GB2102234A

GB2102234A - Image processing

Info

Publication number: GB2102234A
Application number: GB08122626A
Authority: GB
Inventors: John Bravin
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1981-07-22
Filing date: 1981-07-22
Publication date: 1983-01-26
Also published as: GB2102234B

Abstract

A visual display unit (V.D.U.), 13, has a curved image surface which causes a distorted representation of the object image, 17, to be displayed (as illustrated at 20). In the situation where a hard copy, 21, of the image is required in order, for example, to take detailed measurements this distortion can introduce unacceptable errors. The invention employs an image processing system, 11 of the ARGS (Advanced Radar Graphics System) type, to produce a version of the image that is predistorted (illustrated at 24) such that the image as viewed on the screen is an undistorted representation of the object image. <IMAGE>

Description

SPECIFICATION Image processing This invention relates to image processing, and concerns in particular methods of processing data to be used to form an image upon a distorting surface so that the resulting image appears in fact to be undistorted.

It is often the case that an information system is required to provide a visual display by causing a viewable image to appear upon a suitable surface.

A typical example of one such system provides a visual display using television-type apparatus (more generally, a cathod ray tube, CRT); in such a system the image-defining data is used to control a beam of electrons focussed onto a luminescent screen (the control being primarily as regards the intensity of the beam and where it strikes the screen). For convenience the following discussion is mainly in terms of information systems using CRT image display apparatus, though it will be appreciated that many of the points made are equally applicable to other sorts of imaging apparatus.

Although it is possible to construct CRT visual display units (VDUs) with flat image surfaces (and so to arrange the mechanical and electronic design of the system that the image as formed on the VDU is true and undistorted), so that the image as viewed appears undistorted, such VDUs (and their ancilliary equipment) are relatively expensive.Accordingly, most CRT VDUs have somewhat curved, but cheaper, image surfaces~ they bulge outwards slightly -- and so the image as actually viewed is necessarily distorted (a flat image is screened on a curved surface; the result on viewing is a little distortion, usually of the "barrel" variety) This distortion, which is over and above any distortion arising from the incorrect forming of the image upon the image surface in the first place, can be most inconvenient, and is especially so when a "hard copy" of the image is taken photographically - for, say, accurate measurement of the imaged object - because the hard copy is necessarily a copy not of the true image but of the image-as-viewed ... that is, of the distorted image.

At this point it is perhaps necessary to define certain terms in order to reduce the possibility of confusion. By the term "object image" used hereinafter is meant what an ideal imaging system would shown image of the object perfect in every detail (the term thus corresponds to the basic image data before it has actually been formed into a viewable image). By the term "displayed image" used hereinafter is meant the image as actually formed by the display unit upon the image surface. By the term "viewed image" used hereinafter is meant the displayed image as it appears when viewed (or when photographed, say). By the term "recorded image" used hereinafter is meant the viewed image in the form of a record such as a photograph, or the output of a vidicon camera (not necessarily recorded onto another medium such as magnetic tape).It will be apparent that the object image is by definition undistorted, that the displayed image can be distorted (because of faulty formation on the image surface), that the viewed image can be, and usually is, distorted (or further distorted) because it is an essentially flat "concept" derived from an essentially curved "object" (the curved image surface), and that the recorded image can be yet further distorted because of faults or inadequacies in the recording equipment.

It will be appreciated that the recorded image, i.e., a photograph of the displayed image (or, indeed, the image formed on the target of a vidicon tube viewing the displayed image), is a manifestation of the flat "concept" that is the viewed image - and that it will of necessity be distorted because it is flat while the displayed image, being on a curved surface, is itself curved.

Considerable effort has been expended in designing hard copy systems incorporating optical apparatus that can take in the viewed image and give out something approximating the object image, this output then being photographed to provide a recorded image which is a relatively true, undistorted picture of the originaltobject.

Since, however, every CRT VDU is different (as regards the distortion it imparts), either a large number of these not-inexpensive optical apparatus are needed, or much time has to be spent modifying one apparatus to fit the VDU presently being used. This is highly unsatisfactory, and the present invention seeks to supply an alternative distortion correction arrangement that relies upon the use of a digital image processing system to provide a version of the object image pre-distorted in such a way as to counteract at least the distortion arising as it is "converted" via the displayed image to the viewed image. Desirably, indeed the inventive arrangement enables any recording equipment distortions to be counteracted as well.

There are currently available a number of digital image processing systems - that known as ARGS (Advanced Raster Graphics System), from SIGMA, is an example - in which the image input is electronically "divided" into a very large number (a 4096 by 4096 array in the ARGS) of very small discrete picture elements ("pixels"), and a microprocessor is then used to control, in accordance with a chosen program, how these individual pixels are re-combined to give the output image. Within limits, these systems can in effect manipulate the input image in any conceivable way, outputting something that is wholely or partly different in shape, size or colour.

The present invention employs such an image processing system to provide a version of the object that is pre-distorted, in accordance with measurements of the amount of apparent distortion exhibited by the viewed image, so that the displayed image is itself distorted but in a sense such as to counteract the subsequent apparent distortion of the viewed image, with the result that the viewed image - and, more specifically, the recorded image (a photograph or other hard copy thereof) - is in fact an undistorted representation of the object image.

In one aspect, therefore, this invention provides a method of reducing the apparent distortion of the viewed image derived from a displayed image shown on a display unit, and, if appropriate the recorded image further derived therefrom, in which the image data supplied to the display unit is first so modified as to form a displayed image which is pre-distorted in such a way as to counteract the distorting effect(s) of the display unit and if appropriate the recording equipment.

The method of the invention may be employed to reduce almost any kind of distortion~ geometric, contrast, brightness, colour - likely to be met, and can do so whether the distortion arises from the display unit or from the "viewing system" (an expression intended to encompass both the recording equipment and the human eye). Primarily, however, it will be of use for combating geometric distortion (as shown in a "flat" photograph of the image displayed on a "curved" screen) and colour distortion (as shown by the incorrect colour response of either or both the screen phosphors of a CRT display unit and the photographic film in the recording equipment).

The method of the invention relies upon the use of an image processing system that allows the object image data to be manipulated - as regards image shape and colour, say - in any desired way. As stated above, that image processing system known as ARGS is such a system. The ARGS is commercially available (from SIGMA), and needs no detailed description. Briefly, however, it uses a number of 16-bit microprocessors, interfaced with a 128K internal memory, to enable a very high degree of image manipulation (and may be interfaced to a host computer, such as a DEC PDP-1 1 or VAX 1 1/780, for determining the nature of this manipulation).It has features such as: a video processor running a 32-bit transformatior table; the ability to be user-controlled in a hign level language such as BASIC; a 4096 x 4096 pixel array which can be sub-divided into independently-controllable bit planes or smaller modules; zoom, pan and scroll capability; 16 million colours; and outputs for independently driving several high definition (1000 line) colour video units.

Before operating the method of the invention it is of course necessary to determine the distortion expected from the display unit (and the recording equipment) so that the correct degree and type of pre-distortion can be applied. This is most conveniently done using the apparatus itself to display, view and record a standard test image (fed directly to the image processing system as data in digital format in immediately useable form, bypassing any input conversion devices likely to introduce distortion themselves), the comparing the recorded test image with the object test image to derive a measure of how the two differ (this comparison is best done on a digital basis, the recorded test image being digitised by, say, a video camera coupled to an A-D convertor, and compared with the original digital format test image).There may then be applied to - and stored in - the image processing system a set of rules - a program - defining how the data for any object image to be displayed and viewed using that particular combination of apparatus is first to be pre-distorted. The comparison of the object test image with the recorded test image is best done by computer (a DEC PDP-1 1 series minicomputer is generally sufficient for this job), employing digitized versions of both images, the computer itself then producing the predistortion program to be stored in and applied by the image processing system.Though the software necessary for causing the computer to generate the required pre-distortion program will have to be written to suit each system - and probably each combination of processing system, display unit type, and recording equipment - nevertheless the general concepts behind such software are standard, and need no further discussion here.

The invention extends, of course, to any recorded image, specifically in the nature of a photograph, whenever produced using the distortion-reducing method of the invention.

The invention is now described, though by way of illustration only, with reference to the accompanying drawings in which: Figure 1 shows, in schematic, block-diagram form, the calibration of a complete imaging system in accordance with the invention; and Figure 2 shows the subsequent use of the system of Figure 1.

For convenience, bot Figures 1 and 2, and the following description, relate solely to the correction of the geometrical distortion of the imaging system. It should not be overlooked, however, that the invention may be employed to correct other types of distortion.

The apparatus shown in Figure 1 comprises an image processing system (1 1), a test image generator (12), a visual display unit (13), a 35 mm photographic camera (14), an image digitizer (15) and a computer (16). The test image generator 12 is feeding the image processing system 11 with an object image composed of a standard test pattern (17: a simple orthogonal grid) which the processing system passes on, initially unchanged (18) to the VDU 13 were it is shown as the displayed image (19: it is here assumed for convenience that the VDU 13 is capable of faithfully displaying the input image - thus, that the input and displayed images are identical~ though this need not be the case). The displayed image is then viewed by the camera 14, which "sees", and photographs, a distorted form thereof, namely the viewed image (20: shown exhibiting rather exaggerated barrel distortion); the formed photograph (21) is the recorded image, and displays the same distortion as observed in the viewed image 20 (it is assumed for convenience that the photograph has no intrinsic distortion).

The recorded image 22 as evidenced by the photograph 21 is then fed back into the image processing system 11 via the image digitizer 15 (it is further assumed that this causes no additional distortion), and the two images now held by the processing system - the object image 17 and the digitized recorded image 22~are then passed to the computer 16; the computer compares the two, and, in accordance with its program, provides an output which constitutes a set of instructions to the image processing system 1 1 to pre-distort the image 176 to be displayed such that it corresponds to an oppositely-distorted version thereof.These instructions are for convenience shown as that oppositely-distorted verion (24); if applied as a pre-distortion to the test image 17 when it is output as the VDU input image 18, this degree of opposite distortion will exactly counteract the distortion of the object image 1 7 as it becomes the viewed image 20.

This application of the pre-distortion 24 specified by the computer 1 6 is not shown in Figure 1 (to prevent confusion it is imagined that the loop is "broken" along the dashed line 25, somewhere between the image processing system receiving the pre-distortion 24 and outputting the VDU input image 18), but it is shown in Figure 2.

In Figure 2 it is imagined that the apparatus of Figure 1 has already been used to calibrate the display system (1 1, 13), and that the image processing system 11 is now programmed to construct a pre-distorted version of its object image input 1 7, before outputting it as 18 to the VDU 13, in accordance with the pre-distortion instructions 24 of the computer 16. For simplicity, the object image 1 7 being input to the image processing system is still represented by the standard grid test pattern, but the "image" 18 output by the processing system is now suitably predistorted. The displayed image 9 shown on the VDU 13 is identically pre-distorted, and as a result the viewed image 20 is re-distorted, the two distortions exactly cancelling out so that the viewed image 20 (and any recorded image, as the photograph 21) appears to be a completely undistorted version of the object image 1 7.

Claims

1. A method of reducing the apparent distortion of the viewed image derived from a displayed image shown on a display unit, and, if appropriate, of the recorded image further derived therefrom, in which the image data supplied to the display unit is first so modified as to form a displayed image which is pre-distorted in such a way as to counteract the distorting effect(s) of the display unit and if appropriate the recording equipment.

2. A method as claimed in claim 1, in which the visual display unit is a Cathode Ray Tube device.

3. A method as claimed in either of the preceding claims, in which the image data is modified as required using an image processing system of the type described hereinbefore in connection with the ARGS.

4. A method as claimed in claim 3, in which the distortion expected from the display unit (and the recording equipment) is first determined using the apparatus itself to display, view and record a standard test image, the recorded test image then being compared with the object test image to derive a measure of how the two differ, there then being stored in and applied by the image processing system a set of rules defining how the data for any object image to be displayed and viewed using that particular combination of apparatus is first to be pre-distorted.

5. A method as claimed in claim 4, in which the standard test image is fed directly to the image processing system as data in digital format in immediately useable form, bypassing any input conversion devices likely to introduce distortion themselves, and the comparison is done on a digital basis, the recorded test image being first digitised and then compared with the original digital format test image.

6. A recorded image in the nature of a photograph, whenever produced using a distortion-reducing method as claimed in any of the preceding claims.

6. A method as claimed in claim 5, in which the comparison is done by computer, the computer itself then producing the pre-distortion rules to be stored in and applied by the image processing system.

7. A distortion-reducing method as claimed in any of the preceding claims and substantially as described hereinbefore.

8. A recorded image in the nature of a photograph, whenever produced using a distortion-reducing method as claimed in any of the preceding claims.

New claims or amendments to claims filed on 18 October 1 982 Superseded claims 1-8 New or Amended Claims

1. A method of reducing the apparent distortion of the viewed image derived from a displayed image shown on a display unit, and, if appropriate, of the recorded image further derived therefrom, in which an image processing system of the type described hereinbefore in connection with the ARGS is used to supply image data to the display unit, and the thus supplied image data is first so modified by the image processing system as to form a displayed image which is pre-distorted in such a way as to counteract the distorting effect(s) of the display unit (and if appropriate the recording equipment); and in which the distortion expected from the display unit (and if appropriate the recording equipment) is first determined using the apparatus itself to display, view and record a standard test image, the recorded test image then being compared with the object test image to derive a measure of how the two differ, there then being stored in and applied by the image processing system a set or rules defining how the data for any object image to be displayed and viewed using that particular combination of apparatus is first to be predistorted.

3. A method as claimed in either of the preceding claims, in which the standard test image is fed directly to the image processing system as data in digital format in immediately useable form, bypassing any input conversion devices likely to introduce distortion themselves, and the comparison is done on a digital basis, the recorded test image being first digitised and then compared with the original digital format test image.

4. A method as claimed in claim 3, in which the comparison is done by computer, the computer itself then producing the pre-distortion rules to be stored in and applied by the image processing system.

5. A distortion-reducing method as claimed in any of the preceding claims and substantially as described hereinbefore.