Specifying Centre and Extent

An alternative form for the subscript expression involves specifying the centre and extent of the region required along each dimension, as follows

name( p$\sim$q, r$\sim$s, ... )

where `p$\sim$q', `r$\sim$s', (etc.) specify the centre and extent. Thus,

name(100$\sim$11,200$\sim$5)

would refer to an 11$\times$5-pixel region of an image centred on pixel (100, 200).

If the value before the delimiting `$\sim$' is omitted, it will default to the index of the central pixel in that dimension (rounded downwards if there are an even number of pixels). If the value following the `$\sim$' is omitted, it will default to the number of pixels in that dimension. Thus,

image( $\sim$100, $\sim$100)

could be used to refer to a 100$\times$100-pixel region located centrally within an image, while

image( 10$\sim$, 20$\sim$ )

would specify a section which is the same size as the original image, but displaced so that it is centred on pixel (10, 20).