NAME
gdaladdo - gdaladdo builds or rebuilds overview images
SYNOPSIS
gdaladdo [-r {nearest,average,gauss,average_mp,average_magphase,mode}]
[-ro] [--help-general] filename levels
DESCRIPTION
The gdaladdo utility can be used to build or rebuild overview images
for most supported file formats with one over several downsampling
algorithms.
-r {nearest,average,gauss,average_mp,average_magphase,mode}:
Select a resampling algorithm.
-ro:
(available from GDAL 1.6.0) open the dataset in read-only mode, in
order to generate external overview (for GeoTIFF especially).
filename:
The file to build overviews for.
levels:
A list of integral overview levels to build.
Mode (available from GDAL 1.6.0) selects the value which appears most
often of all the sampled points. average_mp is unsuitable for use.
Average_magphase averages complex data in mag/phase space. Nearest and
average are applicable to normal image data. Nearest applies a nearest
neighbour (simple sampling) resampler, while average computes the
average of all non-NODATA contributing pixels. Gauss resampling
(available from GDAL 1.6.0) applies a Gaussian kernel before computing
the overview, which can lead to better results than simple averaging in
e.g case of sharp edges with high contrast or noisy patterns. The
advised level values should be 2, 4, 8, ... so that a 3x3 resampling
Gaussian kernel is selected.
Selecting a level value like 2 causes an overview level that is 1/2 the
resolution (in each dimension) of the base layer to be computed. If the
file has existing overview levels at a level selected, those levels
will be recomputed and rewritten in place.
Some format drivers do not support overviews at all. Many format
drivers store overviews in a secondary file with the extension .ovr
that is actually in TIFF format. The GeoTIFF driver stores overviews
internally to the file operated on, unless the -ro flag is specified.
External overviews created in TIFF format may be compressed using the
COMPRESS_OVERVIEW configuration option. All compression methods,
supported by the GeoTIFF driver, available here. (eg --config
COMPRESS_OVERVIEW DEFLATE). The photometric interpretation can be set
with --config PHOTOMETRIC_OVERVIEW {RGB,YCBCR,...}, and the
interleaving with --config INTERLEAVE_OVERVIEW {PIXEL|BAND}.
To produce the smallest possible JPEG-In-TIFF overviews, you should use
:
--config COMPRESS_OVERVIEW JPEG --config PHOTOMETRIC_OVERVIEW YCBCR --config INTERLEAVE_OVERVIEW PIXEL
Most drivers also support an alternate overview format using Erdas
Imagine format. To trigger this use the USE_RRD=YES configuration
option. This will place the overviews in an associated .aux file
suitable for direct use with Imagine or ArcGIS as well as GDAL
applications. (eg --config USE_RRD YES)
EXAMPLE
Create overviews, embedded in the supplied TIFF file:
gdaladdo -r average abc.tif 2 4 8 16
Create an external compressed GeoTIFF overview file from the ERDAS .IMG
file:
gdaladdo --config COMPRESS_OVERVIEW DEFLATE erdas.img 2 4 8 16
Create an external JPEG-compressed GeoTIFF overview file from a 3-band
RGB dataset:
gdaladdo --config COMPRESS_OVERVIEW JPEG --config PHOTOMETRIC_OVERVIEW YCBCR
--config INTERLEAVE_OVERVIEW PIXEL rgb_dataset.ext 2 4 8 16
Create an Erdas Imagine format overviews for the indicated JPEG file:
gdaladdo --config USE_RRD YES airphoto.jpg 3 9 27 81
AUTHORS
Frank Warmerdam <warmerdam@pobox.com>, Silke Reimer
<silke@intevation.de>