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Menu:   Processing > Filters… > Edges->Roberts  
Script:   roberts  
Use this function to detect fine edges and their directions on an image by applying the Roberts cross operator. The filter represents a finite approximation to the derivative of intensity by calculating the absolute differences in two diagonal directions in the 2x2 neighborhood. The filter is based on the following kernels:

roberts1 roberts2

The method used for combining the two differentials depends on the Mode selected. If an overflow occurs, the result is clipped to the minimum or maximum pixel value for the given image type.

If you choose this command, the following options will become available:

Displays the input image frame number. If you want to process another image, type or select the corresponding value.  

Displays the number of the frame in which the output image will be created. Depending on the Preferences. ImageWarp will set it either to the first available value or to the Input frame number. Type or select another value if you want the output image to be created in a different frame.  

Select this check box to watch the Preview of the output image.  
Select this check box to invert the resulting image. The edges will be presented as dark lines on a bright background.  
Lets you choose the mode in which the two differentials are combined:  
Select Sum to have the target pixel substituted with the sum of the absolute values of the pixels in two differential images:  
Select Max to to have the target pixel substituted with the maximum absolute value from two differential images.  
Select Sqrt to have the target pixel substituted with the square root of the sum of the squares of the pixels in two differential images.  
Select Phase to have the target pixel substituted with the angle of the gradient direction:  
The angle is measured from the positive X-axis counterclockwise. For the images of the integer types the angle is scaled to the full gray scale, and the zero value indicates the absence of the gradient. For the floating point and complex images the angle is reported in radians, while the absence of the gradient is reported as negative 1.