Using Hugin to create list of chessboard corners
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### Install hugin:

    apt-get install hugin


### Procedure:

1. Start `hugin`.

2. Menu "Interface", select "Advanced" (or "Expert").

3. In the "Advanced" window, menu "Edit", select "Add Image" to load the 
   camera image.

   In the popup, select Lens Type: Normal (Rectilinear), HFOV(v): 60 
   degrees. This does not actually have any effect for this procedure.

4. Switch to the Control Points tab, which shows the same image twice.

5. At bottom-right:

   - Select Zoom: 200%
   - Uncheck "Auto Fine-tune"
   - Check "Auto Add"
   - Uncheck "Auto-estimate"

6. Then for each chessboard inner corner:

   - Click mouse in *left* image near the correct place, will show "new" 
     label. A small crosshair will be shown on the pixel, and an 
     enlargement will be shown next to it.

     While keeping the mousepointer over the left image, use the arrow 
     keys to finetune. This provides 1/3rd subpixel accuracy.

   - Then in the *right* image, click somewhere arbitrary -- but it 
     works best to make a rows/columns grid there as well.

     This will immediately add a control point number (starting at 0) 
     and draw a line.

   - Then at the bottom-center, select Mode: "Normal". This will stop 
     drawing the connecting lines, which would become very slow.

   Repeat this for all inner corners. Start with the first row left to 
   right, then the second row left to right, then the third row, etc.

   For a 9x7 chessboard, there will be 8 inner corners per row, and 6 
   rows. First row will get labels 0 -- 7, second row 8 -- 15, etc.

7. Menu "File", select "Save As", in .pto format.

8. Exit hugin.

9. Finally convert hugin .pto file to (partial) input for 
   `camcalibplanes`:

       ./pto2gridpix 8 6  < file.pto  > file.json

   ("8 6" is for a 9x7 chessboard)


### Partial results

To add a few points to an incomplete `opencv-findcorners` result:

- Just use hugin for the missing/wrong points, then use `pto2gridpix` 
  with "fake" small grid size. Edit the resulting list into the 
  `opencv-findcorners` result manually.

  For example: when `opencv-findcorners` refuses to detect the leftmost 
  column, just specify a smaller chessboard size for it. This will still 
  cause it to number the detected pixels from column 0 and upwards. Then 
  use hugin to make a list for the leftmost column only, and edit this 
  into the opencv-findcorners output as "-1" column. The row/column 
  numbers have no absolute meaning, they only specify a relative 
  distance to neighbors.

  Beware: `opencv-findcorners` may produce upside-down or sideways 
  output. In such cases, adjust any hugin addition correspondingly.