Auto exposure breaks when object is too dark

[GitGhillie]

Bevy version

Tested on main commit 2aed777

Relevant system information

Windows 10

AdapterInfo { name: "AMD Radeon RX 7900 XT", vendor: 4098, device: 29772, device_type: DiscreteGpu, driver: "", driver_info: "", backend: Dx12 }

What you did

I have an object with just a base texture and a roughness map, exported from Blender to a gltf and then loaded in Bevy.
I'm using the AutoExposurePlugin with pretty much default settings.

What went wrong

If I place the camera near a dark corner of the mesh, facing the dark corner, the image becomes darker instead of brighter.
You can see what I mean in this video: https://discord.com/channels/691052431525675048/743663924229963868/1242088583108624444

Additional information

Removing the roughness map in Blender and setting the roughness to a fixed value 'fixes' the issue.
You can try it out in this fairly minimal reproduction: https://github.com/GitGhillie/bevy_repro/tree/auto-exposure

The camera will spawn roughly here:

Move the camera into the dark corner where the arrow is and you should be able to reproduce the effect.

[alice-i-cecile]

FYI @Kurble, in case you have any ideas or are interested in putting together a fix :)

[Kurble]

Increasing the AutoExposureSettings::range to -16.0..=16.0 will also fix the issue for this scene. It can be quite tricky to get auto exposure right, we might need some diagnostic tools for it, such as a widget to render out the histogram.

When all samples are out of range (too dark), the shader now targets 0.0 (no correction) as exposure, I think it should maybe target range.start instead so I'll put up a fix for that.