pfsalign(1)

NAME
pfsalign - Align image stack

SYNOPSIS
pfsalign [--reference <index>|-r <index>] [-c (min|max)|--crop (min|max)] [-s val|--sensitivity val] [-f|--fail-no-match] [--display-matches|-d] [--verbose] [--help]

DESCRIPTION
Use this command to align image stack, for example for HDR merging. The command will use a feature-point-based matching method to find a homographic transformation that maps all images in the stack to the coordinates of the reference image. Then, it will output transformed (aligned) images, all of the same resolution. The resolution of input images may vary. The command can eliminate a substantial misalignment due to camera rotation (along any axis) and slight movement.

The method is essentially similar to the one presented in the paper: Tomaszewska, Anna, and Radoslaw Mantiuk. "Image Registration for Multi-Exposure High Dynamic Range Image Acquisition." In International Conferences in Central Europe on Computer Graphics, Visualization and Computer Vision, pp. 49-56, 2007. However, the code is not based on the authors' implementation.

The images are matched pairwise in the same order as they are send to the command. Make sure that all consecutive image pairs contain overlapping parts. When aligning multiple exposure, use pfsinme command, which sorts exposures according to the shutter speed and thus improves matching between image pairs. The command will assume no misalignment (or fail with -f option) if there is no overlap, or it cannot find a sufficient number of features in one of the pairs.

Homographic alignment will not eliminate all misalignment artefacts. In particular, it will not eliminate ghosting due to moving objects. The alignment algorithm assumes that the effect of parallax is negligible. Therefore, if the camera moves rather than rotates and there are objects close to the camera, they will most likely remain misaligned.

OPTIONS

--reference <index>, -r <index>
    Index of the reference image. The first image has index 1. Default: 1 (first image).

--display-matches, -d
    Display matching key-points for each image pair. This option is useful for inspecting what went wrong with key-point matching.

--crop min|max, -c min|max
    Crop either to the image that contains all pixels (max) or to the image that contains only the pixels that overlap in all exposures (min). The latter option will eliminate black (empty) pixels from all exposures.

--sensitivity value, -s value
    Higher sensitivity will result in detecting more feature points and potentially a better alignment but also longer computation time. "value" parameter must be in the range from 1 to 10. Default: 5.

--fail-no-match, -f
    If the option is present, the program will exit with a fail status when no match was found for one or more of image pairs. Otherwise, the program will assume that the failed image pair is perfectly aligned (assume identity matrix as a homograhy).

EXAMPLES

pfsin img1.exr img2.exr | pfsalign -v -r 2 | pfsview

    Align two EXR images using the second image (img2.exr) as a reference. Then, preview the result.

pfsinme *.CR2 | pfssize  --maxx 1200 | pfsalign -v -c min  | pfshdrcalibrate -r linear -v --bpp 16 | pfsout result.exr

    Align a stack of RAW image exposures (from Canon camera), resize them so that the width is equal or less 1200 pixels, align exposures, and merge them to an HDR image.

SEE ALSO
pfsin(1)

BUGS
The current implementation is rather inefficient in terms of memory management and can consume a considerable amount of memory. This is due to the pfstools architecture, which requires that all aligned frames must be loaded into memory. It should be possible to align 3-4 10 mega-pixels images for 1.5GB of available memory.

Please report bugs and comments to the pfstools discussion group (http://groups.google.com/group/pfstools).