SARTEX

Description

SARTEX creates a set of texture images from a single channel in an input image. The radar-specific measures are based on pixel-value statistics in a window surrounding each pixel. If necessary, you can use the extracted texture measures as input features with CATALYST Professional classification algorithms.

Parameters

sartex(file, dbic, texture, dboc, flsz, mask, imagefmt)

Name	Type	Caption	Length	Value range
FILE *	str	Input detected SAR image	1 -
DBIC *	List[int]	Detected-SAR channel	1 - 1
TEXTURE *	str	List of SAR texture measures	1 -
DBOC *	List[int]	Texture measure channels	1 - 4
FLSZ	List[int]	Filter size (pixels, lines)	0 - 2	3 - 101 Default: 7,7
MASK	List[int]	Area mask	0 - 4	Xoffset, Yoffset, Xsize, Ysize
IMAGEFMT	str	Image units	0 - 3	Amplitude | Power | Decibel Default: Amplitude

* Required parameter

Parameter descriptions

FILE

The name of the input SAR image file containing the channel to analyze.

The output channels are also written to the file.

DBIC

The input SAR channel to analyze for texture measure.

The channel must be detected data, and not complex numbers.

TEXTURE

The list of texture measures to calculate.

For more information about the measures, see Algorithm.

DBOC

The list of output channels to contain the calculated texture measures.

The input channel cannot be used as an output channel. The output channels must be 32-bit real.

The total number of output channels must correspond to the number of texture measures you specify.

Do not specify duplicate output channels.

FLSZ

The window size to use to extract the texture measures for each input pixel.

The window must be an odd size so that the pixel being processed is centered in the window. The value you specify must be one or two odd integers ranging from 3 through 101. The default value is 7,7.

This parameter is optional.

MASK

The window or bitmap that defines the area of the input raster to process.

Only pixels under the bitmap are processed. The rest of the image remains unchanged.

If you do not specify a value, the entire input SAR channel is processed. When you specify only a single value, it represents the number of the bitmap segment in the input file. If the specified bitmap is empty (all pixels are set to zero), a warning is displayed and processing does not occur.

If you specify four values, they define the x and y offsets and the x and y dimensions of a rectangular window identifying the area to process. Xoffset, Yoffset define the upper-left starting pixel coordinates of the window. Xsize is the number of pixels that define the window width. Ysize is the number of lines that define the window height.

This parameter is optional.

IMAGEFMT

The image units of the input SAR image.

Amplitude is the square root of power. Most radar images are stored in amplitude format.

The input values will be converted to the format required by the selected texture measures (power and amplitude).

This parameter is optional.

Details

SARTEX creates a set of texture images from a single channel in an input image. The radar-specific measures are based on pixel-value statistics in a window surrounding each pixel. If necessary, you can use the extracted texture measures as input features with CATALYST Professional classification algorithms.

Texture is an important characteristic used to identify objects or regions of interest in an image. Unlike spectral features, which describe the average tonal variation in the various bands of an image, textural features contain information about the spatial distribution of tonal variations within a band.

The SAR-specific texture measures that SARTEX applies account for radar-image formation and statistical properties of radar speckle; therefore, it is essential that filtering has not been applied previously on the input channel.

For each selected texture measure, SARTEX produces a channel in which pixel values represent the value of this measure in the input image. The textural measures are derived from a histogram of pixel values in a rectangular window you specify. The window slides over the input image, and computed texture measures are assigned to the central pixel of the consecutive positions of the window.

The output images contain the raw texture measures, which may vary in ranges of values. To avoid loss of important information, always save in 32-bit real channels. After assessing the distribution of texture-measure values by running the HIS algorithm, you can scale the images to the range you want by running the SCALE algorithm.

With the MASK parameter, you specify the area within the input channel to process. Only pixels under the mask area receive output textures. The remaining output pixels are not modified.

SARTEX cannot process pixels on or near the edges of the image, because all the required pixels are not within it. Therefore, when an entire image is processed with a rectangular window that is x-pixels by y-lines, the first and last (X+1)/2 pixels per line will be the same and the first and last (Y+1)/2 lines in the output image will be the same. Each will contain a replicated value of the closest processed pixel. Avoid these regions when selecting training data for supervised classification. If a partial image is processed, the input image buffers are extended towards the margins as far as possible, so that all pixels in the selected area receive computed textures; however, pixels closer than a half-window to the image edge will contain replicated values.

Texture measures extracted by SARTEX complement measures extracted by running the TEX algorithm, based on a gray-level co-occurrence matrix (GLCM), and with the HISTEX algorithm, based on the histogram of pixel values in a window. To conduct additional analysis, such as standard segmentation and classification of the image, you can combine the three types of measures. In this case, the best and most effective combination depends on land-cover characteristics, and may require some testing to establish. When classification methods account for properties of radar speckle, use only the texture measures extracted with SARTEX.

The optimum window size depends on image characteristics (terrain roughness, land-cover type, imaging parameters). With a large window, blocky artifacts may appear in texture images if a small, bright target is enclosed fully by the window. In most cases, use a window size that is small to moderate.

Example

Create all four 32-bit real texture-measure images from a 16-bit ERS-1 image on channel 11 of the irvine.pix file, for the whole image. The channel is in the amplitude format.

from pci.pcimod import pcimod
from pci.sartex import sartex

file	=	'irvine.pix'
pciop	=	'ADD'
pcival	=	[0,0,0,4]	# add four 32R channels

pcimod( file, pciop, pcival )

dbic	=	[11]	# radar image
texture	=	"1,2,3,4"	# select required texture
dboc	=	range(12,16)
flsz	=	[3,3]	# filter size in pixels and lines
mask	=	[]	# process entire image
imagefmt	=	'AMP'	# amplitude format

sartex ( file, dbic, texture, dboc, flsz, mask, imagefmt )

Algorithm

With SAR images, a single pixel conveys little information about the underlying ground cover. Interpretation occurs only by looking at the values of many pixels. The statistics that apply best to a given area depend on the actual image-formation process.

Texture measures computed by SARTEX are based on Chapter 8 of the book by Oliver and Quegan cited in References.

The measures are extracted in a window with pixels (K) and lines (L), sliding over successive pixels of the image.

Depending on the texture measures and the format of the input image you specify, the input pixel values may need to be converted to radar power (intensity), and the VA measure converted to amplitude. The conversion is performed according to the following expressions.

Amplitude (AMP) is converted to power (POW) as follows:

POW = AMP^2

Power in decibels (DB) is converted to power POW as follows:

POW = EXP10( DB / 10.0 )

Power (POW) is converted to amplitude (AMP) as follows:

AMP = SQRT( POW )

If the pixel power is zero or negative, the pixel is excluded from the accumulated sums for the current window. If all pixels in a window are excluded, the pixel is assigned a zero value for all selected texture measures.

References

Oliver, C. and S. Quegan (1998). Understanding Synthetic Aperture Radar Images. Artech House, Boston, London.