A simple subscript expression may be given along with each channel specification to change the pixels being accessed. These expressions are given in square brackets after the channel specification, and use the special symbols "@x" and "@y" to represent the particular pixel and line locations when the expression is evaluated for each point.
For example, the following Modeling expression will shift image channel 1 upwards by one pixel:
%1 = %1[@x, @y+1]
If the Modeling expression refers to a pixel outside of the image, the edge pixels are copied outward and used instead. This allows filters to be created and used, which can be applied over an entire image, including the edges. The subscript expressions used must be linear and uni-variate. When no subscript expression is given, it is equivalent to the specification [@x,@y].
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