Using the Trend process

The Trend process changes the radiometry of an image based on a correction surface, which can be used as a simple method to remove trends in images. Brightness is the trend that is removed using this process. Brightness trends are often seen in aerial and satellite imaging where one side of the image is brighter than the other. When you want to mosaic several images with a brightness trend, the mosaic has an unattractive appearance with all the different bright and dark areas. By removing the uneven brightness in each image, you can create a consistent-looking mosaic.

The Trend process can be applied to no more than four bands.

When using the Brightness method, it is the result of the trend equation, which is added to the pixel values.

Two surface types can be defined for the Brightness method:

• Plane Surface—This defines a simple plane correction surface.

  dV = A(column - column0) + B(row - row0) + C

• Simple Curve along X—This defines a curve that is nonlinear along the x-axis.

  Band value = band value + (amplitude*((CenterColumn-i)*2/ImageWidth)Curvature)

The values of Simple Curve X Center, amplitude, and curvature form a curve used to create trends that increase or decrease toward the edges of an image.

These parameters are generally determined by external analysis of the trends within the image. Tools to determine these parameters are not included.

Trend process parameters

Tab	Field	Description
General	Alias	Specifies an alias for this process, which uniquely identifies the process in a process chain.
	Process Method	Specifies which property of the image is modified by the process. Currently, this has only one option, Brightness.
	Surface Type	The name of the surface to be generated. This surface can be either a plane or simple curve along the x-axis.
	Output only the Surface	Outputs only the surface. This option is checked mostly for analysis.
Plane—Coefficients	Column Coefficient for Band 1–4	Column coefficient for bands 1 (A) to 4 (A).
	Row Coefficient for Band 1–4	Row coefficient for bands 1 (B) to 4 (B).
Plane—Origins	Column Origin for Band 1–4	Column origin for bands 1 (Column0) to 2 (Column0).
	Row Origin for Band 1–4	Row origin for bands 1 (Row0) to 4 (Row0).
Plane—Constants	Constant for Band 1–4	Constant for bands 1 (C) to 4 (C).
Simple X Curve—Center of the Curve	Center for Band 1–4	Pixel column for bands 1 to 4, respectively, at which local extrema (trough or crest) of the curve will be placed.
Simple X Curve—Amplitude of the Curve	Amplitude for Band 1–4	Value for bands 1 to 4, respectively, of the surface at the left and right ends of the image.
Simple X Curve—Curvature of the Curve	Curvature of Band 1–2	Curvature of 1.0 defines a linear V-shaped surface; a value of 2.0 defines a U-shaped surface.

Trend process parameters