Changes between Version 69 and Version 70 of WKTRaster/SpecificationWorking03

Timestamp:

Apr 24, 2011, 11:10:02 AM (13 years ago)

Author:

Bborie Park

Comment:

—

WKTRaster/SpecificationWorking03

@@ -139 +139 @@
 {{{
 ST_MinMax(rast)
+}}}
+
+Due to the time it may take to do on-the-fly determination of min/max for large rasters (say 10000 x 10000), an alternative that sacrifices accuracy for speed is required.  The following functions sample a percentage of the raster in a methodical randomized manner.  The algorithm used for sampling is...
+
+1. select the larger dimension of the width and height.  compute the number of pixels to sample in each "row" of the larger dimension
+
+2. pick pixels from each "row" of the larger dimension in an incremental rolling manner where each increment is randomly determined.
+
+The functions are:
+
+1. ST_ApproxMinMax(rast raster, nband int, ignore_nodata boolean, sample float8) -> record
+
+  sample: a value between 0 and 100 indicating the percentage of the raster band's pixels to consider when determining the min/max pair.
+
+{{{
+ST_ApproxMinMax(rast, 3, FALSE, 10)
+
+ST_ApproxMinMax(rast, 1, TRUE, 50)
+}}}
+
+2. ST_ApproxMinMax(rast raster, ignore_nodata boolean, sample float8) -> record
+
+  assumes that nband = 1
+
+{{{
+ST_ApproxMinMax(rast, FALSE, 1)
+
+ST_ApproxMinMax(rast, TRUE, 2.5)
+}}}
+
+3. ST_ApproxMinMax(rast raster, sample float8) -> record
+
+  assumes that nband = 1 and ignore_nodata = TRUE
+
+{{{
+ST_ApproxMinMax(rast, 25)
+}}}
+
+4. ST_ApproxMinMax(rast raster) -> record
+
+  assumes that nband = 1, ignore_nodata = TRUE and sample = 10
+
+{{{
+ST_ApproxMinMax(rast)
 }}}