[raster] Convex hull of an empty raster

[dustymugs]

What is the convex hull of an empty raster (raster = NULL or width/height = zero)?

For a NULL raster…

SELECT ST_ConvexHull(NULL::raster)

 st_convexhull 
---------------
 
(1 row)

For a raster with width or height set to zero…

SELECT ST_ConvexHull(ST_MakeEmptyRaster(1, 0, 0, 0, 1, -1, 0, 0, 0))

ERROR:  RASTER_convex_hull: Could not get raster's convex hull

Instead of an error, shouldn't the answer be NULL?

[strk]

Good question. For geometry ConvexHull(EMPTY) == EMPTY. The type of EMPTY (in geometry) is taken from the type of input, but I think that's pretty arbitrary. For Raster if you take the EMPTY path I'd use POLYGON EMPTY, as I think a raster's convexhull is _always_ polygonal in other cases, is that right ?

[dustymugs]

For rasters, it may be that the convex hull is _NOT_ polygonal in three cases.

Case 1 and Case 2: raster width OR height is zero (but other dimension is NOT zero)

Possible Answer: LINESTRING along the non-zero dimension from upper-left

Case 3: raster width AND height is zero

Possible Answer: POINT of upper-left.

Granted, I'm just as happy returning POLYGON EMPTY as it is keeping it simple. But I wonder if raster should be as arbitrary as geometry. I wonder what ArcGIS, ENVI and ERDAS says…

[pracine]

I tend to opt for the 3 cases.

[robe]

I'd vote for POLYGON EMPTY

what other dimension are you talking about? I thought raster spatial foot print is strictly 2D.

[dustymugs]

I used dimension to describe width or height of the grid. raster spatial footprint is strictly 2D (for now).

[pracine]

We defined an empty raster by width and height = 0 but still an empty raster can have a georeference and hence being like a unidimensional point somewhere. If only one of width or height is = 0 then it is more like a line (hence bidimensional and still somewhere)…

[dustymugs]

Fixed for -trunk in r10810

[dustymugs]

Fixed for 2.0 in r10817. Final solution was the three case example above (plus one case for NULL raster = NULL convex hull)