I would recommend creating a new tool for straightening out the horizon: bezier curves. When dealing with photos shot handheld under strange conditions, eg. 11km above the earth or a long horizontal pano of the horizon at the beach, where the pano tends to exaggerate the earth's curve, using the Verticals Tool to straighten out the horizon is cumbersome. Being able to draw a bezier curve from one end of the horizon to the other would be a very quick and easy way to get the horizon straight. Once it's straight we could leave it at that, or we could use another tool to BEND it the way WE like it. A perfectly straight horizon isn't natural when you can see 100km left and right, but the curve that results right after stitching some panos like that is far too bent. That's why having those two tools would be ideal - one to aid in easier straightening out of a long unnaturaly curved horizon, and the other to aid in curving a pano to our exact specifications.
I know that doing that is somewhat possible using the verticals tool, but the result isnt good and its a tedious job. The Automatic Horizon tool doesn't help at all in such cases.
I would also recommend adding a tool like Verticals Tool only working horizontally, so Horizontals Tool. Using the dotted yellow line isn't always practical. Or instead of adding a new tool, we could change the way Verticals Tool behaves: if the angle of the line is greater than 45° (for example), then it switches from vertical to horizontal functionality. In practice it would work like this: when I drag a line from the left edge to the right of a skew horizon, instead of making that horizon vertical and me having to click on rotate right 90°, it would understand that what Im straightening is horizontal and keep it horizontal but just straighten it. If you changed the way it works, the name would also have to change from Verticals Tool to eg. Straighten. If you would like to see such a tool in action, you can take a look at the free RAW converter RawTherapee, I think it works quite similarly. Don't download from the main page, a newer verstion is linked to in the forum: http://www.rawtherapee.com/forum/viewto … =1847#1847
Straightening the horizon is somewhat ambiguous...
As far as I understand it, the intent of corresponding APP tools is to place the "should-be-vertical" axis of the projection (any projection mode) in the best possible orientation.
For planar panos any useful change is then possible using moderate changes of yaw and/or pitch and/or roll (using Yaw-Pitch-Roll tool.)
This is a necessary step for cylindrical and spherical (equirectangular) projections because when this axis is not vertical the resulting image have a sinusoidal shape and is wavy. For a very large amount of such panos (for any 360° pano) having the vertical axis as close as possible to the actual vertical gives the best possible result. Happily enough APP makes it possible to crop the pano so that the horizon is not in the middle of the view.
11km above the earth or a long horizontal pano of the horizon at the beach are exceptional situations where a problem should arise only when the camera is high enough for the earth curvature being visible : I never experienced that... Another more common situation where one could want to make changes after the normal placement of the horizon is when the source images were taken in a downward direction and the horizontal FOV of the pano is limited : in this case I found that a moderate change of the panorama pitch can help.
This is not an easy matter, I did my best to share what I know about that and hope it can help
You're right, and I want to help users who find themselves in situations where for one reason or other the horizon gets displayed wrong and using the currently available tools (v1.4) is quite cumbersome.
I will post some photos with which I have horizon problems during the next several days so you can all take a look and reflect.