# Align a stack of photos

There are occasions where it is necessary to align a set of otherwise identical pictures:

- Aligning red, green and blue channels to correct chromatic aberration.
- Aligning photos taken over a period of time to create a time-lapse movie.
- Aligning bracketed shots to create a single HDR or contrast blended image.
- Aligning photos taken at different focus distances to merge into a single extended Depth of Field image.

Panorama tools is particularly useful for this process since it allows sub-pixel alignment and has a sophisticated lens correction model for applying distortion - Even photos taken years apart with different cameras can be aligned perfectly.

Most tools for HDR generation such as photomatix have some level of automatic alignment, so this may be sufficient for most purposes. Otherwise the hdrprep perl script automates the process described below:

## Aligning with hugin

- Start up a new hugin project for each series and load the images. Set the Field of View, lens parameters and projection type, ie. if your lens is a fisheye, set this for both the input and output projection.

- Create a few hundred control points between each pair of consecutive photos with the g key in the control point tab.

- Fine tune all points (available in the Edit menu)

- Delete any control points with a correlation less than 90% (i.e. open the control point list (F3 or the Show Control Points icon in the toolbar), press the select by distance button, enter -0.9, and hit the delete button).

- Optimise positions (ctrl-T), select points by distance again but this time enter 0.2. Optimise again.

- Adjust the field-of view in the stitching tab to something slightly smaller than the input size so there are no transparent edges.

- Stitch to multiple TIFF format.

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