Batch merge handheld images

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Problem

Shooting for HDR requires the bracketed images to be perfectly aligned. Usually this is done using a sturdy tripod. But sometimes this isn't possible and images need to be aligned later. For not so wide rectilinear images this requires a simple image shift. PTGui and hugin allow for not linking bracketed shots to the same place in order to compensate for shifts and several HDR programs do a simple alignment as well.

Very wide angle and fisheye lenses are more difficult. Since handheld shooting means a possible displacement in all three spatial directions and fisheyes cover up to 180° or more a back or forth displacement effectively means a different viewpoint. Any attempts to align such images should take this into account. However, since handheld shooting always can cause parallax errors between the bracketed image a perfect alignment is not always possible.

Solution

Best results can be achieved if all possible image distortions are allowed. Panotools based stitchers allow for optimization of Yaw, Pitch and Roll, field of view, lens correction parameters and center shift. The most recent versions also allow for viewpoint correction (called translation in hugin). There should be plenty of control points throughout the whole image. All this possibilities together usually allow for a very good alignment.

hugin

align_image_stack

Hugin comes with a command line tool called align_image_stack. Since version 2011.5 it allows for optimization of camera position, too. For a fisheye lens you need to specify parameters -e (fisheye lens), -f HFOV (where HFOV is the approximate horizontal field of view in degrees), -m (Field of View), -d (distortion), -i (center shift) and -x -y -z (camera position, since version 2011.5 only).

The hugin windows bundle also includes a batch file called enfuse_align_droplet.bat which calls align_image_stack and passes the aligned images to enfuse for merging. You need to open the file with a text editor and put the correct parameters for your lens in the line starting with 

set align_additional_parameters=

hugin GUI

A way to align images directly inside hugin GUI is oulined in Align a stack of photos.

PTGui

Aligning images in PTGui is a bit tricky, but it has the advantage of the Batch Builder and is relatively fast.

create a template

To create a template for the Batch Builder please follow the steps carefully (assuming advanced mode in PTGui pro):

  1. open PTgui and load a single image with the pixel size you will process.
  2. on Lens settings tab choose Lens type Rectilinear (normal lens), enter an arbitrary value below 180 as Horizontal field of view (if your lens' HFoV is below 180° you can use that value) and click Restore defaults in Advanced section.
  3. on Panorama Settings tab choose Rectilinear ('flat').
  4. open Pano Editor (Ctrl+E) and choose Edit->Fit Panorama
  5. on Create Panorama tab uncheck Link width and height and choose Set optimum size -> Maximum size. Width and height should now be identical to your source image. Check Link width and height again.
  6. back on Lens Settings tab choose Lens type Fullframe fisheye and enter correct HFoV (if not already present).
  7. on Panorama Settings tab choose Fullframe and enter same FoV like on Lens Settings tab (if not already present).
  8. go to Create Panorama tab and enter the width of your source image (only if you changed FoV in previous step). Then uncheck Link width and height and enter height of your source image.

    All this is necessary because PTGui doesn't support Fit panorama for output projection Fullframe. Without those counter-intuitive steps it wouldn't be possible to have the exact same output dimensions and projection like input. The following steps prepare the template for the batch builder:

  9. on Source Images tab add as many images as you will probably want to align at most. This can be arbitrary images of same pixel size or the same image as the first, it doesn't matter.
  10. on Lens Settings tab check Lens and Shift for all but for Image 0 (double click the column header for all images).
  11. on Optimizer tab advanced check Viewpoint, FoV, a, b, c, HShift and VShift for all but Image 0. Uncheck anything under Optimize globally.
  12. on Exposure/HDR tab check Enable HDR stitching and Exposure fusion. Click Reset for anything else.
  13. on Project Settings tab:
    • in 'Align Images' behaviour section uncheck anything but Generate control points and Optimize project.
    • under HDR defauls choose Enable HDR mode but do not link the images and check Do this without asking.
    • check anything in Batch Stitcher actions
    • under File naming, Project file save as choose <prefix first image filename> and enter _fused. Panorama save as Same as project name and save in Source image folder for both. Check Store relative path...
    • in Template behaviour section check anything but Image parameters and Image masks.
    • uncheck anything under Batch builder behaviour and Miscellaneous.
  14. on Create Panorama tab choose TIFF, 16 bit, no compression, no alpha as LDR file format and Exposure fused panorama under Output.
  15. choose File->Save as template from the menu and give it a reasonable name.

For later modification it is advisable to save as a normal project, too.

Use the template

Your source images need to be in a folder with no supplemental images or files. In order to have enough control points you need to go to the PTGui menu Tools->Options->Control Point Generator and enter a high number (f.e. 2000) for Generate at most ... control points per image pair and an even higher one (f.e. 20000) for ... per project. Don't forget to reset these numbers to the previous values before you stitch normal panoramas again. Close PTGui in order to have this values saved.

To actually use the template

  1. open PTGui
  2. start the batch builder from Tools->Batch Builder menu and Click Generate new projects.
  3. choose Method: Multiple panoramas per folder with a fixed number of images, choose source folder and enter number of images (maximum will be the number of images you created the template with). After confirmation the Batch Builder will show the projects with image thumbnails.
  4. under the preview choose your template from Use template dropdown.
  5. once you click Generate Projects PTGui creates a project file for each set of images and a batch list for the Batch Stitcher, which either can be processed immediately or later.

Use with SNS-HDR lite

It happens that SNS-HDR lite command line is partially compatible with PTGui plugin interface. It can be used instead of enblend to process images. To do this go to PTGui menu Tools->Options->Plugins. In the Enblend section and browse to the SNS-HDR.exe you previously installed. Under Command line parameters enter 

-da -natural -tiff16

-da disables SNS-HDR internal alignment, -natural is a style preset. Call SNS-HDR.exe directly (double click desktop shortcut) to get information about other command line parameters.

Remember to revert this if you actually want to use enblend for normal panoramas.

Modify the above template creation steps to create a template as follows:

  1. on Exposure/HDR tab uncheck Enable HDR stitching
  2. on Project Settings tab under HDR defauls choose Do not enable HDR mode and enter a different prefix (f.e. _SNS) for Project file: save as...
  3. on Create Panorama tab choose TIFF, 16 bit, no compression, no alpha as File format and Blended panorama only for Layers. In Advanced section choose Blend using: Enblend plugin
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