[postgis-devel] Regular raster blocking (was: Real extent feature lost after metadata as views)

[Mateusz Loskot]

On 6 December 2012 12:38, Sandro Santilli <strk at keybit.net> wrote:

> On Thu, Dec 06, 2012 at 12:14:10PM +0000, Mateusz Loskot wrote:
> > On 6 December 2012 11:45, Sandro Santilli <strk at keybit.net> wrote:
>
> > > Why the distinction between internal and rightmost/bottom blocks ?
> >
> > Your suggestion allows partially missing tiles in the middle of tiled
> coverage.
> > List of some possible situations that need to be considered:
> > Spatial extent for the tiled coverage is calculated, and it represents
> > a real extent.
> > Spatial extent of partial tile in the middle of coverage is smaller
> > than extent of coverage grid cell,
> > leaves gap in extent. Gap in extent is different to gap in data.
>
> Extent never has gaps, it's always the minimum rectangle containing
> all the tiles in the table.
>

Of course, I should have wrapped it with 'gap'.



> I do can see that omitting or reducing tile sizes near the boundary
> of the whole extent might effectively change that extent, but that's
> the case also when only allowing it on the right or on the bottom.
>
> If you want the extent to remain immutable there should be an explicit
> extent constraint (only computed when missing).


I'm not suggestion it's immutable.
AFAIU Pierre's modified proposal, the overall extent is computed
based on the actual tile set.


> > If partial tiles are allowed, one may ask why larger tiles shouldn't be
> and it
> > raises question about relevance of the overlapping constraint.
>
> Larger tiles shouldn't be allowed because they would cross block boundary.
>
>   - Each tile belongs to exactly _one_ block
>     (so no tile crosses block boundary)
>   - Every block contains at most _one_ tile



We have the following situations possible:
1) tile is missing (implicit NODATA tile of width == max_blocksize_x
    and height == max_blocksize_y, UL of cell == UL of tile)
2) tile fills cell (width == max_blocksize_x and height == max_blocksize_y,
    UL of cell == UL of tile)
3) tile partially fills cell (width < max_blocksize_x and height <
max_blocksize_y)

Where is UL corner of a partial tile in the case 3) allowed?

Best regards,
-- 
Mateusz Loskot, http://mateusz.loskot.net
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