[GRASS-dev] Re: [GRASS-user] problems using r.proj with large data set

[Glynn Clements]

Dylan Beaudette wrote:

> Interesting work Gylnn. Not having tried your module yet, I am wondering: does 
> the segmenting result in any sort of artifacts in the output raster? i.e. I 
> have seen artifaces related to segmenting with the v.surf.rst module before, 
> when optimal settings are not used. 

No. The projection algorithm is unchanged; the only difference is in
the mechanism used to retrieve the contents of a specific source cell.

The normal r.proj essentially does:

	FCELL val = ibuffer[row][col];

OTOH, with r.proj.seg the code is more like:

	typedef FCELL block[BDIM][BDIM];

	#define HI(i) ((i) / BDIM)
	#define LO(i) ((i) % BDIM)

	...

	block *b = cache->blocks[HI(row)][HI(col)];

	if (!b) /* block not in memory */
		b = cache->blocks[HI(row)][HI(col)] = read_block(cache, row, col);

	FCELL val = b[LO(row)][LO(col)];

The above isn't actual code; for a start, BDIM is a power of 2, and
the HI/LO macros use shift/mask instead of division/remainder. Also,
the details are hidden inside macros to simplify the application code,
which actually looks like:

	FCELL *p = CPTR(cache,row,col);

The main point to note is that interpolation isn't affected by segment
boundaries, which I suspect is probably what was happening with
v.surf.rst.

Assuming that it works out okay, I'll probably end up merging
r.proj.seg into r.proj, with the macro handling the details of whether
to read from a tile cache (large maps) or from a single block of
memory (small maps).

Actually, there is one minor difference in the observable behaviour of
the two programs. r.proj.seg doesn't include the half-baked
null-handling hack used by r.proj for the bilinear and bicubic
methods; it just propagates the nulls.

Consequently, r.proj.seg will tend to enlarge null regions slightly
when using bilinear or bicubic interpolation, whereas r.proj tends to
fill them in (with values whose accuracy is rather dubious). If you
want null regions filled, run r.fillnulls first.

BTW, in the course of writing r.proj.seg, I discovered that
method=bilinear was seriously broken (it had the row/column swapped,
essentially flipping each source cell along the main diagonal). The
fact that no-one noticed until now suggests that either no-one used
method=bilinear, or they didn't pay much attention to the results.

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