GB2206011A - Overlap compensation in digital image generation - Google Patents

Overlap compensation in digital image generation Download PDF

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Publication number
GB2206011A
GB2206011A GB8714324A GB8714324A GB2206011A GB 2206011 A GB2206011 A GB 2206011A GB 8714324 A GB8714324 A GB 8714324A GB 8714324 A GB8714324 A GB 8714324A GB 2206011 A GB2206011 A GB 2206011A
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Prior art keywords
image
original image
correlation
swath
apparatus
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Granted
Application number
GB8714324A
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GB8714324D0 (en )
GB2206011B (en )
Inventor
Lindsay W Macdonald
Michael Ernest Hicks
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Crosfield Electronics Ltd
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Crosfield Electronics Ltd
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Classifications

    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/04Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa
    • H04N1/06Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using cylindrical picture-bearing surfaces, i.e. scanning a main-scanning line substantially perpendicular to the axis and lying in a curved cylindrical surface
    • H04N1/0671Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using cylindrical picture-bearing surfaces, i.e. scanning a main-scanning line substantially perpendicular to the axis and lying in a curved cylindrical surface with sub-scanning by translational movement of the main-scanning components
    • H04N1/0678Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using cylindrical picture-bearing surfaces, i.e. scanning a main-scanning line substantially perpendicular to the axis and lying in a curved cylindrical surface with sub-scanning by translational movement of the main-scanning components using a lead-screw or worm
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/04Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa
    • H04N1/06Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using cylindrical picture-bearing surfaces, i.e. scanning a main-scanning line substantially perpendicular to the axis and lying in a curved cylindrical surface
    • H04N1/0657Scanning a transparent surface, e.g. reading a transparency original
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/04Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa
    • H04N1/19Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using multi-element arrays
    • H04N1/191Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using multi-element arrays the array comprising a one-dimensional array, or a combination of one-dimensional arrays, or a substantially one-dimensional array, e.g. an array of staggered elements
    • H04N1/1911Simultaneously or substantially simultaneously scanning picture elements on more than one main scanning line, e.g. scanning in swaths
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/04Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa
    • H04N1/06Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using cylindrical picture-bearing surfaces, i.e. scanning a main-scanning line substantially perpendicular to the axis and lying in a curved cylindrical surface
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N2201/00Indexing scheme relating to scanning, transmission or reproduction of documents or the like, and to details thereof
    • H04N2201/04Scanning arrangements
    • H04N2201/0402Arrangements not specific to a particular one of the scanning methods covered by groups H04N1/04 - H04N1/207
    • H04N2201/0414Scanning an image in a series of overlapping zones

Abstract

In an apparatus for generating a digital representation of an original image 2 by scanning detectors relative to the image in a plurality of scans, there may be imperfect alignment of the scans resulting in overlaps in the reproduced image. To overcome this the image representative signals are stored, 16, and adjacent scans are compared in a processor 15 to detect correlation. Overlapping signals from one scan are discarded. It is envisaged that relative displacement and rotation of scans may be compensated for, using generally known correlation techniques. Calibration marks may be added to the image (Fig. 6) to provide boundary conditions or starting values. <IMAGE>

Description

DIGITAL IMAGE GENERATION The invention relates to methods and apparatus for generatiny a digital representation of an original image.

It is common practice in the printing industry to generate digital representations of original images to enable output scanning systems to produce visual representations of those images in the form of colour separations and the like and to enable detailed modifications to be made to the images before they are printed. In order to generate the initial digital representation, an original image is typically scanned in a series of scan lines positioned side by side across the original image. A typical conventional means is the drum scanner, in which the image is secured on the surface of a rotating drum and a detection means traverses at a steady speed parallel to the axis of rotation of the drum. Light transmitted through or reflected from the image is passed through a small circular aperture and is converted by a light-sensitive detecting element into an electrical signal.The scanning spot traces out a helical path over the surface of the drum.

Recently arrays of sensors have become available, in particular the charge-coupled device (CCD) array.

Linear arrays of up to about 3500 elements, and two-dimensional arrays of up to about 400 by 300 elements are currently economically viable. For high quality scanning in the graphic arts industry, however, spatial resolutions of up to 600 elements per inch are frequently required. Scanning at this resolution with a linear CCD array of 3500 elements would limit the image width to less than 6 inches. Wider images can be scanned either by butting multiple arrays together or by making multiple passes over the image with a single array.

Butting multiple CCD arrays together, apart from the increased expense, leads to problems in butting their respective image outputs. Various solutions have been proposed, such as optical beam-splitting as disclosed in our published European Patent Application 140529 or optical minification, as disclosed in US Patent 4,323,925.

With a single CCD array, the butting problem becomes one of registering successive sub-images, each sub-image resulting from a scan of the array across the original image. The requirement to reposition the array on successive scanning passes to an accuracy of better than half the pixel size (ie. 1/1200th inch) imposes severe demands upon the design of the mechanical and optical components of the scanning carriage. The human eye is very sensitive to certain types of registration error in reproduced images, and even small errors in the butting of scan lines may lead to unacceptable visual artefacts in the rastered image.

In accordance with the present invention, apparatus for generating a digital representation of an original image comprises a support member on which the original image is positioned; irradiating means for irradiating the original image; detection means for sensing radiation which has been reflected from or transmitted through the original image; means for causing relative scanning movement between the support member and the detection means whereby the detection means receives radiation from a series of overlapping regions of the original image; and processing means responsive to signals generated by the detection means to generate digital representations of the overlapping regions of the image and to correlate the overlapping regions to generate a digital representation of the original image.

The invention deals with the problems mentioned above by purposely scanning the image so as to generate a series of overlapping subsidiary images. The duplication of information in adjacent pairs of such subsidiary images is then removed and the adjacent images positioned correctly with respect to one another using a correlation method to reconstruct a seamless representation of the image.

Typically, the subsidiary images defined by the regions will comprise two dimensional arrays of pixels (one such array for each colour component in the case of a coloured image).

Conveniently, the step of scanning the image comprises causing relative movement between an array of radiation detectors and the original image whereby the array scans or traverses the original image in a series of overlapping swaths defining respective subsidiary images.

Typically, the radiation detector array will comprise a one dimensional array although in certain cases a two dimensional array of detectors could be moved in a stepwise manner in the scanning direction.

Generally, the radiation will comprise optical radiation but the invention could also be used at non-optical wavelengths such as infra-red.

Cross-correlation is a known technique which is frequently associated with template matching, where a known pattern is sought in the signal stream from a sensor device. The template pattern is placed in successive positions over the signal stream, and a "goodness of fit" metric computed at each position.

Where this metric yields the highest value, the fit is best. Typical applications are in radar signal analysis ("signatures" of aircraft); fingerprint classification (extraction of minutia locations); and optical character recognition (identification of written letterforms).

Descriptions of the technique are given in many standard texts, for example "Digital Image Processing" by Gonzalez & Wintz, published by Addison Wesley 1977 page 67-70; or "Computer Image Processing & Recognition" by Ernest L.Hall, published by Academic Press 1979, pages 480-484.

In the present invention the cross-correlation processing may take various forms, depending upon the relative orientation and degree of overlap of the regions or subsidiary images. The simplest case occurs when each sub-image is substantially the same size and scale and when the accuracy and mechanical stability of the scanning system is such that the registration of sub-images in a direction perpendicular to the (linear CCD) array is significantly better than one pixel. In this case, correlation between adjacent sub-images is achieved simply by one-dimensional movement of the overlapping sub-images relative to one another until the best fit of corresponding pixel values is achieved. The sub-images are then combined on a line by line basis by discarding the duplicated information in the region of the overlap.

The invention may be extended to more complex forms of two-dimensional cross-correlation where the sub-images need to be registered in both axes, or to correct for parallelogram distortion or rotational errors. It may also be extended to cases where the sub-images have differing size or scale.

Preferably, the method comprises generating a full set of overlapping subsidiary images and subsequently carrying out the correlation step. Alternatively, the method could comprise generating a first subsidiary image and carrying out the correlating step after the generation of each subsequent subsidiary image.

In certain cases, it may be necessary to add correlation marks around the original image to aid the correlation step particularly if there is little variation in the image content in the overlapping regions. An example of suitable correlation marks would be a series of crossed, mutually perpendicular black lines on a white background.

The invention can be extended for use with the simultaneous scanning of an original image using more than one detector array, particularly where each array is sensitive to one particular colour component. Similarly it can be extended to colour sequential scanning with light of different colour components. In these cases it is preferable for the calibration marks to be effective for all colour components.

In order to achieve accurate correlation between overlapping subsidiary images, the degree of overlap should be signficiant, typically in the order of 1-2% of the width of the subsidiary image. This would mean an overlap of 30-60 pixels when a CCD array of 3000 elements is used.

It should be noted that the subsidiary images do not necessarily correspond to a single scan of the original image by a sensor array but could be generated by other combinations of pixels after the scanning step has been completed. For example, each line of pixels corresponding to a single sampling operation could comprise a subsidiary image which is correlated with an adjacent, laterally offset line of pixels.

An example of scanning apparatus for generating a digital representation of an original image in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a block diagram of the apparatus; Figures 2A-2C are enlarged, schematic views of parts of two subsidiary images before (2A, 2B) and after (2C) correlation; Figure 3 illustrates overlapping pixels of subsidiary images requiring more complex correlation; Figures 4 and 5 illustrate schematically two further ways in which subsidiary images may overlap; and, Figure 6 illustrates the use of calibration marks.

The invention is applicable to a wide variety of scanning apparatus including flat bed scanning apparatus.

For the purposes of this description, however, a conventional cylinder scanning apparatus is shown in Figure 1 comprising a transparent cylinder 1 on which is mounted an original image 2. The cylinder 1 is rotated in the direction of an arrow 3 by a motor 4. A light source 5 generates a light beam 6 which is guided through the cylinder 1 and is reflected by a mirror 7 through the original image 2 (in the form of a transparency) onto an analyse head 8. The analyse head 8 is mounted on a lead screw 9 rotatably driven by a stepping motor 10. In use, the cylinder 1 is rotated at a moderate speed by the motor 4 while the analyse head 8 is held stationary.

After one full rotation of the cylinder, a swath around the circumference has been scanned. The lead screw 9 is then rotated by motor 10 to advance the analyse head 8 to a new scanning position further along cylinder 1. After synchronising with the rotation of cylinder 1 the next circumferential swath is scanned.

A dichroic beam splitter 11 is mounted within the analyse head 8 to split the incoming light signal into subsidiary signals corresponding to the three colour components cyan, yellow, and magenta. The subsidiary beams impinge on respective linear, CCD arrays 12-14 within the analyse head 8. Each array 12-14 contains a large number, typically in the order of 3000, of light sensors and each sensor generates an electrical signal corresponding to the total light impinging upon the sensor during a sampling period. These signals are passed periodically to a processor 15 and are representative of the colour component content of individual pixels in the original image 2. The processor 15 initially stores the signals in a temporary store 16 and, after processing the signals in the manner to be described below, stores a final digital representation of the original image 2 in an image store 17.

The analyse head is arranged to scan the original image 2 in a series of overlapping swaths, each swath having a dimension in the direction of movement of the analyse head corresponding to the length of a CCD array 12-14. The amount of advance of the analyse head 8 is chosen such that each successive swath overlaps the preceding swath by a small number of pixels, typically 30-60. Initially, the digital representations of each swath are separately stored in the temporary store 16.

In practice, each region of the original coloured image 2 will be represented by three swaths, one for each colour component, but for the present purposes, only a single colour component will be considered.

Figures 2A and 2B illustrate small portions of two overlapping swaths 18 19. The digital content of some of the pixels of these swaths 18, 19 is indicated in Figure 2. In this case, the accuracy of positioning of the analyse head 8 and the rotation of the cylinder 1 is better than the dimension of a single pixel so that exact overlapping of pixels can be achieved without further computation. In this example, the scanning direction (direction of rotation of the cylinder 1) is indicated by an arrow 20 while the direction of movement of the analyse head 8 is in the direction of an arrow 21. For convenience of illustration, the region of overlap between the swaths is shown as exactly 3 pixels over the full height of the swath.

Initially, the swath 19 will have been detected and stored in the temporary store 16 followed by the swath 18. The processor 15 then reviews adjacent sections of these two swaths 18, 19 and will determine that there is a correlation between the endmost three columns of pixels of each swath by comparing the pixel contents of these columns. It will be seen in Figure 2 that a line 22 in the swath 19 has pixel contents 40, 20 ... while a line 23 in the swath 18 also has pixel values 40, 20....

Similar comparisons confirm that this is indeed the correct correlation and the processor 15 will then generate a consolidated image (Figure 2C) in which the first three columns of pixels in the swath 18 are discarded and the swath 19 is butted with the subsequent columns of the swath 18. It is intended that the correlation process be applied to every scan line of the swath independently allowing adaptation for positional instability of the analyse head 8.

A more complex correlation is needed in the case shown in Figure 3. In this case, the adjustment resolution is greater than one pixel and so there is no exact correspondence between pixels of adjacent swaths 24, 25. In this case, the processor 15 carries out a fairly simple cross-correlation between the swaths 24, 25 and will determine in this case that the overlapping pixels of the swath 25 are positioned about the junction between pixels in the swath 24. Thus, the endmost pixels 26, 27 of the swath 25 have an average value of 30 corresponding to the content of the pixel 28 in the swath 24. The processor 15 then resamples the content of the swath 25 to generate pixels which are precisely registered with the pixels of the swath 24 and then consolidates the two swaths in a similar manner to that shown in Figure 2.

More complex cross-correlations can be carried out where there is displacement of one swath relative to another not only in the direction of movement of the analyse head 8 but also in the scanning direction.

If the two axes of movement are'not perpendicular, the situation is further complicated and this results in swaths of the form shown in Figure 4 or Figure 5.

Figure 4 shows the case where the CCD array is not exactly perpendicular to the direction of scan across the image, resulting in a form of parallelogram distortion of the swath. In this case the corresponding lines of successive swaths do not contain corresponding lines of pixel data. Cross-correlation along the axis of the swath will then also be necessary to determine the true correspondence. A similar case occurs where the swaths are not distorted but are not exactly synchronised with the cylinder rotation, resulting in a relative displacement.

Figure 5 shows the worst case where the swaths are slightly rotated relative to one another. The correspondence of pixels in the region of overlap then needs to be determined by two-dimensional cross-correlation followed by two-dimensional resampling and interpolation of all the pixel data in the swath, in order to correct for the rotational displacement.

Problems may arise with the basic cross-correlation technique for a number of reasons. First, there may be insufficient high-frequency detail in the region of overlap of the sub-images to give a significant measure of correlation. Such "flat" areas are not uncommon in many images. Second, noise, especially in the CCD array and digitising circuitry, can reduce the discrimination of the correlation process. Third, where repetitive or similar detail occurs near the true overlap position, multiple correlation peaks can occur. These can be very troublesome producing ambiguity or false matches.

The system can therefore incorporate smoothing and adaptive prediction of the amount of overlap for each scan line. Such techniques are known in the field of signal processing, see for example "Digital Image Processing" by William K.Pratt, published by Wiley 1978, pages 637-648. They rely on the fact that the amount of overlap, and hence the peak cross-correlation position, changes only slowly from one line to the next, and hence can be predicted with reasonable confidence. When a meaningful cross-correlation cannot be made, for any of the reasons listed above, the position can be estimated from previous values and trends.

To provide boundary conditions, or starting values, for the prediction process, calibration marks may be placed around the periphery of the image by optical or electronic means.

Figure 6 illustrates an example of this in which a set of calibration marks 30 in the form of perpendicular black lines of different sizes have been added around the edge of the original image 2. This original image 2 is then scanned in a series of four overlaping swaths 31-34 and correlation between these swaths is achieved by using the calibration marks to "train" the correlation process.

Claims (5)

1. Apparatus for generating a digital repre3entation of an original image, the apparatus comprising a support member on which the original image is positioned; irradiating means for irradiating the original image; detection means for sensing radiation which has been reflected from or transmitted through the original image; means for causing relative scanning movement between the support member and the detection means whereby the detection means receives radiation from a series of overlapping regions of the original image; and processing means responsive to signals generating by the detection means to generate digital representations of the overlapping regions of the image and to correlate the overlapping regions to generate a digital representation of the original image.
2. Apparatus according to claim 1, wherein the radiation detector array comprises a one dimensional array.
3. Apparatus according to claim 1 or claim 2, wherein the radiation comprises optical radiation.
4. Apparatus according to any of the preceding claims, wherein the processing means is adapted to cross-correlate the overlapping regions to generate a digital representation of the original image.
5. Apparatus for generating a digital representation of an original image substantially as hereinbefore described with reference to the accompanying drawings.
GB8714324A 1987-06-18 1987-06-18 Digital image generation Expired - Fee Related GB2206011B (en)

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GB2206011A true true GB2206011A (en) 1988-12-21
GB2206011B GB2206011B (en) 1991-06-05

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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4916530A (en) * 1988-09-02 1990-04-10 Itek Graphix Corp. High resolution halftone dot generator system including LED array
EP0591974A2 (en) * 1992-10-08 1994-04-13 Sharp Kabushiki Kaisha Image processing apparatus
WO1998008338A2 (en) * 1996-08-21 1998-02-26 Philips Electronics N.V. Composing an image from sub-images
EP1018634A2 (en) * 1995-08-24 2000-07-12 Vexcel Imaging GmbH Method for scanning an object image and a reseau
DE19946332A1 (en) * 1999-09-28 2001-04-05 Agfa Gevaert Ag Method and apparatus for detecting digital photographic films
EP1610166A1 (en) 2004-06-24 2005-12-28 Fujifilm Electronic Imaging Limited Method and apparatus for forming a multiple focus stack image

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0019777A1 (en) * 1979-05-29 1980-12-10 International Business Machines Corporation Electronically abutting parts of an electronic image produced by linear arrays of photosensitive elements
US4675745A (en) * 1983-09-19 1987-06-23 Canon Kabushiki Kaisha Image reading apparatus

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS6364941B2 (en) * 1979-08-15 1988-12-14

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0019777A1 (en) * 1979-05-29 1980-12-10 International Business Machines Corporation Electronically abutting parts of an electronic image produced by linear arrays of photosensitive elements
US4675745A (en) * 1983-09-19 1987-06-23 Canon Kabushiki Kaisha Image reading apparatus

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
JP A 60064568 *
NOTE: JP A 60/064568 AND US 4675745 ARE EQUIVALENT; *

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4916530A (en) * 1988-09-02 1990-04-10 Itek Graphix Corp. High resolution halftone dot generator system including LED array
EP0591974A2 (en) * 1992-10-08 1994-04-13 Sharp Kabushiki Kaisha Image processing apparatus
EP0591974A3 (en) * 1992-10-08 1995-03-22 Sharp Kk Image processing apparatus.
US5481375A (en) * 1992-10-08 1996-01-02 Sharp Kabushiki Kaisha Joint-portion processing device for image data in an image-forming apparatus
US5625720A (en) * 1992-10-08 1997-04-29 Sharp Kabushiki Kaisha Joint-portion processing device for image data in an image-forming apparatus
US5654807A (en) * 1992-10-08 1997-08-05 Sharp Kabushiki Kaisha Joint-portion processing device for image data in an image-forming apparatus
EP1018634A2 (en) * 1995-08-24 2000-07-12 Vexcel Imaging GmbH Method for scanning an object image and a reseau
EP1018634A3 (en) * 1995-08-24 2000-08-02 Vexcel Imaging GmbH Method for scanning an object image and a reseau
WO1998008338A2 (en) * 1996-08-21 1998-02-26 Philips Electronics N.V. Composing an image from sub-images
WO1998008338A3 (en) * 1996-08-21 1998-06-04 Philips Electronics Nv Composing an image from sub-images
DE19946332A1 (en) * 1999-09-28 2001-04-05 Agfa Gevaert Ag Method and apparatus for detecting digital photographic films
DE19946332C2 (en) * 1999-09-28 2001-11-08 Agfa Gevaert Ag Method and apparatus for detecting digital photographic films
EP1610166A1 (en) 2004-06-24 2005-12-28 Fujifilm Electronic Imaging Limited Method and apparatus for forming a multiple focus stack image

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GB8714324D0 (en) 1987-07-22 grant
GB2206011B (en) 1991-06-05 grant

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Effective date: 20020618