# 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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