Patent GB-1375991-A (GB)
Title: Apparatus for thinning characters
IPC: G06V30/20
Filed: 1971-11-09
Abstract: 1375991 Character recognition systems PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 9 Nov 1971 [12 Nov 1970] 51965/71 Heading G4R A character to be machine read is imaged on a two dimensional array of points and each image point is inspected in turn to determine whether or not it is redundant. Redundant image points are converted to background points, the resulting image being a thinned representation of the character. The thinning operation takes place in two successive modes each consisting of at least one cycle. The character, which may be handwritten, is scanned by a flying spot scanner or other means, the signals from the scanner passing to a matrix store which may comprise a number of shift registers and in which the character image is simulated as an array of signals, e.g. as the numeral 4 in Figure 2. After smoothing the image and filling voids by known means thinning takes place according to the first mode. In mode 1 each image point is inspected to determine whether or not it is an edge point which can be removed without causing a gap in the image. To do this the image undergoes processing in a first sub-cycle logic, Fig. 10 (not shown), in which each image point in turn is notionally placed at the centre of a 3 x 3 matrix of points. Four tests take place in each of which the number of image points in a respective edge row or column of the matrix is determined. If in any one of these tests there are less than two image points and if, in the remaining rows or columns of the matrix, there are more than 3 image points the centre image point is marked, by associating with it further data to show it is an edge point of the image. The marked points are shown as O or X in Fig. 4. In a second sub-cycle each marked point is examined to determine whether its removal would create a gap in the image. A logic circuit (Y4-Y15, Fig. 12, not shown) examines a marked point with respect to the 8 surrounding points of a 3 x 3 matrix. Removal takes place of those points marked 0 in Fig. 4, whereas those markod X are retained, assuming scanning of the image from left to right and top to bottom. A marked point is removed or reinstated prior to examination of the next marked point. If less than a certain number of marked bits are removed in the second sub-cycle the image is now processed according to a second mode, but if more than that number has been removed the image is again processed according to the first mode. In the example shown the image points remaining after the end of the second sub-cycle (points A and X in Fig. 4) are processed again according to the mode 1 sub-cycles. In the second first sub-cycle image points O and X, Fig. 5, are marked as edge points. In the second sub-cycle those marked O are removed and those marked X are retained. When considering whether a marked point is to be removed or retained in any second sub-cycle special consideration is given to any 2 x 2 block of marked points which has no image points at the 5 points contiguous to two adjoining sides of the block. Such a block is shown in the broken line square in Fig. 5. If special consideration were not given all the marked image points in the block would be removed, resulting in undue erosion of the character stroke element. Accordingly, when the top left point of the block is considered the marking of the top right point is invalidated so that it will not be tested to see if its removal would cause a gap. when the latter test is performed on the remaining markedbits the two left hand points of the block are removed. The operation now switches to mode 2 in which all the remaining points, A and X in Fig. 5, are treated as though they were marked points. The logic circuit (Y4-Y15) which was used in the second sub-cycle of the first mode now results in the removal of the boxed point A, which is redundant since its removal does not cause a gap. In addition a further part of the circuit (Y16-Y18) is now operative to prevent removal of end points whose removal, while not causing a gap to appear, would cause undue erosion, or even total removal, of stroke elements of the thinned image. This is done by preventing removal of an image point if it has only one neighbouring image point in the surrounding eight points.