PostGIS Raster : Extending PostgreSQL for The Support of ... - CoDE

Therefore, when the -k option is applied for a raster image, it implies that:
• The multidimensional matrix of cells representing raster image is partitioned into small blocks
(or tiles) for large-scale storage and optimal retrieval and processing.
• All imported blocks have the same width and height.
• All blocks do not overlap and their upper left corners follow a regular block grid.
• The global extent of the raster layer is rectangular and not rotated.
There is, however, no mechanism to enforce these criteria and adding, modifying or deleting a row
from the table might break this regular blocking. So some blocks can be missing in a regular blocked
table. Missing blocks are assumed to be filled with the proper nodata value of each band when they
are displayed in a map.
4.11 Pyramids or Overviews
4.11.1 Concept
The Pyramids concept is used to reduce resolution representations of a raster image to improve
performance in a way that Pyramids can speed up the display of raster data by retrieving only the
data at a specified resolution that is required for the display. For instance, the level resolution of image
is related to the amount of time that an application needs to retrieve and display it, particularly over
the Web. The lower the image resolution, the faster it can be displayed. For example, in the case
where the image details are not important as user has "zoomed out" considerably, loading a lower
resolution image is more adequate than loading entire image with a very high resolution.
Figure 4.29: Pyramids (or Overviews) in PostGIS Raster [24].
When users zoom in, levels with lower resolutions are required to draw successively smaller areas.
However, by using pyramids, performance is maintained because the database server can choose the
most appropriate pyramid level (or the most appropriate resolution) of the raster data to quickly
display it on the screen. In PostGIS Raster, at each level resolution, a copy of image is created and
maintained as a separate raster. So without pyramids, the entire raster must be read from disk and
resampled to a smaller size each time. This is called display resampling process.
Pyramid levels represent reduced resolution images that require less memory space. A pyramid
level of 1 indicates the original raster data, there is no reduction in the image resolution and no
change in the storage space required. Values greater than 1 indicates increasingly reduced levels of
image resolution and reduced storage space requirements. Each successive level of the pyramid is
downsampled at a scale of 4:1 as in Figure 4.30. Pyramids only need to be built once per raster. After
that, they are accessed each time the raster data is viewed. The larger the raster data, the longer it
takes to create the set of pyramids. This also means that more time is saved in the long run.
4.11.2 Creating Pyramids
In PostGIS Raster, a pyramid can be created at the same time as loading a raster image:
python raster2pgsql.py -r *.jpg -t raster_table -s 26986 -l 4 -k 100x100 -I
-o raster_overview_4.sql
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