JP2002312781A - Image processor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily and quickly perform image processing for emphasizing contrast in a picture photographed by an image sensor having logarithmic output characteristics for enlarging a dynamic range. SOLUTION: This image processor is provided with a conversion table for converting image data outputted from an image sensor into image data for emphasizing the change of brightness in a plurality of luminance areas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus for enhancing the contrast of an image captured by an image sensor.

[0002]

[Prior art]

In general, when detecting a white line on a road by photographing the front of a vehicle, a photograph may be taken of the tunnel exit (entrance) from inside (outside) of the tunnel, welding, laser processing, or the like.
When shooting a subject that has a very bright part (highlight part) and a dark part (shadow part) around it, such as when monitoring work conditions such as plasma processing and thermal spraying, it is particularly dynamic. An image sensor with a wide range is required.

Conventionally, in order to expand the dynamic range, a sensor current flowing through a photodiode serving as a photoelectric conversion element in accordance with the amount of incident light has a logarithmic characteristic in a weak inversion state by utilizing characteristics of a sub-threshold region of a transistor. An image sensor having a logarithmic output characteristic using an optical sensor circuit that converts a signal into a signal and outputs a sensor signal corresponding to the converted voltage signal has been developed (Japanese Patent Laid-Open No. 2000-329616). reference).

[0005]

The problem to be solved is that an image sensor provided with a logarithmic output characteristic in order to expand a dynamic range can capture an image from a very bright part to a dark part. In contrast, the contrast is insufficient because the luminance is logarithmically compressed.

For example, in a case where an image captured by the image sensor is displayed on a monitor screen and visually evaluated, a process for increasing the contrast of the image is performed by a logical image processing method using a computer. In such a case, there is a problem that it takes time to perform the processing, and it is not possible to perform an image analysis in real time.

[0007]

SUMMARY OF THE INVENTION In an image processing apparatus according to the present invention, image processing for enhancing contrast in an image photographed by an image sensor having a logarithmic output characteristic for expanding a dynamic range is facilitated. In order to perform the processing promptly, a means for converting the image data into image data that emphasizes a change in brightness in a plurality of luminance regions using a conversion table of image data output from the image sensor is adopted.

[0008]

FIG. 1 shows a configuration example of a photosensor circuit used for each pixel of an image sensor according to the present invention.

[0009] The optical sensor circuit is provided with a photodiode PD as a photoelectric conversion element for generating a sensor current corresponding to the amount of incident light Ls, and a sensor current flowing through the photodiode PD, which is weakened by utilizing characteristics of a sub-threshold region. A transistor Q1 for converting to a voltage signal Vpd with a logarithmic characteristic in an inverted state, and the converted voltage signal Vp
It comprises a transistor Q2 that amplifies d and a transistor Q3 that outputs a sensor signal Vo at the pulse timing of the read signal Vs.

In the optical sensor circuit, when the incident light Ls is incident on the photodiode PD with a sufficient amount of light, a sufficient sensor current flows through the transistor Q1, and the resistance value of the transistor Q1 is not so large. As a result, the optical signal can be detected with a sufficient response speed that does not cause an afterimage as an image sensor.

However, the incident light L of the photodiode PD
Since the transistor Q1 is set to operate such that when the amount of s light decreases and the sensor current flowing through the transistor Q1 decreases, the resistance value of the transistor Q1 increases by one digit when the current flowing therethrough decreases by one digit. As the resistance value of Q1 increases, the time constant with the parasitic capacitance C of the photodiode PD increases, so that it takes time to discharge the charge accumulated in the parasitic capacitance C. Therefore, as the amount of incident light Ls decreases, an afterimage is observed over a long period of time.

Therefore, the saturation time of the voltage signal Vpd corresponding to the sensor current when the light quantity of the incident light Ls of the photodiode PD is small becomes long, so that the sensor signal Vo at the pulse timing of the read signal Vs as shown in FIG. Is read out, an output of a level as large as in the beginning appears as an afterimage. In FIG. 4, Vpd 'indicates a voltage signal inverted and amplified by the amplifying transistor Q2.

In such an optical sensor circuit, prior to reading out the sensor signal Vo, the drain voltage VD of the transistor Q1 is set lower than a steady value for a predetermined time and stored in the parasitic capacitance C of the photodiode PD. When the amount of incident light Ls is small, a voltage signal corresponding to the amount of incident light at that time is immediately obtained even if a sudden change occurs in the sensor current by discharging and initializing the accumulated electric charge. However, no afterimage is caused.

FIG. 2 shows a time chart of signals of respective parts in the optical sensor circuit at that time. Here, t1
Indicates the timing of initialization, and t2 indicates the timing of optical signal detection. Drain voltage VD of transistor Q1
Is switched from a steady value (high level H) to a low voltage (low level L) as the predetermined time tm, for example, 5 μsec when the reading speed for one pixel is about 100 nsec.
It is set to about c. In the figure, T is a photodiode PD
Shows the accumulation period of the parasitic capacitance C, and the accumulation period T
Is 1/30 sec (or 1/60 sec.) For NTSC signals.
sec).

In such a device, when the drain voltage VD of the transistor Q1 is switched to the low level L at the time of initialization, the potential difference between the gate voltage VG and the drain voltage VD at that time is a threshold of the transistor Q1. If it is larger than the value, the transistor Q1 will be in a low resistance state.
As a result, the potential on the source side at that time becomes equal to the drain voltage VD (source voltage = drain voltage in an n-MOS transistor), and the junction capacitance C of the photodiode PD is discharged.

When the drain voltage VD is switched to the steady high level H after the elapse of the time tm and the optical signal is detected, the potential on the source side becomes the drain voltage V
If the potential difference between the gate voltage VG and the drain voltage VD at that time is larger than the threshold value, the MOS transistor Q1 enters a low resistance state, and the junction capacitance C of the photodiode PD is charged. Be started.

As described above, if the parasitic capacitance C is charged after discharging and initializing the junction capacitance C of the photodiode PD prior to detection of an optical signal, a certain time has elapsed from the timing of the initialization. The output voltage (terminal voltage of the photodiode PD) Vpd at the time becomes a value corresponding to the amount of incident light Ls. That is, after the initialization, a discharge characteristic with a constant time constant following the change in the amount of incident light Ls can be obtained.

At this time, if left for a long time, the drain voltage V
The current supplied from D through the transistor Q1 and the current flowing through the photodiode PD are the same, but if there is no charge left before, the same discharge characteristics are always obtained, so that the afterimage does not occur.

Therefore, if the optical signal is detected for a predetermined time after the initialization, it is possible to obtain a sensor signal Vo having no afterimage corresponding to the amount of the incident light Ls.

FIG. 3 shows the output characteristics of the sensor signal Vo with respect to the sensor current flowing through the photodiode PD according to the amount of incident light in such an optical sensor circuit.
It shows a logarithmic output characteristic when the sensor current is large, but shows a photodiode PD when the sensor current is small.
, The response of the parasitic capacitance C is delayed, and a substantially linear non-logarithmic output characteristic is shown. In the figure, WA indicates a non-logarithmic response area, and WB indicates a logarithmic response area.

FIG. 5 shows an image sensor in which a plurality of pixels are arranged in a matrix by using such an optical sensor circuit as a pixel unit, and the sensor signal of each pixel is read out in time series. One configuration example is shown.

The image sensor has a basic configuration in which, for example, 4 × 4 pixels D11 to D44 are arranged in a matrix, and a pixel column for one line is provided from the pixel column selection circuit 1. Selection signals LS1 to LS4 sequentially output
And a selection signal DS1 sequentially output from the pixel selection circuit 2 for each pixel in the selected pixel column.
The corresponding switches SW1 to SW4 in the switch group 3 are sequentially turned on by .about.DS4 so that the sensor signal Vo of each pixel is read out in time series. In the figure, reference numeral 4 denotes a power supply for the gate voltage VG of the transistor Q1 in each pixel, and reference numeral 6 denotes a power supply for the drain voltage VD.

In such an image sensor, when selecting a pixel column for each one line, the drain voltage VD of the transistor Q1 of each pixel in the selected pixel column is set at a predetermined timing in a normal state. A voltage switching circuit 5 for switching between a high level H and a low level L at the time of initialization is provided.

The operation of the thus constructed image sensor according to the present invention will be described below with reference to a time chart of signals of respective parts shown in FIG.

First, when the pixel column selection signal LS1 becomes high level H, the corresponding D11, D12, D13,
The first pixel row composed of D14 is selected. And L
During a certain period T1 in which S1 is at the high level H, the pixel selection signals DS1 to DS4 sequentially become the high level H, and the sensor signals Vo of the pixels D11, D12, D13, and D14 are sequentially read.

Next, when the next row LS2 goes high when the pixel row selection signal LS1 goes low, a second pixel row consisting of D21, D22, D23 and D24 is selected. . Then, during a certain period T1 in which LS2 is at the high level H, the pixel selection signals DS1 to DS4 sequentially become the high level H, and the sensor signals Vo of the pixels D21, D22, D23, and D24 are sequentially read.

Similarly, the pixel column selection signals LS3 and LS4 continuously go to the high level H, and the corresponding third
And the fourth pixel column are sequentially selected, and LS3 and LS
4 during a certain period T1 in which the pixel selection signals DS1 to DS4 are at the high level H, respectively.
, The sensor signals Vo of the pixels D31, D32, D33, D34 and D41, D42, D43, D44 are sequentially read.

At the time when the pixel column selection signal LS1 falls to the low level L after the period T1, each pixel D11, D12,.
Each pixel is initialized by switching the drain voltage VD1 of D13 and D14 from the high level H to the low level L for a predetermined time T2, and the sensor signal in the next cycle performed after the elapse of one cycle period T3. To read the data.

Next, when the pixel column selection signal LS2 falls to the low level L after the period T1, each pixel D21, D2 in the second pixel column selected at that time.
2, each pixel is initialized by switching the drain voltage VD1 of D23, D24 from the previous high level H to the low level L for a predetermined time T2, and in the next cycle performed after the elapse of one cycle period T3. Prepare for reading sensor signals.

Similarly, when the pixel column selection signals LS3 and LS4 fall to the low level L after the period T1, respectively, the drain voltages VD3 corresponding to the third and fourth pixel columns selected at that time, respectively. Is switched to the low level L to initialize each pixel, to prepare for the reading of the sensor signal in the next cycle performed after the elapse of one cycle period T3.

Here, the pixel column selection signal LSX (X
= 1 to 4) falls to the low level L after the T1 period, the drain voltage VDX is switched to the low level L to perform initialization. The initialization timing is the pixel column selection signal LSX. During the idle period T4 of the pixel column selection in the low level L state.

The timing of the generation of the signals of the respective parts as described above is determined by driving the pixel column selecting circuit 1, the pixel selecting circuit 2, and the voltage switching circuit 5 under the control of an ECU (not shown). ing.

As described above, by initializing each pixel at an appropriate timing according to the reading scan of the sensor signal of each pixel, it is possible to reduce the excess and deficiency of the accumulation time of the entire image sensor.

Then, an image sensor having a logarithmic output characteristic having a wide dynamic range without an afterimage can be realized.

The image processing apparatus according to the present invention can easily and quickly perform a process for enhancing the contrast of an image captured by the image sensor having the logarithmic output characteristic configured as described above. Therefore, a means for converting the image data into image data that emphasizes a change in brightness in a plurality of luminance regions by using a conversion table of image data output from the image sensor is adopted.

FIG. 7 shows an example of the configuration of an image processing apparatus for that purpose.

The A / D converter 8 converts a sensor signal (analog signal) Vo of each pixel as image data output in time series from the image sensor 7 into a digital image signal DD1, DD1
From the memory 9 in which a predetermined output conversion table is set in advance in accordance with the digital image signal DD
2 is output.

According to such a configuration, no matter what output characteristics the image sensor 7 has, an arbitrary output can be achieved by using the memory 9 in which the output conversion table is set. It can be converted to characteristics.

FIG. 8 shows an example of the output conversion table set in the memory 9.

In this case, the sensor signal V of each pixel as image data output in time series from the image sensor 7
o is converted to 12 bits by the AD converter 8 (4096 gradations)
Of the digital image signal DD1.
The entire luminance region over 4096 gradations is continuously divided into a plurality of equal regions, and the change in brightness in the plurality of luminance regions is continuously emphasized.

Here, the entire luminance area is equally divided into four luminance areas A1 to A4 with an area width of 1024 gradations.
Changes in brightness in the plurality of brightness regions A1 to A4 are continuously emphasized.

By converting a digital image signal DD1 of an image captured by the image sensor 7 into a digital image signal DD2 using a conversion table as shown in FIG. 8, a luminance distribution such as a contour line of a map can be obtained.

Now, when the image photographed by the image sensor 7 has dark surroundings and increases in brightness toward the center as shown in FIG. 9, for example, the processing converted by the conversion table is performed. As shown in FIG. 10, the changed image becomes an image in which the change in brightness in each of the divided luminance areas A1 to A4 is emphasized like a contour line.

Similarly, FIG. 11 shows an image obtained by photographing a laser welded portion by the image sensor 7, and FIG. 12 shows an image obtained by processing the image using a conversion table.

As described above, by displaying an image in which the change in brightness is emphasized like a contour line on the monitor screen, the luminance of each part can be easily visually recognized.

As a procedure for measuring the luminance of each part in the image, the gradation of each part emphasized like a contour line of the image obtained by processing the image photographed by the image sensor 7 by the conversion table is set in the conversion table. Estimate from Then, using the previously measured performance characteristics of the image sensor 7 as shown in FIG. 13, the luminance of each part is calculated by comparing the gradation and the luminance estimated from the processed image.

Thus, according to the present invention, it is possible to easily and quickly evaluate an image.

When dividing the entire luminance area into a plurality of areas for each luminance area, by increasing the number of divisions,
Changes in brightness in a plurality of brightness regions can be further subdivided and emphasized.

Further, as shown in FIG. 14, the luminance region of the dark part which does not require the analysis of the luminance distribution state is widened, and the luminance region of the dark part which requires the analysis of the luminance distribution state is subdivided. If, for example, the width of the luminance region dividing the entire luminance region is set at random, the analysis of the luminance distribution state can be efficiently performed according to the purpose.

Further, as shown in FIG. 15, it is also possible to intermittently divide the entire luminance area into a plurality of areas so as to intermittently emphasize changes in brightness in the plurality of luminance areas.

At this time, a plurality of intermittently divided luminance areas are converted into intermediate gradations.

When the image photographed by the image sensor 7 shown in FIG. 16 is processed by using the conversion table shown in FIG. 15, as shown in FIG. An image is obtained in which each of the luminance regions in which the change is intermittently emphasized is expressed by a halftone.

Further, according to the present invention, since the image sensor 7 which initializes each pixel prior to photographing and suppresses the occurrence of an afterimage is used, high-speed photographing becomes possible (up to 400 frames per second). Therefore, for example, the analysis of the luminance distribution state of the illumination following the flicker of the fluorescent lamp can be performed in real time.

[0054]

As described above, in the image processing apparatus according to the present invention, the means for converting the image data outputted from the image sensor into image data which emphasizes a change in brightness in a plurality of luminance regions using the conversion table. It is possible to easily and quickly perform image processing for enhancing contrast in an image taken by an image sensor having a logarithmic output characteristic in order to expand a dynamic range. Has advantages.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram showing a configuration example of an optical sensor circuit serving as a pixel unit of an image sensor used in the present invention.

FIG. 2 is a time chart of signals of each part in the optical sensor circuit.

FIG. 3 is a diagram showing output characteristics of a sensor signal with respect to a sensor current flowing through a photodiode according to the amount of incident light of the optical sensor circuit.

FIG. 4 is a diagram illustrating output characteristics of a sensor signal read at a predetermined timing when the amount of incident light in the optical sensor circuit is small when initialization is not performed.

FIG. 5 is a block diagram illustrating a configuration example of an image sensor according to the present invention.

FIG. 6 is a time chart of signals of various parts in the image sensor.

FIG. 7 is a block diagram showing an embodiment of an image processing apparatus according to the present invention.

FIG. 8 is a diagram illustrating an example of conversion characteristics of input and output data in an output conversion table set in a memory according to the embodiment.

FIG. 9 is a diagram illustrating an example of an image captured by an image sensor.

10 is a diagram showing an example of the state of an image obtained by processing the captured image shown in FIG. 9 according to the present invention.

FIG. 11 is a diagram illustrating another example of an image captured by an image sensor.

12 is a diagram showing an example of the state of an image obtained by processing the captured image shown in FIG. 11 according to the present invention.

FIG. 13 is a diagram illustrating actual output characteristics of an image sensor with respect to a luminance distribution of a shooting target object.

FIG. 14 is a diagram illustrating another example of conversion characteristics of input and output data in the output conversion table.

FIG. 15 is a diagram showing still another example of conversion characteristics of input and output data in the output conversion table.

FIG. 16 is a diagram showing still another example of the image captured by the image sensor.

17 is a diagram showing an example of the state of an image obtained by processing the captured image shown in FIG. 16 according to the present invention.

[Explanation of symbols]

 7 Image sensor 8 AD converter 9 Output conversion table memory

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Hideo Seki 1-10-1 Shinsayama, Sayama-shi, Saitama Honda Engineering Co., Ltd. F-term (reference) 5B047 BB04 CB05 DA01 DB01 5B057 BA02 BA12 BA28 CA08 CA12 CA16 CB08 CB12 CB16 CE11 5C072 BA07 BA15 EA08 UA01 5C077 LL19 MM02 PP15 PQ23

Claims (6)

[Claims] An image processing apparatus for enhancing the contrast of an image captured by a image sensor, wherein the image processing apparatus uses a conversion table of image data output from the image sensor to enhance a change in brightness in a plurality of luminance regions. An image processing apparatus comprising means for converting data into data. 2. The entire luminance area is divided into a plurality of continuous areas.
2. The image processing apparatus according to claim 1, wherein a change in brightness in a plurality of luminance regions is continuously emphasized. 3. The brightness in a plurality of brightness areas is divided into a plurality of brightness areas continuously so that a brightness area in a dark area is widened and a brightness area in a bright area for which contrast is to be enhanced is subdivided. 2. The image processing apparatus according to claim 1, wherein the change of the image is continuously emphasized. 4. An image processing apparatus according to claim 1, wherein the entire luminance area is intermittently divided into a plurality of areas, and a change in brightness in the plurality of luminance areas is intermittently emphasized. 5. The image processing apparatus according to claim 4, wherein a plurality of intermittently divided luminance areas are converted into halftone image data. 6. The image processing apparatus according to claim 1, wherein an image captured by an image sensor having a logarithmic output characteristic is processed.