Hugin is a free software for editing panoramic photos with powerful features. Among other things, it can be used to erase the tripod in your 360 photo or add a watermark, or level the horizon. However, it's not the most user-friendly interface. But you can check out this video tutorial on leveling the horizon.
Here's the video:
Hugin is available for download here. Thank you very much to Adrien Monnoyer of Tamia Reality for the video!
Comparison between Hugin and Flexify
Straighten your 360 photos with this app!
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