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    Correction of image distortion and perspective - part 2

    More corrections...

    The image obtained at the end of part one is corrected from barrel distortion, and vertical lines are no longer oblique. But camera was not perpendicular to rear wall of the room, so horizontal lines of this wall are not horizontal in corrected image. If we modify yaw angle of the image in the panorama, we can easily modify this perspective too.

    Here are the additions to part 1 to do that.

    Step 5

Control points tab:

  1. Check "Align vertically" and add a few control points pairs placed on lines that should be horizontal.
  2. Here I added points on ceiling line, on floor tiling and on rear wall shelves.

    Caution: "align vertically" type CPs mean that in the resulting panorama these points should be on the same horizontal. For spherical or cylindrical panoramas, horizontal lines in real life are NOT horizontal in generated panorama. Using "align vertically" type CPs will not work in that case (the only circumstance where you can apply these CPs without problem is on a flat horizon, sea for example)


    Step 6

Optimizer tab:

  1. Add the optimization of Yaw, then click on "optimize" button


    Step 8

Panorama settings tab:

  1. Verify image in Panorama Editor, then tune parameters
  2. As the image is shifted on left side, I had to increase fov to 90°. Note that rectilinear images get heavily distorted above 100°, you shouldn't go higher with this projection.
  3. As image is proportionnaly smaller in panorama, I also had to increase its size.

     

    The results, after panorama creation and cropping:


    Original image


    Raw result


    Cropped result

    PTgui script

    PTstitcher (optimizer) script

    PTstitcher (create panorama) script

     

    Fisheye images

    The techniques described previously apply to fisheye images, which can be handy for tight spaces. As an example I shot my bathroom with a Coolpix 990 + fisheye FC-E8 mode 2 (full frame).

    The differences with part 1 are in Lens and Panorama settings tabs:

    Step 2

Lens settings tab:

  1. Select Full frame fisheye lens type
  2. Insert horizontal field of view - exact value is not critical. For CP990 mode 2, hfov is around 90° in portrait mode, around 150° in landscape mode.


    Step 3

Panorama settings tab.

Once again, it is of little use to increase fov above 90-100° for rectilinear projection: you may be able to get your entire image, but with too much distortion. You can have a look here to the image generated with a 110° fov.

    I placed numerous controls points on verticals and horizontals. As you can see if you try my script, optimisation results are "not so good" (though straighten image looks fine). This is caused by off-center CCD in Coolpix. Note that this center shift exists with non-fisheye images too, but has much less importance for optimisation because there is little distortion.

Two way to improve this:

  1. If you know the offset values for your camera, you can fill in global shift values in lens parameters
  2. If you don't know them, check "Use individual shift parameters" and
  3. Optimize for d and e parameters

     

    The results, after panorama creation and cropping:


    Original image


    Raw result


    Cropped result

    PTgui script
    PTstitcher (optimizer)     script
    PTstitcher (create panorama)     script

    ...and with d/e optimisation:

    PTgui script
    PTstitcher (optimizer)     script
    PTstitcher (create panorama)     script

      

Panoramic PhotographyPhoto GalleryLego & PhotographyLego Mindstorms and TechnicMindstorms SensorsMindstorms NXTHome