Applying OLAP Pre-Aggregation Techniques to ... - Jacobs University

3.2 Geo-Raster Operations 49
Query 3.2.12. Return the smallest element of all cell values in the NRG raster image
shown in Fig. 3.11(a).
Results are shown in Fig. 3.11.
min cells(A) = COND min,sdom(A),i (A[i])
(a) Original NRG raster
(b) Output result
Figure 3.11. Result of a Minimum Aggregate Operation
Histogram
A histogram provides information about the number of times a value occurs across a
range of possible values. For an 8-bit raster up to 256 different values are possible.
As an example consider the following query:
Query 3.2.13. Calculate the histogram for a 2D raster A with 8-bit integer pixel
resolution.
The query can be computed as follows:
Results are shown in Fig. 3.12.
Diversity
MARRAY sdom(A),g (count cells(A = g[0])) (3.1)
The diversity operation returns the different classifications in a raster. For example,
consider the following query:
Query 3.2.14. Given the classifications in an 8-bit gray raster image, return true (1)
for those classes whose total number of cells are greater than 0.