Image processing apparatus

Image processing apparatus in which a reconstructed digital representation of a raster scanned image is interpolated from a plurality of sub-bands in the two-dimensional spatial frequency domain and having aliasing distortion and ringing distortion, the aliasing distortion in said second direction being greater than in said first direction, said raster scanned image having a resolution in a first direction and a resolution in a second direction, said resolution of said raster scanned image in said first direction being greater than said resolution of said raster scanned image in said second direction such that a portion of the ringing distortion in said first direction is confined to a smaller region of said reconstructed image than a portion of the ringing distortion of equal magnitude in said second direction of said reconstructed image, said image processing apparatus comprising:

• interpolation filter means having a first filter group comprising a first plurality of multi-tap digital interpolation filters, each having a filter length, for filtering in said first direction and a second filter group comprising a second plurality of multi-tap digital interpolation filters, each having a filter length, for filtering in said second direction, said filter lengths of said first plurality of multi-tap digital interpolation filters having a mean which is greater than the mean of said filter lengths of said second plurality of multi-tap digital interpolation filters such that the aliasing distortion is reduced in said first direction and the ringing distortion is reduced in said second direction without significantly increasing the aliasing distortion in said second direction.

A method of filtering a digital representation of a raster scanned image into a plurality of sub-bands in the two-dimensional spatial frequency domain, said raster scanned image having a resolution in a first direction and a resolution in a second direction, said raster scanned image corresponding to a reconstructed image having a resolution in the first direction and a resolution in the second direction and having aliasing distortion and ringing distortion, the aliasing distortion in said second direction being greater than in said first direction, said resolution of said raster scanned image in said first direction being greater than said resolution of said raster scanned image in said second direction such that a portion of the ringing distortion in said first direction is confined to a smaller region of said reconstructed image than a portion of the ringing distortion of equal magnitude in said second direction of said reconstructed image, said method comprising the steps of:

• (I) filtering in said first direction with a first plurality of multi-tap digital decimation filters, each having a filter length;
• and
• (II) filtering in said second direction with a second plurality of multi-tap digital decimation filters, each having a filter length, said filter lengths of said first plurality of multi-tap digital decimation filters having a mean which is greater than the mean of the filter lengths of said second plurality of multi-tap digital decimation filters such that the aliasing distortion is reduced in said first direction and the ringing distortion is reduced in said second direction without significantly increasing the aliasing distortion in said second direction.

A method of interpolating a reconstructed digital representation of a raster scanned image from a plurality of sub-bands in the two-dimensional spatial frequency domain and having aliasing distortion and ringing distortion, the aliasing distortion in said second direction being greater than in said first direction, said raster scanned image having a resolution in a first direction and a resolution in a second direction, said resolution of said raster scanned image in said first direction being greater than said resolution of the raster scanned image in said second direction such that a portion of the ringing distortion in said first direction is confined to a smaller region of said reconstructed image than a portion of the ringing distortion of equal magnitude in said second direction of said reconstructed image, said method comprising the steps of:

• (I) filtering in said first direction with a first plurality of multi-tap digital interpolation filters, each having a filter length;
• and
• (II) filtering in said second direction with a second plurality of multi-tap digital interpolation filters, each having a filter length, said filter lengths of said first plurality of multi-tap digital interpolation filter having a mean which is greater than the mean of said filter lengths of said second plurality of multi-tap digital interpolation filters such that the aliasing distortion is reduced in said first direction and the ringing distortion is reduced in said second direction without significantly increasing the aliasing distortion in said second direction.