## External tools

### Alignment options

Luminance HDR can optionally use align_image_stack command line tool bundled with Hugin, a panorama stitcher, for the alignment of slightly misaligned images. When several differently exposed images are taken in the process of creating a High Dynamic Range image, the images are not perfectly aligned, and the quality of the HDR image created will suffer from this. align_image_stack uses the tools available to hugin to optimize the roll, pitch and yaw, as well as some parameters of the lens geometry, to align the images.

align_image_stack doesn't have a lot of relevant options, so we list them here:

-a prefix
  Output aligned images as prefix_xxxx.tif
-e
  Assume input images are full frame fish eye (default: rectilinear)
-t num
  Remove all control points with an error higher than num pixels (default: 3)
-f HFOV
  Approximate horizontal field of view of input images, use if EXIF info not complete
-m
  Optimize field of view for all images, except for first. Useful for aligning focus stacks with slightly different magnification.
-c num
  Number of control points (per grid) to create between adjacent images (default: 8)
-l
  Assume linear input files
-s scale
  Scale down image by 2^scale (default: 1 [2x down-sampling])
-g gsize
  Break image into a rectangular grid (gsize x gsize) and attempt to find num control points in each section (default: 5 [5x5 grid])
-v
  Verbose, print progress messages. Repeat for higher verbosity

By default Luminance HDR just defines prefix for aligned images: `-a aligned_` and for verbose printing `-v`.