. Both require reading the PNG different format using different colorspace. There is simply no reliable way to automatically decide what the loaded image file contains just based on the format.

@Rawalanche You can (and should) set the color space to Non-Color for normal maps. I've tested that this setting isn't changed by saving the image.

In #74610#888448, @rjg wrote:
@Rawalanche Setting the color space to Non-Color will accomplish that and I've tested that this setting isn't changed by saving the image.

Why should non-color be one specific settings that behaves differently to others, and why should non-color not change when re-saving loaded file but change when another file is selected as image texture sources? This is a horrible minefield or random, under the hood exceptions.

This comment was removed by @rjg

Changed status from 'Needs Triage' to: 'Confirmed'

Added subscriber: @brecht

It simply should not change the colorspace in this case. There are some cases where it should (e.g. saving OpenEXR as PNG with view transform applied), but this is not one of them.

Added subscriber: @ManuelGrad

@rjg Since when are .png sRGB only?
It stores RGB values and how to interpret is not up to the container format as far as i know, no?

@ManuelGrad It seems that you're right, the png format defines iCCP chunks that would allow to use an embedded ICC profile. Although the sRGB chunk is the most common way of storing the data. Sorry for causing any confusion, I'm currently a bit ill and not quite up to speed.

Added subscriber: @fatboyzzfatboyzz

I confirm this bug in blender 3.1.2.
Happens when saving baked texture.

Saving .png file with texture node 'Color Space' set to 'Linear'.
Result in changing the 'Linear' automaticlly to 'sRGB'.
Color looks correct in 'sRGB' but wrong when change back to 'Linear'.

Saving .exr file won't change 'Color Space' automaticlly and everything works fine.
The result color of '.png + srgb' is same as '.exr + linear'.

Seems no way to save linear texture to .png.

Also note this bug happens after texture bake.
I tested by using same source texture then bake to both .png and .exr.
If you simply open a .png set to 'Linear' then save, blender won't change 'Color Space' automaticlly.

Added subscriber: @EAW

This issue was referenced by 5baa3ecda6

Changed status from 'Confirmed' to: 'Resolved'

This will be fixed in 3.3, since it involves a deeper change that is too risky for 3.2.

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Saving image from image editor resets Image Texture color space #74610