Changes between Version 43 and Version 44 of WKTRaster/SpecificationWorking03

Timestamp:

03/14/11 13:22:29 (14 years ago)

Author:

pracine

Comment:

—

WKTRaster/SpecificationWorking03

@@ -201 +201 @@
 For now ST_MapAlgebra expressions refer only to the pixel being computed. e.g. "rast * 100". The original plan was to allow refering to neighbour pixels using two coordinated relative to the pixel being computed. e.g. "rast[-1,0] * 100" where rast[-1,0] refer to the value of the pixel one pixel to the left of the pixel being computed. However this syntax might prove to be hard to use when many neighbours are to be used.
 
-An alternative syntax would involve another function name (e.g. ST_MapAlgebraNgb) and a way to define a neighbour rectangular region around the computed pixel (e.g.: "2,2" meaning a rectangle encompassing the two neighbour pixels in each direction) and a function to call with this matrix of pixel values. A complete example might look like:
-
-SELECT ST_MapAlgebraNgb(rast, "ST_Mean", 2, 2, "ignore") 
+An alternative syntax would involve another function name (e.g. ST_MapAlgebraNgb or ST_MovingWindow) and a way to define a neighbour rectangular region around the computed pixel (e.g.: "2,2" meaning a rectangle encompassing the two neighbour pixels in each direction) and a function to call with this matrix of pixel values. A complete example might look like:
+
+ SELECT ST_MapAlgebraNgb(rast, band, pixeltype, "ST_Mean", 2, 2, "ignore") 
 
 So this would mean "for each pixel, compute the average of all the 1 + 8 + 16 = 25 pixels surrounding the current pixel and "ignore" pixels with nodata values."
@@ -222 +222 @@
 A more sophisticated version would pass a georeferenced raster instead of just a value matrix so that summarizing function could use this geoereference (e.g. to determine a value from the whole coverage (with ST_Value) when the neighbours are out of the bound of the raster). Passing a raster would allow existing raster functions (like the summarizing function which are to come). Only the optional "what to do with nodata values" could be needed and some additional parameters. In this case the example become:
 
-SELECT ST_MapAlgebraNgb(rast, 2, 2, "ST_Mean", "ignore")
+ SELECT ST_MapAlgebraNgb(rast, band, pixeltype, 2, 2, "ST_Mean", "ignore")
 
 and the dimensions do not have to be passed to the summarizing functions since it could deduce them from ST_Width & ST_Height.
 
+An even more sophisticated version should get a raster table and a raster column as parameters and try to search for neighbour in the whole raster coverage when out of bound pixels are part of the neighbourhood. e.g.:
+
+ SELECT ST_MapAlgebraNgb("mycoveragetable", "myrastercolumn", band, pixeltype, 2, 2, "ST_Mean", "ignore")
+
 Three difficulties must be solved to implement this function:
 
- * The matrix values assigment must be optimized when passing from one pixel to the other.
+ * The construction of the matrix must to be passed to the summarizing functions must be optimized when passing from one pixel to the other.
  * We must see how it is possible to call a PL/pgSQL function from a C function
  * We must see how to pass a variable number of parameter to a PL/pgSQL function