Browse Prior Art Database

Transform Domain Scaling for Non-Commensurable Ratios

IP.com Disclosure Number: IPCOM000115634D
Original Publication Date: 1995-Jun-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 4 page(s) / 171K

Publishing Venue

IBM

Related People

Feig, E: AUTHOR [+3]

Abstract

Scaling objects that have been transformed from the spatial domain to a transform domain has been a long standing problem. Prior art demonstrates scaling factors that are fractional multiples (commensurable ratios) of the spatial domain block size. A method of applying a more general scaling factor to an object in the transform domain, before it is transformed back into the spatial domain is described herein. The increasing proliferation of Joint Photographic Experts Group (JPEG) and Motion Picture Experts Group (MPEG) is creating a strong requirement for such scaling techniques.

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Transform Domain Scaling for Non-Commensurable Ratios

      Scaling objects that have been transformed from the spatial
domain to a transform domain has been a long standing problem.  Prior
art demonstrates scaling factors that are fractional multiples
(commensurable ratios) of the spatial domain block size.  A method of
applying a more general scaling factor to an object in the transform
domain, before it is transformed back into the spatial domain is
described herein.  The increasing proliferation of Joint Photographic
Experts Group (JPEG) and Motion Picture Experts Group (MPEG) is
creating a strong requirement for such scaling techniques.

      Lossy JPEG compression employs a spatial-to-frequency domain
transform known as the Discrete Cosine Transform (DCT).  An image is
segmented into 8 * 8 blocks of samples.  A two dimensional DCT is
then applied to the 8 * 8 blocks to generate 64 DCT coefficients.
These coefficients are quantized, then entropy coded, with control
information in the form of JPEG marker segments.  Decompression
involves parsing the data stream for the control information, entropy
decoding the quantized DCT coefficients, dequantizing the DCT
coefficients, and finally applying an Inverse Discrete Cosine
Transform (IDCT) to produce a reconstructed image.  The details of
this process, defined by JPEG, are documented in the ISO/IEC
International Standard (IS) 10918-1, "Digital Compression and Coding
of Continuous-tone Still Images, Part 1: Requirements and
Guidelines", 1993, ITU-TSS Recommendation T.81:1994.

      Often, it is desirable to scale the image for purposes of
printing or displaying on a CRT.  There are many techniques, well
known in the prior art, for scaling an image in the spatial domain,
as well as for scaling an image while it is being transformed from
the transform or frequency domain back to the spatial domain.
However,
these techniques have draw backs.

      Fine details are lost in the process of scaling down.  This
translates into the loss of high frequency components in the
transform or frequency domain being discarded, which in turn results
in reduced computational complexity.

      To apply spatial domain scaling, the image must be fully
transformed back to the spatial domain.  This imposes the full cost
of performing the inverse transform, which is computationally
intensive.  Then, the spatial domain scaling must be applied.  The
total number of operations required to produce the scaled image is
greater than the number of operations to generate the unscaled image.
As a result, the user pays a performance penalty for having the image
scaled.  One advantage to this scaling technique is that any scaling
factor can be applied.

      Applying the scaling while the image is being transformed from
the transform domain back to the spatial domain, called
transform-to-spatial domain scaling, or in the case of a DCT,
DCT-to-spatial domain scaling, offers significant per...