GCD

Description

Generates a difference image from two input images.

Parameters

Name	Type	Length	Value range
Input2: Input reference channel/layer *	Raster port	1 - 1	1 -
Input: Input image channel/layer *	Raster port	1 - 1	1 -
Output: Output difference channel/layer *	Raster port	1 - 1	1 -

* Required parameter

Parameter descriptions

Input2: Input reference channel/layer

Specifies the reference image channel or layer. This reference image channel must be the same size and datatype (8-bit unsigned) as DBIC_IN (Input).

Input: Input image channel/layer

Specifies the input image channel or layer. This image channel must be the same size and datatype (8-bit unsigned) as DBIC_REF (Input2).

Output: Output difference channel/layer

Specifies the image channel or layer to receive the GCD image data. This image channel must have the same dimensions as DBIC_IN (Input) and DBIC_REF (Input2), and must be 32-bit real (32R).

Details

GCD performs a Change Detection comparison of two input images, with the output going to a 32-bit real image of the same size. Input is restricted to 8-bit unsigned images, of the same X/Y (pixels and lines) dimensions.

GCD performs its change detection on a global scale, meaning it calculates difference on the whole image. The least squares minimization parameters, B0 and B1, are calculated once on the whole image (globally), then using the following formula:

FILI_DIF = Minimize || FILI_IN - (B1 * FILI_REF + B0) ||

The difference image is calculated.

Algorithm

Global Change Detection obtains a difference image from two input images, I1(Input) and I2 (Input2), using the the following formula:

I1 -  (b1 * I2 + b0)            (1)

The coefficients b0 and b1 are calculated using a Least Square Minimization, namely:

Min || I1 -  (b1 * I2 + b0) ||  (2)

The coefficients b0 and b1 are calculated from the entire image databases, I1 (Input) and I2 (Input2), before the equation (1) is applied, to obtain the difference image.