NAME

fill -- Baade-Lucy background "filling" algorithm.

USAGE

fill input output

DESCRIPTION

This task implements the Baade-Lucy method of filling "pits" (pits are defined as pixels far below the average background) by reshuffling local flux.

The algorithm seeks to reduce high-frequency noise in the background, without loosing high-frequency information on the sources, and enforcing strict flux conservation. The method achieves optimum performance only on images that have already been corrected for any large scale (low-frequency) structures in the background distribution.

The following step by step algorithm description is taken directly from Baade-Lucy's paper (see REFERENCES below):

1. Find all pixels with a flux below a given threshold (called "recipients"). For each one of them:

o Examine within a given radius (r) all pixels with a flux above the threshold (these are called "donors").

o Find all pixels within r of the donor with a flux below the threshold (i.e., further "recipients").

o Compute how much flux a donor can contribute to all its recipients while (i) contributing equal amounts to all its recipients; (ii) not letting its own flux drop below the threshold.

o Take the minimum of all possible contributions; subtract it from the donor.

2. Credit all donations to the recipient.

The implementation used in this task adopts a simplification: instead of looking for pixels a given distance r from the relevant pixel; it looks for pixels inside a square box with size pixels on a side, centered in the relevant pixel. This reduces the computation time by a significant amount.

The task can process an image template or list as input. In this case, the output is either a matching list of images or a directory.

PARAMETERS

input [file name template]

Input 2-dimensional image section, template or list of images.

output [file name template]

Output file name, list or directory. Output images are always type real, regardless of input image type.

(size = 3) [integer, min=3]

Size of filtering box. Always an odd number.

(threshold = INDEF) [real]

Pixels below this value will be filled up; pixels above it will "donate" flux.

(niter = 1) [integer, min=1]

Number of iterations.

(verbose = no) [boolean]

Print file names and iteration count?

EXAMPLES

1. Fill pixels below 50 units in image input. Use a 3 X 3 box size and iterate twice.

pl> fill  input.hhh output.hhh size=3 threshold=50 niter=2

BUGS

REFERENCES

The algorithm is described in:

Grosbel, P., F. Murtagh and R. Warmels (editors), "Proceedings of the 1st ESO/ST-ECF Data Analysis Workshop," Garching, April 17-19 1989, pp. 169-172.

This task was written by I.Busko

SEE ALSO