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Tips & Tricks for manual calibration with PTGui

PTGui is the professional choice for panoramic photography creation, and our favorite tool to create calibrations for VideoStitch products. It shines with its convenient user interface that allows easy editing of control points and tweaking of all optimization parameters.

This article points out some tips that are useful when creating camera array calibrations for VideoStitch Studio and Vahana VR

Choose carefully the calibration scene you work with

When extracting images from VideoStitch Studio or Vahana VR to work in PTGui, choosing a good calibration scene is the key to successful stitching. It should provide enough details in overlapping areas for adding control points. The following images were extracted from the same Videostitch Studio project :

The first set was extracted at 00:00:00, the blue sky doesn’t offer any visible detail to add control points between images 1 and 4 :

After seeking a better frame and extracting the images from VideoStitch Studio again, we can easily add control points were they were missing

For Vahana VR, you can for example move your cameras or rotate them to get the overlap area on another part of your scene.

When extracting images from Studio or Vahana VR to overwrite a previous frame set, PTGui will update automatically.

This allows to easily combine control points from different frames.

Generate control points in a specific image area

This is probably the first step if you are getting started with PTGui : adding control points where the images don’t stitch properly, in order to provide PTGui with the information it needs to adjust images parameters.

PTGui offers multiple options to automatically generate control points : between image pairs, and between all images. These commands are all available in the main window’s “Control Points” menu.

But what if we want to generate control points automatically in a specific area ?

Of course, there is a very convenient way to do this :

  • In the control points tab, while holding the “Shift” key down, click & drag your mouse to define a selection.
  • Right click on this selection, and choose ‘add control points here’ to generate more control points within this area.

The selection can also be used to remove multiple control points at once. Just hit your keyboard’s ‘delete’ key.

Optimize parameters wisely

By default, PTGui provides a simplified interface to it’s optimizer. But stepping into the advanced settings it is possible to get finer control.

Start, with the default setting, by optimizing globally lens fov, the lens b distortion parameter, and image yaw, pitch and roll. ( b distortion is referred to as ‘Medium’ lens distortion in the optimizer ‘simple’ interface ).

Optimizing Shift for each image individually is another trick specific to 360 video rigs. Make sure to check “use individual lens parameters” in the “lens settings” tab enable this feature.

When you have good alignment with ‘Medium’ lens distortion, optimizing ‘heavy’ lens distortion ( lens a and c parameters) isn’t necessary, but may improve the results but require that your control points are well spread over the entire overlapping areas.

Disabling yaw, pitch, & roll optimization on one of the images ensures the panorama global orientation is preserved during optimization.

Re-initialize your project when optimization fails

If PTGui’s optimizer keeps giving you erroneous results, don’t forget to re-initialize your project. This will get you back on tracks most of the time

 

Follow the control points assistant

The “control point assistant” window can help you improve results by giving you suggestions on improving the project.

You can access it anytime by clicking on the control points table button.

Make sure that all overlapping image pairs have control points.

Remove worst control points

Sometimes, a few misplaced control points can introduce optimization error. Remove these control points can help to achieve a better optimization, and thus better visual output.

PTGui has a dedicated function to remove the worst control points. It can only be used after a first optimization has been done, since the ‘worst control points’ are those that yield the most optimization error.

Control points can also manually be removed directly in the control point lists : the ones that show the most ‘distance’ can usually be considered outliers

To locate and view a control point in the image, just double click on it in the list, it will display and be selected instantly in the control point tab.

Draw straight lines with the mask tool

VideoStitch Studio and Vahana VR automatically imports PTGui pro ‘s red masks (it doesn’t take the green mask).

Did you know, it is possible to draw straight lines with the mask tool ?

  • Simply choose the pencil to draw red masks, and click where you want the line to start.
  • Press and hold the “Shift” key down, and click where you want the line to stop.

Change the “Align Image” behavior in your templates

PTGui pro users can change the “align images” button behavior in the “project settings” tab. These settings are also saved in PTGui templates, so that when this project is applied as a template, it will keep the “align images” button behavior.

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