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##### regionmanager.difference - Function

1.1.2 Create the difference of two world regions

Description

This function (short-hand name diff) creates a region which is the difference of two world regions. The order of the regions is important.

The difference consists of all pixels masked-on in the first region and not masked-on in the second region.

Arguments

 Inputs region1 The first world region allowed: record Default: region2 The second world region allowed: record Default: Region tool comment A comment stored with the region allowed: string Default: String
Returns
record

Example

- ia.open(’hcn’)
- csys = ia.coordsys()
- rg.setcoordinates(csys.torecord())
-
- blc = "10pix 10pix"
- trc = "60pix 60pix"
- r1 = rg.wbox(blc,trc,[0,1])
-
- blc = "50pix 50pix"
- trc = "80pix 80pix"
- r2 = rg.wbox(blc, trc, [0,1])
-
- r3 = rg.difference(r1, r2)                        # r1 - r2
-
- ia.statistics(region=r1)                          # Some output discarded
Selected bounding box [10, 10] to [60, 60]
No pts   = 2601
-
- ia.statistics(region=r3)                          # Some output discarded
Selected bounding box [10, 10] to [60, 60]
No pts   = 2480
-
-
- r4 = rg.difference(r2, r1)                        # r2 - r1
-
- ia.statistics(region=r2)                          # Some output discarded
Selected bounding box [50, 50] to [80, 80]
No pts   = 961
-
- ia.statistics(region=r4)                          # Some output discarded
Selected bounding box [50, 50, 1] to [80, 80, 64]
No pts   = 840

We use pixel units and boxes in this example to make it clear what is
happening.  The two regions overlap in the top right corner area of
region {\stf r1} by an area of $11\times11=121$~pixels.  Therefore, the
difference region {\stf r3} has $2601-121=2480$~pixels in it.  For
difference region {\stf r4}, the region of overlap is the bottom left
corner area of region {\stf r2} and still contains 121~pixels.  We expect
$961-121=840$~pixels in the differnce region.

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