GB2325810A - Image processing with different detection and processing of white and dark areas - Google Patents

Abstract

An imaging system comprises a photodetector 508, a DC gain adjusting amplifier 515, at least one A - D converter 525, 530, a data mask 510, 520 and a gamma corrector 545, 550 for each A - D converter, and an adder 555 for combining the outputs of the gamma corrector(s). The photodetector is controlled by read out pulses (Figure 6) whose period is short (63) for detecting white portions of the image and long (66) for detecting dark portions. The period of the long and short pulses may be in the ratio 10:1 and switching between the periods occurs at line or pixel frequency. The data mask(s) use reference signals 502, 503 and 504, 505 to eliminate signals outside the reference range and to separately process the dark and white parts of the image. The intensity of the light source may also be switched with the photodetector control pulses. A single A-D converter/mask/gamma corrector path may be used (Figure 8) with corresponding switching of the reference values and gamma correction function. The system is said to increase the linear portion of the transfer characteristic (Figure 7), the non-linearity of which is normally corrected only by the gamma corrector.

Description

APPARATUS OF AN IMAGE PROCESSING USING - - - DISTINCTIVE WEIGHTED CONTROL The present invention relates to an image system, and more particularly to the system for image scanning system in an image system, using distinctive weighted control signal.

An image system makes use of focusing a reflected light beam through a photodetector to generate an image signal for further image processing, storing and displaying. Among various applications such as image scanners, camera recorders or facsimile machines everywhere in the modern world, there are two primary functions performed in these machines, image signal taking and analogue-to-digital converting.

For example, the block diagram of an image scanner in the prior art is shown in Fig. 1 . It includes a dull control signal source 9, a light source 10, a glass surface 11, a mirror 12, a lens 13,a charge coupled device (CCD)14, a pre-processing system which is usually implemented by a fixed dc-gain voltage amplifier 15, an analogue-to-digital converter (ADC)16, and a postprocessing system 17. The waveform of the output signal, shown in fig. 2, of a dull control signal source 9 is fed to COD 14. The width of every pulse is all the same and all the time intervals are equal. For example, time interval 23 equals to time interval 27, and equals other time intervals. This system operates in the way that COD 14 converts the light emitted by light source 10, a text or a picture firstly reflected by the glass surface 11 and secondly reflected by the mirror 12 to an image signal.

- Note that when the front edge 20 of pulse 21 is fed to the COD 14, the COD 14 pours out all the charges that cumulated in the previous time interval and then begin to produce and cumulate charge until the front edge 34 of pulse 35 is fed to the COD 14. Accordingly the COD 14 pour out all the charges that cumulated in time interval 23, located between front edge 30 and front edge 24, and begin to produce and accumulate charge until next front edge arrives. Thus the optical signal is transformed into electrical image signal.

The fixed dc-gain voltage amplifier 15 adjust the dc-gain of the electrical image signal and then feed it to the ADC 16. The reference input REF+ and REFrelates to the maximum and minimum value of gray level of the image system.

If the voltage of the signal processed by fixed dc-gain voltage amplifier 15 is smaller than REF-, no matter how small the signal is, the output signal is set corresponding to the smallest gray level of the image system. As the signal input to fixed dc-gain voltage amplifier 15 is larger then REF+, the output signal is set corresponding to the largest gray level of the image system.

Contrast adjustment by a Gamma characteristic is performed by the postprocessing means 17, and then obtained the output signal which can be further processed or displayed. Apparently, if the scope of REF+ and REF- is too small, the scope of the gray level used to construct an image is narrow, thus the quality of the output image is poor.

It is known that the output characteristic of a COD or the transfer curve of an OP is linear only in one region as shown in Fig. 3A. If the element is a CCD, the horizontal coordinate represents light flux, and the vertical coordinate represents the output voltage. If the element is an OP, the horizontal coordinate represents input voltage, and the vertical coordinate represents the output voltage. In Fig. 3A the region b is linear, yet regions a and c are nonlinear. Because the pixels with small light flux are in region a, the pixels in region a represents the dark part of an image. The pixels with large light flux are in region c, the pixels in region c represents the white part of an image.

- The transfer curve of the system is shown in Fig. 3B, which is obtained by inverting Fig. 3A. The horizontal coordinate of Fig. 3B represents input density, and the vertical coordinate of Fig. 3B represents the output density. It is obvious that if the dark part or the white part of an image is input to the image system, the output image is distorted in image density, because regions a and c are not linear. If the linear region b is larger, the image quality of the output image would be better.

It is therefore a primary object of this invention to provide an improved method for acquiring a system which has a transfer curve with larger range of linear region. To enlarge the linear region of the transfer curve of the image system without changing the specification of COD or OP, the exposure period of COD must be weighted and the reference level of ADC must be rectified according to the weighted exposure period. Therefore, the present invention separately process the signals in small light flux and in large light flux. The reference level of the ADC is set related to the weighted exposure period in the present invention, thus the number of reference levels are more than that of the prior art.

Accordingly, the present invention provides a method for image processing in an image system. The present invention includes a weighted control signal source for generating pulses of different time intervals; and a pre-processing system for converting an optical image to an multi-weighted signal which has various exposure period controlled by the output signal of the weighted control signal source. A multi-sampling system is used for converting the multi-weighted signal to the multi-sampled digital signal. The present invention also includes post-processing system for generating image codes by processing the multi-sampled digital signal, and reference level generating system for generating reference level value according to the time intervals between the output signal of the weighted control signal source, wherein the reference level value is used as the reference level when producing a digital signal by taking samples of an analog signal.

In the first preferred embodiment, the pre-processing system mentioned above includes charge coupled device (CCD) for converting the optical image to an electric signal and a dc-gain voltage amplifier adjusting the dc-gain of the electric signal and the dc gain voltage amplifier includes an operational amplifier.

The multi-sampling system includes a sampling system for converting an analog signal to the digital signal according to the reference inputs of the sampling system related to the time intervals of the output of the weighted control signal source. In addition the multi-sampling system include an adder.

Notice that there are two ways to implement the present invention, that is, line division and point division. To carry out the present invention in line division, the sampling system contains many ADCs, wherein the ADC has the reference inputs. The reference inputs are the criterion that the masking means used to selectively pass partial content of gray levels of the digitalized electrical signal. Also including a masking system for selectively pass partial content of gray levels of the digitalized electrical signal. In other words, the signal produced related to different exposure period can pass different masking systems.

To implement the present invention in point division, the sampling system contains a ADC, which has the reference inputs fed from the reference generating system and a multiplexer for sequentially selecting a digit from a sequence of data. Also including a masking system for selectively pass partial content of gray levels of the digitalized electrical signal. Accordingly, the reference inputs alternates with the period of the weighted control signal source. Similarly, the signal with different exposure period can pass the masking system with responsive to the reference inputs.

The post-processing system includes a Gamma table for mapping the multi-sampled digital signal to the mapped signal according to Gamma characteristics to smooth the quality of the produced image and an adder for adding the digital signals of different exposure period.

In the drawings Fig. 1 schematically illustrates the block diagram of the image scanner in the prior art.

Fig. 2 illustrates the waveform of the output of the control signal source in the prior art.

Fig. 3A illustrates the output characteristic of a charge coupled device (CCD).

Fig. 3B illustrates the input output transfer curve of the image system in the prior art.

Fig. 4 illustrates the input output transfer curve of the image system with linear section extended by the method of distinctive weighted control.

Fig. 5 schematically illustrates the functional block diagram of the image scanner of the first preferred embodiment of the present invention, which represents the method of space division of the present invention. Because the element before charge coupled device is all the same with that of the prior art, and they have nothing to do with the present invention, the elements are not shown in this figure.

Fig. 6 illustrates the waveform of the output of the weighted control signal source in the present invention.

Fig. 7 illustrates the input output transfer curve of the image system in the first preferred embodiment in the present invention.

Fig. 8 schematically illustrates the functional block diagram of the image scanner of the second preferred embodiment of the present invention, which represents the method of time division of the present invention. Because the element before charge coupled device is all the same with that of the prior art, and they have nothing to do with the present invention, the elements are not shown in this figure.

To improve the quality of the output image, it is important to increase the density range of the image scanner because it will make possible in which every pixel has more choice in shades. The important factors to the quality of the output image of the image scanner is described such as the density range, bit number of the image signal and Gamma characteristic.

If the density range of the system is large, the difference of the output image of the system would be displayed. Assume that the bit number of the image signal is 8 then the signal-to-noise ratio S/N: (22)/1=256 so density range D=log256=2.4. It is clear that the larger the bit number of image signal is, the larger the density range is. Therefore, more shades can be used to construct the output image. According to the characteristic of the input image signal obtained in a exposure period, the post-processing system get a suitable Gamma characteristic of a specific value, which can adjust the characteristic of an output signal. Besides, the scope of linear section is an important factor to the output image of an image system.

Signal sampling is utilized In the prior art. Whereas the multisampling is utilized in this architecture to make it possible to increase the bit number of image signal and make the signal-to-noise ratio of system and OOD satisfy the equation : [(N1S)5+(N1S)c) < 10. To satisfy the equation, signal-tonoise ratio of OCD is increased by way of extending exposure period. It is well known that there are many kinds of depth in gray level in a picture simultaneously. When the exposure period extended, both white and dark part of the picture have longer exposure period, so the white part reflects more light to the COD resulting in the saturation of CCD. As explained above the, present invention uses the separate processing technology to process pixels of different density with different exposure period.

Because the separate processing technology is used in the present invention, the exposure period can be controlled to fit the dark part and white part of the image, and the nonlinearlity of the transfer curve can be improved by increasing the light flux of the dark part as well as decreasing the light flux of the white part of the image. Refer to Fig. 3A, the pixel in the dark part of the input image have little light flux, such as X, is transferred by nonlinear section a. Whereas the pixels in the dark part of the input image, have much light flux, such as Y, is transferred by nonlinear section c. Since section a and c of the transfer curve are nonlinear, the pixels of the output image of the light flux within section a and c are distorted in density. To make the pixels within sections a and c transferred by a linear section, the light flux X must be increased, and the light flux Y must be decreased to the section between P and Q. The pixels mentioned above can be transferred by linear section b. If the pixels in dark part and white part can be processed by different exposure period, the pixels in dark part and in white part can be transferred to linear section simultaneously. The changed transfer curve of the system in the present invention is shown in Fig. 4.

The first preferred embodiment using weighted multi-sampling, separate processing, distinctive weighted control and image compounding, is now described in detail as follows. The apparatus can also be used for image processing, storing and displaying in the application, such as image scanners, camera recorders facsimile machines.

A functional block diagram of the first preferred embodiment in the present invention used in image scanner is shown in Fig. 5. It includes a weighted control signal source 501, a COD (Charge Couple Device)508, a pre-processing system which is implemented as a DC gain voltage amplifier 515, an analogue-to-digital converter (ADC) 525, a data mask 510 which is implemented as a program or a hardware's output signal that eliminate the signal of a image beyond the range of the input reference 502and 503 of the ADC 525 according to the gray level of the input signal, an adder 535, a postprocessing system which is implemented as a Gamma table 545 performed by software or hardware, a ADC (analogue-todigital converter) 530,a data mask 520, an adder 540, a post-processing system which is implemented as a Gamma table 550 performed by software or hardware and a bit offset adder 555.

The waveform of the output signal, shown in fig. 6, of the weighted control signal source 501 is fed to COD 508. The short time intervals are mutually equal , and the long time intervals are mutually equal respectively For example, time interval 63 equals to time interval 601 , time interval 66 equals to time interval 605 , so dose other time intervals.

The optical image focused on COD 508. COO 508 converts the optical image focused on it to an image signal 509. Note that when the front edge 60 of pulse 61 is fed to the COD 508, the COD 508 pour out all the charges that accumulated in the previous time interval and then begin to produce and accumulate charge until the front edge 64 of pulse 65 is fed to the COD 508. Accordingly the COD 508 pour out all the charges that cumulated in time interval 63, located between front edge 60 and front edge 64, and begin to produce and cumulate charge until next front edge arrives. Thus the optical signal is transformed into electrical image signal and the signal gained in this way is used to acquire the white part of the image signal. To acquire the dark part of the image signal 509, the exposure period is set 10 times that of the white part by the way that COD 508 poured the charges accumulated during the time interval 66.

DC gain voltage amplifier 515 amplify the DC voltage of signal 509 to get signal 511. ADC 525 use the reference level offered by reference input 502 and 503 to convert signal 511 to a digital signal, and feed it to data mask 510 to get signal 513. Adder 535 process signal 513 to perform the multisampling and generate the dark part signal 536 which is a 9 bits digital signal and is the signal in the dark part of the image. The post-processing system is implemented by a Gamma table 545 and a bit offset adder 555 to focus on the resolution of the dark part of the image. The Gamma characteristic of the Gamma table is chosen of the Gamma value larger than 1.

In the white part, the signal 511 is fed through ADC 530 and data mask 520 to adder 540 to perform the function of multi-sampling, and a digital signal is generated and called in this preferred embodiment i.e. the white-part signal 541. Wherein ADC 530 use the reference level offered by reference input 504 and 505 to convert signal 511 to a digital signal, and feed it to data mask 520 to get signal 521. The post-processing system is implemented by a Gamma table 550 and a bit offset adder 555, to focus on the resolution of the white part of the image, the Gamma characteristic of the Gamma table is chosen of the Gamma value smaller than 1.

The output signal of Gamma table 545 according to dark part signal 536 is dark part signal 546, and the output signal of Gamma table 550 according to white-part signal 541 is white-part signal 551. The dark part signal 546 and the white-part signal 551 are added by bit-offset adder 555 to generate an image signal. The above mentioned weighted control signal source 501 with the waveform of the output signal as shown in Fig. 6, which has unequal time intervals, can be realized by alternative form of control signal source 9, such as adding Flip-Flops or counters. The technique is well known to those skilled in the art, so the circuit of weighted control signal source 501 is not described.

In this first preferred embodiment of the present invention, the ADC 525 use the reference level offered by reference input 502 and 503 to convert signal 511 to a digital signal, and feed it to data mask 510 to get signal 513.

ADC 530 use the reference level offered by reference input 504 and 505 to convert signal 511 to get signal 521. The data mask 510 and 520 eliminate the pixels of the gray level exceeds the preset value of reference level in data mask 510 and 520. The preset value is related to the reference input by an equation: B=255x104 (1) Where B represents the boundary of the dark part and the white part in gray level and D represents the density. For example, if the static value that nonlinearlity occurred when the input density is deeper than 1.0 D, that is to say that when the dark part of input image signal is fed to this traditional system, the output would be distorted. The present invention use input density 1.0 D as a boundary to separately process the dark part and the white part.

According to equation (1) the gray level 25.5 is used as a boundary of dark part and white part.

In the first preferred embodiment, the input reference 502 and 503 are respectively set as the reference level corresponding to densities equal to 1.0 D and the dark reference level corresponding to densities equal to 4.0 D, when the weight is 10. The input reference 504 and 505 are respectively set as the white reference level corresponding to densities equal to 0.0 D and the dark reference level corresponding to densities equal to 4.0 D when the weight is 1. The weight is used to determine the exposure period of COD 508, in other words, the time interval is determined. In this first preferred embodiment, the time interval 66 is 10 times that of time interval 63 as shown in Fig. 6. The gray level used in data mask 510 and 520 to separate dark part and white part of the image is thus set as 25.5. The reference level of data mask of the system in the first preferred embodiment, which smaller than 25.5 is set as the dark part and the gray level larger than 25.5 is set as the white part.

The transfer curve of this example of the first preferred embodiment is shown in Fig. 7. Wherein the dashed curve represents the transfer curve of the traditional system, the solid curve represents the transfer curve of the first preferred embodiment in the present invention. It is obvious that the input density where the nonlinearlity occur is changed from 1 .OD to z D, thus the linear section is enlarged.

The present invention which is implemented by time division is described in the second preferred embodiment. Refer to Fig. 8, there are a weighted control signal source 801, a COD (Charge Couple Device)808, a pre-processing system which is implemented as a DC gain voltage amplifier 815, a analogue-to-digital converter (ADC) 825, an multiplexer 820, an adder 835, a data mask 810 which is implemented as a program or a hardware's output signal that eliminate the signal of a image beyond the range of the input reference 802and 803 of the ADC 825 according to the gray level of the input signal, a post-processing system which is implemented as a Gamma table 845 performed by software or hardware, and an adder 855.

The waveform of the output signal, shown in fig. 6, of the weighted control signal source 801 is fed to COD 808. The operation of weighted control signal source 801, COD 808, DC gain voltage amplifier 815 and analogue-todigital converter (ADC) 825 is the same as that of the first preferred embodiment. .

DC gain voltage amplifier 815 amplifies the DC voltage of signal 809 to get signal 811. ADC 825 use the reference level offered by reference input 802 and 803 to convert signal 811 to a digital signal 817, and feed it to multiplexer 820. As signal 811 contains signals of different weight1 ADC 825 take sample of signal 811 with the reference input 802 and 803 change with the weight (time intervals in Fig. 6) synchronously.

When time interval 63 pass COD 808, reference input 802 and 803 are set as the white reference level corresponding to densities equal to 0.0 D and the dark reference level corresponding to densities equal to 4.0 D respectively, thus the weight is 1. When time interval 66 pass COD 808, the input reference 802 are set as the reference level corresponding to densities equal to 1.0 D and the input reference 803 remains the dark reference level corresponding to densities equal to 4.0 D, thus the weight is 10.

In period 63 of Fig. 6, multiplexer 820 select the signal of weight 1 and send it to data mask 810. The reference level of data mask 810 is now above 25.5, in other words, the pixels of gray level smaller than 25.5 are eliminated by data mask 810. In period 605 of Fig. 6, multiplexer 820 selects the signal of weight 10 and send it to data mask 810. The reference level of data mask 810 is now below 25.5, in other words, the pixels of gray level larger than 25.5 are eliminated by data mask 810. The period is synchronously changed with that of weighted control signal source 801. Thus the dark part signal and the white part signal is separately processed.

Adder 810 and data mask 835 are used together to perform the function of multi-sampling, and the digital signal is thus generated and called in this preferred embodiment i.e. the white part signal or the dark part signal, up to the weight of the signal itself. - The post-processing system is implemented by a Gamma table 845 and a bit offset adder 855. The Gamma characteristic of the Gamma table is chosen of the Gamma value larger than 1, when the dark part signal is processed. Whereas, the Gamma characteristic with the Gamma table is chosen of the Gamma value smaller than 1, when the white part signal is processed. In other words, the the Gamma value of the Gamma table is synchronously alternated to the period of weighted control signal source 801. Thus the whtie part signal and the dark part signal is added to get an image signal.

If the present invention is utilized to operate in a color scanner, the signal of 3 primary colors can be processed by two ways. The first way is line division, which can be implemented by the first and the second preferred embodiment in the present invention. The other way is point division, which can be carryed out by the second preferred embodiment in the present invention.

Another preferred embodiment of the present invention is implemented by replacing the light source of the first and the second preferred embodiment with a multi- intensity light source which is changable in the light intensity. To control the light flux on the CCD, this preferred embodiment utilize a dull control signal source and a multi-intensity light source, and this preferred embodiment can carry out the enlargement of the linear section of the transfer curve of the image system too. Because the light fulx is produced by multiplexing the intensity of light source and the exposure time of CCD, this preferred embodiment utilize a multi-intensity light source and a dull control signal source. The intensity of the light of multi-intensity light source is controlled by the weighted control signal source and a control circuit in the prior art used to control the power of a power source. The exposure time of the COD is controlled by a dull control isgnal source. Thus the image signal is separately processed as described in the first and the second preferred embodiment.

The apparatus for processing the image signal of an image system, provide an improved method for acquiring a system which has a transfer curve with larger range of linear section. To enlarge the linear section of the transfer curve of the image system, the exposure period of COD must be weighted and the reference level of ADC must be rectified according to the weighted exposure period. Therefore, the present invention separately process the signals of little light flux and of much light flux, and with various exposure period, many pixels of the dark part and white part of the image is changed to the linear region of the transfer curve in the present invention.

Although specific embodiments have been illustrated and described it will be obvious to those skilled in the art that various modification may be made without departing from thApirj which is intended to be limited solely by the appended claims.

Claims (18)

1. Apparatus of an image processing using distinctive weighted control, said apparatus comprising: photodetecting means for converting an optical image to an electrical signal, wherein said photodetecting means is controlled by a weigh signal; pre-processing means electrically coupled to photodetecti ng means for adjusting dc gain of the electrical signal; a plurality of sets of analog-to-digital means for distinctly transfering the adjusted electrical signal from an analog form to a digital form, each of said sets of analog-to-digital means having a different pair of reference signals; a plurality of masking means for selectively passing partial content of gray levels of the digitalized electrical signal according to the corresponding pair of reference signals; and adding means for adding the digitalized electrical signals.

2. The apparatus as claim 1, wherein said photodetecting means is a charged coupled device (CCD).

3. The apparatus as claim 1, wherein said plurality of sets of analog-to-digital means comprising a plurality of ADC (analog-to-digital converter).

4. The apparatus as claim 1, wherein said pre-processing means comprising an operational amplifier.

5. The apparatus as claim 1, wherein said plurality of masking means is program of software.

6. The apparatus as claim 1, wherein said plurality of masking means passing partial content of gray levels of the digitalized electrical signal according to the corresponding pair of reference signals.

7. The apparatus as claim 6, wherein the corresponding pair of reference signals are related to said weight signal.

8. The apparatus as claim 1, wherein said adding means comprises an adder.

9. A method for an image processing using distinctive weighted control, said apparatus comprising: converting an optical image to an electrical signal related to a weight signal; adjusting dc gain of the electrical signal; distinctly transfering the adjusted electrical signal from an analog form to a digital form according to a plurality of pair of reference signals, wherein the plurality of pair of reference signals are related to said weight signal; selectively passing partial content of gray levels of the digitalized electrical signal according to the corresponding pair of reference signals; and adding the digitalized electrical signals.

10. Apparatus of an image processing using distinctive weighted control, said apparatus comprising: photodetecting means for converting an optical image to an electrical signal, wherein said photodetecting means is controlled by a weigh signal; pre-processing means electrically coupled to photodetecting means for adjusting dc gain of the electrical signal; analog-to-digital means for distinctly transfering the adjusted electrical signal from an analog form to a digital form, according to a plurality of pair of reference signals, wherein said plurality of pair of reference signals synchronously change with the period of weight signal; multiplexing means for sequentially passing portions of the digitalized electrical signal according to the change of corresponding pair of reference signals; masking means for selectively passing partial content of gray levels of the digitalized electrical signal according to the corresponding pair of reference signals; and adding means for adding the digitalized electrical signals.

11. The apparatus as claim 10, wherein said photodetecting means is a charged coupled device (CCD).

12. The apparatus as claim 10, wherein said analog-to-digital means comprising a plurality of ADC(analog-to-digital converter).

13. The apparatus as claim 10, wherein said pre-processing means comprising an operational amplifier.

14. The apparatus as claim 10, wherein said plurality of masking means is program of software.

15. The apparatus as claim 10, wherein said plurality of masking means passing partial content of gray levels of the digitalized electrical signal according to the corresponding pair of reference signals.

16. The apparatus as claim 15, wherein the corresponding pair of reference signals are related to said weight signal.

17. The apparatus as claim 10, wherein said adding means comprises an adder.

18. A method for an image processing using distinctive weighted control, said apparatus comprising: converting an optical image to an electrical signal related to a weight signal; adjusting dc gain of the electrical signal; distinctly transfering the adjusted electrical signal from an analog form to a digital form according to a different pair of reference signals, wherein the different pair of reference signals are related to said weight signal; sequentially passing portions of the digitalized electrical signal according to the change of corresponding pair of reference signals; selectively passing partial content of gray levels of the digitalized electrical signal according to the corresponding pair of reference signals; and adding the digitalized electrical signals.