Why is this software changing my brightness?

https://imgur.com/a/QGM1crb

Hey man you need to learn about smooth lighting

Fabric API ingido rendering may be doing it. It should only be very minor and not effect gameplay in awayway.

It hasnt been fully fixed yet, as Mojang have said more changes are planned for the lighting, so its not worth the time required just for it to break again.

If its unplayable can you send a screenshot showing that please.

Try set smooth lighting in your minecraft settings. It appears your smooth lighting level is low. If you change it higher the brightness should look better.

It's still incorrect at any smooth lighting level. Blocks with custom models have no AO I believe. And there's like no shading at all with smooth lighting off.

I have opened a PR that should fix the issues, It may be worth doing a side-by-side with vanilla to ensure it is correct in all situtations.

There's someone else reporting issues with smooth lighting off.

No smooth lighting:

Minimal smooth lighting:

Maximum smooth lighting:

This is pretty bad.