JP4941424B2 - Imaging apparatus and image processing program - Google Patents

Description

  The present invention relates to an imaging apparatus and an image processing program.

2. Description of the Related Art Conventionally, a phenomenon is known in which a dark portion gradation of image data is darkened by photographing a subject having a large luminance difference. Therefore, in the invention of Patent Document 1, the tone is compressed by increasing the gain of the dark portion gradation, and the blackening of the dark portion gradation is improved.
Japanese Patent No. 2663189

  By the way, with regard to the gradation conversion process, a gradation conversion process using a so-called S-shaped gradation curve is widely known in order to enhance contrast. In the gradation conversion process using the S-shaped gradation curve, the contrast can be enhanced. On the other hand, the contrast of the bright part gradation may be lowered or the dark part gradation may be crushed darkly. there were.

  The present invention has been made in view of the above problem, and performs gradation conversion processing and dark part gradation correction capable of improving the bright part gradation and the dark part gradation while maintaining the apparent contrast. For the purpose.

  An image pickup apparatus according to the present invention includes an image pickup unit that picks up a subject image to generate image data, a first shooting mode that does not correct dark portion gradation of the image data, and correction of dark portion gradation of the image data. A selection unit for selecting one of the second shooting modes for performing the gradation conversion, and when the first shooting mode is selected, gradation conversion processing is performed according to a first input / output characteristic, and the second shooting mode is selected. When the shooting mode is selected, a second input having a characteristic for realizing an output level lower than the first input / output characteristic for the same level input and a characteristic for changing contrast is provided. A gradation conversion processing unit that performs gradation conversion processing according to output characteristics, and, when the second shooting mode is selected, gradation conversion processing according to the second input / output characteristics by the gradation conversion processing unit The darkened image data And a correcting unit performing the correction to improve the lightness of the gray scale.

  Of the second input / output characteristics, the characteristic that changes the contrast may have a characteristic that weakens the contrast in an intermediate portion of a gradation.

  The second input / output characteristic includes a gradation curve having a characteristic for realizing an output level lower than that of the first input / output characteristic with respect to an input of the same level, and a level having a characteristic for changing the contrast. One having a characteristic that realizes an output level lower than the first input / output characteristic for an input of the same level, and a characteristic that changes the contrast. It may be defined by a gradation curve.

  And a determining unit configured to determine a lightness improvement amount of the dark portion gradation by the correction unit according to the second input / output characteristic, wherein the correction unit performs dark portion gradation of the image data according to the lightness improvement amount. May be corrected.

  Further, the determining unit obtains a local contrast adjustment degree indicating a contrast in a local region of the image according to a characteristic that changes the contrast among the second output characteristics, and based on the obtained adjustment degree, You may determine the brightness improvement amount.

  In addition, a configuration obtained by converting the configuration related to the above invention into an image processing program for realizing image processing on image data to be processed by a computer is also effective as a specific aspect of the present invention.

  According to the present invention, it is possible to perform gradation conversion processing and dark part gradation correction capable of improving the bright part gradation and the dark part gradation while maintaining the apparent contrast.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiments, a single-lens reflex type electronic camera will be described as an example of the camera of the present invention.

  FIG. 1 is a diagram illustrating a configuration of an electronic camera 1 according to the present embodiment. As shown in FIG. 1, an electronic camera 1 includes a photographing lens 2, an aperture 3, a quick return mirror 4, a sub mirror 5, a diffusion screen 6, a condenser lens 7, a pentaprism 8, a beam splitter 9, an eyepiece lens 10, and an imaging lens. 11, a photometric sensor 12, a shutter 13, an image sensor 14, and a focus detection unit 15.

  The photometric sensor 12 is, for example, a five-part photometric sensor. The imaging element 14 is a semiconductor device such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor). For example, the focus detection unit 15 detects a focus state of the photographic lens 2 by performing phase difference type focus detection. The electronic camera 1 detects the focus state of the photographic lens 2 by performing contrast-type focus detection based on the luminance detected by the photometric sensor 12.

  The electronic camera 1 further includes a monitor 16 such as a liquid crystal monitor that displays an image generated by imaging, and a control unit 17 that controls each unit. The control unit 17 includes a memory (not shown) therein, and records a program for controlling each unit in advance.

  When not photographing, that is, when photographing is not performed, the quick return mirror 4 is disposed at an angle of 45 ° as shown in FIG. The light beam that has passed through the photographing lens 2 and the diaphragm 3 is reflected by the quick return mirror 4 and guided to the eyepiece lens 10 through the diffusion screen 6, the condenser lens 7, the pentaprism 8, and the beam splitter 9. The user confirms the composition by viewing the subject image through the eyepiece 10. On the other hand, the light beam split upward by the beam splitter 9 is re-imaged on the imaging surface of the photometric sensor 12 via the imaging lens 11. Further, the light beam transmitted through the quick return mirror 4 is guided to the focus detection unit 15 via the sub mirror 5.

  On the other hand, at the time of photographing, the quick return mirror 4 is retracted to the position indicated by the broken line, the shutter 13 is opened, and the light flux from the photographing lens 2 is guided to the image sensor 14.

  FIG. 2 is a functional block diagram of the electronic camera 1 of the present embodiment. 2, the electronic camera 1 includes a timing generator 20, a signal processing unit 21, an A / D conversion unit 22, a buffer memory 23, a bus 24, a card interface 25, and a compression / decompression unit in addition to the configuration of FIG. 26, each part of the image display unit 27 is provided. The timing generator 20 supplies output pulses to the image sensor 14. The image data generated by the image sensor 14 is temporarily stored in the buffer memory 23 via the signal processing unit 21 (including a gain adjustment unit corresponding to the imaging sensitivity) and the A / D conversion unit 22. The buffer memory 23 is connected to the bus 24. The bus 24 is connected to the card interface 25, the control unit 17, the compression / decompression unit 26, and the image display unit 27 described in FIG. The card interface 25 is connected to a removable memory card 28 and records image data on the memory card 28. The control unit 17 is connected to a switch group 29 (including a release button (not shown)) of the electronic camera 1, a timing generator 20, and a photometric sensor 12. Further, the image display unit 27 displays an image or the like on the monitor 16 provided on the back surface of the electronic camera 1.

  The electronic camera 1 also includes a gradation non-compression mode that does not correct dark part gradation of image data, and a gradation compression mode that performs dark part gradation correction. Which mode is used for shooting may be selected in advance by the user via the switch group 29 or may be automatically selected by the control unit 17.

  The operation at the time of photographing of the electronic camera 1 having the above-described configuration will be described using the flowchart shown in FIG.

  In step S <b> 1, the control unit 17 determines whether or not the user has instructed to start shooting via the switch group 29. If the control unit 17 determines that an instruction to start photographing is given, the process proceeds to step S2.

  In step S <b> 2, the control unit 17 controls each unit and captures a subject image with the image sensor 14 to generate image data. Image data generated by the imaging device 14 is temporarily stored in the buffer memory 23 via the signal processing unit 21 and the A / D conversion unit 22.

  In step S3, the control unit 17 reads the image data from the buffer memory 23 and performs normal image processing. Normal image processing includes white balance adjustment, interpolation processing, color tone correction processing, and the like. Since the specific method of each process is the same as that of a well-known technique, description is abbreviate | omitted.

  In step S4, the control unit 17 determines whether or not the gradation compression mode is set. If the control unit 17 determines that the gradation compression mode is set, the process proceeds to step S7 described later. On the other hand, if it is determined that the gradation compression mode is not set (the gradation non-compression mode is set), the control unit 17 proceeds to step S5.

  In step S5, the control unit 17 selects the gradation curve G1. The gradation curve is used for gradation conversion processing performed in step S6 described later. The control unit 17 records in advance three types of gradation curves (G1 to G3) shown in FIGS. 4 and 5 in an internal memory (not shown).

  In the gradation non-compression mode, the control unit 17 selects a gradation curve G1 shown in FIG. The gradation curve G1 is a gradation curve used for normal gradation conversion processing similar to the known technique.

  In step S6, the control unit 17 performs gradation conversion processing on the image data subjected to image processing in step S3 according to the gradation curve G1 selected in step S5. The details of the gradation conversion process are the same as those of the known technique, and thus description thereof is omitted. And the control part 17 progresses to step S12 mentioned later.

  In step S7, the control unit 17 selects the gradation curve G2. As shown in FIG. 4, the gradation curve G2 has a characteristic that realizes a lower output level with respect to the same level of input as compared with the gradation curve G1 described above. More specifically, the gradation curve G2 has a characteristic that realizes an output level lower than the input level in the dark part of the gradation as compared with the gradation curve G1 described above. Therefore, compared to the above-described gradation curve G1, the dark portion of the gradation has a smaller slope than the gradation curve G1. Further, the gradation curve G2 has a characteristic of further enhancing the contrast in the bright part of the gradation as compared with the gradation curve G1 described above. Therefore, compared to the above-described gradation curve G1, the bright part of the gradation has a larger slope than the gradation curve G1.

  In step S8, the control unit 17 performs gradation conversion processing on the image data subjected to image processing in step S3 according to the gradation curve G2 selected in step S7. The details of the gradation conversion process are the same as those of the known technique, and thus description thereof is omitted.

  In step S9, the control unit 17 selects the gradation curve G3 shown in FIG. As shown in FIG. 5, the gradation curve G3 has a characteristic of weakening the contrast in the middle portion of the gradation as compared with the 1: 1 input / output (see the dotted line in FIG. 5).

  In step S10, the control unit 17 performs the second gradation conversion process on the image data subjected to the first gradation conversion process in step S8 according to the gradation curve G3 selected in step S9. The details of the gradation conversion process are the same as those of the known technique, and thus description thereof is omitted. Note that the gradation is softened at the intermediate portion of the gradation by performing the gradation conversion processing according to the gradation curve G3 described above.

  In step S11, the control unit 17 performs gradation compression processing on the image data that has been subjected to the second gradation conversion processing in step S10.

  FIG. 6 is a diagram showing the gradation compression parameter fg. The parameter fg has a gain corresponding to the brightness Y of the image as shown in FIG. The parameter fg increases as the brightness Y decreases (as the neighborhood including the processing pixel becomes darker). On the contrary, the parameter fg approaches 1 as the brightness Y increases (the neighborhood range including the processing pixel is brighter).

  The gradation compression calculation in each pixel R [x, y], G [x, y], B [x, y] is performed by the following equations 1 to 5.

式１中のＹは、注目画素の輝度値を示す。また、式１中のｋｒ，ｋｇ，ｋｂは所定の係数である。また、式２中のＬｐｗは、注目画素周りのローパスフィルタであり、このローパスフィルタは、図７に示す特性を有する。また、式２中のｋｙは、所定の調整係数である。また、式３〜式５中のｆｇは、上述したパラメータｆｇに対応する。

Y in Equation 1 indicates the luminance value of the target pixel. Further, kr, kg, and kb in Equation 1 are predetermined coefficients. In addition, Lpw in Equation 2 is a low-pass filter around the pixel of interest, and this low-pass filter has the characteristics shown in FIG. Further, ky in Equation 2 is a predetermined adjustment coefficient. Further, fg in Expressions 3 to 5 corresponds to the parameter fg described above.

  The adjustment coefficient ky is an adjustment coefficient for adjusting the contrast in the local region of the image (hereinafter referred to as “local contrast”). As is clear from Equation 2, when ky changes, the weight for combining the luminance value of the target pixel and the low-pass value around the target pixel changes. For example, when the adjustment coefficient ky = 1, the improvement amount of the parameter fg is determined by only the luminance value of the target pixel according to Equations 3 to 5. On the other hand, when the adjustment coefficient ky = 0, the improvement amount of the parameter fg is determined by only the low-pass value around the pixel of interest using Equations 3 to 5. As a tendency, the local contrast decreases as the adjustment coefficient ky increases, and the local contrast is enhanced as the adjustment coefficient ky decreases. As described in step S10, the gradation is softened by performing the gradation conversion processing according to the gradation curve G3. Therefore, the local contrast is adjusted by appropriately setting the adjustment coefficient ky. The visual contrast can be maintained. The adjustment coefficient ky may be a fixed value or a fluctuation value. Here, for example, it is assumed that ky is a fixed value of about 0.5.

  Here, the relationship between local contrast and gradation compression processing will be described with reference to FIG. FIG. 8 is a schematic diagram showing the behavior of the luminance value near a certain target pixel. As shown in FIG. 8, when the dark portion raising process by the gradation curve is performed on the original image A, the dark portion gradation blackout of the processed image B can be improved, but the contrast is lowered.

  On the other hand, as described above, during the gradation compression process on the original image A, the local contrast is emphasized by the adjustment coefficient, thereby improving the blackout of the dark portion gradation of the processed image C. It is possible to enhance (or maintain) the apparent contrast (see arrow C in FIG. 8). This effect can be obtained because in human vision, it is local to perceive contrast.

  In step S12, the control unit 17 records the image data subjected to the gradation compression process in step S11 or the image data subjected to the gradation conversion process in step S6 on the memory card 28 via the card interface 25, A series of processing ends. Note that before the image data is recorded on the memory card 28, an image compression process (such as a JPEG compression process) may be performed as necessary via the compression / decompression unit 26.

  As described above, according to the present embodiment, it is possible to select the first shooting mode in which the dark gradation correction of the image data is not performed and the second shooting mode in which the dark gradation correction is performed. When the first shooting mode is selected, gradation conversion processing is performed according to the first input / output characteristics, and when the second shooting mode is selected, the first level is input for the same level input. The gradation conversion process is performed in accordance with a second input / output characteristic having a characteristic for realizing an output level lower than the input / output characteristic and a characteristic for changing contrast. When the second shooting mode is selected, a correction for improving the brightness of the dark portion gradation of the image data subjected to the gradation conversion processing according to the second input / output characteristic by the gradation conversion processing portion is performed. Do. Therefore, in the region of interest, gradation conversion processing and dark part gradation correction that can improve the bright part gradation and the dark part gradation while maintaining the apparent contrast can be performed. Furthermore, according to the present embodiment, it is possible to reduce the phenomenon that the hue is changed by the gradation conversion processing using the S-shaped gradation curve as in the prior art.

  Further, according to the present embodiment, of the second input / output characteristics, the characteristic that changes the contrast has a characteristic that weakens the contrast in the middle portion of the gradation. Accordingly, since the result of the gradation conversion process can be kept dark, the brightness of the entire image can be maintained before and after the gradation compression process.

  Further, according to the present embodiment, the degree of adjustment of the local contrast indicating the contrast in the local region of the image is obtained according to the characteristic that changes the contrast among the second output characteristics, and the brightness is determined based on the obtained degree of adjustment. Determine the amount of improvement. Therefore, even if the gradation is softened by the gradation conversion process, the dark area gradation can be corrected so that the apparent contrast is maintained in the region of interest.

  In the present embodiment, as described with reference to the flowchart of FIG. 3, the case where the gradation conversion process is performed twice using the gradation curve G2 and the gradation curve G3 has been described as an example. A configuration in which gradation conversion processing is performed only once using one gradation curve having a characteristic that realizes an output level lower than G1 for the same level of input and a characteristic that changes contrast. Also good. That is, as shown in the flowchart of FIG. 9, in step S21 to step S26, the control unit 17 performs the same processing as in step S1 to step S6 of the flowchart of FIG.

  In step S27, the control unit 17 selects the gradation curve G4. As shown in FIG. 10, the gradation curve G4 is a gradation curve having both the characteristics of the gradation curve G2 and the characteristics of the gradation curve G3 described with reference to the flowchart of FIG.

  In step S28, the control unit 17 performs gradation conversion processing on the image data subjected to image processing in step S23 according to the gradation curve G4 selected in step S27.

  In step S29 to step S30, the control unit 17 performs the same processing as in step S11 to step S12 in the flowchart of FIG. By adopting such a configuration, an effect similar to that of the present embodiment can be obtained.

  In the present embodiment, the case where the gradation curve G3, which is a gradation curve used for gradation conversion processing in the gradation compression mode, and the local contrast adjustment coefficient ky are fixed is described as an example. The gradation curve G3 and the adjustment coefficient ky may be changed in conjunction with each other. As described above, the local contrast decreases as the adjustment coefficient ky increases, and the local contrast is enhanced as the adjustment coefficient ky decreases. Therefore, the gradation curve G3 is set so that the contrast becomes stronger in the middle portion of the gradation as the adjustment coefficient ky becomes larger, and the gradation curve G3 so that the contrast becomes weaker in the middle portion of the gradation as the adjustment coefficient ky becomes smaller. Should be set. FIG. 11 shows an example of the gradation curve G3 when the adjustment coefficient ky is changed.

  The gradation curve G3 and the adjustment coefficient ky may be changed according to the shooting mode (for example, “portrait mode”, “landscape mode”, etc.), or changed according to the contrast level of the image. It is good also as a structure, and it is good also as a structure changed according to the adjustment mode of an image. Further, the gradation curve G3 and the adjustment coefficient ky may be changed according to the image determination result by scene analysis or face recognition. As described above, by appropriately changing the gradation curve G3 and the adjustment coefficient ky, it is possible to perform gradation conversion processing and dark portion gradation correction optimized for an image.

  In addition, a plurality of gradation curves as shown in FIG. 11 may be selected and used from those recorded in advance, or the control unit 17 may appropriately select the gradation curve. It is good also as a structure adjusted and used.

  In this embodiment, the gradation curve used for gradation conversion processing may be changed according to the influence of exposure correction. For example, when the brightness of the entire image can be maintained to some extent by exposure correction, the amount of change in the gradation curve used in the gradation compression mode may be reduced.

  In the above-described embodiment, the example in which the technique of the present invention is realized in the electronic camera 1 has been described. However, the present invention is not limited to this. For example, the present invention can be similarly applied to a compact type electronic camera, a movie camera for taking a moving image, and the like.

  Further, the image processing apparatus described in the present embodiment may be realized as software by a computer and an image processing program. In this case, what is necessary is just to make it the structure which implement | achieves part or all of the process after step S4 demonstrated with the flowchart of FIG. 3 with a computer. Or what is necessary is just to set it as the structure which implement | achieves part or all of the process after step S24 demonstrated with the flowchart of FIG. 9 with a computer. In order to be realized by a computer, information such as whether or not the image is in the gradation compression mode may be supplied to the computer together with the image data. Such information can be supplied using the EXIF information of the image data. By adopting such a configuration, it is possible to perform the same processing as in the present embodiment.

  The present invention can be implemented in various other forms without departing from the spirit or main features thereof. For this reason, the above-described embodiments are merely examples in all respects and should not be interpreted in a limited manner. The scope of the present invention is indicated by the scope of claims, and is not restricted by the text of the specification. Further, all modifications and changes belonging to the equivalent scope of the claims are within the scope of the present invention.

It is a figure which shows the structure of the electronic camera 1 of this embodiment. It is a functional block diagram of electronic camera 1 of this embodiment. It is a flowchart which shows the operation | movement at the time of imaging | photography of the electronic camera 1 of this embodiment. It is a figure explaining a gradation curve. It is another figure explaining a gradation curve. It is a figure explaining the parameter fg of gradation compression. It is a figure explaining a low pass filter. It is a figure explaining the relationship between local contrast and a gradation compression process. It is another flowchart which shows the operation | movement at the time of imaging | photography of the electronic camera 1 of this embodiment. It is another figure explaining a gradation curve. It is another figure explaining a gradation curve.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Electronic camera, 2 ... Shooting lens, 14 ... Image sensor, 17 ... Control part

Claims (10)

An imaging unit that captures a subject image and generates image data;
A selection unit that selects one of a first shooting mode that does not correct the dark portion gradation of the image data and a second shooting mode that corrects the dark portion gradation of the image data;
When the first shooting mode is selected, gradation conversion processing is performed according to the first input / output characteristics, and when the second shooting mode is selected, the input for the same level is performed. A gradation conversion processing unit that has a characteristic that realizes an output level lower than the first input / output characteristic and that performs gradation conversion processing according to a second input / output characteristic that has a characteristic of changing contrast;
Correction for improving the lightness of the dark portion gradation of the image data that has been subjected to the gradation conversion processing according to the second input / output characteristic by the gradation conversion processing section when the second shooting mode is selected An imaging apparatus comprising: a correction unit that performs the following. The imaging device according to claim 1,
Of the second input / output characteristics, the characteristic that changes the contrast has a characteristic that weakens the contrast in an intermediate portion of a gradation. The imaging device according to claim 1,
The second input / output characteristic includes a gradation curve having a characteristic for realizing an output level lower than that of the first input / output characteristic for an input of the same level, and a gradation curve having a characteristic for changing the contrast. Or a gradation having a characteristic that realizes an output level lower than the first input / output characteristic for an input of the same level and a characteristic that changes the contrast An imaging device characterized by being defined by a curve. The imaging device according to claim 1,
In accordance with the second input / output characteristic, a determination unit that determines a brightness improvement amount of a dark portion gradation by the correction unit,
The image pickup apparatus, wherein the correction unit corrects a dark portion gradation of the image data according to the lightness improvement amount. The imaging apparatus according to claim 4,
The determination unit obtains a local contrast adjustment degree indicating a contrast in a local region of the image according to the contrast changing characteristic among the second output characteristics, and improves the brightness based on the obtained adjustment degree. An imaging apparatus characterized by determining an amount. An image processing program for realizing image processing on image data to be processed by a computer,
The image data is generated by any one of the image data and a first shooting mode in which the dark portion gradation of the image data is not corrected and a second shooting mode in which the dark portion gradation of the image data is corrected. An acquisition step for acquiring information indicating whether or not
When the image data is generated in the first shooting mode, gradation conversion processing is performed according to the first input / output characteristics, and the same is applied when the image data is generated in the second shooting mode. The gradation conversion processing is performed in accordance with a second input / output characteristic having a characteristic that realizes an output level lower than the first input / output characteristic with respect to an input of the level and a characteristic that changes contrast. Key conversion processing step;
When the image data is generated in the second shooting mode, the brightness of the dark gradation of the image data that has been subjected to the gradation conversion processing according to the second input / output characteristic in the gradation conversion processing step An image processing program characterized in that a correction step for performing correction to improve the image quality is realized by a computer. The image processing program according to claim 6,
Among the second input / output characteristics, the characteristic that changes the contrast has a characteristic that weakens the contrast in an intermediate portion of a gradation. The image processing program according to claim 6,
The second input / output characteristic includes a gradation curve having a characteristic for realizing an output level lower than that of the first input / output characteristic for an input of the same level, and a gradation curve having a characteristic for changing the contrast. Or a gradation having a characteristic that realizes an output level lower than the first input / output characteristic for an input of the same level and a characteristic that changes the contrast An image processing program characterized by being defined by curves. The image processing program according to claim 6,
According to the second input / output characteristic, comprising a determination step of determining a brightness improvement amount of the dark portion gradation in the correction step,
In the correction step, the dark portion gradation of the image data is corrected according to the lightness improvement amount. The image processing program according to claim 9.
In the determining step, a degree of adjustment of a local contrast indicating contrast in a local region of the image is obtained according to the characteristic of changing the contrast among the second output characteristics, and the brightness is improved based on the obtained degree of adjustment. An image processing program characterized by determining an amount.

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