JP4043574B2 - Digital camera and recording / reproducing method of digital camera - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a digital camera and a recording / reproducing method for recording digital image data in the digital camera and reproducing an image based on the recorded data.
[0002]
[Prior art]
Using a solid-state imaging device (eg, CCD) instead of film, the luminance signal obtained by imaging the subject is converted into digital image data and recorded on a recording medium (eg, built-in memory, hard disk, memory card, etc.) Digital still cameras (hereinafter referred to as “digital cameras” or “cameras”) are rapidly spreading today.
[0003]
The signal output of the digital camera is, for example, digital image data represented by a digital value of 8 bits (0 to 255). This data is taken into a personal computer (PC) and subjected to predetermined image signal processing. An image is displayed on a CRT display device or output as a print image (hard copy).
[0004]
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. When the exposure becomes excessive, high brightness (highlight) image information is lost. Conversely, underexposure causes a noise problem (S / N degradation). For example, regarding S / N degradation, shot noise proportional to the 1/2 power of the incident light quantity to the CCD and dark current noise of the CCD independent of the incident light quantity are dominant as noise components. Noise is generated dramatically.
[0005]
In addition, since the object reflectance is assigned to the image data having a limited number of bits, when converted to the object density, the density resolution of the low-brightness (shadow) image part density becomes coarser as the shadow part, and the noise is further expanded. .
[0006]
On the other hand, conventionally, in a film type camera, automatic exposure control is known as a method for performing appropriate exposure control. For example, it is a method of controlling the aperture and shutter speed of the camera based on the weighted average luminance and peak luminance. In video cameras that capture moving images, a method of performing gain control on an image signal based on the weighted average luminance and peak luminance is often used. Furthermore, a method has been proposed in which the conversion condition is changed depending on the imaging state when converting subject luminance (brightness) information into an image signal (see Japanese Patent Laid-Open Nos. 8-190121 and 6-253176).
[0007]
[Problems to be solved by the invention]
By the way, when image data obtained by imaging with a camera is displayed on a CRT display device without performing image processing, the main image has a fixed value (for example, the 30% point close to the reflectance of the face is 8 bits). It is desirable that the image data has a value of 148). In this case, since the dynamic range of the digital camera is narrow, image data corresponding to the luminance of the high luminance (highlight) portion and the background image is likely to be saturated, but this is not a problem in practice. On the other hand, when image data from a digital camera is subjected to predetermined image processing and printed out (hard copy) by a digital printer or the like, all of the subject luminance from the shadow portion to the highlight portion is recorded as image data. It is desirable that As described above, it is necessary to change the correspondence relationship of the image data with respect to the subject luminance in accordance with the intention of what kind of display medium the image data obtained by capturing with the digital camera is output. For this purpose, it is necessary to change the exposure control method in accordance with each intention, and it is desirable to change the gradation when converting the luminance information into image data.
[0008]
However, even if any of the above exposure control methods is applied to a digital camera, such a request cannot be satisfied. Further, the gradation of the image data cannot be changed according to the above request by the methods disclosed in the above Japanese Patent Laid-Open Nos. 8-190121 and 6-253176. In particular, the high reflectance part of the image signal that is higher than white (referred to as “specular white”) is not saturated, and the data value of the main image part is kept at a predetermined value. None of the subject information from the shadow part to the high brightness part including specular white can be captured and recorded.
[0009]
The present invention has been made in view of the above circumstances, and it is possible to select and record an optimal recording method in advance according to which output device outputs image data recorded by a digital camera. Digital turtle La It is intended to provide.
[0010]
[Means for Solving the Problems]
The digital camera according to the present invention uses a luminance signal obtained by imaging a subject. Read for image output by output device A digital camera that converts digital image data into a recording medium and records the digital image data corresponding to the main image portion luminance of the subject to a predetermined value, and corresponds to the reference luminance of the subject One of the methods for recording so that the reference image data, which is digital image data, is a predetermined value. According to the preselected output device The image recording method selecting means for selecting is provided.
[0011]
In this digital camera,
Detecting means for detecting the reference luminance and the main image portion luminance;
First exposure control means for performing exposure control so that a luminance signal corresponding to the reference luminance detected by the detection means becomes a predetermined value; and
A second exposure control unit that performs exposure control so that a luminance signal corresponding to the main image portion luminance detected by the detection unit has a predetermined value;
It is preferable that the image recording method selection unit selects one of the first exposure control unit and the second exposure control unit.
[0012]
Also Digital camera according to the present invention Take Detection means for detecting an image feature value from an image, processing condition determination means for determining a gradation processing condition for converting a luminance signal into digital image data based on the image feature value detected by the detection means, Gradation processing means for converting a luminance signal into digital image data based on the gradation processing conditions determined by the processing condition determination means, and recording means for recording the determined gradation processing conditions together with digital image data; The further Having Is desirable .
[0014]
In the above, “reference brightness” indicates a reference value of subject brightness, for example, “maximum reference brightness” or “minimum reference brightness”, which is near the maximum value or the minimum value of subject brightness. This is a luminance value serving as a reference on the maximum side or the minimum side according to a predetermined condition. For example, it may be a value corresponding to the maximum luminance and the minimum luminance of the subject luminance, or a histogram of values corresponding to the subject luminance (for example, luminance signal, image data, etc.) is obtained, and the maximum luminance side, A limit luminance value that occupies a predetermined ratio on the minimum luminance side may be used, or a predetermined value from a histogram corresponding to the subject luminance (for example, 0.3% lower than the maximum luminance value on the histogram) (Brightness, brightness of 0.3% higher brightness from minimum brightness value) may be used. Furthermore, the conditions for determining the maximum reference brightness and the minimum reference brightness may be changed depending on the color and the subject portion.
[0015]
The “main image portion luminance of the subject” is the luminance corresponding to a human face, the average luminance of a main image region excluding a large area without contrast (for example, sky or dark portion), and the like. Further, it may be an average image value in the center of the screen or an average image value in a predetermined area including the 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, it shows facial features such as the average image value of the skin color area of the predetermined area including the center part of the screen or the focus point, or the area where the skin color area of the predetermined area including the center part of the screen or the focus point is substantially circular. An average image value from the region can also be used.
[0016]
The “image feature amount” represents the feature of the subject image information, and is, for example, the “maximum reference luminance”, “minimum reference luminance”, or “main image portion luminance”. Further, the value is not necessarily a value based on luminance, and any value may be used as long as it can represent the characteristics of subject image information such as subject reflectance.
[0017]
【The invention's effect】
The digital camera according to the present invention records the digital image data corresponding to the luminance of the main image portion of the subject to a predetermined value, and the reference image data that is digital image data corresponding to the reference luminance of the subject to the predetermined value. Since any one of the recording methods can be selected and recorded, it is possible to select an optimum recording method in advance according to the characteristics of the image output device, and to record the appropriate output. An image (display on CRT, hard copy, etc.) can be obtained.
[0018]
Further, the exposure control for imaging can also be configured so that an optimum method can be selected in accordance with the recording method, and a more preferable output image can be obtained. For example, when obtaining a captured image, it is possible to select whether the specular / white portion and the background image are not saturated (output as a print), or the main image portion is reproduced uniformly (mainly PC output).
[0019]
Further, it is more preferable to perform gradation processing as necessary. For example, the dynamic range of the image signal can be compressed according to the scene at the time of shooting and restored at the time of reproduction, so that the entire subject luminance can be recorded in an appropriate reproduction range.
[0020]
Furthermore, by recording the gradation processing conditions and image feature quantities together with the digital image data, the gradation processing conditions are read out during image reproduction, and the image is read with reference to the read gradation processing conditions. By reproducing, it becomes possible to restore to an appropriate image. For example, by recording gradation processing conditions together with data corresponding to the brightness of all subjects, it is possible to control the reproduction of images according to the necessity of background type and preferences during image reproduction. . In addition, as is well known, since a high-quality print can be obtained on a silver halide photosensitive material using a digital printer or the like, digital data can be recorded by recording image data carrying the entire subject brightness in the same way as a photographic film. Correction by image processing and high image quality processing are possible.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0022]
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 images a subject to obtain an image signal (including a luminance signal) S1, a digital image corresponding to the luminance of the subject main image portion. Image recording for selecting either a method for recording data to be a predetermined value or a method for recording reference image data that is digital image data corresponding to the reference luminance of the subject in the captured image to have a predetermined value Method selection means 10, recording means 54 for recording the digital image data D1 output from the image recording method selection means 10 on a recording medium (not shown), each part of this camera (particularly gradation processing means 40, recording means 54, CCD control means) 58) is centrally controlled.
[0023]
The image recording method selection means 10 is an image feature quantity detection means 12 for detecting the reference luminance (maximum reference luminance, minimum reference luminance, etc.) and the luminance of the main image portion from the image signal S1 obtained by the CCD imaging means 52, and image features. Output of quantity detection means 12 (maximum reference luminance Y _max Or main image area brightness Y _main The exposure control means 14 for controlling the exposure of the image pickup based on max / main in the figure), the level based on the reference luminance (indicated as max, min in the figure) or the main image portion luminance (indicated as main in the figure). The processing condition determining means 16 for determining the tone processing conditions, and the gradation processing means 40 for converting the image signal S1 into the digital image data D1 in accordance with the selected image recording method. In the exposure control means 14, the output of the image feature quantity detection means 12 is the maximum reference luminance Y _max Is the maximum reference brightness Y _max And the output of the image feature quantity detection means 12 is the main image portion luminance Y. _main In case of the main image area brightness Y _main The exposure control of imaging is performed so that the luminance signal S1 corresponding to the above becomes a predetermined value, and this output is input to the CPU.
[0024]
The processing condition determination unit 16 is configured to detect the maximum reference luminance Y detected by the image feature amount detection unit 12. _max , Minimum reference brightness Y _min And the main image portion luminance Y detected by the main image portion luminance detecting means 12. _main One of the recording methods is selected so that the image data corresponding to the above becomes a predetermined value, and further, the gradation processing condition J is determined, and the condition J is input to the gradation processing means 40. The gradation processing means 40 converts the image signal S1 into digital image data D1 while performing gradation processing according to the condition J.
[0025]
The digital camera according to the present invention is not limited to the one having the above configuration. For example, a mode as shown in FIG. 2A in which the part (the part surrounded by the broken line indicated by a in FIG. 1) composed of the image feature amount detection means 12 and the exposure control means 14 is also conceivable. The image recording method selection means 20 of the digital camera shown in FIG. 2A is a reference brightness detection means for detecting a reference brightness (maximum reference brightness, minimum reference brightness, etc.) from the image signal S1 obtained by the CCD imaging means 52. 21, main image portion luminance detecting means 23 for detecting the luminance of the main image portion, maximum reference luminance Y detected by the reference luminance detecting means 21 _max Main image portion luminance Y detected by main image portion luminance detecting means 23, exposure control means 22 for controlling the exposure of the imaging so that the luminance signal corresponding to is a predetermined value _main The exposure control means 24 for controlling the exposure of the image pickup so that the luminance signal corresponding to the predetermined value is selected, and the output of the exposure control means 22 and the exposure control means 24 is selected and the selected output is input to the CPU 56 Means 25 and processing condition determining means 26 for determining gradation processing conditions based on the reference luminance or the main image portion luminance.
[0026]
Further, the portion (the portion surrounded by the broken line indicated by a in the drawing) composed of the exposure control means 22 and 24 and the selection means 25 in FIG. 2A can be further modified as shown in FIG. It is. In FIG. 2B, the maximum reference luminance Y _max Or main image brightness Y _main A selection means 27 for selecting one of the above and an exposure control means 28 for performing exposure control based on the selected luminance are employed. The exposure control means 28 indicates that the output of the selection means 27 is the maximum reference luminance Y _max Is the maximum reference brightness Y _max And the output of the selection means 27 is the main image portion luminance Y so that the luminance signal S1 corresponding to _main In case of the main image area brightness Y _main The exposure control of imaging is performed so that the luminance signal S1 corresponding to the above becomes a predetermined value, and this output is input to the CPU.
[0027]
In any of the configurations described above, the selection signal C1 is input from the CPU 56 to the image feature amount detection means 12, the selection means 25 and 27, and the selection signal C5 is input to the recording means 54. Further, the control pulse P2 is inputted from the CPU 56 to the CCD control means 58, and the CPU 56 receives the control signal C3 from the selection means 15, and finally the control pulse (driving the CCD is inputted) to the CCD image pickup means 52. P1 is output from the CCD controller 58.
[0028]
Next, the operation of the digital camera having the above configuration will be described with reference to FIGS.
[0029]
The flowchart shown in FIG. 4 shows the flow of basic processing by the CPU 56 focusing on the selection of exposure control of the digital camera adopting the configuration shown in FIG. Various processing steps are indicated by symbols with a # mark in the figure, and are referred to in the following description. Even if the configuration of the digital camera adopts the configuration shown in FIG. 2, the basic processing is not changed, and thus the description of the operation thereof is omitted.
[0030]
First, specify what output device the image data captured and recorded by the digital camera is output (or what image quality is output) (Step 1; # 1 in the figure, and so on) .) Next, pre-photometry (imaging) is performed prior to the main photometry (imaging) (# 2), and the image feature amount (specifically, according to a predetermined definition) is determined from the photometric value measured by the pre-photometry. Maximum reference brightness, minimum reference brightness, and main image brightness). Next, an exposure control method corresponding to the first designation is selected (# 3, # 4), exposure control is performed based on the selected control method (# 5), and the main photometry (imaging) is performed ( # 6). The subject luminance information (luminance signal) obtained by this photometry is converted into digital image data (# 7). At this time, gradation processing conditions are determined based on the image feature amount (details will be described later), and the luminance signal S1 is converted into 8-bit digital image data in accordance with these conditions and recorded on a recording medium. The CPU 56 controls the flow of this series of processing. Details will be described below.
[0031]
When displaying an image captured by a digital camera on a CRT display device using a personal computer or the like, even if the brightness of the highlight portion or the background image is saturated, the main image portion is rather at a predetermined level. An image is desirable. Also, when outputting as a hard copy to a simple printer, the density range of the hard copy is narrow, and the reproduction of the highlight portion and the background image is not so important. In such a case, the output of the image feature amount detection means 12 is received from the main image portion luminance Y in response to the selection signal C1 from the CPU 56 so that exposure control is performed based on the main image portion luminance. _main As input to the exposure control means 14.
[0032]
On the other hand, when a digital printer processes an image signal from a camera and outputs a high-quality image as a hard copy (especially a silver salt print), it is required that all subject luminance is recorded as image data. . In such a case, the output of the image feature amount detection means 12 is received from the maximum reference luminance Y in response to the selection signal C1 from the CPU 56 so that the exposure control is performed based on the maximum reference luminance. _max As input to the exposure control means 14. In this way, almost all of the subject brightness can be recorded as image data in accordance with the effect of gradation processing in the gradation processing means 40 described later, and the read image data is automatically set up in the printer during image reproduction. It is processed by various image processing such as a function, a gradation correction function, and a dodging function, and can be output as a more preferable image.
[0033]
In this way, when first specifying what output device the image data captured and recorded by the digital camera is to be output, the selection signal C1 is sent from the CPU 56 to the image feature quantity detection means 12, and the selection signal C5 is used to determine the processing conditions. Input to means 16 (# 1).
[0034]
When imaging of the subject is started by pre-photometry, distance measurement is first performed to focus on the subject, the captured image is focused, and subject luminance is measured (# 2). This photometry may be carried out using a dedicated photometric sensor, or may be carried out using the image signal S1 obtained by the CCD imaging means 52 (in this case, rather than photometry). Imaging is appropriate.) The following description will be made on the assumption 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.
[0035]
Next, image feature amounts are detected for exposure control and gradation processing condition determination (# 4a, 4b).
[0036]
First, a method for detecting the reference luminance when the image feature amount detection means 12 has selected the mode for detecting the reference luminance will be described. Using the image signal (which can be considered as a luminance signal) S1 obtained by the CCD image pickup means 52, the image feature quantity detection means 12 detects the maximum reference brightness and the minimum reference brightness (# 4a). This reference luminance is detected by creating a histogram of luminance (image signal S1 in this example). FIG. 3 shows an example of a 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 value of the luminance Y corresponding to the image signal S1, the luminance histogram Can also be considered. In FIG. 3A, in the case of an overexposed state, the image signal S1 is indicated by a histogram a. Here, the luminance signal values corresponding to the maximum reference luminance and the minimum reference luminance are limit values on the high luminance side and the low luminance side at which the frequency ratio on the histogram occupies 1% (the hatched portion in the figure). Then, the maximum reference brightness is Y _max1 , Minimum reference brightness is Y _min1 It becomes. Conversely, in the case of underexposure, the image signal S1 is shown by the histogram b, and the maximum reference luminance is Y _max2 , Minimum reference brightness is Y _min2 It becomes. Note that the maximum reference brightness and the minimum reference brightness are the brightness (brightness) of the subject, and the values do not change before and after the exposure control. However, the image signal S1 obtained by the CCD imaging means 52 is before and after the exposure control. Since the values change, FIG. 3 in which the axis of the image signal S1 is fixed is shown as if the values of the maximum reference luminance and the minimum reference luminance are changed. In the following, when there is no need to distinguish between them, the maximum reference luminance Y is collectively collected. _max0 , Y _max1 , Y _max2 Y _max , Minimum reference brightness Y _min0 , Y _min1 , Y _min2 Y _min It describes.
[0037]
Further, the maximum reference brightness and the minimum reference brightness are not limited to the above example, and may be values corresponding to the maximum brightness and the minimum brightness of the subject brightness. Also, a predetermined value from the histogram of the value corresponding to the subject luminance (for example, the luminance on the low luminance side from the maximum luminance value on the histogram by 0.3%, the luminance on the high luminance side from the minimum luminance value), Also good. Furthermore, the conditions for determining the maximum reference brightness and the minimum reference brightness may be changed depending on the color and the subject portion.
[0038]
Maximum reference luminance Y detected in this way _max Is input to the exposure control means 14. The exposure control means 14 determines the maximum reference brightness Y _max The luminance signal level corresponding to _STD Thus, exposure control is performed by controlling the aperture or shutter speed (# 5). Further, exposure control may be performed by a so-called electronic shutter by controlling the timing of the CCD drive pulse. The predetermined value S1 _STD It is desirable to set the level so that specular white can be imaged to some extent (that is, not saturated).
[0039]
When exposure control is performed based on the maximum reference brightness, the image signal S1 is shown by the histogram c, and the maximum reference brightness is Y _max0 , Minimum reference brightness is Y _min0 It becomes. That is, by performing exposure control as described above based on the maximum reference luminance, levels (0 to S1) at which almost all subject luminance can be processed as the image signal S1. _STD ).
[0040]
Next, a method for detecting the main image portion luminance when the image feature amount detecting means 12 has selected the mode for detecting the main image portion luminance will be described. Using the image signal (which can be considered as a luminance signal) S1 obtained by the CCD imaging means 52, the image feature quantity detecting means 12 detects the luminance of the main image portion (# 4b). The detection of the main image portion luminance is also detected by creating a histogram in the same manner as described above.
[0041]
In the case of an overexposed state in FIG. 3B (histogram a), if the luminance signal value corresponding to the main image portion luminance is the limit value on the high luminance side of the shaded portion in the histogram diagram, the main image portion luminance is Y _main1 It becomes. On the contrary, in the case of underexposure, the image signal S1 is indicated by the histogram b, and the main image portion luminance is Y _main2 It becomes. In the following, when there is no need to distinguish between the main image portion luminance Y _main It describes.
[0042]
Main image portion luminance Y detected in this way _main Is input to the exposure control means 14. The exposure control means 14 determines the main image area luminance Y _main The luminance signal level corresponding to _main Exposure control is performed by controlling the aperture or shutter speed in the same manner as described above (for example, at a level such that the value of the digital image data is 128) (# 5).
[0043]
When exposure control is performed based on the main image portion luminance, the image signal S1 is represented by a histogram c, and the main image portion luminance is Y _min0 It becomes. In FIG. 3B, almost all of the subject brightness falls within a level (0 to 255) that can be processed as the image signal S1 as in the case of the reference brightness. Depending on whether such an image is used and on which image data the main image portion is assigned, a range that cannot be handled as the digital image data D1 may be generated in the high luminance portion or the low luminance portion. However, even in such a case, it is possible to record without being saturated as digital image data D1 finally obtained by performing gradation processing described later.
[0044]
When the exposure level is determined based on the preset exposure control method in this way, the main photometry (imaging) is performed (# 6), and the image signal S1 obtained by the main photometry (imaging) is converted into the gradation processing means. 40 is converted into 8-bit digital image data D1 (# 7). At this time, gradation processing is performed in accordance with gradation processing conditions described later. The image data D1 is input to the recording means 54 and recorded on a recording medium (not shown).
[0045]
Next, the operation of gradation processing in the gradation processing means 40 will be described in detail with reference to FIG. The flowchart shown in FIG. 5 shows the flow of processing focusing on the selection of gradation processing conditions.
[0046]
Image feature amounts (maximum reference luminance, minimum reference luminance, and main image portion luminance) are detected from the photometric values measured by the pre-photometry (# 1) (# 2). Next, whether or not to change the gradation processing condition is selected (# 3), and when the gradation processing condition is selected to be changed (soft gradation in this example), a predetermined gradation changing process is performed. Is applied (# 4b), and finally required gradation processing conditions are determined (# 5). When “no change in gradation processing condition” is selected (# 4a), a predetermined gradation processing condition is determined (# 5).
[0047]
Next, after performing exposure control based on the control method as described above (# 6), the main photometry (imaging) is performed (# 7). The subject luminance information (luminance signal) obtained by this photometry is converted into digital image data (# 8). At this time, the luminance signal S1 is converted into 8-bit digital image data according to the gradation processing conditions determined as described above, and is recorded on the recording medium. The CPU 56 controls the flow of this series of processing. Details will be described below.
[0048]
As described above, when exposure control is performed based on the main image portion luminance, for example, as shown in FIG. 6A, the subject luminance range is wide, and a histogram of the image signal S1 after exposure control is shown. In the case of the shape shown by d in 6 (A), much information on the low luminance (shadow) side and high luminance (highlight) side is discarded and does not appear as a signal. On the other hand, in the case where the range of the subject brightness is narrow and the histogram of the image signal S1 after exposure control is as shown by e in FIG. 6A, a large margin can be made on the shadow side and the highlight side, The range of signals that can be handled cannot be used effectively.
[0049]
Therefore, the image data corresponding to the luminance of the main image portion is assigned to a predetermined value (for example, data value 128), and the gradation is adjusted as a whole so that the image data value corresponding to the luminance on the highlight side is not saturated. It should be soft (softening). At this time, the shadow side may be softened so as not to be assigned to low image data.
[0050]
Specifically, the main image portion luminance Y of the subject _main Main image portion data D which is digital image data corresponding to _main Is obtained by the processing condition determining means 16, and this main image portion data D _main And maximum reference brightness Y _max Maximum reference image data D which is digital image data corresponding to _max , Main image data D _main And minimum reference brightness Y _min Minimum reference image data D which is digital image data corresponding to _min And the reference image data D based on the comparison result. _max , D _min The gradation processing condition may be determined so that the value falls within the range of 0 to 255. At this time, main image data D _main Alternatively, it may be determined which of the low-luminance side and the high-luminance side is wide as a data range, and the gradation processing condition is changed only for one of them.
[0051]
In this way, the processing condition determination means 16 changes a predetermined gradation processing condition (for example, a standard gradation of gamma value 0.45), and sets a new gradation processing condition J. Input to the key processing means 40. The gradation processing means 40 converts a luminance signal carrying subject luminance into digital image data in accordance with the input gradation processing condition J. Note that the gradation processing condition may be changed by selecting one of a plurality of predetermined gamma values, or by obtaining a new gamma value according to the processing conditions.
[0052]
Also, when performing gradation processing of image data, the gradation processing conditions are provided as a look-up table (LUT) so that the image signal S1 is converted into digital image data, and then data conversion is performed based on the LUT. You may make it perform. In this case, it is desirable that the image signal S1 is once 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 function expression or a conditional expression using the maximum reference image data, the minimum reference image data, and the main image portion data. A desired LUT may be selected according to the copy data. Further, the gradation of the image signal S1 may be converted and then converted into digital image data.
[0053]
The image data D1 subjected to gradation processing in this way is input to the recording means 54, and this data D1 is recorded on the recording medium. At this time, it is preferable to record the gradation processing condition J or the image feature quantity based on the gradation processing condition J together with the digital image data and reproduce the image according to the gradation processing condition or the image feature quantity in an image reproduction system described later.
[0054]
The gradation processing condition changing process as described above is performed using the maximum reference luminance Y _max The same applies when exposure control is performed based on the above. That is, the maximum reference luminance Y _max When the exposure control is performed properly and the subject brightness is converted into digital image data, and the image data D1 is indicated by the histogram a or b in FIG. Even if gradation processing is not performed, the image data is converted into digital image data at an appropriate level (that is, data values 0 to 255) from the shadow side to the highlight side. However, the subject luminance histogram is not always like this. For example, as shown in FIG. 6B, in the case where the range of the subject luminance is wide and the histogram of the image signal S1 after exposure control is as shown by d in FIG. The information is discarded and does not appear as a signal. Conversely, in the case where the range of the subject brightness is narrow and the histogram of the image signal S1 after exposure control is as shown by e in FIG. 6B, there is a large margin on the shadow side, which can be handled. The signal range cannot be used effectively. Thus, when the range of the subject brightness is narrow or wide, the shadow side is not converted into digital image data at an appropriate level (that is, the shadow side is not effectively used).
[0055]
Therefore, as shown in the histograms d and e in FIG. 6B, when the subject luminance range is narrow or wide, the minimum reference luminance Y _min Is detected and the minimum reference luminance Y is detected. _min Also, the gradation of the image data may be converted so as to correspond to about 0 level of the image signal S1. By doing so, the image signal S1 becomes as shown by the histogram f, and the entire subject brightness is always set to the reproducible level (0 to S1) regardless of the subject brightness range. _STD ) Can fit in the last minute.
[0056]
Here, “0 level” is not limited to the minimum value 0 of the digital image data, but is a minimum value determined from the viewpoint of an image reproduction density range (for example, an optical density that can be normally reproduced is 2.0). For example, a value of about the 10th place may be used instead of 0. Hereinafter, the minimum reference luminance Y _min “0 level” corresponding to “10” also includes “10 level” in the case of the above example (about the 10th value).
[0057]
For example, the maximum reference luminance Y after exposure control is performed _max0 , Minimum reference brightness Y _min0 (Or maximum reference brightness Y _max0 , Minimum reference brightness Y _min0 Digital image data corresponding to the maximum and minimum reference image data D _max , D _min ; D _max , D _min Will be described) using predetermined values (maximum reference image data D). _max Near 255, minimum reference image data D _min If it is determined that it is not within the predetermined value, both reference image data D _max , D _min Gradation processing conditions (for example, the value of gamma γ) are determined so as to be within the range from 0 to 255. At this time, the gradation processing conditions may be changed depending on the degree of deviation from the predetermined value.
[0058]
Both reference image data D _max , D _min Are compared (actually a difference is taken), and both reference image data D are _max , D _min The gradation of the image data may be converted by setting gradation processing conditions that fall within the range of 0 to 255. For example, the gradation may be softened when the difference is large, and the gradation may be hardened when the difference is small.
[0059]
As described above, exposure control is appropriately performed based on the maximum reference luminance, and predetermined gradation processing is performed, so that digital image data corresponding to the subject luminance from the specular white to the shadow portion is approximately. Data values fall within the range of 0 to 255.
[0060]
Next, with reference to FIG. 7, a specific description will be given of how gradation processing is performed in accordance with subject brightness.
[0061]
FIG. 7 shows the relationship between the subject reflectance and the image data (QL (Quantum Level) value). The CCD output value corresponding to the subject reflectance 100% (white) is the image data (QL value) 255, and the subject reflectance 0%. The CCD output value corresponding to (black) is shown corresponding to the image data (QL value) 0. Here, the image data (QL value) is 8-bit image data after gradation processing is performed.
[0062]
Here, even when the subject reflectivity is 100%, the subject brightness varies, there are subjects with a reflectivity of 100%, and the subject with the reflectivity of 100% is not saturated by the exposure control of the camera, and the CCD output value The relationship between the CCD output value and the image data when photometry (imaging) can be performed is shown.
[0063]
In general, the subject reflectance cannot be measured even though the subject brightness can be measured. However, for example, a certain percentage of specular white images having a high reflectance of 100% or more exist in all images, and the maximum brightness excluding them Is assumed to be 100% reflectance (for example, 1%, 3%, etc.), the subject reflectance can be estimated from the subject brightness. FIG. 7 shows the relationship between the subject reflectance and the image data (QL value) by estimating the subject reflectance from the subject luminance from such a viewpoint.
[0064]
Since the QL value of a digital camera is generally proportional to the subject's luminance E 1 / 2.sup.2, the standard gradation processing condition is that the CCD output value corresponding to the subject's luminance is the 0.45th gradation (in the figure). A line indicated by A) is used.
[0065]
If the subject brightness difference (that is, the difference between the maximum reference brightness and the minimum reference brightness) is greater than or equal to a predetermined value, or the subject reflectivity is 100% or less when the subject reflectivity at the maximum reference brightness is 100% or less, the power is 0.45 or less. Are used. At this time, subject luminance and reflectance may be determined by converting to image data, and the gradation may be changed.
[0066]
If the subject brightness difference is large, a gradation of 0.45 or less, for example, 0.40 is used. On the contrary, if the subject luminance difference is small, a gradation (line indicated by B in the figure) of 0.45th power or higher, for example, 0.50th power may be used.
[0067]
When the subject luminance difference is equal to or larger than a predetermined value, the maximum reference luminance and the minimum reference luminance are set to 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). A gradation value may be determined for and gradation processing may be performed.
[0068]
Alternatively, a partial gradation change on the highlight side or the shadow side may be performed depending on whether the main image portion is on the highlight side or the shadow side. For example, the gradation processing may be performed only on the highlight side or the shadow side while basically setting the gradation to the power of 0.50 as indicated by C in the figure.
[0069]
Next, a recording / reproducing method of the digital camera for obtaining a reproduced image from the digital image data recorded by the digital camera according to the present invention described above will be described.
[0070]
FIG. 8 shows the configuration of an embodiment of an image playback system for realizing the recording and playback method of the digital camera according to the present invention. The image reproduction system captures a subject (not shown) and records digital image data D1 carrying subject image information on a recording medium 3, and reads out the image data D1 recorded on the recording medium 3. And an image reproducing device 2 for obtaining a reproduced image.
[0071]
The image reproducing device 2 reads the image data D1 from the recording medium 3 and inputs the data D1 to the image data reverse converting means 2c, and reads the gradation processing condition J from the recording medium 3 and reversely converts the image data. The gradation processing condition input means 2b for inputting the processing condition J to the means 2c and the image data whose gradation has been restored by the image data reverse conversion means 2c are received, and an image carried on the image data (particularly the main image) 2), density conversion / color correction means 2d for correcting image data so that the image has an appropriate density and color, and color conversion means 2e for further improving color reproducibility by receiving the output of density / color correction means 2d, color conversion It comprises a CRT display device that receives the output of the means 2e to convert the image data into a visible image, and an image output means 2f such as a printer.
[0072]
The operation of the image reproduction system having the above configuration will be described below.
[0073]
Digital image data D1 carrying the subject brightness imaged by the digital camera 1 is recorded on a recording medium. The digital camera 1 may be any digital camera according to the present invention described above. The image data input means 2a reads the digital image data D1 from the recording medium 3, and inputs the read image data D1 to the image data reverse conversion means 2c. When a digital camera that also records the gradation processing condition J is used, the gradation processing condition input means 2b reads the gradation processing condition J from the recording medium 3, and the image data reverse conversion means 2c receives the gradation processing condition J. The read gradation processing condition J is input. When a digital camera that records image feature values is used, the gradation processing condition input means 2b reads the image feature values from the recording medium 3, and the image data reverse conversion means 2c reads the read image. What is necessary is just to input a feature-value.
[0074]
When the gradation processing condition J (or image feature amount; hereinafter, “image feature amount” is omitted) is input, the image data reverse conversion means 2c reads out the image data based on the gradation processing condition J. The gradation is inversely converted so that the gradation of D1 becomes a predetermined gradation (for example, a gamma value of 0.45 which is a standard gradation). If the gradation processing condition J is not input, it is not necessary to perform inverse gradation conversion.
[0075]
The image data input from the image data reverse conversion means 2c to the density / color correction means 2d is corrected so that the image (particularly the main image portion) carried in the image data has an appropriate density and color. The amount of density correction depends on the accuracy of exposure control at the time of image capture by the camera 1. If the accuracy of exposure control is high, the density correction here is small. Further, the color correction is for correcting color variation due to the type of light source at the time of imaging, the type of camera 1, etc., the color temperature of strobe light, and the like.
[0076]
Image data whose density and color are corrected by the density / color correction means 2d is input to the color conversion means 2e. In the color conversion means 2e, color conversion for matching the color reproduction system of the camera 1 and the color reproduction system of the printer, color conversion processing by a color matrix or LUT for improving saturation and improving color reproducibility is performed. The
[0077]
The image data subjected to the color conversion processing by the color conversion means 2e is input to the image output means 2f and displayed on the CRT display device or output as a print image (hard copy).
[0078]
In this manner, the processing condition J recorded together with the image data D1 is also read, and the read image data D1 is subjected to gradation processing in accordance with the gradation processing conditions having characteristics opposite to those at the time of recording. The original gradation state is restored and an appropriate reproduced image can be obtained. Further, it is possible to obtain a more preferable image output by further correcting the density and color or improving the color reproduction system.
[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 and 2B are circuit block diagrams showing a configuration in which a part of the digital camera is changed.
FIGS. 3A and 3B show examples of image signal histograms. FIGS.
FIG. 4 is a flowchart showing a flow of processing focusing on exposure control selection of a digital camera according to the present invention.
FIG. 5 is a flowchart showing a flow of processing focusing on selection of gradation processing conditions of a digital camera according to the present invention.
FIGS. 6A and 6B show examples of histograms of image signals.
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 image reproduction system that realizes a recording / reproduction method according to the present invention.
[Explanation of symbols]
1 Digital camera
2 Image playback device
3 recording media
10,20 Image recording method selection means
12 Image feature detection means
14,22,24,28 Exposure control means
16,26 Processing condition determination means
21 Reference luminance detection means
23 Main image area brightness detection means
25,27 Selection method
40 gradation processing means
52 CCD imaging means
54 Recording means
56 CPU
58 CCD control means

Claims (3)

In a digital camera that converts a luminance signal obtained by imaging a subject into digital image data that is read for image output by an output device and records the digital image data on a recording medium.
A method of recording so that digital image data corresponding to the luminance of the main image portion of the subject has a predetermined value, and recording so that reference image data that is digital image data corresponding to the reference luminance of the subject has a predetermined value. A digital camera comprising image recording method selection means for selecting one of the methods according to a preselected output device . The image recording method selection means is
Detecting means for detecting the reference luminance and the main image portion luminance;
First exposure control means for performing exposure control so that a luminance signal corresponding to the reference luminance detected by the detection means becomes a predetermined value; and
Second exposure control means for performing exposure control so that a luminance signal corresponding to the main image portion luminance detected by the detection means becomes a predetermined value;
2. The digital camera according to claim 1, wherein one of the first exposure control means and the second exposure control means is selected. The digital camera according to claim 1 or 2 ,
Detecting means for detecting an image feature value from the captured image;
Processing condition determining means for determining a gradation processing condition for converting a luminance signal into digital image data based on the image feature value detected by the detecting means;
Gradation processing means for converting a luminance signal into digital image data based on the gradation processing conditions determined by the processing condition determination means; and
A digital camera comprising recording means for recording the determined gradation processing conditions together with digital image data.

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