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It's the lookout tower below. You can't shoot from a single place - you take pictures from four different positions, distant a few meters one from another.

I know there logically must be problems on nearby trees (distant only tens to hundreds of meters) but I'd accept enblend retouching/blurring here.

What do I need to prevent are bumps on the horizon. There shouldn't be any as it is tens of kilometers away!

Note 1: I hoped I could add control points manually (without help of Hugin's CPFind) - on the horizon only - but the results have still been poor.

Note 2: Taken without a tripod. I assume it's not a problem though as I've taken tens of other panoramas handheld and they've been stitched perfectly in Hugin.

Lookout Tower enter image description here

user681768917
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2 Answers2

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It's hard to tell, but it looks like in the stitched image your camera might have been aimed slightly above the horizon in at least 2 of the unstitched shots. By aiming up a little bit, the horizon at the widest part of your image "curls upwards" somewhat. This effect is similar to barrel distortion, and is more prominent with wider fields of view (shorter focal length lenses).

The solution is two-fold:

  1. Make sure your camera is absolutely level to the horizon. Use a bubble/spirit level on your tripod to determine this. In this case, it's the up/down tilt axis that is the most critical, that you're trying to resolve.

  2. Stitch together more shots, rather than fewer. The problem you want to avoid is distortion at the edges of the field of view. So increase the overlap for each successive shot (i.e., don't rotate as much between each shot, taking more shots). One way to do this is to rotate your camera to portrait orientation, and take your shots. You will be using the short axis of your sensor frame, thus requiring more shots to cover the same final stitched field of view as you would if used your camera in landscape orientation.

scottbb
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  • While my comment to the question aimed at a similar explanation, it is important to note that any distortions must not happen even if the camera is aimed above the horizon. My comment suggested that maybe correction of the lens distortion is not perfect. But actually, this is improbable. In other words: If the stitching process is working properly, you ca aim your camera at whatever you want. That's the point in stitching after all. – Torsten Bronger Feb 08 '16 at 09:55
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    @TorstenBronger the stitching process can only do so much. Because several (all? not sure) of the shots are from different vantage points, there will definitely be substantial parallax errors that Hugin will have to try to smooth over. Stitching software generally assumes either pure rotation between shots (cylindrical or spherical projection), or pure translation (dolly movement). Substantial deviations from those assumptions just makes the stitcher work harder. At that point, you begin to need conformal mapping to make perspective distortion corrections. – scottbb Feb 08 '16 at 13:28
  • But not for the horizon line, which is at infinity. – Torsten Bronger Feb 08 '16 at 15:13
  • @TorstenBronger correct, but stitching is perpendicular (-ish) to the horizon, and the stitch line below the horizon will have more and more parallax error the further below the horizon you look, for the OP's pre-stitched shots. In that case, considering the OP has both pan and "dolly" movement between shots, by eliminating the vertical tilt axis from the shots, Hugin should be able to do a better job of stitching. – scottbb Feb 08 '16 at 18:29
  • Whatever is affected on his images, the horizon line is also affected, and this is at infinity, so the cause cannot be parallax effects. – Torsten Bronger Feb 08 '16 at 20:12
  • Thanks - I'm going to try those more shots as suggested - this was taken at 24mm equiv. as portraits. There also is a problem with the roof - I needed to shoot under a bit to avoid it. – user681768917 Feb 08 '16 at 20:34
  • Do you think that moving gradually along the edges of the lookout tower after each shot - thus splitting the distance into smaller steps could also help? – user681768917 Feb 08 '16 at 20:39
  • Not really. Because you are not rotating around the no-parallax-point, you will get parallax errors, no doubt. But as Torsten has pointed out, at the horizon, there is no parallax error (it's effectively at infinity). – scottbb Feb 08 '16 at 22:02
  • @TorstenBronger right, the parallax is only in the near field, diminishing rapidly towards the horizon. But because the stitch points at the horizon do not meet at the same slope, going back to my original thesis: it appears that the camera wasn't perfectly tilt-level for a couple of the shots. – scottbb Feb 08 '16 at 22:04
  • @user681768917 you marked this answer as correct --- but are you sure? – Torsten Bronger Feb 09 '16 at 05:20
  • @scottbb the stitching software makes a big optimisation fit, and the roll angle of the camera is one parameter in it. It is eliminated. Which leads me to ... – Torsten Bronger Feb 09 '16 at 05:23
  • ... @user681768917: Which parameters did you include in your stitching? In particular, was the roll angle one of them? – Torsten Bronger Feb 09 '16 at 05:24
  • @TorstenBronger Only those included in the camera EXIF info (retrieved automatically). The most important parameter here is focal lenght/FOV, I guess. – user681768917 Feb 10 '16 at 11:17
  • @user681768917 My wording was not clear, sorry. I mean: Which parameters did you include in the optimization. If "roll" is not one of them, you would get such bumps. – Torsten Bronger Feb 10 '16 at 12:11
  • @TorstenBronger: I don't know. The Hugin's stitching script can be exported to a text file and it contains quite a lot of numbers but I'm not sure if any of them is what are you asking about. I don't know how to set optimisation parameters in Hugin, even in the "Expert" mode, sorry. – user681768917 Feb 10 '16 at 22:02
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My guess is that you're not taking care to keep your camera level/consistent in pitch. When you change the orientation of the camera between member shots, you can get errors like this. I'd highly recommend shooting with a two or three-axis spirit level on the hotshoe. Most cameras will only show you if you're level in roll, not in roll and pitch.

inkista
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