Image forming apparatus

Abstract: The present invention provides an image forming apparatus for reproducing an image from an original, wherein a character portion and an intermediate contrast portion of an original is discriminated, different density converting tables are used to the character portion and the intermediate contrast portion. When the original includes both the character portion and the intermediate contrast portion, the density converting table for character portion is applied to the character portion and the density converting table for intermediate contrast portion is applied to the intermediate contrast portion to form the image.

Description: The present invention relates to an image forming apparatus such as an electrophotographic copying machine, facsimile, printer and the like, and more particularly, it relates to an image forming apparatus for reproducing an original having an image including a character portion and an intermediate contrast portion such as photograph, picture and the like. Conventionally, in a technical field wherein a multi-gradient image is read and a corresponding signal is outputted, an image forming apparatus having an automatic density converting function that the density of an image is detected and the density of an output image is controlled on the basis of the detected density value has been proposed. However, if any original to be read includes an image having, for example, both a character portion and a photograph portion, the simple density conversion results in an unsuitable density output. That is to say, with respect to the image of the original including both the character portion and the photograph portion, when the same or similar density conversion is effected regarding the character portion and the photograph portion, even if the reproduction of intermediate contrast of the photograph portion is important, such photograph portion is subjected to the large or heavy density conversion, with the result that the gradient of contrast is deteriorated, thus making the reproduction of the image having the intermediate contrast impossible. This problem can be solved by discriminating the character portion from the photograph portion by using an editor having the function for designating the image areas on the original with the image forming apparatus. However, in this case, there arise problems that the manipulation of the editor is troublesome and that the apparatus becomes expensive due to the addition of the editor. JP-A-63040461, JP-A-63040462 and JP-A-63040472 disclose an image forming apparatus for reproducing an image from an original including a CCD image. The present invention aims at obtaining a faithful reproduction image from an original image including both a character portion and an intermediate contrast portion such as a photograph. The present invention also aims to automatically obtain a sharp image whilst retaining density-light portions of an original even if an image of the original has different density distribution. According to the invention there is provided an image forming apparatus comprising: an image sensor for reading an original image; designating means for designating character portions and intermediate contrast portions of the original image; determining means for determining on the basis of the image sensor output a first density conversion table for use in relation to a character portion of the original image and a second density conversion table for use in relation to an intermediate contrast portion of the original image; and printing means for reproducing areas designated as character portions using the first density conversion table to convert the image sensor output and reproducing areas designated as intermediate contrast portions using the second density conversion table to convert the image sensor output; characterised in that the designating means comprises discriminating means for using image contrast data obtained from the image sensor output to discriminate between said character portions and intermediate contrast portions of the original image; and in that the determining means is adapted to determine said first and second density conversion tables on the basis of maximum density and minimum density present in the original image, prior to the discrimination made by discrimination means. The present invention also provides an image processing apparatus comprising: input means for inputting signals representative of an original image; designating means for designating character portions and intermediate contrast portions of the original image; determining means for determining on the basis of the input signals a first density conversion table for use in relation to a character portion of the original image and a second density conversion table for use in relation to an intermediate contrast portion of the original image; conversion means for converting input signals in areas designated as character portions using the first density conversion table and converting input signals in areas designated as intermediate contrast portions using the second density conversion table; and output means for outputting the converted signals for printing; characterised in that the designating means comprises discriminating means for using image contrast data obtained from the input signals to discriminate between said character portions and intermediate contrast portions of the original image; and in that the determining means is adapted to determine said first and second density conversion tables on the basis of maximum density and minimum density present in the original image, prior to the discrimination made by the discrimination means. The present invention further provides methods as set out in claims 8 and 13. In the accompanying drawings:- Fig. 1 is a block diagram showing an image forming apparatus according to the present invention; Fig. 2 is a graph showing a density converting curve regarding an image having the contrast; Fig. 3 is a graph showing a density converting curve regarding an image having no contrast; Fig. 4 is a flow chart showing an image forming method in the present invention; Fig. 5 is a flow chart showing the details of steps S7 and S8 in the flow chart of Fig. 4; Fig. 6 is a graph showing a frequency distribution of the minimum density; Fig. 7 is a graph showing a frequency distribution of the maximum density; Fig. 8 is a table showing the contents of steps S3 to S7 in the flow chart of Fig. 5; Fig. 9 is a density converting table; Fig. 10 is a view showing the density converting method; Fig. 11 is an explanatory view showing a character detecting method by separating an image area; and Fig. 12 is a graph showing a density converting curve according to another embodiment of the present invention; The present invention is directed to obtain a sharp image having different density-light areas regarding a background portion, character portion and photograph portion of an original, by detecting the maximum density and the minimum density of the original and by performing the different density conversions regarding the character portion and the photograph portion, for example, as shown in Fig. 2. Further, when the original has a light character portion or a photograph portion (for example, the original consists of a graph sheet), it is possible to remove the fog of the original background and to control the image quality not to make the character portion or the photograph portion excessively dense. The present invention will now be explained with reference to a copying machine embodying the invention. Fig. 1 shows a block diagram of a copying machine according to an embodiment of the present invention. A processing portion shown by a square broken line 15 performs the signal processing for the density conversion. An original inputting portion 1 is constituted by a so-called CCD linear sensor which inputs the density of the original. An inputted analogue signal is converted into a digital signal by means of an A/D converter 2. A bus switch 3 is controlled by a CPU (central processing unit) 10 to select whether the digital signal of the original is inputted to a density converting circuit 4 or whether the digital signal is inputted to a minimum density detector 6 and a maximum density detector 7. The minimum density detector 6 detects the minimum density of the density data included in one line in a main scanning direction of the CCD, and sends the detected result to the CPU 10. Similarly, the maximum density detector 7 detects the maximum of the density data included in one line, and sends the detected result to the CPU 10. Density converting circuits 41 and 42 perform the density converting operation on the basis of the density converting coefficiency set by the CPU 10. The calculation of the density converting coefficiency will be described later. A read only memory (ROM) 9 stores the processing sequence for the CPU 10. Two kinds of density converting tables made by the CPU 10 are stored in a random access memory (RAM) 8. An image area separating circuit 12 is designed to discriminate whether a present pixel in question in one line read by the CPU 10 is a character pixel or a photograph pixel, and to send character/ photograph switching signal to a switch 11. On the basis of this character/photograph switching signal, the switch 11 connects the character density converting circuit 41 to a D/A converter 13 if the signal corresponds to the character signal, and connects the photograph density converting circuit 42 to the D/A converter 13 if the signal corresponds to the photograph signal. The D/A converter 13 converts the digital signal density-converted by the density converting circuit 41 or 42 into an analogue signal. A printer 14 is an image outputting device having the express ability for density-light gradient and outputs an image signal converted by the processing portion 15. Now, a method for determining the density converting curves and a method for forming the image by the use of the density converting curve will be described in accordance with Fig. 4. In a step S1, the density of the original in one line in the main scanning direction of the CCD from the original inputting portion 1 is read. In a step S2, the minimum density of one line is detected by the minimum density detector 6 and the detected result is sent to the CPU 10. In a step S3, the maximum density of one line is detected by the maximum density detector 7 and the detected result is sent to the CPU 10. The above sequences are repeated until the density of all of the lines of the original is detected. Meanwhile, in a step S4, the CPU 10 makes or produces frequency distribution graphs (Figs. 6 and 7) of the minimum density value and of the maximum density value sent from the steps S2 and S3, respectively. After all lines are finished (step S5), the CPU 10 calculates a minimum density reference value (WS) and a maximum density reference value (BS) from Figs. 6 and 7 in a step S6. The minimum density reference value (WS) is obtained as a density value having the maximum frequency of the frequency distribution graph of Fig. 6 regarding the one-line minimum density value, and the maximum density reference value (BS) is obtained as a density value having the maximum frequency of the frequency distribution graph of Fig. 7 regarding the one-line maximum density value. In steps S7 and S8, the CPU 10 calculates the density converting coefficiency applied to the character portion and the density converting coefficiency applied to the photograph portion, respectively. Now, the method for calculating the density converting coefficiency will be explained with reference to Fig. 5. First of all, a value (BS - WS) is calculated in a step S71. In a step S72, if the value (BS - WS) is greater than a predetermined value a , steps S73 and S74 are executed, whereas, if the value is smaller than the predetermined value (i.e., if the difference in density between the original background and the character portion or the photograph portion is small), steps S75 and S76 are executed. The steps S73 or S75 applying the density converting curves for the character portion, and the steps S74 or S76 applying the density converting curve for the photograph portion. In the steps S73, S74, S75 and S76, the density converting curves are applied in accordance with the features of the respective images. In Fig. 8, the feature of the image, the preferable reproduction character required according to such character, and the density converting curve applied in this embodiment to carry out such reproduction are shown. For example, regarding the step S73, since the value (BS - WS) is greater than the predetermined value (step S71), i.e., since the density converting curve in the character portion of the image of relatively large contrast is applied, the reproduction character required is such that the fog of the original background is completely removed and the character is enhanced. Accordingly, as shown in Fig. 8, it is seen that the above requirement is met by selecting a value of the input density of (WS + b) as a minimum value of the output density and by selecting a value of the input density of (BS - c) as a maximum value of the output density and by applying the density converting curve obtained by connecting between the minimum and maximum values with respect to the gradient between the minimum and maximum values. Next, regarding the step S74, similar to the step S73, since the density converting curve in the photograph portion of the image of relatively large contrast is applied, the reproduction character required is such that the fog of the original background is completely removed and the reproduction is effected while retaining the density-light of photograph. Accordingly, as shown in Fig. 8, it is seen that the above requirement is met by selecting a value of the input density of (WS + b) as a minimum value of the output density and by selecting a value of the input density of BS as a maximum value of the output density and by applying the density converting curve obtained by connecting between the minimum and maximum values with respect to the gradient between the minimum and maximum values. Next, regarding the step S75, since the value (BS - WS) is smaller than the predetermined value, i.e., since the density converting curve in the character portion of the image of small contrast is applied, the reproduction character required is such that the fog of the original background is completely removed and the light character of low contrast is not excessively enhanced. Accordingly, as shown in Fig. 8, it is seen that the above requirement is met by selecting a value of the input density of (WS + b) as a minimum value of the output density and by selecting a value of the input density of BS as a maximum value of the output density and by applying the density converting curve obtained by connecting between the minimum and maximum values with respect to the gradient between the minimum and maximum values. Next, regarding the step S76, similar to the step S75, since the density converting curve in the photograph portion of the image of small contrast is applied, the reproduction character required is such that the fog of the original background is completely removed and the difference of density-light is not excessively enhanced". Accordingly, as shown in Fig. 8, it is seen that the above requirement is met by selecting a value of the input density of (WS + b) as a minimum value of the output density and by selecting a value of the input density of (BS + d) as a maximum value of the output density and by applying the density converting curve obtained by connecting between the minimum and maximum values with respect to the gradient between the minimum and maximum values. Here, by setting the predetermined value a compared by the value (BS - WS) in the step S71 as a = b + c + const., the inclination in the curves shown in Fig. 8 are prevented from reversing. Since the above-mentioned density converting curves can be expressed by the information of two points connected by the straight line, in the steps S73 - S76, the variables WSt, BSt, WSp and BSp are calculated on the basis of Fig. 5, these variables being sent to step S81 and S82 as parameters. In the steps S81 and S82, the contents to be treated are the same, but the parameters used are different. In the step S81, the density converting table for character is made by using the variables WSt and BSt, whereas, in the step S82, the density converting table for photograph is made by using the variables WSp and BSp. In the illustrated embodiment, as shown in Fig. 9, the density converting tables are constituted by density converting table data of 256 bits for character and density converting table data of 256 bits for photograph, these tables being designated as Tt(n) and Tp(n), respectively. 0th address WSp address in the density converting table Tt(n), and 0th address WSp address in the density converting table Tp(n) are occupied by zero (0). Whereas, (BSt + 1) address 255th address in the density converting table Tt(n), and (BSp + 1) address 255th address in the density converting table Tp(n) are occupied by 255. In the remaining intermediate addresses in the density converting tables, the values obtained by linear-interpolating between 0 and 255 are set. Fig. 9 shows the density converting table for character when WSt = 20 and BSt = 230, and the density converting table for photograph when WSp = 20 and BSp = 250. In this manner, the density converting tables are calculated by the CPU 10 and are stored in the RAM 8. Thereafter, in a step S9, the CPU 10 sets the density converting coefficiency for character to the density converting circuit 41 and sets the density converting coefficiency for photograph to the density converting circuit 42. The density converting circuits 41, 42 can convert the input density of 256 gradients into the output density of 256 gradients on the basis of the look-up table shown in Fig. 10. Thereafter, the normal image formation is performed. In this case, the original is read by one line by one line by the CCD in a step S10. In a step S11, an image area separating treatment for separating the character portion and the photograph portion in the image is effected. This is achieved by discriminating whether the present pixel in question is a character pixel or is a photograph pixel. Such discrimination can be performed by using the following method. That is to say, the image area separating circuit 12 has a plurality of buffers corresponding to few lines of CCD, where the data are stocked. By using such data, as shown in Fig. 11, vertical, lateral and oblique density differences (inclinations) of the present pixel in question are sought. If the inclinations are greater than a predetermined value, it is considered that such pixel is a vertical, lateral or oblique edge of the character. And, if the pixels are continuous in the vertical, lateral or oblique direction of the present pixel in question, is assumed or considered that the pixel is the character pixel. If the character pixel is determined in accordance with the above method, in a step S12, the sequence goes to a step S13. That is to say, the image area separating circuit 12 sends a signal to the switch 11 to connect the D/A converter 13 to the density converting circuit 41 for character. If it is not judged that the pixel is not the character pixel, the sequence goes from the step S12 to a step S14. That is to say, the image area separating circuit 12 sends a signal to the switch 11 to connect the D/A conveter 13 to the density converting circuit 42 for photograph. In this way, all of the pixels in one line are outputted in a step S15, and thereafter, the image formation is finished after all lines have been outputted in a step S16. By outputting the image by using the above-mentioned image forming apparatus according to the present invention, regarding the original having both a character portion and a photograph portion, a good image wherein the character portion is emphasized and the density-light of the photograph portion is moderately retained by the proper density conversion of the character and photograph portions could be obtained. Next, a second embodiment of the present invention will be explained. In the above-mentioned first embodiment, if the character original has the characteristic of WS = 0 and BS = 255, the density converting curve for character becomes a solid line in Fig. 12, so that the character portion cannot be emphasized adequately. Even if such case occurs, such disadvantage is eliminated by selecting the density converting curve for character as a curve having an upward convex shown a broken line in Fig. 12 rather than the straight line as previous embodiment. However, in this case, in order to calculate the density converting coefficiency in the step S81, since two or more calculating equations must be used, the calculation becomes complex. As mentioned above, according to the present invention, the density conversion required to reproduce the character is effected regarding the character portion and density conversion required to reproduce the photograph is effected regarding the photograph portion, by automatically discriminating the character and photograph portions. Thus, even if there are different density distributions in the original, a sharp image can be obtained with retaining the density-light of the original without using any special devices.

Claims: An image forming apparatus comprising: an image sensor (1) for reading an original image; designating means for designating character portions and intermediate contrast portions of the original image; determining means (6, 7, 10) for determining on the basis of the image sensor output (i) a first density conversion table for use in relation to a character portion of the original image and (ii) a second density conversion table for use in relation to an intermediate contrast portion of the original image; and printing means (11,13,14) for reproducing areas designated as character portions using the first density conversion table to convert the image sensor output and reproducing areas designated as intermediate contrast portions using the second density conversion table to convert the image sensor output; characterised in that the designating means comprises discriminating means (12) for using image contrast data obtained from the image sensor output to discriminate between said character portions and intermediate contrast portions of the original image; and in that the determining means (6, 7, 10) is adapted to determine said first and second density conversion tables on the basis of maximum density (BS) and minimum density (WS) present in the original image, prior to the discrimination made by the discrimination means. An image forming apparatus according claim 1, wherein said discriminating means (12) determines a portion of the original image as a character portion when the rate of change of density of said portion exceeds a predetermined value. An image forming apparatus according to any preceding claim, further comprising means (6,7,10) for determining the overall contrast of the original image in order to discriminate low and high contrast original images on the basis of the density values of the original, and the determining means (10) is adapted to determine the first density conversion table so that characters contained in a low-contrast original image are not reproduced with enhanced contrast, whereby said printing means (14) reproduces original images of low contrast containing both character and intermediate contrast portions such that the character portions are less enhanced than had the original image been of high-contrast. An image forming apparatus according to any preceding claim wherein said determining means operates on the basis of the sensor output during a pre-scan performed by the image sensor. An image forming apparatus according to any preceding claim wherein the discrimination performed by the discriminating means is conducted synchronously with printing by the printing means. An image processing apparatus comprising: input means (2) for inputting signals representative of an original image; designating means for designating character portions and intermediate contrast portions of the original image; determining means (6, 7, 10) for determining on the basis of the input signals (i) a first density conversion table for use in relation to a character portion of the original image and (ii) a second density conversion table for use in relation to an intermediate contrast portion of the original image; conversion means (4) for converting input signals in areas designated as character portions using the first density conversion table and converting input signals in areas designated as intermediate contrast portions using the second density conversion table; and output means for outputting the converted signals for printing; characterised in that the designating means comprises discriminating means (12) for using image contrast data obtained from the input signals to discriminate between said character portions and intermediate contrast portions of the original image; and in that the determining means (6, 7, 10) is adapted to determine said first and second density conversion tables on the basis of maximum density (BS) and minimum density (WS) present in the original image, prior to the discrimination made by the discrimination means. An image processing apparatus according to claim 6, further comprising means (6,7,10) for determining the overall contrast of the original image in order to discriminate low and high contrast original images on the basis of the density values of the original, and the determining means (10) is adapted to determine the first density conversion table so that characters contained in a low-contrast original image are not reproduced with enhanced contrast, whereby said conversion means converts original images of low contrast containing both character and intermediate contrast portions such that the character portions are less enhanced than had the original image been of high-contrast. An image forming method comprising the steps of: reading an original image with an image sensor; designating character portions and intermediate contrast portions of the original image; determining on the basis of the image sensor output (i) a first density conversion table for use in relation to a character portion of the original image and (ii) a second density conversion table for use in relation to an intermediate contrast portion of the original image; and printing areas designated as character portions using the first density conversion table to convert the image sensor output and areas designated as intermediate contrast portions using the second density conversion table to convert the image sensor output; characterised in that the determining step determines said first and second density conversion tables on the basis of maximum density (BS) and minimum density (WS) present in the original image, prior to the designating step; and in that the designating step uses image contrast data obtained from the image sensor output to discriminate between said character portions and intermediate contrast portions of the original image. An image forming method according claim 8, wherein a portion of the original image is discriminated as a character portion when the rate of change of density of said portion exceeds a predetermined value. An image forming method according to any either of claims 8 or 9, further comprising the step of determining the overall contrast of the original image in order to discriminate low and high contrast original images on the basis of the density values of the original; wherein the determining step is adapted to determine the first density conversion table so that characters contained in a low-contrast original image are not reproduced with enhanced contrast, and whereby said printing step reproduces original images of low contrast containing both character and intermediate contrast portions such that the character portions are less enhanced than had the original image been of high-contrast. An image forming method according to any of claims 8 to 10 wherein said determining step operates on the basis of the sensor output during a pre-scan performed by the image sensor. An image forming method according to any of claims 8 to 11 wherein the discrimination of character and intermediate contrast portions is conducted synchronously with printing. An image processing method comprising the steps of: inputting signals representative of an original image; designating character portions and intermediate contrast portions of the original image; determining on the basis of the input signals (i) a first density conversion table for use in relation to a character portion of the original image and (ii) a second density conversion table for use in relation to an intermediate contrast portion of the original image; converting input signals in areas designated as character portions using the first density conversion table and converting input signals in areas designated as intermediate contrast portions using the second density conversion table; and outputting the converted signals for printing; characterised in that the determining step determines said first and second density conversion tables on the basis of maximum density (BS) and minimum density (WS) present in the original image, prior to the designating step and in that the designating step uses image contrast data obtained from the input signals to discriminate between said character portions and intermediate contrast portions of the original image. An image processing method according to claim 13, further comprising the step of determining the overall contrast of the original image in order to discriminate low and high contrast original images on the basis of the density values of the original; wherein the determining step is adapted to determine the first density conversion table so that characters contained in a low-contrast original image are not reproduced with enhanced contrast; and whereby said conversion step converts original images of low contrast containing both character and intermediate contrast portions such that the character portions are less enhanced than had the original image been of high-contrast.

Inventors: MIZOGUCHI YOSHITO, MIZOGUCHI, YOSHITO, MIZOGUCHI, YOSHITO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: H04N1
IPC codes: G03G15, H04N1