GB2035611A - Method of tracing an outline - Google Patents

Abstract

A method for tracing most of an outline in a picture, the outline being a boundary with different light values on its two sides, with a tracing head. First, a plurality of checkpoints P on the boundary R are chosen, in order around the boundary, the last being the same as the first. Small regions S are selected around the checkpoints. Then the tracing head 16 is positioned at the first checkpoint, and is moved, without tracing, to the intersection of the boundary and the edge of the small region around the first checkpoint. Then the tracing head is moved along the boundary towards the second checkpoint, while tracing, until it reaches the edge of the small region around the second checkpoint. The tracing head is then moved to the third and successive checkpoints in the same manner until the boundary has been traced. <IMAGE>

Description

SPECIFICATION Method of tracing an outline This invention relates to a method of tracing the outline of a picture and following it, and in particular, relates to a method for making a mask for photocopying.

In printing, masks are used in many ways. For example, when a catalogue of goods is printed, in most cases it is made up from various photographs of the goods in different settings. Therefore, it is necessary to remove the backgrounds from these photographs before assembling them to form the catalogue. Therefore, a reproduction image is obtained by preparing a film of which the necessary part is transparent and the rest opaque, laying this film on the original picture, and reproducing the combination photographically. According to circumstances, on the other hand, it may be desirable to provide such a mask in which the necessary part is opaque, and the rest transparent.

Such a mask has heretofore been made by hand.

For example, in the same manner as when tracing a drawing, a transparent film sheet is laid on the original picture, and the outline of the part to be masked is drawn. The outside or the inside of the outline is then blotted out by opaque ink, to obtain the cutting mask.

This kind of hand work presents no problems, if the object to be masked has a simple outline, but if the object to be masked has a complicated, convoluted, or irregular outline, great skill and trouble, and a lot of time, are required. Hence, it is quite difficult to prepare an accurate cutting mask quickly for photographic plate making.

Another method of producing such a cutting mask is to reproduce a photofilm of the desired size, and then to directly obliterate the portion to be blanked off with opaque ink. This method again requires hand work, and suffers from cost and accuracy problems.

Another method has been used for making such a mask, in which a piece of peel-off laminate film made of a colorless transparent film and a colored transparent film which has a safe light color such as red is laid on the original picture, and then the transparent color film is cut along the outline of the object in the picture. The unnecessary part of the transparent color film is then peeled off, to obtain the cutting mask. This method skips over the process of blotting out with opaque ink, but the cutting is done by hand by a thin-bladed cutter, and therefore the outline of the image must still be followed by hand. So this method, too, is tedious, slow, unreliable and inaccurate, and expensive.

An automatic method for preparing such a mask uses an electronic color separation scanner, or a color scanner for plate making. In this case, the background of the object to be masked in the picture is set to a specific colour, prior to photographing the picture. This picture is then scanned for colour separation by the colour scanner, and the specific colour of the background is detected and exposed on a recording film to prqvide the cutting mask. This method is accurate and automatic, but is only applicable to an original picture of which the background has been specifically set to a given colour, and cannot be used for an ordinary picture. Further this method requires the use of a costly colour scanner.

A machine has been developed for automatically tracing the outlines of pictures, and cutting a mask as it goes, as illustrated in Fig. 1 of the accompanying drawings. This machine is a known prior art, and the known way of using it is described below. This description is necessary for a proper understanding of the present invention, which is based upon this machine, and ones similar thereto.

The invention seeks to provide a method of tracing an outline which at least partially avoids these aforementioned difficulties, and which is adapted to be performed at least partially by machine, so as to relieve the operator of substantial work.

The present invention is in fact a new way of controlling the machine of Fig. 1, explained in its structure, and in its conventional operation, below. This method of control is adapted to be performed by machine, although it does involve some steps best done by an operator. An automatic digital computer is well adapted for performance of many of the steps of the method, which will be explained with reference to a flowchart of the sort conventionally used in discussing computer programs. It should be particularly noted, however, that the use of a computer is not essential to the practice of the method of the present invention; it could perfectly practicably be performed by hand, possibly using a flowchart to aid the memory.

According to the present invention, there is provided a method of tracing an outline being a boundary of different light values within a picture, which comprises: (a) selecting points on the boundary as checkpoints, and selecting small regions around them; (b) selecting one point as source point and a second point to be destination point (c) positioning a tracing head at the source point, moving it in an approximately straight line towards the destination point until it reaches the edge of the small region around the source, and allowing the tracing head to move to the intersection of the boundary and edge of the small region; (d) moving the tracing head from the said intersection along the boundary towards the destination point until it reaches the edge of the small region around the destination point; and (e) repeating steps (c) and (d) with the destination point substituted for the source point and the next successive checkpoint as the destination point, until all the boundary has been traced.

The present invention will be more fully understood from the following description of a preferred embodiment thereof, and from the following strictly non-limiting drawings, in which: Figure 1 is a perspective view of a machine which can be used to practice a method of the present invention; Figure 2 is a schematic view of a set of photoelectric detecting elements used in the machine of Figure 1; Figure 3 is two views of the elements of Figure 2, during the tracing process; Figure 4 is a view of an outline with a right angle bend in it, and of the error made in tracing it accord ing to a prior art method; Figure 5 is a schematic view of a picture pattern which is liable to cause another kind of tracing error; Figure 6 is a schematic view of a picture pattern to be traced, for illustrating the method of the present invention; and Figure 7 is a flowchart for an embodiment of the method of the present invention.

in Figure 1 of the accompanying drawings is shown in perspective view a conventional machine abovementioned, as far as its physical parts (not its control system) is concerned. On a frame 1 is mounted a piece of frosted glass 2, which is illuminated by a light source underneath it and not shown.

A pair of guide rails 3 and 4 are fixed on opposite sides of the frame 1 in parallel, and on these guide rails slide sliders 5 and 6, respectively. Between the sliders 5 and 6 extend a second pair of guide rails 10 and 11, which are parallel, and on these guide rails 10 and 11 slides a carriage 15, which therefore is movable over the glass 2 in both the X- and Y- directions. Movement of the carriage in the one direction is controlled by a motor 9 which, via gears 8, drives a screw rod 7 which propels the slider 5 along the guide rail 3. Movement of the carriage 15 in the other, orthogonal, direction, is controlled by a motor 14, mounted on the assembly of the sliders 5 and 6 and the guide rails 10 and 11, which, via gears 13, drives a screw rod 12 which propels the carriage 15 along the guide rails 10 and 11.From the carriage on its two sides project two arms 1 5a and 1 sub. The arm 15a carries an optical head 16 at its end, facing downwards towards the glass 2, and the arm 15b carries a recording head 17 at its end, again facing downwards towards the glass 2.

Thus the optical head 16, which acts as a tracing head, follows the outline 19 of the picture 18, and the recording head 17, which cuts the mask, move always together, being physically both mounted on the carriage 15.

The recording head 17 has a heat pen 20, and is adapted to melt and cut the colour film layer of a peel-off film 21.

The optical head 17 includes a number of photoelectric elements, and is adapted to detect variations in the light received by it.

The machine whose structure is described above is essentially the same as the upper half part of the machine described in U.S. Patent Application No.

930,382 and United Kingdom Patent Laying-Open Publication No. 2,006, 426 A, in regard to the sliders 5 and 6, the drive means for them, and the optical and recording heads. However, the machine therein described was provided with further manual means for moving the carriage, independently, in the X- and the Y- directions, and these two sorts of movement, manual and the automatic movement provided by the drive means, operated together so as to trace the picture outline in a particular manner. Reference is made to the above sources. In particular, the manner in which the optical head scanned the contour to be followed, and controlled the drive means, was quite the same in that prior machine, and in the known machine of Fig. 1, according to its conventional known method of operation.However, a brief sum mary will now be given of this conventional operational mode of the machine of Fig. 1 (which is notthe mode of operation, in full, of the machine of the above-listed proposals). Reference is made to Fig. 2 of the accompanying drawings.

Fig. 2 shows the structure of the detector Z of the Fig. 1 machine. It has eight photoelectric detectors spaced around eight sectors of a circle, denoted by A-H, each having substantially the same photoelectric performance. The output from these detectors is used to control the movements of the drive motors.

In practical use, of course, an automatic digital or analog computer is used for processing the signals from the detectors, in order to produce the control signals forthe drive motors; however, other means could in principle be used, and the possibility of control by hand, based on visual observation of visual displays indicating the strengths of the signals from the detectors, and possibly while following by eye a flowchart for helping the memory, is notto be excluded.

In more detail, when a picture corresponding to an image of a light spot as projected through an original picture 18 laid on the glass 2 is focused on the detectors, then, when the boundary X between a light and a dark area-or, in any event, between areas emitting light of different qualities-passes across the detectors, then irregularity between the outputs of the detectors will occur. From the pattern of this irregularity, the orientation and the offset direction of the outline between the areas can be determined. Reference should be had to the above-identified sources for the details. For example, in Fig. 2 the hatched portion indicates a darker area, and the unhatched portion a lighter area.The detectors A, B, C, and D are emitting their maximum signals, the detectors F and G are emitting rather low signals, and the detectors E and H are emitting signals of intermediate intensity. Various different combinations of the output signals A-H are worked out on an ongoing basis, and various different comparisons are made therebetween, and the orientation of the boundary X, and its position, are determined on the basis of these comparisons and combinations. The position of the optical head. 16 is controlled by controlling the motors 9 and 14, so as to keep the boundary X passing underthe center of the detector, as near as prac ticalele. Thereby exact tracing of the outline X can be ensured. And, in synchronism with the tracing of the outline of the original, the heat pen 20 is moved across the film 21 placed on the glass 2, so as to cut a mask.

This system is automatic and simple. However, it has two severe disadvantages, which in practice render it of limited value for actual pictures. These may be summarized as follows, referring to Figs. 3-6 of the accompanying drawings: 1) When the outline X to be followed is curved sharply, errors in the following become serious.

Figs. 3a and 3b show two outlines X to be followed, one of which has a sharp right-angle bend X' in it, and the other of which has a gentle bend X" in it. The center of the detectorZ is assumed to be moving upwards in the figures. In these two cases, the output signals of the detectors are substantially the same, and hence they cannot be discriminated between, at least initially. The effect of this is that, when the boundary X to be followed is curved as in Fig. 4, the detector instead follows a curvaceous path, as shown by "x", cutting off the corner. In fact, if the curvature of any part of the boundary is greater than the size of the detector, no substantial problem occurs, but if the boundary's radius of curvature becomes smaller than the radius of the detector, inaccuracies appear.

A way to escape this inconvenience would be to make the diameter of the detector smaller. This cannot be taken very far, however, in view of maintaining precision of control of the position of the detector.

Another way would be to enlarge the image formed before impinging it on the detector. Again, this results in problems of control.

2) The second defect is more fundamental. If the area whose outline is to be followed is composed of two or more areas of dissimilar light values, then there is a danger that, when the optical head reaches the division between two of those areas, it may take the wrong path. Fig. 5 shows an example of this. The rectangle JKLQ is very light; the rectangle MNOP is intermediate in density; and the background is dark.

It is desired to trace the outline, starting at J, of JKLMNOPQJ. However, depending on the densities of the light, intermediate, and dark areas, there is a danger that after the detector has traced the line JKLM, it may err, and pursue the line MP rather than the line MN, as it should.

Another example of this is shown in Figure 6, in which a hexagonal shaped area P1P2PsP4P5P6 ought to be traced out. However, this area is composed of three areas of different light values: P1P2WP6, P2P3P4W, and P4P5P6W. (The variation in light values is not shown on the drawing; and the circles around the points P126 are irrelevant to the present discussion). It is desired to trace around the outside of the hexagon; but there is a danger that the optical head will err at one of the points P2, P4, or P6, and wander along to the point W.

Another method of cutting this mask that might be tried would be to subject the whole image to encoding on a coordinate system, and control the movement of the tracing of the outline by digital computer. However, if for example the contour of Figure 6 isto be so traced, because the sides are curved, very complicated programming and complex calculations are required, and this is not practicable.

Although this problem has been discussed in relation to cutting a mask for photo-printing, it is wider in its scope, and relates to automatic tracing of outlines in general, which is done in many different kinds of machine.

Referring particularly to Figures 6 and 7, which respectively show a pattern R to be traced and a flowchart of the method, which is an embodiment of the present invention, for tracing it, the method of the present invention, which is a new method of operating the machine of Figure 1, which has detectors as in Figure 2 will be explained.

First, the pattern to be traced is scrutinized, and a set of points which are likely to give trouble, according to the problems (1) and (2) explained above, are selected. These will hereinafter be called checkpoints. Hopefully, all the troublesome points will be included in this set. In the example shown in Fig. 6, the checkpoints P1, P3, and P5 have been selected according to problem (1), because at these points the outline to be traced bends sharply, and the checkpoints P2, P4, and P6 have been selected according to problem (2), because at them subboundaries within the main area to be outlined impinge on the boundary to be traced, and there is a danger that automatic tracing might wander down these subboundaries by mistake, thus producing an incorrect mask.As a matter of fact, in the shown example, these checkpoints P2, P4, and P6 would also give trouble because of problem (1), but this is not always necessarily the case.

These checkpoints are notionally arranged in order, with the first one repeated at the end, as P1P2P3P4P5P6P1. Further, a small region around each checkpoint is selected. In the shown embodiment of the method, these areas are circles S. However, this is not strictly necessary, although it is more convenient in practice.

In the present embodiment of the method, the drive motors of the machine of Fig. 1 are connected to a control means of a per known sort, which includes a digital computer, AID and D/A converters, and the like, and also sensing means not shown which detect the position of the carriage 15 relative to the glass 2 are connected to this control means.

The original picture 18 is fixed to the glass 2 under the optical head 16, and the film 21 of the sort explained above is fixed to the glass 2 under the heat pen 20. The checkpoints are mentally selected by the operator, and the optical head is moved to them in order, by manually controlling the motors 9 and 14.

At each checkpoint, a signal is sent by the operator to the control means by a button or the like, and this causes the control means to memorize the location of the checkpoints. This is of course all done with the pen up. The flowchart (Fig. 7) shows this process as steps 31-38. In this case, N = 6, and seven check points are established, the last being the same as the first, in order around the outline.

Exact accuracy in the positioning of the optical head overthe checkpoints in order is not required.

However, it should at least be within the limits of the circle S around the checkpoints.

This operation could be conducted in a different way, for instance by a digitizing table, in which case the steps 31-38 in Fig. 7 may be omitted.

Also, if it is not implicit in the operation of the control program, the limits of the circles S around the points P are set in the control means.

Next, utilizing these checkpoints, the picture out line, or boundary, is mostly trace out on the original picture, and thereby a mask is almost completely cut out of the film.

This is performed as follows. Starting with the first and second checkpoints, the head is moved to the first checkpoint, say P1 as seen in Fig. 6, and then it is moved in a straight line towards the second check point, or approximately in the direction of the sec ondcheckpoint, in any case, until it comes to the edge of the small region around the first checkpoint, in this case, the circle S. This is performed with the heat pen up, so that the film is not cut. Thus the optical-head has attained the point Pm in Fig. 6. This of course may be done by calculating the coordinates of Pm. Next, it is determined whether by now the optical head has reached the small region (circle S) corresponding to the next checkpoint.In general it will not have done so, but if the first and second checkpoints are very close together, then exceptionally their circles will overlap. In this case, no cutting at all of film is done between the first and second checkpoints, and this is left to be done by hand later.

This will happen in the case of a very wiggly outline.

In this case, control passes to the stage, to be described later, when the next pair of checkpoints are dealt with. If, however, the regions of the first and the second checkpoints do not overlap, then the operation in the usual above-describedwell known way of the photoelectric elements of the optical head is allowed to control the motion of the optical head.

This will move the head from the point Pm to the point where the circle S for point P1 intersects the arc Pup2. 19ext, the-pen is lowered, so that movement of it will cut the film. Next, the optical head is again allowed to follow the boundary in the abovedescribed known normal manner, under the usual control by the-control means according to the signals from the photoelectric elements, so that it follows the boundary towards P2. This is repeated, checking all the while thatthe center of the optical head is not within the small region around the second checkpoint.If and when it is, the pen is lifted so that it cuts no longer, and then, if the tracing has not yet reached back to the first checkpoint of all (in which case the process is stopped), then the second and third checkpoints are used instead to the first and the, second checkpoints, and the cycle- is repeated so as to cut most of the boundary between the second and third checkpoints; and so on. This process is shown in the flowchart as steps 39-52.

Thereby, it is easily seen that all the mask is cut out,-exceptthe parts of the boundary which fall in the small regions or circles S around the checkpoints. These delicate areas now have to be cut by hand. To do this, the picture and the film are removed from-the machine and superimposed, and then with a fine cutter the remaining parts ofthefilm are cut. Thereby a finished mask is obtained.

By ensuring that every part of the boundary which is of a very tight radius of curvature is covered by a checkpoint-or rather by its region or circle-and by ensuring that every junction between the main boundary and any subboundary which could confuse the tracing operation is likewise included within the small region of a checkpoint, the operator of the machine may ensure that its automatic operation is used to the fullest, and that no mistakes are made. It is to be noted that these sorts of difficult point are in factvery easily cut by hand on the mask, as they involve either sharp angles or multi-way junctions of lines, and usually do not require the person doing the cutting to follow sweeping curves smoothly, but to make angles, which is very easy, comparatively.

The extra work involved in cutting the checkpoints out on the mask is relatively unimportant, as com pared with the difficulty of cutting a complete mask.

The present invention is not only, in its broadest sense, applicable to cutting a mask, but can be applied to all sorts of path following around an outline in a picture. It is not necessary to the present invention that the checkpoints should later be treated by hand; in a more sophisticated embodiment that the one shown they might be treated mechani- cally in some way. Further, the method of following the outline, in the parts of it which are traced, which in the shown embodiment is by the optical detector and the photoelectric elements, is not crucial to the present invention Therefore, although the present invention has been shown and described with reference to a preferred embodiment thereof, it should not be considered as limited to that, however, or mere and simple generalizations, or other detailed embodiments. Yet further alterations could be made to the form and the detail of any particular embodiment, without departing from the scope of the present invention, which it is therefore desired should be determined solely by the accompanying claims.

Claims (9)

1. A method of tracing an outline being a boundary of different light values within a picture, which comprises: (a) selecting points on the boundary as checkpoints, and selecting small regions around them; (b) selecting one point as source point and a second point to be destination point; (c) positioningatracing head at the source point, moving it in an approximately straight line towards the destination point until it reaches the edge of the small region around the source, and allowing the tracing head to move to the intersection of the boundary and edge of the small regions; (d) moving tEle tracing head from the said intersection along the boundary towards the destination point until it reaches the edge of the small region around the destination point; and (e) repeatirs steps (c) and (d) with the destination point substituted for the source point and the next successive checkpoint as the destination point until all the boundary has been traced.

2. A method as claimed in claim 1 in which tracing is not carried out while the tracing head is within one of the regions around a checkpoint.

3. A method according to claim 1 or 2, wherein the tracing head is the centre of an optical detector, and in steps (c) and (d) the tracing head is aligned on and maintained on the boundary by being automati cally controlled according to signals sent out from the optical detector.

4. A method according to claim 3, wherein the tracing head comprises an arrangement of a plural ity of photodetectors arranged around a tracing point.

5. A method according to any of claims 1 to 4, wherein steps (b)-(e) are performed automatically, the selection of regions is implicit in the automatic performance, and in step (a) the selection of checkpoints is performed manually by moving the tracing head to the checkpoints in turn and signalling when it reaches each of the checkpoints.

6. A method according to claims 1 to 5, wherein a marking point is moved on a recording medium together with the tracing head, and is activated to leave a trace on the recording medium when the tracing head is tracing.

7. A method according to claim 6, wherein the recording medium is a mask film, and the marking point is a cutter.

8. A method according to claim 1 substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

9. A method for tracing part of an outline in an original picture which is a boundary with different light values on the different sides of the boundary, with a tracing point, comprising the steps, in the specified order, of: (a) selecting a plurality of checkpoints on the boundary, in order according to their order around the boundary, the last being the same as the first, and selecting small regions around them; (b) setting a source point to be the first checkpoint, and a destination point to be the second checkpoint;; (c) positioning the tracing point at the source point, and moving it, without tracing, to the first intersection of the boundary and the edge of the small region around the source point, the first intersection being the closest to the second intersection of the edge of the small region around the source point and an approximately straight line from the source point to the destination point, provided that the first intersection is not within the small region around the destination point; (d) moving the tracing point from the first intersection along the boundary in a direction generally awayfrom the source point and generallytowards the destination point, while tracing until the edge of the small region around the destination point, provided that the first intersection is not within the small region around the destination point; ; (e) if there is no checkpoint unused as a destination point, stopping; and otherwise (f) repeating the steps from step (c) inclusive on, with the destination point substituted for the source point, and the next checkpoint substituted for the destination point.