Image reading apparatus

Abstract: An image reading apparatus, for converting an image on a document having a white/black text area into a display signal (S4), includes an image reader that scans pixels forming the image to produce a read signal (S1/S22) having a level corresponding to the brightness density of each of the scanned pixels. The read signal (S22) is compared (26) with a dither matrix data signal (S23) and converted into a dither converted signal (S24/S11). The read signal (S1) is also compared (41) with a threshold value and converted into a binary signal (S31/S12) having a first level representing a white pixel and a second level representing a black pixel. A discrimination circuit (42) receives the binary signal for producing a detection signal (S40) when the white/black text area is scanned. Upon occurrence of the detection signal (S40), the dither converted signal (S11) is mixed with the binary signal (S12) to form the display signal (S4).

Description: This invention relates to image reading apparatus. More particularly, but not exclusively, the invention relates to image reading apparatus suitable for use with facsimiles, copiers, image scanners or the like. Pseudo halftone methods have been employed to reproduce halftone gradations by changing the density of black or white pixels (picture elements). A typical pseudo halftone method is suggested by T.H. Morrin in his paper entitled "A Black-White Representation of a Gray-Scale Picture" published February, 1974, in IEEE TRANSACTIONS ON COMPUTERS, C-23 at pp. 184-186. Another pseudo halftone method is suggested by B.E. Bayer in his paper entitled "An Optimum Method for Two-Level Rendition of Continuous-Tone Pictures" published in 1973, in ICC CONF RECOND at pp. 2611-2615. These pseudo halftone methods, which convert a read image into a display signal using one bit to represent each pixel, permit the use of a simple signal processing technique and their application to printers, facsimiles or the liked displaying white/black images. Although such previously proposed pseudo halftone methods have the capacity for good tone reproduction characteristics, they exhibit poor resolution in some cases, particularly in line-copy reproduction, as described later in greater detail. US Patent US-A-4 495 522 discloses an image reading apparatus for converting an image on a document into a display signal, the image having a first or halftone area having a relatively wide range of density values and a second or line area having a relatively limited range of density values, the apparatus comprising: an image reader for scanning pixels forming the image to produce a read signal having a level corresponding to the brightness density of each of the scanned pixels; means for generating a dither matrix data signal; means for comparing the read signal with the dither matrix data signal to produce a dither converted signal; a binary converter for comparing the read signal with a threshold level to produce a binary converted signal having a first level representing a white pixel and a second level representing a black pixel; detection means for indicating when the first or halftone area is scanned; and output means which is: responsive to the indication that the first or halftone area is scanned to cause output of the dither converted signal as the display signal; and capable of causing output of the binary converted signal as the display signal. More specifically, the apparatus of US-A-4 495 522 divides the image into a predetermined array of blocks and the detection means is responsive to the read signal to decide whether each block comprises a "line picture" (a picture made up of fine lines) or a "photograph" (a picture having a wide range of density regions). The read signal is applied to the binary converter and the output means is operative to supply either the dither matrix data signal or a reference voltage to the binary converter (which acts also as the comparing means), whereby the read signal is converted into a binary signal either according to the dither matrix data signal or without using the dither matrix data signal, depending upon whether it has been detected that the relevant block comprises a photograph or a line picture. US-A-4 495 522 mentions the possibility of removing background data from the read signal. According to the present invention there is provided an image reading apparatus for converting an image on a document into a display signal, the image having a first or halftone area having a relatively wide range of density values and a second or line area having a relatively limited range of density values, the apparatus comprising: an image reader for scanning pixels forming the image to produce a read signal having a level corresponding to the brightness density of each of the scanned pixels; means for generating a dither matrix data signal; means for comparing the read signal with the dither matrix data signal to produce a dither converted signal; a binary converter for comparing the read signal with a threshold level to produce a binary converted signal having a first level representing a white pixel and a second level representing a black pixel; detection means for indicating when the first or halftone area is scanned; and output means which is: responsive to the indication that the first or halftone area is scanned to cause output of the dither converted signal as the display signal; and capable of causing output of the binary converted signal as the display signal; characterised in that: the detection means is responsive to the binary converted signal to produce any one of: a first detection indication when the first or halftone area is scanned; a second detection indication when the second or line area is scanned; and a third detection indication when a background area of the image, namely an area over which the density values are substantially invariant, is scanned; and the output means comprises an AND circuit and: is responsive to the first detection indication to output the dither converted signal as the display signal; is responsive to the second detection indication to mix the dither converted signal and the binary converted signal to form the display signal by supplying the dither converted signal and the binary converted signal to respective inputs of the AND circuit; and is responsive to the third detection indication to output the binary converted signal as the display signal. An embodiment of the invention described hereinbelow provides an improved image reading apparatus which can provide good resolution as well as good halftone reproduction characteristics in line-copy reproduction. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which like references identify like elements throughout the several figures, and in which: Figure 1 is a schematic block diagram of a previously proposed image reading apparatus employing an organisational dither method; Figures 2A, 2B and 2C are diagrams used in explaining a dither conversion process carried out in the apparatus of Figure 1; Figure 3 is a diagram showing an example of an image to be read; Figures 4A and 4B are diagrams showing two different types of dither matrix, namely a concentrated dither matrix and a distributed dither matrix; Figure 5A is a diagram showing a display image produced with the use of the concentrated dither matrix of Figure 4A; Figure 5B is a diagram showing a display image produced with the use of the distributed dither matrix of Figure 4B; Figure 6 is a schematic block diagram of an image reading apparatus embodying the invention; Figure 7 is a circuit diagram showing the detailed arrangement of the apparatus of Figure 6; Figures 8(A) to 8(E) are diagrams used in explaining the operation of a floating binary converter of the apparatus of Figure 6; Figure 9 is a diagram used in explaining the operation of a line data holding circuit of the apparatus of Figure 6; Figure 10 is a diagram showing an example of an image to be read; Figure 11 is a graph showing changes in a read image signal when a halftone area is scanned; Figure 12 is a diagram showing changes in a binary signal when the halftone area is scanned; Figure 13 is a graph showing changes in the read image signal when a white/black text area is scanned; and Figure 14 is a diagram showing changes in the binary signal when the white/black text area is scanned. Before describing an image reading apparatus according to a preferred embodiment of the invention, a previously proposed image reading apparatus will be described briefly with reference to Figure 1 in order to specifically point out the difficulties associated therewith. Referring to Figure 1, the previously proposed image reading apparatus, which can employ an organisational dither method, includes an image scanner 1 by which documents are entered into the system. The scanner 1 comprises a luminous source 2 such as a fluorescent lamp. A light beam emitted from the source 2 is reflected from an original document 3 placed face down on the image scanner 1. The reflected light beam forms a luminous read image signal L1 which is reflected by mirrors 4 and 5. The reflected signal L1 travels through a condenser lens 6 to a photoelectric conversion element 7 such as a charge coupled device. The element 7 converts the signal L1 into an electrical read image signal S1 corresponding to one line segment of picture elements (pixels). The signal S1 is fed to an analog-to-digital (A/D) converter 8 which converts it into a corresponding digital read image signal S2 having 4 bits per one pixel. For example, the digital read image signal S2 may include read image data IM0 of a unit matrix having 4 x 4 pixels, as shown in Figure 2A. The digital read image signal S2 is fed from the A/D converter 8 to one input of a comparator 9 having another input coupled to receive a dither matrix data signal S3 produced by a dither matrix data generator 10. The dither matrix data signal S3 includes dither matrix data IM1 having 16 distributed grey-level (brightness density) threshold values "0" to "15" assigned for the respective 4 x 4 pixels of a unit matrix, as shown in Figure 2B. The comparator 9 compares the read image data IM0 with the dither matrix data IM1 for each of the 4 x 4 pixels. The comparator 9 produces a dither converted display signal S4. The signal S4 is at a high level "H", indicating a white pixel, when the pixel brightness density value included in the read image data IM0 is equal to or greater than the corresponding brightness density threshold value distributed in the dither matrix data IM1, and is at a low level "L", indicating a black pixel, when the pixel brightness density value is less than the corresponding brightness density threshold value. Figure 2C illustrates a black/white pattern IM2 indicated by the dither converted display signal S4 produced as a result of comparisons made in the comparator 9. The dither converted display signal S4 is fed to an automatic feed unit 11 which produces an output display signal S5 having one bit per one pixel. Although the previously proposed organisational dither method is satisfactory for applications where good halftone reproduction characteristics are essential, it reproduces images with poor resolution, particularly in line-copy reproduction. This stems mainly from the fact that the dither matrix data have brightness density threshold values which are different even for pixels having the same brightness density so that they are not detected as the same colour (white or black). Accordingly, the pattern of the read image data IM0 is not reproduced at the corresponding position on the black/white pattern IM2 with high fidelity. Secondly, the manner in which the resolution is reduced is dependent on the manner in which the brightness density threshold values are distributed in the dither matrix data IM1 since a change in the manner where the brightness density threshold values are distributed in the dither matrix data causes a change of the disposition of the white and black pixels in the white/black pattern IM2. Figure 5A shows a converted pattern PT21 produced on a display image IM21 when a cross pattern PT1 on a read image IM01 (Figure 3) is processed according to an organisational dither method employing a concentrated dither matrix IM11, as shown in Figure 4A, where similar brightness density threshold values are concentrated on the dither matrix. The concentrated dither matrix IM11 is arranged to have unit areas ARA each corresponding to 8 x 8 pixels. The unit area is divided into four segments ARA11, ARA12, ARA21 and ARA22 each corresponding to 4 x 4 pixels. The diagonally adjacent segments ARA12 and ARA21 have smaller brightness density threshold values "0" to "31". A pixel having a brightness density value equal to or less than the brightness density threshold value "31" is determined to be a black pixel. The diagonally adjacent segments ARA11 and ARA22 have greater brightness density threshold values "32" to "63". A pixel having a brightness density value equal to or greater than the brightness density threshold value "32" is determined to be a white pixel. With the use of such a concentrated dither matrix IM11 as shown in Figure 4A, the resolution power of the display image is deteriorated to a great extent, although viewers may feel that there are smooth brightness density variations on the display image. This will be described in greater detail. While the pixels which form the cross pattern PT1 on the read image IM01 of Figure 3 have the same brightness density level of 32, only part of the pixels are converted into black pixels on the display image IM21. Accordingly, the pattern PT21 on the display image IM21 is broken. This results in a great extent of deterioration of the resolution power. Figure 5B shows a converted pattern PT22 produced on a display image IM22 when the cross (crossing line) pattern PT1 on the read image IM01 (Figure 3) is processed according to an organisational dither method employing a distributed dither matrix, as shown in Figure 4B, where small and great brightness density threshold values are distributed uniformly over the dither matrix. The distributed dither matrix IM12 is arranged to have unit areas ARA each corresponding to 8 x 8 pixels. Each of the unit areas has a lower brightness values and has a greater brightness threshold value among smaller brightness threshold values. With the use of such a distributed dither matrix IM12 as shown in Figure 4B, only part of the pixels, which form the cross pattern PT1 on the read image IM01 of Figure 3, are converted into black pixels arranged alternately on the display image IM22, as shown in Figure 5B, causing resolution power deterioration. Since the converted pattern PT22 is similar to the pattern PT1 on the read image IM01, the extent of deterioration of the resolution power is smaller than is the case when use is made of the concentrated dither matrix of Figure 4A. It can be seen from a comparison of Figures 5A and 5B that the resolution power with which the image is reproduced is dependent greatly on the arrangement of the dither matrix used in the organisational dither method. As described above, the organisational dither method is satisfactory in reading halftone pictures or the like where tone changes are small over a wide range; however, it cannot provide good resolution in line-copy reproduction (e.g., text matter, drawings, etc.). Figure 6 is a schematic block diagram of an image reading apparatus 20 constituting the preferred embodiment of the invention. The image reading apparatus 20 includes a dither conversion unit COMP1, a binary conversion unit COMP2, a background processing unit COMP3, and a display signal output unit COMP4. The read image signal S1 produced by the photoelectric conversion element 7 has a level corresponding to the brightness density of each of the scanned pixels. The dither conversion unit COMP1, which is coupled to receive the read image signal S1, employs a dither matrix for converting the read image signal S1 into a dither converted signal S11 in substantially the same manner as described with reference to Figure 1. The signal S11 is fed to the display signal output unit COMP4. The binary conversion unit COMP2, which is coupled to receive the read image signal S1, converts the signal S1 into a binary converted signal S12. The signal S12 has a high level representing a white pixel when the read image signal S1 has a level equal to or greater than a threshold level set in the binary conversion unit COMP2 and a low level representing a black pixel when the signal S1 has a level smaller than the threshold level. The binary signal S12 is fed to the display signal output unit COMP4. The binary conversion unit COMP2 includes a discrimination circuit for making a determination, from the signal S12, as to whether the image scanner 1 is scanning a halftone area or a white/black text area. The discrimination circuit causes the display signal output unit COMP4 to pass the dither converted signal S11 to the automatic feed unit 11 when the discrimination circuit detects that the image scanner 1 is scanning a halftone area. The display signal output unit COMP4 mixes the binary converted signal S12 with the dither converted signal S11 and the resulting signal is fed to the automatic feed unit 11 when the discrimination circuit detects that the image scanner 1 is scanning a white/black text area. In this embodiment, the background processing unit COMP3 produces a high-level white background detection signal S13 when the read image signal S1 has a level equal to or greater than the brightness density of a white background. The white background detection signal S13 is fed to the dither conversion unit COMP1 for the purpose of providing sharper white-background reproduction characteristics. The background processing unit COMP3 uses the white background detection signal S13 to detect the fact that the image scanner 1 is scanning a white background. The background processing unit COMP3 produces a background detection signal S14 which is supplied to the binary conversion unit COMP2 which thereby causes the display signal output unit COMP4 to produce the binary converted signal S12 when the image scanner 1 is scanning a white background. The background processing unit COMP3 also produces a low-level black background detection signal S15 when the read image signal S1 has a level equal to or smaller than the brightness density of a black background. The black background detection signal S13 is fed to the dither conversion unit COMP1 for the purpose of providing sharper black-background reproduction characteristics. Referring to Figure 7, the dither conversion unit COMP1 includes an A/D converter 24 for converting the read image signal S1 into 8-bit read pixel data S21 which represent the brightness density of the read pixel. The read pixel data S21 are stored in a buffer 25. The reference S22 designates read pixel data read from the buffer 25. The read pixel data S22 are transferred to input terminals Q0 to Q7 of a comparator circuit 26. The comparator circuit 26 has input terminals P0 to P7 coupled to receive 8-bit dither matrix data S23 produced by a dither matrix data generator 27. The dither matrix data S23 represent the brightness density threshold value. The comparator circuit 26 compares the read pixel data S22 with the dither matrix data S23 and produces a dither converted signal S24. The dither converted signal S24 has a low level representing a black pixel when the read pixel data S22 are smaller than the dither matrix data S23 and a high level representing a white pixel when the read pixel data S22 are equal to or greater than the dither matrix data S23. The dither converted signal S24 is applied to one input of an AND circuit 28 having another input from a background black comparator circuit 65 (described below). The AND circuit 28 has an output connected to one input of an OR circuit 29 having another input coupled to a background white comparator circuit 63 (described below). The OR circuit 29 has an output connected to a timing adjustment circuit 30. The circuit 30, which may comprise a delay circuit, provides a predetermined time delay to the signal outputted from the OR circuit 29 and produces a dither converted display signal S22 which is supplied to the display signal output unit COMP4, more specifically to one input of an OR circuit 31 thereof. The time delay corresponds to the time during which the binary conversion unit COMP2 processes a predetermined number (in this case 10) of pixels to provide coincidence of the time at which the dither converted display signal S11 arrives at the display signal output unit COMP4 and the time at which the binary converted display signal S12 is applied from the binary conversion unit COMP2 to the display signal output unit COMP4. For this purpose, the timing adjustment circuit 30 operates on clock pulses CK1 fed thereto from a clock pulse generator 33. The binary conversion unit COMP2 includes a floating binary converter 41 having a floating threshold value for converting the read image signal S1 into a binary converted signal S31.The binary converted signal S31 has a high level representing a white pixel when the read image signal S1 is equal to or greater than the floating threshold value and a low level representing a black pixel when the read image signal S1 is less than the floating threshold value. The floating threshold value varies according to the level of the read image signal S1. This will now be described in greater detail with reference to Figures 8(A) to 8(E). When the photoelectric conversion element 7 reads a document IM0 containing a black line X2 dividing a wide white area X1 and a white line X4 dividing a wide black area X3, as shown in Figure 8(A), it produces a read image signal S1 as shown by a bold curve in Figure 8(B). The read image signal S1 has signal portions S1 X1 produced for the white area X1, a signal portion S1 X2 produced for the black line X2, signal portions S1 X3 produced for the black area X3, and a signal portion S1 X4 produced for the white line X4. It can be seen from Figure 8(B) that the signal portion S1 X2 cannot decrease to the level of the signal portions S1 X3 produced for the black area X3 and the signal portion S1 X4 cannot increase to the level of the signal portions S1 X1 produced for the white area X1. If the binary converter 41 is arranged to have a fixed threshold value LV1 (Figure 8(B)) crossing the signal portions S1 X2 , the white line X4 cannot be reproduced, although the black line X1 can be reproduced, as shown in Figure 8(C). If the binary converter 41 is arranged to have a fixed threshold value LV2 (Figure 8(B)) which crosses the signal portion S1 X4 , the black line X1 cannot be reproduced although the white line X4 in the wide black portion X3 can be reproduced, as shown in Figure 8D. The present apparatus can eliminate these problems by arranging for the binary converter 41 to have a floating threshold value LV3 which changes between high and low levels according to the level of the read image signal S1, as shown in Figure 8(B). The floating threshold value LV3 has a high level crossing the signal portion S1 X2 when a wide white area is being processed and changes to a low level crossing the signal portion S1 X4 when a wide black area is being processed. Such a floating threshold value may be obtained, for example, by adding the brightness densities of a predetermined number of pixels positioned before and after the present pixel with appropriate weights. With the use of such a floating threshold value, it is possible to reproduce both the black line X2 dividing the wide white area X1 and the white line X4 dividing the wide black area X3, as shown in Figure 8(E). The binary conversion unit COMP2 also includes a successive pixel discrimination circuit 42 which includes a successive pixel data register 43 having an input coupled to the floating binary converter 41. The successive pixel data register 43 operates on clock pulses CK2 fed thereto from the clock pulse generator 33 to store binary pixel data of 10 successive pixels and produce 10-bit data D1-D10 which is supplied to a 10-input AND circuit 44 and also to a 10-input OR circuit 47. The 10-input AND circuit 44 produces an output signal S32 which has a high level representing the fact that all of the 10 successive pixels are white pixels only when all of the data D1-D10 are high. The output signal S32 is applied to an inverter 45 which inverts the signal S32 to produce an inverted output signal S33 which is applied to an output circuit 46. The 10-input OR circuit 47 produces an output signal S34 which has a low level representing the fact that all of the 10 successive pixels are black pixels only when all of the 10-bit data are low. The output signal S34 is applied directly to the output circuit 46 and also to an inverter 48 which inverts the signal S34 and produces an inverted signal S35. The binary converted signal S31 from the floating binary converter 41 is fed also to a line data holding circuit 50 which operates on clock pulses CK3 fed thereto from the clock pulse generator 33 for retaining data corresponding to three line segments of pixels. Assuming now that the line data holding circuit 50 receives binary pixel data LIN 0 (Figure 9) from the floating binary converter 41, it produces binary pixel data LIN 1 (Figure 9) one line before the received binary pixel data LIN 0, binary pixel data LIN 2 (Figure 9) two lines before the received pixel LIN 0, and binary pixel data LIN 3 (Figure 9) three lines before the received binary pixel data LIN 0. These binary pixel data signals LIN 1, LIN 2 and LIN 3 are fed to three inputs of a 4-input AND circuit 51 having another input coupled to receive a lock signal S37 fed thereto from a Q output terminal of a J-K flip-flop circuit 49 which constitutes a half-tone text portion detecting circuit. The 4-input AND circuit 51 produces an output signal which is applied to an inverter 52 which inverts it to produce a signal S36. The signal S36 is applied to J and K input terminals of the J-K flip-flop circuit 49. The signal S36 has a high level when at least one of the four signals is applied to the four inputs of the 4-input AND circuit 51. It is assumed now that the J-K flip-flop circuit 49 is in a cleared state, producing a low output at a Q output terminal thereof and a high output at its Q output terminal. When at least one of the three line data signals LIN 1, LIN 2 and LIN 3 has a low level representing a black pixel, the signal S36 changes to its high level. When, under this condition, the signal S35 changes from its low level to its high level representing the fact that all of the 10 successive pixels are white pixels, the J-K flip-flop circuit 49 is set to produce a high level signal at its Q output terminal and a low level signal at its Q output terminal. The low level signal S37 is applied from the J-K flip-flop circuit 49 to the 4-input AND circuit 51 to hold the signal S36 at its high level. As a result, the J-K flip-flop 49 is retained in its set condition until it is cleared by a background detection signal S14 fed to a clear terminal CLR thereof from the background processing unit COMP3. This is effective to prevent the J-K flip-flop circuit 49 from being set when the image scanner 1 scans 10 successive black pixels, as indicated by the reference K1 in Figure 9. The J-K flip-flop circuit 49 can be set when the scanned image contains four lines of 10 successive black pixels, as shown by the reference K2 in Figure 9. The J-K flip-flop circuit 49 produces a halftone area detection signal S38 which changes to its high level when it is set. The output unit 46 includes an OR circuit 53 having an input connected to the Q output terminal of the J-K flip-flop circuit 49 and another input connected to the 10th-bit output terminal of the successive pixel data register 43. The OR circuit 53 produces the binary converted display signal S12, which is supplied to the display signal output unit COMP4, more specifically to one input of an AND circuit 32 thereof, the AND circuit 32 having another input connected to an output of the OR circuit 31. The binary converted display signal S12 has the same logical level as the binary data D10 fed from the 10th-bit output terminal of the successive pixel data register 43 when the J-K flip-flop circuit 49 is cleared. The binary converted display signal S12 has a high level regardless of the binary data D10 when the J-K flip-flop circuit 49 is set. The output unit 46 also includes an AND circuit 54 having one input coupled to receive the successive white pixel detection signal S33 from the inverter 45 and another input coupled to receive the successive black pixel detection signal S34 from the OR circuit 47. The AND circuit 54 produces an output signal S39 which is supplied to an inverter 55 which inverts it to produce a white/black text area detection signal S40 which is supplied to one input of an AND circuit 56 having another input connected to the Q output terminal of the J-K flip-flop circuit 49. The AND circuit 56 produces a control signal S41 which is supplied to the display signal output unit COMP4, more specifically to another input of the OR circuit 31 thereof. The signal S39 has a high level when the successive 10 pixels contains white and black pixels; that is, when the successive white pixel detection signal S33 has a high level representing the fact that the 10 successive pixels include at least one black pixel and when the successive black pixel detection signal S34 has a high level representing the fact that the 10 successive pixels include at least one white pixel. Under this condition, the white/black text area detecting signal S40 has a low level representing the fact that the image scanner 1 is scanning a white/black text area, retaining the control signal S41 at its low level. The control signal S41 is at its high level when the image scanner 1 is scanning a background; that is, when the image scanned by the image scanner 1 contains neither a white/black text area nor a halftone area. The background processing unit COMP3 includes a background detecting circuit 61 which includes a successive pixel data register 62. The register 62, which may be a 10-bit shift register, operates in accordance with clock pulses CK4 fed thereto from the clock pulse generator 33 to store binary pixel data of successive 10 pixels fed thereto from the white background comparator circuit 63 and produces 10-bit data D1-D10 which is supplied to a 10-input AND circuit 67. The AND circuit 67 has an output coupled through an inverter 68 to the clear terminal CLR of the J-K flip-flop circuit 49. The white background comparator circuit 63 has input terminals Q0 to Q7 through which the read pixel data S22 are transferred to the white background comparator circuit 63 and input terminals P0 to P7 through which 8-bit white background data S51 are transferred thereto from a white background data holding circuit 64. The white background data S51 represent the brightness density of the white background obtained during a preparatory scanning operation made prior to a scanning operation to read the image IM01 from the document. The white background comparator circuit 63 compares the read pixel data S22 with the white background data S51 and produces a white background detection signal S13. The white background detection signal S13 has a high level representing that the read pixel has a brightness density equal to or greater than the white background only when the read pixel data S22 are equal to or greater than the white background data S51. The white background detection signal S13 is applied to the successive pixel data register 62 and also to the another input of the OR circuit 29. The background processing unit COMP3 also includes the black background comparator circuit 65, which has input terminals Q0 to Q7 through which the read pixel data S22 are transferred to the black background comparator circuit 65 and input terminals P0 to P7 through which 8-bit black background data S52 are transferred thereto from a black background data holding circuit 66. The black background data S52 represent the brightness density of the black background obtained during the preparatory scanning operation made prior to the scanning operation to read the image IM01 from the document. The black background comparator circuit 65 compares the read pixel data S22 with the black background data S52 and produces a black background detection signal S15. The black background detection signal S15 has a low level representing that the read pixel has a brightness density equal to or smaller than the black background only when the read pixel data S22 are equal to or smaller than the black background data S52. The black background detection signal S15 is applied to the another input of the AND circuit 28. The operation of the image reading apparatus will be described. It is now assumed that a document IM0 containing a halftone area IM012 and a white/black text area IM013 disposed on a background area IM011 is scanned along a scan line SCAN, as shown in Figure 10. The display signal S4 produced from the display signal output unit COMP4 is the dither converted display signal S11 when the halftone area IM012 is scanned, is the dither converted display signal S11 mixed with the binary converted display signal S12 when the white/black text area IM013 is scanned, and is the binary converted display signal S12 when the background area IM011 is scanned. The read image signal S1 is fed from the image scanner 1 to the A/D converter 24 and also to the floating binary converter 41. The A/D converter 24 converts the read image signal S1 into digital form having 8-bit brightness density data S23 for each pixel. The converted digital data S21 are transferred to and read from the buffer 25 to form the read pixel data signal S22. The floating binary converter 41 compares the read image signal S1 with a floating threshold level to make a determination whether the scanned pixel is a white or black pixel and produces a binary converted signal S31 having binary pixel data which represent the result of the determination. The binary converted signal S31 is fed to the successive pixel data register 43 which latches the new pixel data D10 along with old pixel data D9 to D1 for the successive 9 pixels scanned prior to the new pixel. The binary converted signal S31 is also applied to the line data holding circuit 50 which latches the new pixel data and produces old pixel data LIN 1, LIN 2 and LIN 3 for the old pixels one line, two lines and three lines before the new pixel. The old pixel data signals LIN 1, LIN 2 and LIN 3 are fed to the 4-input AND circuit 51. When the image scanner 1 is scanning the background area IM011 along a portion SCAN1 of the scan line SCAN under a normal condition where the read pixel data signal S22 is greater than the black background data S52 read from the black background data holding circuit 66 and is smaller than the white background data S51 read from the white background data holding circuit 64, the white background detection signal S13 has a low level and the black background detection signal S15 has a high level. The dither comparator circuit 26 compares the read pixel data signal S22 with the dither matrix data S23 read from the dither matrix data holding circuit 27 to produce a dither converted signal S24. The dither converted signal S24 has a high level when the read pixel data signal S22 is equal to or greater than the dither matrix data S23 and a low level when the read pixel data S22 are smaller than the dither matrix data S23. Under this condition, the dither converted signal S24 is transmitted through the AND circuit 28 and the OR circuit 29 to the timing adjustment circuit 30 which thereby produces a dither converted display signal S11 delayed in time relative to the dither converted signal S24, as described previously. The successive pixel discrimination circuit 42 detects the fact that the image scanner 1 is scanning the background area IM011 when all of the pixel data D1 to D10 have a high level representing a white pixel. The OR circuit 47 produces a high-level signal S34. The high-level signal S34 is applied to the inverter 48 which inverts it to form a low-level signal S35. The low-level signal S35 has no affect on the J-K flip-flop circuit 49. Accordingly, the J-K flip-flop circuit 49 is held in its clear state, producing a low-level signal S38 at its Q output terminal and a high-level signal S37 at its Q output terminal. The low-level signal S38 is applied to permit the high-level pixel data signal D10 to pass through the OR circuit 53 so as to form a high-level binary signal S12 which is applied to the AND circuit 32. The AND circuit 44 produces a high-level signal S32. The high-level signal S32 is applied to the inverter 45 which inverts it to form a low-level signal S33. The low-level signal S33 causes the AND circuit 54 to produce a low-level signal S39. The low-level signal S39 is applied to the inverter 55 which inverts it to a high-level signal S40 which is applied to the AND circuit 56. Since the signal S37 applied from the Q output terminal of the J-K flip-flop circuit 49 has a high-level, the AND circuit 56 produces a high-level control signal S41, causing the OR circuit 31 to block the dither converted signal S11 and the AND circuit 32 to pass the binary converted display signal S12 so as to form the display signal S4. When the image scanner 1 is scanning the halftone area IM012 along a portion SCAN2 of the scan line SCAN, a great change appears only unusually in the read image signal S1 produced by the photoelectric conversion element 7. For example, Figure 11 shows changes in the read image signal S1 produced when a halftone picture of a woman's face present on a white background is scanned by the image scanner 1 moving along a scan line crossing her nose. The image scanner 1 scans the halftone picture in the following order: a dark background image portion L2, a left hair image portion L3, a nose image portion L4, a left face image portion L5, a right face image portion L6, a right hair image portion L7, a shining hair image portion L8 and a dark background image portion L9. The read image signal S1 has a very high level only when the white background area L1 or L10 is scanned. Under this condition, the binary signal S31 produced by the floating binary converter 41 is a high-level signal when the white background area L1 is scanned, a low-level signal when the dark background image portion L2 or the left hair image portion L3 is scanned, a high-level signal when the nose image portion L4 or the left face image portion L5 is scanned, a low-level signal when the right face image portion L6 or the right hair image portions L7 is scanned, a signal changing repetitively between high and low levels when the shining right hair image portion L8 is scanned, a high-level signal when the dark background image portion L9 is scanned, and a high-level signal when the white background area L10 is scanned, as shown in Figure 12. It is to be noted that, when a halftone area is scanned, the binary converted signal S31 changes alternately between its high and low levels normally at intervals of time longer than the time required for the image scanner 1 to scan 10 or more pixels, as shown in Figure 12, where the references B1, B3, B5, ... BE designate the intervals during which the binary signal S31 has a low level and the reference W2 and W4 designate the intervals during which the binary signal S31 has a high level. Accordingly, it may be said that the image scanner 1 is scanning a halftone area when the binary converted signal S31 remains at its low level for a time period longer than the time required for the image scanner 1 to scan 10 or more successive pixels. When the image scanner 1 starts scanning the halftone area IM012 along the scan line portion SCAN2, the white background comparator circuit 63 produces a low-level white background detection signal S13 and the black background comparator circuit 65 produces a high-level white background detection signal S15. The dither comparator circuit 26 compares the read pixel data signal S22 with the dither matrix data S23 read from the dither matrix signal generator 27 and produces a dither converted signal S24. Under this condition, the dither converted signal S24 passes through the AND circuit 28 and the OR circuit 29 to the timing adjustment circuit 30, which thereby produces a dither converted display signal S11 delayed in time relative to the dither converted signal S24, as described previously. At a time when the successive pixel data register 43 accumulates 10 successive black pixel data, that is when all of the pixel data D1 to D10 have a low level representing a black pixel, the successive pixel discrimination circuit 42 detects the fact that the image scanner 1 is scanning the halftone area IM012. The signal S34 produced by the OR circuit 47 changes from its high level to its low level. The low-level signal S34 is applied to the inverter 48 which inverts it to form a high-level signal S35 which sets the J-K flip-flop circuit 49 since at least the present line data signal LIN 0 has a low level. Accordingly, the J-K flip-flop circuit 49 produces a high-level signal S38 at its Q output terminal and a low-level signal S37 at its Q output terminal. The high-level signal S38 causes the OR circuit 53 to block the low-level pixel data signal D10. The high-level signal S12 is applied from the OR circuit 53 to the AND circuit 32. The low-level signal S37 from the Q terminal of the J-K flip-flop circuit 49 causes the AND circuit 56 to produce a low-level control signal S41. The low-level control signal S41 permits the dither converted display signal S11 to pass through the OR circuit 31. Since the AND circuit 32 has a high-level input from the OR circuit 53, it permits the dither converted display signal S11 to pass through the AND circuit 32 so as to form the display signal S4. When the image scanner 1 starts scanning the light portion W2 (Figure 12) following the dark portion B1 (Figure 12), the white background comparator circuit 63 produces a low-level white background detection signal S13 and the black background comparator circuit 65 produces a high-level white background detection signal S15. The dither comparator circuit 26 compares the read pixel data signal S22 with the dither matrix data S23 read from the dither matrix signal generator 27 and produces a dither converted signal S24. Under this condition, the dither converted signal S24 passes through the AND circuit 28 and the OR circuit 29 to the timing adjustment circuit 30. The timing adjustment circuit 30 supplies a dither converted display signal S11 to the OR circuit 31. The dither converted display signal S11 is the same as the dither converted signal S24, except that it is delayed in time relative to the dither converted signal S24. At a time when the successive pixel data register 43 accumulates 10 successive white pixel data, that is when all of the pixel data D1 to D10 have a high level representing a white pixel, the signal S34 produced by the OR circuit 47 changes from its low level to its high level. The high-level signal S34 is applied to the inverter 48 which inverts it to form a low-level signal S35. The low-level signal S35 has no effect on the J-K flip-flop circuit 49. Accordingly, the J-K flip-flop circuit 49 remains in its set state, producing a high-level signal S38 at its Q output terminal and a low-level signal S37 at its Q output terminal. Under this condition, the binary data signal D10 is blocked by the OR circuit 53 and the dither converted display signal S11 passes through the OR circuit 31 and the AND circuit 32 so as to form the display signal S4. When the image scanner 1 starts scanning the dark portion B3 (Figure 12) following the light portion W2 (Figure 12), black pixel data are written into the successive pixel data register 43. At a time when the successive pixel data register 43 accumulates 10 successive black pixel data, that is when all of the pixel data D1 to D10 have a low level representing a black pixel, the signal S34 produced by a the OR circuit 47 changes from its high level to its low level. The low-level signal S34 is applied to the inverter 48 which inverts it to form a high-level signal S35. The high-level signal S35 triggers the J-K flip-flop circuit 49. Since the lock signal is a low-level signal, causing the AND circuit 51 to produce a low-level signal so that the J-K flip-flop circuit 49 has high-level inputs at its J and K input terminals, the J-K flip-flop circuit 49 remains in its set state. Under this condition, the binary data signal D10 is blocked by the OR circuit 53 and the dither converted display signal S11 passes through the OR circuit 31 and the AND circuit 32 so as to form the display signal S4. It should be apparent that the display signal S4 outputted from the display signal output unit COMP4 is the same as the dither converted display signal S11 when the image scanner 1 is scanning the halftone area IM012. When the image scanner 1 starts scanning the background area IM011 along the a portion SCAN3 of the scan line SCAN, the white background comparator circuit 63 produces a high-level white background detection signal S13 since the read pixel data S22 have a brightness density equal to the white background data S51 read from the white background data holding circuit 64. The high-level white background detection signal S13 is applied to the successive pixel data register 62. At a time when all of the data accumulated in the successive pixel data register 62 have a high level, the AND circuit 67 produces a high-level signal. This high-level signal is applied to the inverter 68 which inverts it to form a low-level background detection signal S14. The low-level signal S14 is applied to the clear terminal CLR of the J-K flip-flop circuit 49, causing the J-K flip-flop circuit 49 to change from its set state into its cleared state, and thus producing a low-level signal S38 at its Q output terminal and a high-level signal S37 at its Q output terminal. The low-level signal S38 is applied to the OR circuit 53 which thereby passes the pixel data signal D10 which forms a high-level binary converted display signal S12. The high-level binary converted display signal S12 is applied to the AND circuit 32. The high-level signal S37 is applied to the AND circuit 56. Under this condition, the successive pixel data register 43 has accumulated 10 successive white pixel data; that is, all of the pixel data D1 to D10 have a high level representing a white pixel. Accordingly, the AND circuit 44 produces a high-level signal S32. The high-level signal S32 is applied to the inverter 45 which inverts it to form a low-level signal S33. The low-level signal S33 causes the AND circuit 54 to produce a low-level signal S39. The low-level signal S39 is applied to the inverter 55 which inverts it to form a high-level signal 40. The high-level signal S40 is applied, along with the high-level signal S37, to the AND circuit 56, which thereby produces a high-level control signal S41. The high-level control signal S41 is applied to the OR circuit 31 which thereby blocks the dither converted display signal S11 and produces a high-level signal, causing the AND circuit 32 to pass the binary converted display signal S12 so as to form the display signal S4. When the image scanner 1 is scanning the white/black text area IM013 along a portion SCAN 4 of the scan line SCAN, great changes appear in the read image signal S1 produced by the photoelectric conversion element 7. For example, Figure 13 shows at (B) changes in the read image signal S1 produced when the image scanner 1 scans a white/black text area having different line elements SL1 to SL9 as shown in Figure 13 at (A). Accordingly, the binary converted signal S31 produced by the floating binary converter 41 changes alternately between high and low levels at intervals of time shorter than the time required for the image scanner 1 to scan 10 or more pixels, as shown in Figure 14, where the references B11, B13, B15, B17, ... B1E designate the intervals during which the binary converted signal S31 has a low level and the references W12, W14 and W16 designate the intervals during which the binary converted signal S31 has a high level. It may be said that the image scanner 1 is scanning a white/black text area when the binary converted signal S31 changes alternately between high and low levels at intervals of time shorter than the time required for the image scanner 1 to scan 10 or more pixels; that is, when the successive pixel data register 43 accumulates white and black pixel data. When the image scanner 1 is scanning the white/black text area IM013 along the scan line portion SCAN4, the OR circuit 47 produces a high-level signal S34. The high-level signal S34 is applied to the AND circuit 54 and also to the inverter 48, which inverts it to form a low-level signal S35. The low-level signal S35 has no effect on the J-K flip-flop circuit 49. Accordingly, the J-K flip-flop circuit 49 remains in its cleared state producing a low-level S38 at its Q output terminal and a high-level signal S37 at its Q output terminal. The low-level signal S38 is applied to the OR circuit 53 which thereby passes the new pixel data D10 to form the binary converted display signal S12. The binary converted display signal S12 is applied to the AND circuit 32. Under this condition, the AND circuit 44 produces a low-level signal S32. The low-level signal S32 is applied to the inverter 45, which inverts it to form a high-level signal S33. The high-level signal S33 is applied, along with the high-level signal S34, to the AND circuit 54 to cause it to produce a high-level signal S39. The high-level signal S39 is applied to the inverter 55 which inverts it to form a low-level signal S40. The low-level signal S40 is applied to the AND circuit 56 to cause it to produce a low-level control signal S41. The low-level control signal S41 is applied to the OR circuit 31 which thereby passes the dither converted display signal S11 to the AND circuit 32. Accordingly, the output unit COMP4 mixes the dither-converted and binary-converted display signals S11 and S12 to form the display signal S4. In this embodiment, the white background comparator circuit 63 produces a white background detection signal S13 as a result of comparison of the pixel data S22 with the white background data S51 read from the white background data holding circuit 64. The white background detection signal S13 is applied to the OR circuit 29. Accordingly, the dither converted signal S24, which is produced from the dither comparator circuit 26 as a result of comparison of the pixel data signal S22 with the dither matrix data signal S23 read from the dither matrix data holding circuit 27, is replaced by a high-level white background detection signal S13 when the pixel data S22 are equal to or greater than the white background data S51. This is effective to provide sharper white-background reproduction characteristics. In addition, the black background comparator circuit 65 produces a black background detection signal S15 as a result of comparison of the pixel data S22 with the black background data S52 read from the black background data holding circuit 66. The black background detection signal S15 is applied to the AND circuit 28. Accordingly, the dither converted signal S24 is replaced by a low-level black background detection signal S15 when the pixel data S22 is smaller than the black background data S52. This is effective to provide sharper black background reproduction characteristics. When the image scanner 1 starts scanning the background area IM011 along a portion SCAN5 of the scan line SCAN, the successive pixel data register 43 stores successive white pixel data while the J-K flip-flop circuit 49 remains in its cleared state. At a time when all of the pixel data accumulated in the successive pixel data register 43 have a high-level representing a white pixel, the signal S34 produced by the OR circuit 47 remains at its high level and the signal S32 produced by the AND circuit 44 changes to its high level. Under this condition, the J-K flip-flop circuit 49 remains in its cleared state, supplying a low-level signal S38 to the OR circuit 53 which thereby passes the pixel data D10 to form a binary signal S12. The binary signal S12 is applied to the AND circuit 32. The high-level signal S32 is applied to the inverter 45 which inverts it to form a low-level signal S33, causing the AND circuit 54 to produce a low-level signal S39. The low-level signal S39 is applied to the inverter 55 which inverts it to form a high-level signal S40. The high-level signal S40 is applied, along with the high-level signal from the Q output terminal of the J-K flip-flop circuit 49, to the AND circuit 56. Accordingly, the AND circuit 56 produces a high-level control signal S41. The high-level control signal S41 is applied to the OR circuit 31 which thereby blocks the dither converted display signal S11 and produces a high-level signal, causing the AND circuit 32 to pass the binary converted display signal 12 so as to form the display signal S4. It is, therefore, apparent from the foregoing that the above-described image processing apparatus can provide good halftone reproduction characteristics by producing the dither converted display signal S11 to form the display signal S4 when the image scanner 1 scans the halftone area IM012 on the document. In addition, the image processing apparatus mixes the dither converted display signal S11 with the binary converted display signal S12 to form the display signal S4 when the image scanner 1 scans the white/black text area IM013. The dither converted display signal S11 provides good halftone reproduction characteristics and the binary converted display signal S12 provides good resolution in line copy reproduction (e.g., text matter, drawings, etc.).

Claims: An image reading apparatus for converting an image on a document into a display signal (S4), the image having a first or halftone area (IM012) having a relatively wide range of density values and a second or line area (IM013) having a relatively limited range of density values, the apparatus comprising: an image reader (1) for scanning pixels forming the image to produce a read signal (S1/S22) having a level corresponding to the brightness density of each of the scanned pixels; means (27) for generating a dither matrix data signal; means (26) for comparing the read signal (S22) with the dither matrix data signal (S23) to produce a dither converted signal (S24/S11); a binary converter (41) for comparing the read signal (S1) with a threshold level to produce a binary converted signal (S31/S12) having a first level representing a white pixel and a second level representing a black pixel; detection means for indicating when the first or halftone area (IM012) is scanned; and output means (COMP 4) which is: responsive to the indication that the first or halftone area (IM012) is scanned to cause output of the dither converted signal (S11) as the display signal (S4); and capable of causing output of the binary converted signal (S12) as the display signal (S4); characterised in that: the detection means is responsive to the binary converted signal (S31) to produce any one of: a first detection indication (S38-HIGH, S41-LOW) when the first or halftone area (IM012) is scanned; a second detection indication (S38-LOW, S41-LOW) when the second or line area (IM013) is scanned; and a third detection indication (S38-HIGH, S41-HIGH) when a background area (IM011) of the image, namely an area over which the density values are substantially invariant, is scanned; and the output means (COMP 4) comprises an AND circuit (32) and: is responsive to the first detection indication (S38-HIGH, S41-LOW) to output the dither converted signal (S11) as the display signal (S4); is responsive to the second detection indication (S38-LOW, S41-LOW) to mix the dither converted signal (S11) and the binary converted signal (S12) to form the display signal (S4) by supplying the dither converted signal (S11) and the binary converted signal (S12) to respective inputs of the AND circuit (32); and is responsive to the third detection indication (S38-LOW, S41-HIGH) to output the binary converted signal (S12) as the display signal (S4). Apparatus according to claim 1, wherein the detection means is operative to produce a detection signal (S38) signifying that the first or halftone area (IM012) is scanned when the binary converted signal (S31) changes alternately between its first and second levels with the intervals of time between successive changes being longer than a time required for the image reader (1) to scan a predetermined number of successive pixels. Apparatus according to claim 2, wherein the detection means includes means (43) for storing the binary converted signal (S31) at predetermined time intervals to accumulate binary pixel data for the predetermined number of successive pixels, and means for producing the detection signal (S38) signifying scanning of the first or halftone area (IM012) when all of the accumulated binary pixel data represent white or black pixels. Apparatus according to claim 2, wherein the detection means is operative to produce a background detection signal (S41) when the read signal (S1) remains at a background level for a time longer than the time required for the image reader to scan the predetermined number of successive pixels, said first to third detection indications comprising respective combinations of production and non-production of the first-mentioned detection signal (S38) and the background detection signal (S41). Apparatus according to any one of the preceding claims, wherein the binary converter (41) includes means for varying the threshold level in accordance with the level of the read signal (S1). Apparatus according to any one of the preceding claims, which includes means (64) for producing a white background level (S51), means (63) for comparing the read signal (S22) with the white background level (S51) to produce a white background detection signal (S13) when the read signal is equal to or greater than the white background level, and means (29) for replacing the dither converted signal (S24) with the white background detection signal (S13). Apparatus according to any one of the preceding claims, which includes means (66) for generating a black background level (S52), means (65) for comparing the read signal (S22) with the black background level (S52) to produce a black background detection signal (S15) when the read signal is smaller than the black background detection signal, and means (28) for replacing the dither converted signal (S24) with the black background detection signal (S15).

Inventors: KIMURA KEN C O PATENTS DIVISIO, KIMURA, KEN C/O PATENTS DIVISION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1988
Main classification: H04N1
IPC codes: H04N1