[GRASS-user] identifying diagonal clumps

[Glynn Clements]

Milton Cezar Ribeiro wrote:

> I tryed your suggestion and notice that the F8 solution
> simulate a dilatation of 1pixels on everyside of patches.
> But, if I am not wrong, when I have a patch separated by
> 2 pixels, the F8 solution will join these patches on only one.
> 
> But I can preserve the patches separated, and only
> consider as one patches those pixels that can be joined
> on the rule 8. I don´t know if I am so clear, but I would
> like to request a little more time of you on this regard.
> 
> I run the folowwing commands:
> 
> r.mapcalc "F8 = (A*A[-1,-1]) * (A*A[-1,0]) * (A*A[1,-1]) * (A*A[0,-1]) * (A*A[0,1]) * (A*A[-1,1]) * (A*A[1,1])"
> r.clump input=F8 output=F8_clump

The r.mapcalc command will perform "thinning". The result will be one
if all nine cells (the centre and all eight surrounding cells) are
one, and zero if any of the nine cells are zero).

[Actually, you're missing A[1,0].]

For "fattening", you should use e.g.:

r.mapcalc "F8 = A[-1,-1] || A[-1,0] || A[-1,1] || A[0,-1] || A || A[0,1] || A[1,-1] || A[1,0] || A[1,1]"

[For 0/1, multiplication is equivalent to the "&&" operator (logical
conjunction).]

Another option, which should only fill in the cells necessary to make
r.clump recognise diagonal adjacency, is:

r.mapcalc "AF = A || A[0,1] && A[1,0] || A[0,-1] && A[1,0] || A[0,1] && A[-1,0] || A[0,-1] && A[-1,0]"

This will only set the centre cell if two diagonally-adjacent
neighbours are both set.

It may even suffice to use:

r.mapcalc "AF = A || A[0,1] && A[1,0] || A[0,-1] && A[1,0]"

This will only fill the cell on one side of a pair of diagonally
adjacent cells.

-- 
Glynn Clements <glynn at gclements.plus.com>