Changes between Version 25 and Version 26 of WKTRaster/SpecificationWorking01

Timestamp:

May 14, 2009, 9:11:04 AM (15 years ago)

Author:

mpd

Comment:

Added RASTER_OVERVIEWS section

WKTRaster/SpecificationWorking01

@@ -268 +268 @@
 
 It is permissable to for regular_blocking rasters to omit some blocks/tiles (sparse rasters) in which case the missing tiles should be assumed to be all nodata or zero valued. 
+
+----
+
+== '''RASTER_OVERVIEWS Metadata Table''' ==
+
+If the RASTER_COLUMNS "regular_blocking" value is true then "all blocks are equal sized, abutted and non-overlapping, started at top left origin", plus additional constraints. This regular blocking capability raises the possibility of having very large contiguous raster coverages (made up of many individual WKTRaster-s) which, in turn, raises potential performance problems. Other raster formats counter this by having overviews; a concept that is already [http://www.gdal.org/gdaladdo.html supported] by [http://www.gdal.org/ GDAL].
+
+The following metadata table provides external applications and possibly internal tools the ability to discover tables that contain overview raster data, and to learn various information about those datasets. 
+
+|| '''Column Name''' || '''Type''' || '''Constraints''' || '''Comments''' ||
+|| o_table_catalog || character varying(256) || NOT NULL || ||
+|| o_table_schema || character varying(256) || NOT NULL || ||
+|| o_table_name || character varying(256) || NOT NULL || ||
+|| o_column || character varying(256) || NOT NULL || Together define the column containing overview data ||
+|| r_table_catalog || character varying(256) || NOT NULL || ||
+|| r_table_schema || character varying(256) || NOT NULL || ||
+|| r_table_name || character varying(256) || NOT NULL || ||
+|| r_column || character varying(256) || NOT NULL || Together define the base raster coverage ||
+|| out_db || boolean || NOT NULL || false if internal tiles, true if tiles are references to files outside the database ||
+|| overview_factor || integer || NOT NULL || The integral factor of the overview, e.g. 2 means an overview is 1/2 resolution of the base raster coverage ||
+
+The following restrictions apply:[[BR]]
+
+ 1. blocksize_x and blocksize_y for the overview are the same as that of the original, base raster coverage.
+ 1. pixelsize_x and pixelsize_y for the overview are calculated from the original, base raster coverage pixelsize_x and pixelsize_y (using overview_factor).
+ 1. Overviews should not be registered in the raster_columns metadata table.
+ 1. The raster_overviews metadata table can have more than one entry for any given base raster coverage, but they must all have different overview_factor values, e.g. 2, 4, 8, 16, etc.
+
+No provision is provided, or suggested, for creating, updating, or deleting overviews. 
+
+==== '''Pros''' ====
+
+ 1. Overviews and base raster data tables are independent of each other, so overviews do not muddy the water for casual queries ("SELECT * FROM RASTER_COLUMNS").
+ 1. Overview and base raster data metadata will always match.
+
+==== '''Cons''' ====
+
+ 1. Two metadata tables instead of one.
+ 1. Overviews have to share blocksizes with the base raster column.
+
+=== '''Alternative Overviews Metadata''' ===
+
+An alternative design is to extend the RASTER_COLUMNS metadata table with the following columns:
+
+|| '''Column Name''' || '''Type''' || '''Constraints''' || '''Comments''' ||
+|| or_table_catalog || character varying(256) || NOT NULL || ||
+|| or_table_schema || character varying(256) || NOT NULL || ||
+|| or_table_name || character varying(256) || NOT NULL || ||
+|| or_column || character varying(256) || NOT NULL || Together define the original ("or_") raster coverage ||
+|| overview_factor || integer || NOT NULL || The integral factor of the overview, e.g. 2 means an overview is 1/2 resolution of the base raster coverage ||
+
+The overview_factor column could be removed by enforcing a divide by two rule, each overview being half the resolution of its parent.
+
+Original rasters would be queried like this:
+
+ SELECT * FROM raster_columns WHERE overview_factor = 0
+
+Overview rasters would be queried like this:
+
+ SELECT * FROM raster_columns WHERE or_table_name = 'myraster'
+
+==== '''Pros''' ====
+
+ 1. Only one metadata table.
+ 1. Overviews can have blocksizes independent of their base raster data.
+
+==== '''Cons''' ====
+
+ 1. A casual metadata query ("SELECT * FROM RASTER_COLUMNS") will return overviews and base raster data.
+ 1. Metadata for base data and overviews could conflict, e.g. different SRID-s, mis-matched pixelsizes, etc.
+
+----
 
 == '''Specification Comments following the Code Sprint''' ==