Image analysis – Image transformation or preprocessing – Changing the image coordinates
Reexamination Certificate
1998-09-04
2001-09-04
Johns, Andrew W. (Department: 2621)
Image analysis
Image transformation or preprocessing
Changing the image coordinates
Reexamination Certificate
active
06285803
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The invention relates to a method and apparatus for generating digital image data. More particularly, this invention relates to digital image generation with, for example, a digital copying machine or a free-standing scanner apparatus.
In apparatus of this kind, a document having an image thereon is scanned with an electro-optical converter to generate digital image data which contain a grey value for each image element or pixel. This is usually done with a CCD array, on which a part of the document in the form of a line is projected by an optical imaging system. By moving either the document or the imaging system in a direction perpendicular to the scanning line of the document, the document is scanned completely, line-by-line, in accordance with a usually rectangular raster of pixels.
The spatial density or resolution of the pixels is, in practice, on the order of some hundreds of pixels per inch, usually specified as “dpi” (dots per inch).
2. Description of Related Art
Digital image data of the kind described above can be processed in the spatial domain, but it has also been proposed to process such data in the frequency domain. The latter domain is for instance particularly suited for filtering operations, in which disturbing elements, which often have frequencies outside the range of those of the image information, can be removed easily from the digitised image.
In this connection, also the combined use of image data generated from one image by two scanners each having a resolution different from the other, is proposed.
For instance, U.S. Pat. No. 5,121,445 describes a method aimed at removing moiré patterns. Patterns of this kind occur when a rastered image is scanned with a scanner, due to the fact that the raster frequency of the image interferes with the scanning frequency (resolution) of the scanner. The interference causes frequency domain sidebands on either side of the spectrum of the original non-rastered image. Also, the entire spectrum, including the sidebands, is periodically repeated as a result of the discrete scanning by the scanner. These sidebands contain information on both the rastering and the scanning. The known method comprises eliminating the side bands as far as possible by comparing the spectra of the image data from the two scanners, so that the original non-rastered image can be reconstructed. Thus, this method not only removes the moiré but also the raster. Effectively, therefore, information present in the scanned image is destroyed.
SUMMARY OF THE INVENTION
An object of the present invention is to retain as far as possible all the information in the scanned image. Another object is to increase the dpi of a scanned image.
For a good description of an image it is of course desirable that pixels should have a high spatial density. However, high-resolution systems are expensive, both because of the requirements relating to the CCD array and the requirements relating to the optical imaging system. Therefore, there is a demand for a scanner system which generates image data at high resolution and yet is constructed from simple and hence inexpensive components. The present invention meets this demand.
To this end, the method according to the invention includes the following steps:
1) generating first digital image data by scanning the image with a scanner having a first resolution f
1
;
2) generating second digital image data by scanning the image with a second scanner having a second resolution f
2
higher than f
1
; and
3) combining the first and second digital image data to form third digital image data, which describes the image with a third resolution f
3
which is higher than f
1
and higher than f
2
.
According to one embodiment, step 3) includes the following sub-steps:
a) transforming the first digital image data into a first spectrum, and transforming the second digital image data into a second spectrum, which spectra have an amplitude on a spectral axis;
b) superposing in a predetermined manner versions of the first and second spectrum shifted over the spectral axis, to give a first combination;
c) deriving a third spectrum from the first combination; and
d) re-transforming the third spectrum, the result being interpreted as the third digital image data.
The invention is based on combining the image data of two scanners, each of which has a relatively low resolution different from the other, to give image data of a higher resolution. This combination is possible in the frequency domain. The digital image data are therefore first transformed thereto with a Fourier transform, whereafter they are processed further. Alternatively, it is possible to subject the image data to a cosine transform and carry out the combination in the cosine domain.
These resolutions are preferably selected such that
f
3=
f
1+
f
2−
G
(
f
1,
f
2)
where G(a,b) is the highest common divisor of a and b. On the basis of theoretical considerations, this value of f
3
is the highest frequency at which the results of the method still form a reliable estimate of the original image, i.e., still correspond to the original image, if the latter were scanned at the resolution f
3
. There is, therefore, no point in making f
3
higher, because it does not yield any more information. On the other hand, it is also undesirable to make f
3
lower, because then the information present in the image data is not used to maximum effect.
As a marginal condition for the values of f
1
and f
2
, their ratio must be a rational ratio because it is only in that case that the theory on which the processing of the digital image data from the scanner is valid. However, this does not form any limitation, because this is always the case as a result of the discrete form of the scanners.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
REFERENCES:
patent: 5029228 (1991-07-01), Nonoyama et al.
patent: 5121445 (1992-06-01), Tsujiuchi et al.
patent: 5408270 (1995-04-01), Lim
patent: 5572608 (1996-11-01), Edgar
patent: 5633745 (1997-05-01), Chen et al.
patent: 5638164 (1997-06-01), Landau
patent: 9527627A (1995-10-01), None
Patent Abstracts of Japan, vol. 18, No. 570 (E-1623), Oct. 31, 1994 & JP 06 209463A (GC Technol KK) Jul. 26, 1994.
Couwenberg Wilhelmus J.
Jacobs Johannes W. M.
Azarian Seyed
Johns Andrew W.
Oce--Technologies B.V.
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