Basic Information on immask

Task: immask
Purpose: Mask an image dataset
Categories: image analysis

IMMASK is a MIRIAD task which allows you to mask an image
dataset, or find out the number of masked pixels in an image.

Masking is directly done to the ``mask'' item of an image dataset, i.e.
the actual image data are not modified, only the mask.
A pixel with a TRUE mask value is considered a good pixel.

To set all pixels in an image to FALSE:
    immask in=ngc_289_6cm flag=false logic=and

To mask out the pixels in a region:
    immask in=ngc_289_20cm region=@cgcurs.region flag=false logic=and

To undo all masking, delete the mask item from the header:
    delhd in=ngc_289_6cm/mask

To mask all the pixels EXCEPT a chosen region (eg you want to blank out
the nasty-looking edges of a CCD image)
    immask in=myimage flag=good region=@good.region logic=not

To change the masking, you must specify a value for the LOGIC keyword.
Otherwise IMMASK merely reports the current numbers of masked and
unmasked pixels.

See also MATHS for other ways to set the image mask, but note
that un-masking a pixel may then expose a value that is incorrect
or unexpected, e.g. maths will have written 0s for pixels that were 
masked in the process.

Key: in
The name of the input image dataset. No default.

Key: region
Regions which will be masked with the ``flag'' value (see below).
Full region descriptions are supported.
Note that missing image planes will be automatically masked 
to the opposite value set by the ``flag'' keyword below. 
Default: whole image.

Key: logic
The logic of the masking operation. It can have a value of ``AND'',
`OR'' or ``NOT'' which determines how the selected region(s) from the 
region= keyword are masked with the existing mask item in the image:
    OR:     region .OR.   mask
    AND:    region .AND.  mask
    EQV:    region .EQV.  mask
    XOR:    region .XOR.  mask
    NOT:    if (region) .NOT.mask
If no value provided, the program will simply report on the 
total number of pixels already flagged good and bad.
Caution: although XOR is not a valid fortran expression, the
EQV and NEQV boolean operators come close. OR and XOR belong
together: OR=inclusive OR, XOR=exclusive OR.
No default.

Key: flag
The value of the mask inside the selected regions. Can be ``true''
or ``false''. Outside selected region it will be its opposite.
A ``true'' value is considerd a good pixel.
Default: true.

Key: history
Specifies whether the history should be updated. This should
normally never be set to false, though some scripts prefer
to do this manually via ADDHIS, and set this parameter to FALSE.
In reporting mode (no ``logic'' operation supplied) the history
is not updated.
Default: TRUE

Key: options
Valid options: 
   datamin      flag all values with `flag' when they equal the
                value of the 'datamin' keyword in the image header.
Default: none.

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