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ENVI

List of Attributes

List of Attributes

Spectral Attributes


Spectral attributes are computed on each band of the input image. The attribute value for a particular segment is computed from input data band where the segmentation label image has the same value (i.e., all pixels in the same segment contribute to the attribute calculation).

Attribute

Description

Spectral_Mean

Mean value of the pixels comprising the region in band x

Spectral_Max

Maximum value of the pixels comprising the region in band x

Spectral_Min

Minimum value of the pixels comprising the region in band x

Spectral_STD

Standard deviation value of the pixels comprising the region in band x

Texture Attributes


Texture attributes are computed on each band of the input image. Texture attribute computation is a two-step process where the first pass applies a square kernel of pre-defined size to the input image band. The attributes are calculated for all pixels in the kernel window and the result is referenced to the center kernel pixel. Next, the attribute results are averaged across each pixel in the segment to create the attribute value for that band’s segmentation label.

Attribute

Description

Texture_Range

Average data range of the pixels comprising the region inside the kernel (whose size you specify with the Texture Kernel Size parameter in segmentation)

Texture_Mean

Average value of the pixels comprising the region inside the kernel

Texture_Variance

Average variance of the pixels comprising the region inside the kernel

Texture_Entropy

Average entropy value of the pixels comprising the region inside the kernel

Spatial Attributes


Spatial attributes are computed from the polygon defining the boundary of the segment, so band information is not required.

Attribute

Description

Area

Total area of the polygon, minus the area of the holes.

If the input image is pixel-based, the area is the number of pixels in the segmented object. For a segmented object with 20 x 20 pixels the area is 400 pixels.

If the input image is georeferenced, the area is in the map units of the input image. For a segmented object with 20 x 20 pixels, where the input image pixel resolution is 2 meters, the total area is 1600 square meters (400 pixels x 2 meters x 2 meters).

Length

The combined length of all boundaries of the polygon, including the boundaries of the holes. This is different than the Major_Length attribute.

If the input image is pixel-based, the length is the number of pixels. For a segmented object with 20 x 20 pixels, the length is 80 pixels.

If the input image is georeferenced, the length is in the map units of the input image. For a segmented object with 20 x 20 pixels, where the input image pixel resolution is 2 meters, the length is 160 meters (80 pixels x 2 meters).

Compactness

A shape measure that indicates the compactness of the polygon. A circle is the most compact shape with a value of 1 / pi. The compactness value of a square is 1 / 2(sqrt(pi)).

Compactness = Sqrt (4 * Area / pi) / outer contour length

Convexity

Polygons are either convex or concave. This attribute measures the convexity of the polygon. The convexity value for a convex polygon with no holes is 1.0, while the value for a concave polygon is less than 1.0.

Convexity = length of convex hull / Length

Solidity

A shape measure that compares the area of the polygon to the area of a convex hull surrounding the polygon. The solidity value for a convex polygon with no holes is 1.0, and the value for a concave polygon is less than 1.0.

Solidity = Area / area of convex hull

Roundness

A shape measure that compares the area of the polygon to the square of the maximum diameter of the polygon. The "maximum diameter" is the length of the major axis of an oriented bounding box enclosing the polygon. The roundness value for a circle is 1, and the value for a square is 4 / pi.

Roundness = 4 * (Area) / (pi * Major_Length2)

Form_Factor

A shape measure that compares the area of the polygon to the square of the total perimeter. The form factor value of a circle is 1, and the value of a square is pi / 4.

Form_Factor = 4 * pi * (Area) / (total perimeter)2

Elongation

A shape measure that indicates the ratio of the major axis of the polygon to the minor axis of the polygon. The major and minor axes are derived from an oriented bounding box containing the polygon. The elongation value for a square is 1.0, and the value for a rectangle is greater than 1.0.

Elongation = Major_Length / Minor_Length

Rectangular_Fit

A shape measure that indicates how well the shape is described by a rectangle. This attribute compares the area of the polygon to the area of the oriented bounding box enclosing the polygon. The rectangular fit value for a rectangle is 1.0, and the value for a non-rectangular shape is less than 1.0.

Rectangular_Fit = Area / (Major_Length * Minor_Length)

Main_Direction

The angle subtended by the major axis of the polygon and the x-axis in degrees. The main direction value ranges from 0 to 180 degrees. 90 degrees is North/South, and 0 to 180 degrees is East/West.

Major_Length

The length of the major axis of an oriented bounding box enclosing the polygon. Values are map units of the pixel size. If the image is not georeferenced, then pixel units are reported.

Minor_Length

The length of the minor axis of an oriented bounding box enclosing the polygon. Values are map units of the pixel size. If the image is not georeferenced, then pixel units are reported.

Number_of_Holes

The number of holes in the polygon. Integer value.

Hole_Area/Solid_Area

The ratio of the total area of the polygon to the area of the outer contour of the polygon. The hole solid ratio value for a polygon with no holes is 1.0.

Hole_Area/Solid_Area = Area / outer contour area



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