Specifying Fractional Extents

A further form for the subscript expression involves specifying a fractional position along each dimension as a percentage, as follows

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

where `p%$\sim$q%' specifies the centre and extent as percentages, and `r%:s%' specifies a percentage range. Thus,

image(25%:75%,50%:$\sim$50%)

would refer to the central quarter of the image. Both sections are equivalent. The percentages are converted to the nearest pixel centre to decide the centre and extents of the sections.

The rules concerning omitted values before or after the delimiting $\sim$ apply. Thus,

image(40%$\sim$, $\sim$50%)

could be used to refer to a full-width, half-height section centred at the (40%, 50%) fractional position.

A percentage value is not limited to the range 0-100%. In such circumstances the areas beyond the bounds of the NDF are set to bad. For example,

image($\sim$110%, -5%:105%)

would give an enclosing border of bad pixels, extending 5% of the original dimensions. Both sections are equivalent.