NZ233743A - Detecting knots in timber from x-ray density map - Google Patents

Description

233743 r\ Priority Date(s): .A?)*\ \ .W7. ' v? \° ^ Complete Specification! Filed: . «V. ...«•• Class: At ky yiaSS: iTiTTi^ #iTT|t>nft i 2" "DEC 1990 Publication Date: P.O. Journal, No: • • • • o -f £ X>!L' vt^ Under the provisions of Regu- Divided Out of lation 23 (1) the .COEpJStS N. Z . No 226540 Dated 12 October 1988 Specification has been ante-dated & 19 NHLiMlANC Patents Act 1953 Initials COMPLETE SPECIFICATION © A METHOD OF DISCRIMINATING KNOTS IN LUMBER We, MacMILLAN BLOEDEL LIMITED, a Canadian corporation of 1075 West Georgia Street, Vancouver, British Columbia V6E 3R9, Canada, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: -1- (Followed by 1A) o 233743 Background of the Present Invention The concept of analyzing wood using electro-| magnetic radiation has been suggested even well before any | equipment capable of commercially utilizing such a concept | was available, see for example, an article by O.G. Miller j I entitled "Detection of Rot in Wood by Electronic X-Ray | Fluoroscopy" published in the British Columbia Lumberman f of October 1964. The concept of using x-rays for detecting I rot in wood was described as being old at that time and Is refers to an investigation as far back as 1929. The theory suggested that provided rot had progressed sufficiently to | significantly reduce the density of the wood, the x-ray I absorption would also be reduced. The article concludes C I that x-ray fluoroscopy provides a rapid nondestructive I method of making internal inspections of wood for rot j pockets and for metal .

I An article entitled "Defect Detection in Lumber | -State of the Art" by Szymani and McDonald in the November | 1981 issue of Forest Products Journal, reviews the various t techniques for analyzing board and logs. In relation to I f fluoroscopy, reference is made to the above article of < J Miller and to the real time x-ray, television and x-ray i cinetography so that the images can be projected directly through a television network. Also described is a system where an x-ray sensitive device detects the amount of x-rays passing through the specimen and wherein by filtering and amplifying the signal, an indication of the - 1 A - presence or absence of defects could be provided and the output used via computer for lumber grading and sawing decisions. Particular attention is directed in this article to the Scintaflex System which directs, in this case neutrons at a point area on the board and senses the amount of radiation traversing the board to provide an indication of the density at the point.

Various techniques have been used or described to identify knots and/or rot in logs, for example, in the article "Locating Knots by Industrial Tomography - A Feasibility Study" by Taylor et al., published in the Forest Product Journal of May 1984 or in the article "A Computer Vision System that Analyzes CT-Scans of Saw Logs" by Funt and Bryant in a paper given at the IEEE Computer Society Conference on computer vision and pattern recognition.

Funt and Bryant have also published a paper entitled "Detection of Internal Log Defects by Automatic Interpretation of Computer Tomography Images" published in the January 1987 issue of the Forest Products Journal, which describes in detail the analysis of a log cross section histogram of a density map developed by a scan of a log using x-rays and the analysis of this histogram to determine the location of knots and rot.

Optical scanners have also been used to determine the location of surface defects in lumber by differentiating based on surface color.

Automatic lumber processing systems (ALPS) have also been described wherein the information derived from an optical scan of the board is analyzed based on tone, color, texture and pattern recognition to determine the location of surface defects in a piece of lumber and to use the information so generated to provide a sawing solution for sawing of the scanned board to obtain the optimum of recoverable lumber from the scanned board.

An article in IEEE "Transactions on Pattern Analysis and Machine Intelligence" Volume TAMI-5 No. 6 November 1983 entitled "Code Identifying and Locating Surface Defects in Wood - Part of An Automated Lumber Processing System" by Conners et al describes a system wherein an image is produced by optical scanning such as by means of a laser scan and then the image is examined to gauge its tonal properties, i.e. degree of brightness, its texture or pattern qualities and pattern recognition to detect defects in the lumber material and the position of these defects.

An article entitled "ALPS - Potential New Automated Lumber Processings System" by McMillin et al., in Forest Products Journal, Volume 34, No. 1, January 1984 deals with sensing and locating defects in a log and provides an optimum cutting solution based on this information. In this publication a log is processed by scanning using photon tomography and computer reconstruct ion of axial projections from three different angles to locate defects in the log. After sawing the log into boards, the boards themselves are scanned with video cameras and the image information digitized and analyzed for tonal and textural quality and an optimal cutting strategy based on the defect location is then implemented.

The article "A Prototype Software System Locating and Identifying Surface Defects in Wood" by Conners, given at the Seventh International Conference on Pattern Recognition in Montreal, Canada July 30-August 2 1984 and published in the Proceedings Volume 1, provides further details on differentiating clear wood from defective wood in lumber utilizing optical scanning techniques where in the background wane, knots and clearwood are distin-gui shed . 23 3 7 4 3 United States Patent 3,931,501 issued January 6, 1976 to Barr et al discloses yet another scanning technique for determining and designating the surface defects on a piece of wood and then providing a cutting pattern for edging the wood into discreet lumber elements.

Canadian Patent 1,146,051 issued May 10, 1983 to Strandberg et al describes a system for optimizing based on measuring the contours of a piece of timber and sawing based on an optimization program which positions the piece of timber for sawing in the edger and adjusts the edger accordingly.

Canadian Patent 1,125,148 describes an optical sensor detecting irregularities along the lateral edges of a cant and adjusting the position of the cant for sawing to eliminate these irregularities.

It will be apparent from the above that a variety techniques have been suggested and provided in some cases for scanning boards to develop images of the surface of the board which are then analyzed first to determine defects and types of defects and location of these surface defects and then devise an optimum cutting solution for edging of the board and position the same for subsequent edging. These systems primarily utilize optical scanning techniques for providing the image to be analyzed, however it has been proposed to use in some cases electromagnetic radiation by directing a pinpoint of such radiation through the board and sensing the intensity of the radiation after it traverses the board to provide indications of the density.

Thus it is known in real time to provide analysis of a board and devise an optimum cutting solution, i.e. to analyze the images produced by scanning of the board in real time to determine the location of defects and from this generate an optimum cutting pattern. Such real time analysis have been based exclu- 23 3 7 43 sively on optical scanning and analysis of the image so produced. However it will be apparent that optical scanning as such can determine only surface defects and in many cases is limited by the surface of the flitch being scanned so that the rough surface of a rough sawn flitch cannot be processed using this technique.

X-rays or electromagnetic detection techniques have been applied primarily to the analysis of the whole log rather than to a flitch or cant and the image (images) generated have been analyzed for defects and a cutting solution for logs. It has been suggested that x-ray tomography or fluoroscopy may be applied to detect defects in lumber. To date the published information relates to the use of x-rays to inspect wood and have not been of a practical nature, i.e. operated in real time (capable of operating at conventional production speeds).

The present invention provides a method of discriminating knots from other portions of a flitch of lumber, including the steps of generating a density map and histogram of density points of a flitch, analyzing data representative of said density map and said histogram using a one-dimensional (/\^2-G digital filter having its one dimension extending substantially parallel to the longitudinal axis of said flitch and identifying sudden changes in density to indicate the boundary edges of a knot.

For ease of reference, a method of the invention will now be described in combination with a lumber optimizer system that is described and claimed in NZ Application No 226540, from which the present application is divided. 2337 43 Brief Description of the Drawings: Further features, objects and advantages will be evident from the following detailed description taken in conjunction with the accompanying drawings in which: Figure 1 is a schematic side elevation of a flitch processing system; Figure 2 is a schematic illustration of an array of electromagnetic radiation detectors positioned to intercept the line of radiation passing through a board; Figure 3 is a schematic illustration of an edger optimizing system; Figure 4 is a schematic illustration of a trimmer optimizing system.

Description of the Preferred Embodiments As shown in Figure 1 the scanner 10 is composed of basic conveyor 12 which in the illustrated arrangement is a chain type conveyor with lugs 14 onto which the flitches or cants 16 are placed by a suitable means (not shown). The conveyor 12 moves the flitches 16 by engagement of the lugs 14 on one side of each flitch thereby automatically positioning the flitch relative to the conveyor so that the location of each flitch is known, with their spacing preset by the spacing between the lugs 14.

Conveyor 12 moves the flitches one at a time past a profiling station 18 (Figures 1 and 3) having a first sensor which may be any suitable sensor such as an optical scanner adapted to determine the wane and periphery of the board. Such devices are well known, for example the optical sensor sold by Lloyd/Softac of Vancouver, B.C. under the Tradename LS-8600.

After passing through the optical scanner or sensor 18 the flitch then passes into a data acquisition station 20 incorporating an x-ray scanner or a suitable density scanner that will determine the local densities through the f1 itch 16.

Preferably the density scanner will comprise at least one source 26 of x-ray that will be colimated by a colimator generally indicated at 22 to reduce the projected electromagnetic radiation into a flattened fan shape as illustrated at 24 to provide a narrow line of x-rays impinging on the flitch. In Figure 2 four laterally spaced x-ray sources 26 are shown each projecting a line onto the flitch passing through the scanner 20. The lines from each source 26 are arranged substantially end to end in substantially axial alignment thereby to extend substantially the full length of the flitch. If desired the lines need not be axially aligned. The adjacent ends of these lines may overlap slightly, but if so the detectors 32 covered by the overlap will be calibrated accordingly. It is also possible to have small gaps between the adjacent ends of these lines emanating from adjacent emitters 26 which will leave undetected strips across the flitch, for example in the area of the conveyor chains 12.

Electromagnetic radiation is contained within a housing 28 of the scanner 20 by a lead lining and adjacent to the bottom edge, i.e. where the flitches pass therethrough, by a flexible lead lined curtains on the incoming and outcoming sides as indicated at 30. The curtain 30 are deflectable by the flitch entering and leaving the detector 20.

The fan shaped flow of photons 24 for each of the emitters 26 as above described forms a line of photons extending substantially the length of the flitch 16. The line is directed onto an array of side by side x-ray detectors 32 composed of a plurality of side by side scintillators 33 and corresponding photodiodes 34. These photodiodes 34 are each of essentially the same width and are arranged uniformly along the length of the line of photons. The width of these diodes is determined by the number of same positioned in side by side relationship along the length of the line of photons. Generally there will be at least 4 (four) such diodes per inch to obtain a usable image and normally not be more than about 16 (•sixteen) since in normal practice reducing the width of the detectable areas beyond this will be of little practical advantage. Applicant has found the use of 10 diodes per inch provides a very satisfactory resolution.

In the Figure 2 arrangement discreet diodes have been indicated by reference 34 (only some of the diodes indicated) and these diodes are connected by a printed circuit arrangement or the like to an analog to digital converter generally indicated 36 which in turn is connected to an output cables generally indicated at 38 to carry the digitized data collected by each group 35 of the array 32 to a suitable computer 62 as will be described below. 233743 1; •> i < f Each diode has a unique address and their voltage outputs are read and digitized sequentially in groups. Preferably the diodes will be sectioned into discrete groups as indicated at 35. In the illustrated arranged one group is provided for each emitter 26 but this is not essential. The diode outputs of each group 35 is read and digitized sequentially with all of the groups 35 being simultaneously read and digitized. By so dividing the diodes into groups along the line of radiation projected against the flitch the time required to accumulate the relevant data along the full length of the flitch is significantly reduced. For example if four discrete groups 35 are provided and the outputs of the diodes in each group are simultaneously read and digitized only one-quarter of the time necessary to sequentially read and digitize the output of the whole line is necessary (assuming each group 35 contain the equal number of diodes). Thus the diodes will be divided into a suitable number of groups to ensure that the required data is acquired in the available time. In a particular example of the present invention diode output is read and digitized sequentially in groups of 512 diodes at a rate of 100 kilohertz as the flitch traverses the sensors.

In the Figures 1 and 3 embodiment (edger optimizer) after the flitches are sensed or scanned by the scanner 20 they pass along the conveyor 12 over open section 39 and then onto second conveyor 40 (see Figures 1 and 3) which is driven in direction indicated by the arrow 41 and moves the flitch transferred from the conveyor 12 into the positioning station 42 wherein the leading edge of the flitch 16a in Figure 1 is moved into position against adjustable stop pins 44 movable as indicated by the arrows 46 to align the leading edge of the flitch at the desired angle relative to the edger feed conveyor 48. The conveyor 48 is driven by edger feed motor 50. The plurality of rollers such as those schematial1y indicated at 52 are positioned at opposite sides of the conveyor 40 which is relatively narrow compared with the width of the flitch perpendicular to the direction of the travel of the conveyor 40 so that the flitch cannot tip as it is advanced by the conveyor 48. Generally a holddown (not shown) will cooperate with the top of each flitch as it is advanced into the edger 54.

The conveyor 12 transporting the flitches 16 through the two sensor stations 18 and 20 is driven by a suitable feed motor 56 while the advancing conveyor 40 is driven in the direction of arrow 41 by a feed motor 58.

The major data acquisiton, analysis and control systems of the Figures 1 and 3 embodiment are schematically illustrated in Figure 3.

In the embodiment illustrated in Figure 3 the board or flitch surface profile is obtained from the profiling station 18 and fed to the profiling computer 60 via line 61, and data for density mapping is obtained in the data acquisition station 20 and fed to density mapping computer 62 via the communication line 38.

The image analysis computer 64 receives data from computer 62 via line 66 and from computer 60 via lines 68 and 70. The surface profile information from computer 60 is superimposed on the density map from computer 62 to enhance the accuracy of image analyses for defect classification.

The image analysis information which determines the location of defects including knots, rot and board dimensions, etc is then fed to the sawing solution computer 72 via line 74 and used to determine the sawing solution. The determination of the sawing solution also uses input from the computer 60 which is introduced via line 68.

ZO 0 / The above data acquisition and computers are all operated asynchronously and their status reported to a main process control computer 84 which provides the overall control of the process. The process control computer 84 synchronizes the operation of all of the components of the system and has therefore input from the computers 62, 64 and 72 as well as from board detectors 82 (only one shown) detecting the presence of a flitch 16 being introduced into the equipment. Information from detector 82 is fed via line 86 into the process control computer 84. If desired there may be other detectors in other stations, for example, one in each of the stations 18 and 20, section 39, conveyor 40 and position 42.

Each of the computers 60, 62, 64 and 72 is connected to the process control computer 84 by the lines 94, 96, 98 and 100 respectively for transmitting data therebetween. The process computer 84 controls the operation of the various feed motors for the conveyor via line 88, main feed motor 56 of conveyor 12 is connected to line 88 via branch line 90, the feed motor 58 of conveyor 40 via branch line 92 and edger feed motor 50 via branch line 93.

The computer 84 includes computer capacity 76 for controlling the flitch positioner 42 based on input from the various computers including the sawing solution computer 72 and transmits the information for the sawing solution to the positioner 42 via line 80 and the edger setworks 54 via line 78.

In operation of the Figures 1 and 3 embodiment the first flitch or board 16 indicated at 16B in Figure 3 is applied to the conveyor 12 at the infeed end thereof and moved perpendicular to its longitudinal direction in the direction of the arrow 102 first into the optical scanner 18 where its physical profile is scanned so that the wane edges of the flitch are sensed and their positions determined. The flitch is continuously fed through - 11 the scanner 18 at a feed rate determined by the feed motor 56 so that the dimensions and profile of the flitch are easily determined. The flitch is then passed at a similar feed rate through the data acquisition station or scanner 20 where x-rays are passed through the flitch and a density profile detected by the array of detectors 32 on a continuous basis as the flitch is moved there past.

The data acquired in the scanner 18 are transmitted by a line 61 to the data acquisition or profiling computer 60 wherein the profile of the flitch is determined. The information from the scanner 20 is transmitted via line 38 to the data acquisition and density mapping computer 62 wherein a density map of density profile of the scanned flitch is obtained. The density profile is based on the signal intensity per pixel of the image generated which in turn is determined by the signal attenuation of the x-rays passing through the board and being received in a given time period by each of the various detectors of the array 32. Normally the signal will be inverted so that the areas of greatest density will appear in the image as the area of highest grey scale intensity for image analysis.

The image analysis time may be significantly reduced if the images from more than one scan length (equal to the number of pixels across a standard image buffer) can be incorporated onto a single image buffer. This may be accomplished using the information from the wane scanner 18 which determines the maximum width of the board so if the number of pixels across this maximum width is less than 1/2 (one-half) the number of pixels in the height of an image buffer two scan lengths will be incorporated in the same buffer. If the number of pixels across the maximum width of the flitch is less than 1/3 (one-third) the number of pixels in the height of a buffer, three scan lengths may be incorporated in a single - 12 #• JJ r- 23 37 4 3 t I j buffer, and so on. j By packing more than one scan length in a single ! ^ image buffer the processing time for analyzing all of the images is reduced. The number of buffers that must be ; 5 analyzed and processed may well be reduced to in many | cases 1/2 (one-half) and in some cases to 1/3 (one-third) i | thereby reducing the analysis time so that the feed speed (conveyor 12) may be increased.

A histogram of the resultant intensities of the 10 pixels of the grey scale image (sensed densities) is produced by accumulating histograms buffer by buffer and the computer 64 analyzes the cumulated histogram for the intensities (densities) corresponding to good wood density, rot density and low density depicting voids such 15 as holes or dry rot. For example, if relatively dry wood is being processed the density of rot is significantly less than that of good wood and it will be determined on the basis of this lower density, however, in wet wood (i.e. wet wood may have been transported by water) the 20 density of the rot may well be higher than the density of the good wood and the particular density spectrum depicting rot will be higher than that of good wood.

High density areas may also be found by analyz-| ing the frequency information in the image. | 25 Applicant has found that knots may be determined | by sudden changes of density, i.e. a relatively sharp \ CD interface between the high density knot area and the lower | density of a good wood area. It has been found that knots t can be recognized in this manner by using a one-dimension- j 30 al del-2-G digital filter having its one dimension i substantially parallel to the longitudinal axis of the flitch to determine the sudden changes in density as the scan enters and leaves the knot. This technique has been found to accurately detect knots quickly without requiring 35 undue computing capacity. ' 13 " -""} •- . ... _..... jr ... . , .... _ 23 37 In any event data is analyzed to determine which pixels represent knots, which pixels represent good wood and which pixels represent rot. The relative locations of these pixels are known so that the location of rot and knots in the board are determined.

Based on this information plus the information acquired in the detector 18 and processed in the computer 60 which determines the outer edges and the wane of the flitch all of the defects adjacent to the edge of the flitch may be eliminated and the operation of the computer 64 simplified since the outline of the board is determined by computer 60.

The information from the computer 64 and the information from the computer 60 above described are fed to the sawing solution computer 72 which based on the current values for lumber by-grade products is programmed to optimize the value of the material that may be cut from the flitch by grade and volume so the resultant sawing operation will provide a maximum return. The computer 72 supplies the sawing solution to computer 84 which positions the edger saws of the edger via the edger setworks 54 and computes in the computing section 76 the position of the pins 44 and commands the movement of the pins 44 in the positioner 42 accordingly.

The main or controlling computer 84 controls the operations and speed of the system. Each of the computers 60, 62, 64 and 72 feed information to the controlling computer 84 and signal the computer 84 when they have completed their tasks for each flitch as it is passed through the system. The speed of conveyor 12 is governed by motor 56 which is controlled by computer 84 to normally move at maximum speed to move that flitch through the system into the edger 54 as quickly as possible and pass the following flitchs into the system. If the computations of the computers are not completed for a given flitch when " 14 " 233743 that flitch approaches the conveyor 40 the computer 84 slows the speed of or stops conveyor 12 until all operations for that flitch are completed and then speeds up the conveyor 12. This ensures that the line is operated at maximum speed unless an unduly complex operation requiring significantly more time than the average is encountered. If this occurs the line speed (conveyor 12) is slowed or stopped depending on the particular control use.

The flitch 16B when it enters the positioner 42 has been completely analyzed by the computers 60, 62, 64 and 72 and the angle of the flitch to the infeed conveyor 48 has been determined. To position the flitch in the proper orientation the pins 44 (only two shown but more may be used depending on the length of the flitch to be positioned) are adjusted as indicated by the arrows 46 and the flitch is advanced into the positioner 42 by the conveyors 40 driven by motor 58 which is activated when a flitch is in position to move the edge of the flitch against the pins. Conveyor 40 may be simply flat belt type conveyor that can slip relative to the flitch so that when the conveyor 40 pushes the flitch against the most forwardly projecting one of the pins 44 that end of the flitch slides on the conveyor and the flitch is skewed to properly orient its longitudinal axis with the direction of travel of the conveyor 48 to the edger 54 as indicated by the arrow 49.

When the edge of flitch has been positioned against the stops 44 the edger feed motor (50) is activated to move the flitch in the direction of the arrow 49 through the edger 54 to saw the flitch as required.

The above description has been directed to edger system however it may equally well be applied to a trimming system such as that shown in Figure 4. . 15 .

O 23 3 7 In Figure 4 like references numerals have been utilized to indicate like parts of the invention described C^) herein above and these elements will not be described again .

It will be noted that in the Figure 4 embodiment no profile scanner 18 has been incorporated. In this case (2) the data generated by the scanner 20 is conveyed via a line 61A to a data acquisition and board profile computer 10 60A which determines the edge of the board based on a significant change in density generated when the edge of the board traverses the line of radiation and a peak in density that is formed substantially along the line formed where the edge of the wane intersects the plane forming 15 one surface of the flitch. The computer 60A thus determines the location of the edge of the flitch as well as the line of intersection of the wane with the board or flitch surface. This technique of defining edges of the wane surfaces on the flitch cannot determine the slope of 20 the wane edge as can be done with an optical sensor such as that shown in Figure 1 and 3 (sensor 18) thus while the sensor 18 may be eliminated and the board profile determined as described immediately hereinabove described using the computer 60A, it is preferred to utilize an optical 25 scanning system using the optical scanner 18 and the computer 60 as this will permit more accurate determination of a sawing solution, e.g. if the slope of the wane is relatively shallow adjacent its intersection with the planar surface of the flitch it may be acceptable to 30 include more of the wane edge in the cut boards.

The optical scanner 18 and computer 60 may be used in Figure 4 embodiment used in place of the computer 60A or in the Figure 3 embodiment the computer 60A may be used in place of the optical scanner 18 and computer 60. 23 37 43 In the Figure 4 embodiment the' conveyor 12 empties onto an infeed conveyor 200 that carries the flitch (hereinafter referred to as board) into the trimmer 202 having setworks 204. Thus in the Figure 4 embodiment boards will be applied to the conveyor 12 and sensed by the sensor 20 in a similar manner to the flitches fed to the conveyor 12 of the Figure 3 embodiment.

The boards travelling through the sensing station 20 and along conveyor 12 are sensed and the sawing solution determined in essentially the same manner as described above in Figure 3. This information is all fed through the process control computer 84.

The board positioning computing capacity 76A in this case is utilized to control via line 206. The position of end plate 208 which may be moved in and out as indicated by the arrow 210 and to control the operation of the positioning motor 212 via line 214. The positioning motor 212 drives the interconnected rollers 216 to force the lateral end edge at one longitudinal end of a board positioned on rollers 216 into a butting relationship with the plate 208 and thereby position one end of the board.

Simply stated once the sawing solution is known it is necessary to position the board longtitudinal ly relative to the trimmer 202 and to set the trimmer set-works 204 according to the sawing solution which is accomplished by the process control computer 84 and its connecting line 218.

The trimmer conveyor 200 is driven by a feed motor 220 which is controlled from the process computer 84 via line 88 and branch line 222.

In the operation of the system shown Figure 4 once the sawing solution has been delivered to process controller 84 and the board on the conveyor 200 is moved to a position overlying the rollers 216. Plate 208 will normally have been positioned by the board positioning 23 37 43 computer section 76A before the board reaches the conveyor rollers 216. With the plate 208 positioned as required the rollers 216 are activated by the positioning motor 212 to move the board in the direction of the arrow 224 and butt its end edge against the plate 208.

Conveyor 200 moves the board into the trimmer which has had its setworks 204 adjusted so that the saws cut the board to length(s) as required.

While a single conveyor 200 has been shown it will be apparent that two separate conveyors may be used one to deliver the board to the positioning station incorporating the rollers 216 and the plate 208 and a second conveyor dependently operated used to convey the boards through the trimmers 202.

The invention has been described with the flitch being fed substantially perpendicular to its longitudinal axis, but it will be apparent that with suitable modification the flitches may be fed through the sensing equipment with the longitudinal axis substantially parallel to the direction of feed. - 18

Claims (2)

WHAT WE CLAIM IS:

1. A method of discriminating knots from other portions of a flitch of lumber, including the steps of generating a density map and histogram of density points of the flitch, analyzing data representative of said density map and said 2 histogram using a one-dimensional^7 -G digital filter having its one dimension extending substantially parallel to the longitudinal axis of said flitch and identifying sudden changes in density to indicate the boundary edges of a knot.

2. A method according to claim 1, substantially as herein described or exemplified. MacMILLAN BLOEDEL LIMITED By their attorneys HENRY HUGHES LIMITED 19