Image processing
Introduction
Satellite imagery and orthophotos (aerial photographs) are handled in GRASS as raster maps and specialized tasks are performed using the imagery (i.*) modules. All general operations are handled by the raster modules.
- imageryintro: A short introduction to image processing in GRASS 6
- Full GRASS 4.0 Image Processing manual (PDF, 47 pages)
- imagery: Imagery module help pages
- Data import is generally handled by the r.in.gdal module
Screenshots
- The imagery screenshots page
Importing
The wxGUI offers a convenient tool for single map and bulk import:
- see Importing data
Satellite Data
Ocean Color
Sea Surface Temperature (SST)
High resolution data
Orthophotos
Preprocessing
See also NASA LaRC Satellite Overpass Predictor
Geometric preprocessing/Georectification
- Georectification tool is available from the File menu in the GUI.
- i.points, i.vpoints (scanned maps, satellite images)
- i.ortho.photo (aerial images)
A multi-band image may be grouped and georectified with a single set of ground control points (i.group, i.target, i.rectify).
See also the Georeferencing wiki page
Radiometric preprocessing
- use r.mapcalc to apply gain/bias formula
- LANDSAT: you can also use i.landsat.toar
from GRASS AddOns(included since 6.4)
Correction for atmospheric effects
- i.landsat.dehaze: simple dark-object/Tasseled Cap based haze minimization (from GRASS AddOns)
- i.atcorr: more complex correction but based on atmospheric models - see tutorial: Atmospheric correction
Correction for topographic/terrain effects
In rugged terrain, such correction might be useful to minimize negative effects.
- simple "cosine correction" using r.sunmask, r.mapcalc (tends to overshoot when slopes are high)
- In i.topo.corr the following correction methods are implemented: cosine, minnaert, percent, c-factor.
Cloud removal
- with i.landsat.acca
Image classification
See the dedicated Image classification page.
Image segmentation
- i.smap: Performs contextual image classification using sequential maximum a posteriori (SMAP) estimation.
- r.seg: Performs image segmentation and discontinuity detection (based on the Mumford-Shah variational model).
- i.segment: Image Segmentation
Filtering
Fourier Transform
- i.fft, i.ifft
- see also Image destriping
Canonical Component Analysis
Principal Component Analysis
- i.pca
- see also Principal Components Analysis
Texture
A series of commonly used texture measures (derived from the Grey Level Co-occurrence Matrix, GLCM), also called Haralick's texture features are available:
- r.texture: In case of panchromatic maps or limited amount of channels, it is often recommended to generate synthetic channels through texture analysis
See here and here for the formulas to calculate texture.
Time series analysis
- r.series
- see also Time series
- see also Time series development
Enhancements
Radiometric Enhancements
Geometric Enhancements - Image Fusion - Pansharpening
- i.rgb.his and i.his.rgb: can be used for image fusion
- i.fusion.brovey: image fusion of pan-chromatic and color channels
- r.seg which performs image segmentation and discontinuity detection (based on the Mumford-Shah variational model). The module generates a piece-wise smooth approximation of the input raster map and a raster map of the discontinuities of the output approximation. The discontinuities of the output approximation are preserved from being smoothed.
- i.pansharpen: Image fusion algorithms to sharpen multispectral with high-res panchromatic channels (GRASS 7)
Optimal channel selection for color composites
Stereo anaglyphs
- see Stereo anaglyphs
Ideas collection for improving GRASS' Image processing capabilities
Below modules need some tuning before being added to GRASS 6. Volunteers welcome.
Spectral unmixing ideas
- i.spec.unmix: existing GRASS 5 module awaiting complete GRASS 6 update
- Make use of the Spectral Python (SPy) which is a pure Python module for processing hyperspectral image data
Spectral angle mapping ideas
Geocoding ideas
- i.homography: geocoding with lines (instead of points) with homography (as improved i.points; it was formerly called i.linespoints)
- support splines from GDAL (see GRASS_AddOns#Imagery_add-ons)
- New Georectifier: see also http://gama.fsv.cvut.cz/~landa/grass/swf/georect.html
Image matching ideas
- i.points.auto: automated search of GCPs based on FFT correlation (as improved i.points)
- Reference: M. Neteler, D. Grasso, I. Michelazzi, L. Miori, S. Merler, and C. Furlanello, 2005: An integrated toolbox for image registration, fusion and classification. International Journal of Geoinformatics, 1(1), pp. 51-61 PDF
Image classification ideas
- pr: C code for classification problems
- GRASS implementation: i.pr.* source code is available here)
Stereo ideas
This is stand-alone stereo modeling software (DEM extraction etc). Waits for integration into GRASS.
Bundle block adjustment
Needed to orthorectify a series aerial images taken sequentially with overlap. "Histoical" method which is nowadays interesting for UAV flights with octocopters and such.
Automatec GPC search could be done by "auto-sift".
Available: Octave code which prepares input to an i.ortho.photo batch job (contact Markus Neteler).
Lidar LAS format
LAS Tools by M. Isenburg, Howard Butler et al.: http://www.liblas.org
las2txt | r.in.xyz in=- fs=" "
Update: r.in.lidar and v.in.lidar implemented by Markus Metz (GRASS 7)
(see LIDAR)
Improving the existing code
It might be sensible to merge the various image libraries:
- GRASS 6 standard libs:
- lib/imagery/: standard lib, in use (i.* except for i.points3, i.rectify3, see below)
- imagery/i.ortho.photo/libes/: standard lib, in use (i.ortho.photo, photo.*)
- GRASS 5 (! only) image3 lib:
- libes/image3/: never finished improvement which integrated the standard lib and the ortho lib. Seems to provide also ortho rectification for satellite data (i.points3, i.rectify3)
- GRASS 5/6 image proc commands:
- merge of i.points, i.vpoints, i.points3 (see above)
- merge of i.rectify and i.rectify3 (see above)
addition of new resampling algorithms such as bilinear, cubic convolution (take from r.proj or r.resamp.aggreg)(done 10/2010)- add other warping methods (maybe lanczos or thin splines from GDAL?): Addons#i.warp
- implement/finish linewise ortho-rectification of satellite data
Bibliography
- Search for "GRASS GIS Image processing" - Google Scholar
- Search for "GRASS GIS Remote Sensing" - Google Scholar