JPH11196291A - Digital camera and image reproduction system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To record almost all luminance information from a shadow part of an object to a highlight part at prescribed gradation in a digital camera. SOLUTION: A reference luminance detection means 10 detects a maximum reference luminance and a minimum reference luminance of an object from an image signal S1 obtained by a CCD image pickup means 52. Based on the detected maximum/minimum reference luminance, a gradation conversion condition is decided so that the luminance information from the minimum to the maximum luminance is recorded as entire image data. Then based on the maximum reference luminance, an exposure control means 20 applies image pickup control so that a highlight part is not saturated and an image signal S1 after the exposure control is converted into digital image data D1. In this case, according to the gradation conversion condition, gradation conversion is also conducted. The digital image data D1 subject to gradation conversion are recorded on a recording medium by a recording means 54.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital camera and an image reproducing system for reproducing an image based on an image signal (digital image data) recorded by the digital camera.

[0002]

2. Description of the Related Art Instead of a film, a solid-state image sensor (for example, a CCD or the like) is used to convert a luminance signal obtained by imaging a subject into digital image data and convert the signal into a digital image data.
Digital still cameras (hereinafter, referred to as "digital cameras" or "cameras") for recording data in a built-in memory, a hard disk, a memory card, and the like are rapidly spreading today.

The signal output of a digital camera is, for example, digital image data represented by an 8-bit (0 to 255) digital value, and this data is taken into a personal computer (personal computer) and subjected to predetermined image signal processing. Then, an image is displayed on a CRT display device or output as a print image (hard copy).

In this digital camera, since image information representing a subject image is assigned to digital image data of a limited number of bits, there is a problem that the exposure latitude at the time of imaging by the camera is as narrow as about ± 1 EV. Overexposure causes loss of high-brightness (highlight) image information. Conversely, underexposure causes noise (S / N degradation). For example,
As for the S / N degradation, the noise component is CCD
It is known that shot noise proportional to the half of the amount of incident light to the CCD and dark current noise of the CCD which does not depend on the amount of incident light are dominant.

On the other hand, conventionally, in a film type camera, automatic exposure control has been known as a method for performing appropriate exposure control. For example, there is a method of controlling the aperture and shutter speed of the camera based on the weighted average luminance and the peak luminance. In a video camera that captures a moving image, a method of performing gain control on an image signal based on the weighted average luminance and the peak luminance is often used. In addition, subject brightness (brightness)
In converting information into an image signal, a method of changing a conversion condition according to an imaging state has also been proposed (JP-A-8-1901).
21, JP-A-6-253176).

[0006]

Here, a main subject (for example, a face) is set on the basis of a subject reflectivity of about 100% (white).
Is generally designed to provide an appropriate tone reproduction. However, in the method of performing exposure control based on the peak luminance, the reflectance of the main subject is different from the illumination light of the main subject, such as backlight, or a high reflectance of 100% or more, such as over-directed light. Is not reproduced at an appropriate gradation.

In the method of performing exposure control based on the weighted average luminance, a highlight portion or a background image in backlit shooting is saturated. It may not be possible to reproduce the image.

[0008] The method disclosed in Japanese Patent Application Laid-Open No. 8-190121 is a method of selecting a gamma look-up table for suppressing contrast.
When a strobe is used, a desired image cannot be obtained because the contrast is not always high. That is, for example, when the face that is the main image is large due to close-up shooting, the face tends to be an image without contrast, and thus suppressing the contrast results in an even more undesirable image. In addition, strobe light emission during daytime synchro exposure is used to suppress contrast, but in this case, if the contrast is suppressed by a camera, the suppression may be excessive. That is, this method has a problem in that when a strobe is used, the contrast is always controlled to be high.

On the other hand, the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 6-253176 is a method of correcting a gradation by obtaining a correction coefficient of a gradation characteristic based on a feature amount in a captured image. According to this method, the feature amount is a value obtained from the shape of the histogram of the luminance signal.
The type of scene from forward light to backlight is determined based on the number of high-luminance pixels, and gradation conversion is performed by selecting a predetermined gradation correction characteristic so as to obtain an optimum gradation. In this way, it is possible to prevent black and white loss as much as possible in accordance with the imaging scene and to obtain an output image with rich gradation expression. However, in this method, although an attempt is made to prevent black and white loss as much as possible, the main purpose is to obtain an optimum gradation according to a scene.
It cannot completely prevent black and white loss.

When the exposure control method of the above various cameras is used as an exposure control method of a digital camera, a high reflectance portion of an image signal which is higher than white (referred to as "specular
White (specular white) ". ) Easily saturates to the highest data value (for example, 255), which prevents the saturation of the image signal from being prevented. Such specular white exists in many images without intention, such as a still life in a subject, a specular reflection portion of a close-up person, and a specular reflection portion in strobe light emission, and expresses the texture and atmosphere of the subject during image reproduction. This is an important factor above. Also, depending on the accuracy of the exposure control and the variation of the strobe light emission control, a subject or background having a high luminance near white often saturates to the maximum data value, which deteriorates the quality of a reproduced image. .

On the other hand, if the amount of exposure is reduced to make it possible to reproduce a high-luminance portion including specular white, the low-luminance portion (shadow portion) may lose information due to insufficient exposure, or may generate noise even if it is not lost. There is a problem that the image is included.

As described above, even when the conventional exposure control method is applied to a digital camera, it is possible to capture and record all the subject information from a shadow portion to a high brightness (highlight) portion including specular white. Have difficulty.

Further, in the above-described various exposure control methods, generally, as the precision of the exposure control is increased, the main image portion is imaged so as to have a constant value, and the background portion is likely to be saturated with the maximum data value. Therefore, it is difficult for the above-described exposure control method to faithfully reproduce the subject image including the background and the like.

The present invention has been made in view of the above circumstances, and is intended to record and reproduce all possible subject image information including a highlight portion such as a specular white portion or a background of a backlight scene. It is an object of the present invention to provide a digital camera and an image reproduction system that can perform the operation.

[0015]

A digital camera according to the present invention converts a luminance signal obtained by imaging a subject into digital image data and records the digital image data on a recording medium. Detecting means for detecting the image signal so that the detected maximum reference luminance corresponds to a predetermined luminance signal value (a value at which the specular white portion does not saturate, for example, a rated value of 1.0 V _PP ). Exposure control means for performing exposure control, and image signal processing means for converting a luminance signal into digital image data so that a luminance signal corresponding to the maximum reference luminance corresponds to a substantially maximum value of the digital image data. It is characterized by the following.

[0016] The image signal processing means of the digital camera includes a judging means for judging whether or not the maximum reference luminance is within a predetermined value, and a judgment means for judging whether the maximum reference image luminance is out of the predetermined value. It is preferable that the image processing apparatus further includes a gradation conversion means for converting the gradation of the digital image data so as to fall within the predetermined value.

Further, the image signal processing means includes: arithmetic means for obtaining, from the digital image data, main image part data which is digital image data corresponding to the luminance of the main part of the subject; Comparing means for comparing the maximum reference image data, which is digital image data corresponding to the maximum reference luminance, with a predetermined gradation of digital image data from the main image portion data to the maximum reference image data based on the comparison result And a gradation conversion means for converting the gradation of the digital image data so as to achieve the gradation of the digital image data.

A digital camera according to the present invention converts a luminance signal obtained by imaging a subject into digital image data and records the digital image data on a recording medium, and detects a maximum reference luminance and a minimum reference luminance of the subject. Detecting means, exposure control means for performing exposure control so that the detected maximum reference luminance corresponds to a predetermined luminance signal value, and
The luminance signal corresponding to the maximum reference luminance corresponds to a substantially maximum value of the digital image data, and even if the luminance signal corresponding to the minimum reference luminance is small, the luminance signal corresponds to a substantially predetermined minimum value of the digital image data. Image signal processing means for converting a luminance signal into digital image data.

Here, "at least a predetermined minimum value of the digital image data at least" is not limited to the minimum value (for example, 0) of the digital image data, and is not limited to the image reproduction density range (for example, the normal reproduction range). Is the minimum value determined from the viewpoint of 2.0) at an appropriate optical density, for example, a value of about tenth place is used.

[0020] In this digital camera, the image signal processing means is configured to convert the maximum reference image data which is digital image data corresponding to the maximum reference luminance and the minimum reference image data which is digital image data corresponding to the minimum reference luminance. Determining means for determining whether or not the digital image data is within a predetermined value; and determining whether the digital image data falls within the predetermined value when one of the maximum reference image data and the minimum reference image data is outside the predetermined value. It is desirable to include a tone conversion means for converting a tone.

Further, the image signal processing means includes a comparing means for comparing the maximum reference image data with the minimum reference image data, and based on a result of the comparison, a signal from the minimum reference image data to the maximum reference image data. The image processing apparatus may include a gradation conversion unit that converts the gradation of the digital image data so that the gradation of the digital image data becomes a predetermined gradation.

The image signal processing means may include: arithmetic means for obtaining, from the digital image data, main image part data which is digital image data corresponding to the luminance of the main part of the subject; Comparing the maximum reference image data and the minimum reference image data,
Based on the comparison result, the digital image data is set so that the gradation of the digital image data from the main image portion data to the maximum reference image data and from the main image portion data to the minimum reference image data becomes a predetermined gradation. And a gradation conversion means for converting the gradation.

Further, in any of the above digital cameras, it is more preferable that the processing conditions of the image signal processing means be recorded together with digital image data.

Here, the "processing condition" refers to which of the luminances (the maximum reference luminance, the minimum reference luminance, and the luminance of the main part of the subject) is determined based on the luminance or the gradation conversion. It means various processing conditions in the digital camera, such as what the conditions are.

An image reproducing system according to the present invention is based on the digital camera according to the present invention and digital image data recorded by the digital camera (ie,
An image reproducing apparatus for obtaining a reproduced image (by reading recorded digital image data).

In this image reproducing system, if the digital camera records the processing conditions of the image signal processing means together with the digital image data, the recorded processing conditions and the digital image data are read out. It is preferable to obtain a reproduced image based on the read processing conditions and image data.

In the above description, "maximum reference luminance" or "minimum reference luminance" refers to a value near the maximum value or the minimum value of the subject luminance, and the maximum value in accordance with predetermined conditions. Alternatively, the luminance value is a reference value on the minimum side. For example, values corresponding to the maximum brightness and the minimum brightness of the subject brightness may be used. Also, a histogram of values (for example, brightness signal, image data, etc.) corresponding to the subject brightness is obtained, and the maximum brightness side, A limit luminance value occupying a predetermined ratio on the minimum luminance side may be used,
Alternatively, a predetermined value from the histogram of the value corresponding to the subject luminance (for example, 0.
3% low luminance, and 0.3% high luminance from the minimum luminance value). Further, the conditions for determining the maximum reference luminance and the minimum reference luminance may be changed depending on the color and the subject.

The "luminance of the main part of the subject" refers to the luminance corresponding to a human face, the average luminance of a main image area excluding a large-area, non-contrast area (for example, the sky or dark area), and the like. is there. Alternatively, the average image value may be an average image value in a central area of the screen or an average image value in a predetermined area including a focus point. These average image values may be weighted average values obtained by assigning a weight of 1.0 or less to the peripheral area of the area. Furthermore, the average image value of the skin color area in the center area of the screen or the predetermined area including the focus point, or the characteristic of the face such as the area where the skin color area of the center area of the screen or the predetermined area including the focus point is substantially circular, is shown. An average image value from the region can also be used.

[0029]

The digital camera according to the present invention performs exposure control based on the maximum reference luminance to such an extent that the luminance signal of the specular white part or the background part of the backlight scene is not saturated (the same applies to digital image data). Because
Highlights such as the specular white portion and the bright background can be reliably captured as signals.

Further, by changing the gradation of the image data in accordance with the maximum reference luminance, the data from the shadow portion to the highlight portion is recorded so as to be image data of a predetermined number of bits (for example, 8 bits). be able to. That is, almost all subject luminance information can be recorded.

It is also possible to change the gradation of the image data according to the difference between the subject brightness and the brightness of the main part of the subject, and to set a proper data area (for example, a specular white portion near the maximum data value 255). Allows all image data to be recorded.

Further, the tone of the digital image data is changed in accordance with the imaging scene based on the maximum reference luminance, the maximum reference luminance and the minimum reference luminance, or the maximum reference luminance and the luminance of the main image portion, thereby obtaining the main image. It is also possible to reduce the variation in the reproduction density of each part.

Furthermore, by recording the gradation conversion conditions and the like together with the digital image data, the gradation conversion conditions and the like are read out at the time of image reproduction, and the image is read with reference to the read gradation conversion conditions and the like. By reproducing, it is possible to reproduce the background and the like as necessary or to suppress the reproduction.

[0034]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a circuit block diagram showing a basic configuration of a digital camera according to an embodiment of the present invention.
This digital camera receives a control pulse P1 from the CCD control means 58 and captures an image of a subject to obtain an image signal S1.
Image signal obtained by the imaging means 52 and the CCD imaging means 52
A reference luminance detecting means 10 for detecting a reference luminance (a maximum reference luminance, a minimum reference luminance, etc.) from S1, and an exposure control means 20 for performing exposure control of image pickup based on the maximum reference luminance Y _max detected by the reference luminance detecting means 10. An image signal processing unit 40 that performs predetermined gradation conversion based on the maximum reference luminance Y _max and the minimum reference luminance _min detected by the reference luminance detection unit 10 and converts the image signal S1 into digital image data D1; A recording unit 54 for recording digital image data D1 output from the processing unit 40 on a recording medium (not shown), and a CPU 56 for centrally controlling each unit of the camera (especially, the image signal processing unit 40, the recording unit 54, and the CCD control unit 58). Become. A control pulse P2 is input from the CPU 56 to the CCD control means 58,
The CPU 56 receives the control signal C3 from the exposure control means 20 and
Control pulses finally input to the CCD imaging means 52 (horizontal and vertical drive pulses for driving the CCD, etc.)
P1 is output from the CCD control unit 58.

Next, the operation of the digital camera having the above configuration will be described with reference to FIGS.

The flowchart shown in FIG. 4 shows the flow of basic processing by the CPU 56 of the digital camera. It should be noted that various processing steps are indicated by symbols marked with # in the figure, and will be referred to in the following description.

First, prior to the main photometry (imaging), a distance measurement (step 1; # 1 in the drawing; hereinafter, equivalent) and pre-photometry (imaging) (# 2) are performed, and photometry is performed by the pre-photometry. A maximum reference luminance and a minimum reference luminance are detected from the photometric value according to a predetermined definition (# 3). next,
The object luminance information (luminance signal) is converted into digital image data, and a conversion condition for gradation conversion is determined so that the image data falls within the range of 8 bits from 0 to 255 (#
5) Further, exposure control is performed based on the detected maximum reference luminance (# 6). Next, main exposure (image pickup) is performed by controlling the exposure of the camera (# 7), and the obtained image data is converted into 8-bit digital image data according to the above conversion conditions (# 8), and is recorded on a recording medium. . The CPU 56 controls the flow of this series of processing. The details will be described below.

When imaging of a subject is started, first, a distance measurement is performed to focus on the subject, and the captured image is focused (# 1).

Next, pre-metering for measuring the luminance of the subject is performed (# 2). This photometry may be a photometry using a dedicated photometry sensor, or may be a photometry using an image signal S1 obtained by the CCD image pickup means 52 (in this case, it is more than photometry. Imaging is appropriate.). The following description will be made assuming that photometry (imaging) is performed using the image signal S1, but when a photometry sensor is used, it may be considered as a luminance value corresponding to the image signal.

Next, the maximum reference luminance and the minimum reference luminance are detected by the reference luminance detecting means 10 using the image signal (can be regarded as a luminance signal) S1 obtained by the CCD image pickup means 52 (# 3). . This reference luminance is detected by creating a histogram of the luminance (the image signal S1 in this example). FIG. 3 shows an example of the histogram of the image signal S1 with the horizontal axis representing the value of the image signal S1 and the vertical axis representing the frequency. If the horizontal axis represents the luminance value corresponding to the image signal S1, a histogram of the luminance is obtained. Can also be considered. FIG.
In (A), in the case of overexposure, the image signal S1 is represented by a histogram a. Here, the luminance signal values corresponding to the maximum reference luminance and the minimum reference luminance each have a frequency ratio on the histogram of 1% (a hatched portion in the figure), and a high luminance side and a low luminance side limit value. Then, the maximum reference luminance is Y _max1 and the minimum reference luminance is Y _min1 . Conversely, in the case of underexposure, the image signal S1 is represented by the histogram b, and the maximum reference luminance is _Ymax2 and the minimum reference luminance is _Ymin2 . Since the maximum reference luminance and the minimum reference luminance are the luminance (brightness) of the subject, their values do not change before and after the exposure control. However, the image signal S1 obtained by the CCD image pickup means 52 is obtained before and after the exposure control. Since the value is changed, FIG. 3 in which the axis of the image signal S1 is fixed and displayed is merely illustrated as if the values of the maximum reference luminance and the minimum reference luminance were changed. Less than,
If it is not necessary to treat them with particular distinction, the maximum reference luminances Y _max0 , Y _max1 , and Y _max2 are collectively described as Y _max , and the minimum reference luminances Y _min0 , Y _min1 , and Y _min2 are _collectively described as Y _min .

The maximum reference luminance and the minimum reference luminance are not limited to the above examples, and may be values corresponding to the maximum luminance and the minimum luminance of the object luminance. Also, a predetermined value from the histogram of the value corresponding to the subject luminance (for example, the luminance on the low luminance side of 0.3% from the maximum luminance value on the histogram, and the luminance on the high luminance side of 0.3% from the minimum luminance value). Is also good. Further, the conditions for determining the maximum reference luminance and the minimum reference luminance may be changed depending on the color and the subject.

The maximum reference luminance Y thus detected
_max is input to the exposure control means 20. Exposure control means 20
Performs exposure control by controlling the aperture or shutter speed so that the luminance signal level corresponding to the maximum reference luminance Y _max becomes the predetermined value S1 _STD (# 6). Also,
The exposure control may be performed by a so-called electronic shutter by controlling the timing of the drive pulse of the CCD. It is desirable that the predetermined value S1 _STD be at a level at which specular white can be imaged to some extent (that is, the number of saturated pixels is about 1% or less of all pixels).

When the exposure control is performed in this manner, the image signal S1 is represented by a histogram c, and the maximum reference luminance is Y _max0 and the minimum reference luminance is Y _min0 . That is, by performing the exposure control as described above based on the maximum reference luminance Y _max , almost all of the subject luminance is reduced to the image signal.
It will fall within the level (0 to S1 _STD ) that can be processed as S1.

After the exposure level is determined in this way, the main photometry (imaging) is performed (# 7), and the obtained image signal S1 is converted into 8-bit digital image data D1 by the image processing means 40.
(# 8). In this case, S1 _STD level of the image signal S1 corresponding to the maximum reference luminance Y _max gradation conversion is performed such that the data values 255. The image data D1 is input to the recording means 54 and recorded on a recording medium (not shown). When converting the image signal S1 to the image data D1,
It is not necessary to correspond to 255, and a value in the vicinity thereof may be used. Then, even if the luminance of the subject is out of the range of the minimum reference luminance _Ymin to the maximum reference luminance _Ymax , the luminance can be handled as data, and the texture and the like can be faithfully reproduced when the image is reproduced. Will be able to

Next, the operation of gradation conversion in the image processing means 40 will be described in detail with reference to FIG.

As described above, when exposure control is performed based on the maximum reference luminance _Ymax and the subject luminance is converted into digital image data, the image data D1 is represented by the histogram a or b in FIG. Then, when exposure control is performed properly, even without performing gradation conversion,
The image data is converted into digital image data at an appropriate level from the low luminance side to the high luminance side (that is, data values 0 to 255). However, the histogram of the subject brightness is not always such. For example, FIG.
As shown in FIG. 3B, in the case where the range of the subject luminance is wide and the histogram of the image signal S1 after exposure control has a shape as shown by d in FIG. 3B, information on the low luminance side is discarded. And will not appear as a signal. Conversely, in the case where the range of the subject luminance is narrow and the histogram of the image signal S1 after exposure control has a shape as shown by e in FIG. 3B, there is much room on the low luminance side, and The range of possible signals cannot be used effectively. As described above, when the range of the subject luminance is narrow or wide, the low luminance side is not converted into digital image data at an appropriate level (that is, the low luminance side is not effectively used).

Therefore, as shown in FIG.
By adopting a configuration corresponding to various gradation conversion processes as 40, it is possible to make almost all the subject brightness fall within the range from 0 to 255 as digital image data. That is, when the range of the subject luminance is narrow or wide as in the histograms d and e in FIG. 3B, the minimum reference luminance Y _min is detected, and the minimum reference luminance Y _min is also approximately equal to the image signal S1. What is necessary is just to convert the gradation of the image data so as to correspond to the 0 level. By doing so, the image signal S1 becomes as shown by the histogram f, and it is possible to always keep the entire subject brightness within the reproducible level (0 to S1 _STD ) of the image signal S1 regardless of the subject brightness range.

Here, the "0 level" is not limited to the minimum value 0 of the digital image data, but is determined from the viewpoint of an image reproduction density range (for example, an optical density that can be normally reproduced is 2.0). The minimum value may be any value, for example, a value of about tenth place may be used instead of 0. Hereinafter, “0 level” corresponding to the minimum reference luminance Y _min also includes “10 levels” in the case of the above-described example (about the tenth value).

For example, the maximum reference luminance Y _max0 and the minimum reference luminance Y _min0 after the exposure control is performed (or the maximum and minimum reference _values which are digital image data corresponding to the maximum reference luminance Y _max0 and the minimum reference luminance Y _min0). Image data D _max , D
_min ; FIG. 2 shows the reference image data. ) Is input to the judgment means 44, and both reference image data D _max and D _min are inputted.
Are predetermined values (2 for the maximum reference image data D _max) .
It is determined whether or not they are in the vicinity of 55 and the minimum reference image data D _min (near 0) (# 4). When it is determined that they are not within the predetermined value, both the reference image data D _max and D _min are determined. 0
A gradation conversion condition J1 (for example, the value of gamma γ) that falls within the range from # to 255 is determined (# 4). At this time, the condition of gradation conversion may be changed according to the degree of deviation from a predetermined value. The gradation conversion conditions determined in this way
J1 is input to the gradation conversion means 41, and digital image data D0
Is subjected to gradation conversion according to the gradation conversion condition J1 (see FIG. 2A). Image data D2 subjected to gradation conversion
Is input to the recording means 54, and this data D2 is recorded (not shown). Further, whether the gradation conversion condition is determined based on the maximum reference luminance, the minimum reference luminance, or the like, or various processing conditions such as the gradation conversion condition are recorded together with the digital image data, and the image reproduction system described later. The image may be reproduced according to the processing conditions.

The reference image data D _max and D _min are compared (actually, a difference or a ratio is obtained), and the reference image data D _max and D _min are in the range from 0 to 255 in the same manner as described above. The gradation of the image data D0 may be converted by defining a gradation conversion condition J2 that can barely be accommodated (see FIG. 2B). For example, when the difference is large, the gradation may be softened, and when the difference is small, the gradation may be hardened.

Further, the main image part data D _main which is digital image data corresponding to the luminance of the main part of the subject is obtained by the calculating means 48, and the main image part data D _main , the maximum reference image data D _max , the main image part data D _main is compared with the minimum reference image data D _min, and based on the comparison result, both the reference image data D _max and D _min are changed from 0 to 2
Gradation conversion condition J3 that fits within the range up to 55
May be determined to convert the gradation of the image data D0 (see FIG. 2).
(C)). At this time, it may be determined whether the main image portion data D _main is wider on the low luminance side or the high luminance side as the data range, and only one of them may be subjected to gradation conversion.

The main image portion can use the luminance corresponding to a human face, the average luminance of the main image region excluding a large-area non-contrast region (for example, a sky or a dark portion), or the like. Alternatively, the average image value may be an average image value in a central area of the screen or an average image value in a predetermined area including a focus point. These average image values may be weighted average values obtained by assigning a weight of 1.0 or less to the peripheral area of the area. Further, the average image value of the skin color area of the predetermined area including the center of the screen or the focus point, or the area where the skin color area of the predetermined area including the center of the screen or the focus point is substantially circular,
An average image value from an area showing a feature of a face can be used (for example, Japanese Patent Application Laid-Open No. 52-156624,
-346332, JP-A-8-122944).

When performing gradation conversion of image data, a gradation conversion condition is determined by using a look-up table (LUT).
After converting the image signal S1 into digital image data, data conversion may be performed based on this LUT. In this case, it is desirable that the image signal S1 is first converted into digital image data of 8 bits or more (for example, 10 bits), and then converted into 8-bit data. Further, the LUT may be rewritten by a functional expression or a conditional expression based on the maximum reference image data, the minimum reference image data, and the main image portion data. Alternatively, a plurality of LUTs may be prepared and the maximum reference luminance, the minimum reference luminance, the main image A desired LUT may be selected according to the set data. Further, the image signal S1 may be converted into digital image data after converting the gradation.

In the above-mentioned gradation conversion processing, the maximum reference image data D _max and the minimum reference image data D
_It is not necessary to make a determination or the like for both _min and _min, and a determination or the like may be made for only the maximum reference luminance _Ymax . For example, if the maximum reference luminance _Ymax is a large value, it may be determined that the luminance range is wide and the gradation may be softened.

The exposure control processing may correspond to the flash control as shown in FIG. The flow chart shown in FIG. 5 is a modification of the flow chart shown in FIG. 4 so as to be compatible with the flash control. Step # 32 for determining the necessity of the auxiliary light between # 3 and # 4 is performed. A step # 90 of performing strobe light control is added to the branch. In step # 32,
The necessity of strobe light emission is determined from the obtained reference luminance value. When it is determined that the strobe light is necessary, in steps # 4 to # 6, the exposure control amount is set to a predetermined value instead of the processing for exposure control as shown in FIG. After the transition, the strobe light is controlled so that the reference luminance value falls within a predetermined range, and the actual photometry (imaging) is performed. In the case of flash emission, a predetermined gradation conversion condition is used.

When strobe light emission is not required, the process is the same as that shown in FIG.

FIG. 6 shows another example corresponding to the flash control. The flowchart shown in FIG. 6 shows the flow of processing corresponding to the pre-strobe light emission. In the case of strobe light, the same luminance signal cannot always be obtained. Therefore, image data is stored in a memory or the like, and a gradation conversion condition of the image data is determined, and then the image data is converted to image data based on the determined gradation conversion condition. Tone conversion may be performed.

In the process of the flash control shown in FIG. 5, in the case of the flash light emission, a predetermined gradation conversion condition is used. By performing the processing of steps # 2 to # 5 to obtain the gradation conversion conditions in advance,
Even in the case of strobe light emission, appropriate gradation conversion similar to that shown in FIG. 4 can be performed. On this occasion,
By referring to the relationship between the light emission amounts of the pre-strobe light emission and the main flash light emission, appropriate gradation conversion can be performed even if there is a difference between the two light emission amounts. When performing pre-strobe light emission, a so-called red-eye reduction function may be activated.

As described above, by properly controlling the exposure and performing the predetermined gradation conversion, the digital image data corresponding to the subject brightness from the specular white to the shadow portion has an approximate data value of 0. To 255.

Next, with reference to FIG. 7, how the gradation conversion is performed in accordance with the luminance of the subject will be specifically described.

FIG. 7 shows the object reflectance and image data (QL (Qu
antum Level) value is 100%
CCD output value corresponding to (white) is image data (QL value) 2
55, the CCD output value corresponding to the subject reflectance of 0% (black) is shown corresponding to the image data (QL value) 0. Here, the image data (QL value) is 8-bit image data after gradation conversion.

Here, even if the subject reflectance is 100%, the subject brightness is variously different, there is a subject having a reflectance of 100%, and the reflectance of 100% is controlled by the exposure control of the camera.
The relationship between the CCD output value and the image data when the photometry (imaging) can be performed as the CCD output value without the saturation of the object of% is shown.

It is to be noted that, although the luminance of a subject can be normally measured but the reflectance of the subject cannot be measured, for example, a specular white image portion having a high reflectance of 100% or more exists in all images at a certain percentage. By assuming that the maximum luminance excluding is 100% (for example, 1% or 3%), the object reflectance can be estimated from the object luminance. FIG. 7 shows the relationship between the subject reflectance and the image data (QL value) by estimating the subject reflectance from the subject brightness from such a viewpoint.

Normally, the QL value of a digital camera is proportional to the 1 / 2.sup.th power of the luminance E of the subject, so that a standard gradation conversion condition is to set the CCD output value of 0.
Forty-fiveth power gradation (the line indicated by A in the figure) is used.

The subject brightness difference (ie, the difference or ratio between the maximum reference brightness and the minimum reference brightness) is equal to or greater than a predetermined value, or the subject reflectance is less than or equal to a predetermined value when the subject reflectance at the maximum reference brightness is 100%. In this case, a gradation lower than the power of 0.45 is used. At this time, the subject luminance and the reflectance may be converted into image data and determined, and the gradation may be changed.

If the subject luminance difference is large, the power is 0.45 or less,
For example, a gradation of the power of 0.40 is used. Conversely, if the subject luminance difference is small, a gradation of the power of 0.45 or more, for example, the power of 0.50 (the line indicated by B in the figure) may be used.

When the subject luminance difference is equal to or larger than a predetermined value,
A gradation value is determined so that the maximum reference luminance and the minimum reference luminance become predetermined image data values (for example, a value near 255 for the maximum reference luminance and a value near 0 for the minimum reference luminance), and gradation conversion is performed. You may.

Alternatively, depending on whether the main image portion is on the highlight side or the shadow side, partial gradation change on the highlight side or the shadow side may be performed. For example, as in the gray scale shown by C in FIG.
The gradation conversion may be performed only on the highlight side or the shadow side while setting the gradation to the 50th power.

Next, an embodiment of an image reproducing system for obtaining a reproduced image from digital image data recorded by the digital camera according to the present invention described above will be described.

FIG. 8 shows the configuration of an embodiment of an image reproducing system according to the present invention. This image reproducing system captures a subject (not shown) and records digital image data D1 carrying subject image information on a recording medium 3, and reads out image data D1 recorded on the recording medium 3. And an image reproducing device 2 for obtaining a reproduced image. The image reproducing device 2 reads out the image data D1 from the recording medium 3 and performs predetermined signal processing by an image signal processing means 2a to obtain a reproduced image signal (either an analog signal or a digital signal), and a CRT display device 2b And a hard copy by the printer 2c. In addition, as the digital camera 1, the various digital cameras according to the present invention described above can be used.

The processing condition J for the digital camera 1
Is used, the image signal processing means 2a
In the signal processing according to the above, the processing condition J recorded together with the image data D1 is also read, and the signal processing is performed with reference to the read processing condition J so that an appropriate reproduced image can be obtained. become. For example, the gradation conversion conditions at the time of recording are read, and the read image data is read.
By performing gradation conversion on D1 in accordance with gradation conversion conditions having characteristics reverse to those at the time of recording, the original gradation state is restored and an appropriate reproduced image can be obtained. Further, by referring to whether the gradation conversion condition at the time of recording is determined based on the maximum reference luminance, the minimum reference luminance, or the like, more appropriate gradation restoration can be performed.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram showing a basic configuration of a digital camera according to the present invention.

FIGS. 2A to 2C are diagrams for explaining the effect of gradation conversion in an image processing unit.

FIG. 3 is a diagram illustrating an example of a histogram of an image signal.

FIG. 4 is a flowchart showing a basic processing flow of the digital camera according to the present invention.

FIG. 5 is a flowchart illustrating an example of another process.

FIG. 6 is a flowchart illustrating an example of another process.

FIG. 7 is a diagram showing the relationship between subject reflectance and image data (QL value).

FIG. 8 is a diagram showing a configuration of an embodiment of an image reproduction system according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Digital camera 2 Image reproducing device 3 Recording medium 10 Reference luminance detecting means 20 Exposure control means 40 Image signal processing means 52 CCD imaging means 54 Recording means 56 CPU 58 CCD control means

Claims (10)

[Claims] 1. A digital camera for converting a luminance signal obtained by imaging a subject into digital image data and recording the digital image data on a recording medium, wherein: a detecting means for detecting a maximum reference luminance of the subject; Exposure control means for performing exposure control of the imaging so as to correspond to a predetermined luminance signal value, and a luminance signal such that a luminance signal corresponding to the maximum reference luminance corresponds to a substantially maximum value of the digital image data. A digital camera, comprising: image signal processing means for converting image data into digital image data. 2. The image signal processing means determines whether or not the maximum reference luminance is within a predetermined value. The image signal processing means determines whether the maximum reference luminance is outside the predetermined value. 2. A digital camera according to claim 1, further comprising a gradation conversion unit for converting the gradation of the digital image data so as to be within. 3. An image signal processing means for calculating, from the digital image data, main image part data which is digital image data corresponding to the luminance of the main part of the subject; Comparing means for comparing the maximum reference image data, which is digital image data corresponding to the maximum reference luminance, with a predetermined gradation of digital image data from the main image portion data to the maximum reference image data based on the comparison result 2. A digital camera according to claim 1, further comprising: a gradation conversion means for converting the gradation of the digital image data so as to obtain the gradation of the digital image data. 4. A digital camera for converting a luminance signal obtained by imaging a subject into digital image data and recording the digital image data on a recording medium, comprising: detecting means for detecting a maximum reference luminance and a minimum reference luminance of the subject; Exposure control means for performing exposure control so that the maximum reference luminance corresponds to a predetermined luminance signal value, such that a luminance signal corresponding to the maximum reference luminance corresponds to a substantially maximum value of the digital image data, and A digital camera, comprising: image signal processing means for converting a luminance signal into digital image data such that a luminance signal corresponding to a reference luminance is small, but corresponds to a substantially predetermined minimum value of the digital image data. 5. The image signal processing means according to claim 1, wherein maximum reference image data as digital image data corresponding to said maximum reference luminance and minimum reference image data as digital image data corresponding to said minimum reference luminance are within predetermined values. Determining means for determining whether or not there is, and converting the gradation of the digital image data so as to fall within the predetermined value when one of the maximum reference image data and the minimum reference image data is outside the predetermined value. 5. The digital camera according to claim 4, further comprising a gradation conversion unit. 6. A comparison means for comparing the maximum reference image data with the minimum reference image data, wherein the image signal processing means performs a comparison between the minimum reference image data and the maximum reference image data based on a result of the comparison. 5. The digital camera according to claim 4, further comprising: gradation conversion means for converting the gradation of the digital image data so that the gradation of the digital image data becomes a predetermined gradation. 7. An image signal processing means for calculating, from the digital image data, main image part data which is digital image data corresponding to the luminance of the main part of the subject; Comparing the maximum reference image data and the minimum reference image data, and based on the comparison result, digital image data from the main image portion data to the maximum reference image data and from the main image portion data to the minimum reference image data 5. The digital camera according to claim 4, further comprising a gradation conversion unit for converting the gradation of the digital image data so that the gradation of the digital image data becomes a predetermined gradation. 8. The digital camera according to claim 1, wherein processing conditions of said image signal processing means are recorded. 9. An image reproducing apparatus comprising: the digital camera according to claim 1; and an image reproducing apparatus that obtains a reproduced image based on digital image data recorded by the digital camera. system. 10. An image reproducing system, comprising: the digital camera according to claim 8; and an image reproducing apparatus that obtains a reproduced image based on processing conditions recorded on the digital camera and digital image data.