**-I**\ [*file*\|\ *intens*\|\ **+a**\ *azimuth*][**+d**][**+m**\ *ambient*][**+n**\ *args*]
    Apply illumination for the input grid. Several methods are available:

    - Give the *file* name of a co-registered grid with intensities in the ±1 range.
    - Give a constant *intens* to apply everywhere (affects the ambient light).
    
    Alternatively, derive intensities from the main input data grid via a call
    to :doc:`grdgradient` by appending **+a**\ *azimuth*, **+n**\ *args*
    and **+m**\ *ambient* for the arguments needed by that module, or just
    give **+d** to select the default [**+a**\ -45\ **+nt**\ 1\ **+m**\ 0].
    For more specific intensity scenarios run :doc:`grdgradient`
    separately first. If we should derive intensities from another file
    than the main grid, specify that file as the *file* and add suitable
    modifiers [Default is no illumination]. **Note**: If the input grid
    actually is an image then *file* or constant *intens* must be provided
    since derivatives are not available.
