SEE_ALSO

## NAME

overflowx -- Correct 8-bit overflow in FOC images (as far as possible).

## USAGE

`overflowx input1 input2 output`

## DESCRIPTION

In 8-bit mode, an FOC pixel will overflow---cycling back through 0---when the count exceeds 255. If an image is taken with the same conditions, but with a shorter exposure time, then it can be corrected, assuming that the pixels have cycled through 0 no more than one time. Alternatively, in the special case of flat fields we know that the image should not change too rapidly from one pixel to the next, we can therefore attempt to correct an image with overflowed pixels.

There are two modes of operation.
The first uses two images,
and checks the ratio of pixel values.
If the ratio of overflowed
to normal image exceeds `low_thresh`, then we add 255 to first image,
unless
this would give too high a ratio. Negative ratios are interpreted as
multiple of rms statistics (sqrt(count)).

If only one image is given, we proceed along each line and
starting at a specified pixel, if we detect a downward shift by more
than
`low_thresh` we add 255, unless we would end up with a positive shift
greater than `high_thresh`.

## PARAMETERS

- input[file name]
- Name of the image with 8-bit overflow.

- input[file name]
- Name of the optional image without overflow.
If this parameter is not used, then
`input1`is corrected as a flat field image.

- output [file name]
- Output image name.

- (lowthresh = 128.) [real]
- Lower threshold for allowed pixel change.
If
`lowthresh`is negative it is interpreted as the number of standard deviations ( = sqrt(count) in non-overflowed pixel) allowed.

- (highthresh = 128.) [real]
- Upper limit for pixel change.
If
`highthresh`is negative, it is interpreted as the number of stand deviations ( = sqrt(count) in non-overflowed pixel) allowed.

- (startpix = 1) [integer, min=1]
- Starting pixel for processing in each line. This allows the first few
pixels in a line to be ignored. The unprocessed pixels
are just copied from
`input1`to`output`.

- (ratio) [real]
- Expected ratio for counts
`input2`/'input1', for example the ratio of exposure times if other conditions are the same.

## EXAMPLES

1. Correct an image as a flat field, one line at a time.

fo> overflowx inimage "" outimage

2. Here the ratio of the non-overflowed to overflowed images should be 0.5. Those pixels for which the ratio is too much lower than this will be corrected, unless an overcorrection would result.

fo> overflowx ov norm out ratio=0.5

## BUGS

## REFERENCES

This task was written by David Giaretta.

## SEE ALSO

Type "help focutility option=sys" for a higher-level description of
the `focutility` package.