JP2018093295A - Contrast correction device and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress color moire resulting from a color sampling structure.SOLUTION: A contrast correction device 1 includes a frequency decomposition unit 11 for generating a frequency decomposition image by performing frequency decomposition of a video for each color signal, a color sampling structure correction factor determination unit 12 for grasping a Nyquist frequency of the video from the color sampling structure of an image pickup device captured the video, and determining a first correction factor for each frequency band of the color signal, so that the contrast becomes smaller for the lower Nyquist frequency, a contrast correction unit 13 correcting the contrast of the frequency decomposition image for each frequency band on the basis of a correction factor, and generating a contrast correction image for each band, and a frame image recomposition unit 14 performing frequency decomposition of the contrast correction image for each band, and outputting a video subjected to contrast correction.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a contrast correction apparatus and program for correcting the contrast of an image.

  It is known that when sampling an image, if the image contains a frequency component exceeding the Nyquist frequency, moire (folding noise) is generated by the sampling theorem. In order to remove this moiré, an apparatus that first performs a low-pass filter on an input image and then performs image processing such as contrast correction is known (for example, see Patent Document 1).

Japanese Patent Laid-Open No. 09-261481

  In general, a color moving image photographed by a single-plate sensor camera may cause color moire due to a color sampling structure. However, the conventional contrast correction processing does not consider the color sampling structure, and even if a low-pass filter is applied, the color moire caused by the color sampling structure cannot be corrected.

  An object of the present invention made in view of such circumstances is to provide a contrast correction apparatus and program capable of suppressing color moire caused by a color sampling structure.

  In order to solve the above-described problem, a contrast correction apparatus according to the present invention is a contrast correction apparatus that corrects the contrast of an image, and a frequency decomposition unit that generates a frequency-resolved image by frequency-decomposing the image for each color signal. The Nyquist frequency of the video is grasped from the color sampling structure of the image pickup device that picked up the video, and the first correction coefficient for each frequency band of the color signal is determined so that the contrast decreases as the Nyquist frequency decreases. A color sampling structure correction coefficient determination unit; a contrast correction unit that corrects contrast of the frequency-resolved image for each frequency band of the color signal based on the correction coefficient; A frame image reconstruction unit that outputs a video whose contrast is corrected by performing reverse frequency decomposition of the contrast-corrected image. And wherein the Rukoto.

  Further, in the contrast correction device according to the present invention, the video is a frame rate converted video obtained by converting a frame rate, and the contrast is obtained when the frame rate converted video is a video converted to increase the frame rate. Frame rate correction for determining a second correction coefficient for each frequency band of the color signal so that the contrast is reduced when the frame rate conversion image is an image converted to reduce the frame rate. A coefficient determination unit; and the contrast correction unit corrects the contrast of the frequency-resolved image based on the first correction coefficient and the second correction coefficient to generate the contrast correction image for each band. It is characterized by.

  Furthermore, in the contrast correction device according to the present invention, the contrast correction unit corrects the contrast of the frequency-resolved image based on a spatio-temporal contrast sensitivity characteristic indicating visual contrast sensitivity with respect to the spatial frequency when the temporal frequency is changed. It is characterized by doing.

  Further, in the contrast correction apparatus according to the present invention, the frequency resolving unit determines a decomposition rank in the horizontal direction and the vertical direction based on the spatio-temporal contrast sensitivity characteristic.

  Furthermore, in the contrast correction apparatus according to the present invention, the contrast correction unit corrects the contrast of the frequency-resolved image in consideration of spatial anisotropy.

  Furthermore, in the contrast correction apparatus according to the present invention, the frequency resolution unit generates the frequency-resolved image using wavelet decomposition, and the frame image reconstruction unit converts the frequency-resolved image into a frame image using wavelet reconstruction. It is characterized by being inversely transformed into

  In order to solve the above problems, a program according to the present invention causes a computer to function as the contrast correction device.

  According to the present invention, it is possible to suppress color moire and improve the subjective image quality.

It is a block diagram which shows the structural example of the contrast correction apparatus which concerns on the 1st Embodiment of this invention. It is a figure which shows the frequency decomposition image produced | generated by the frequency decomposition part in the contrast correction apparatus which concerns on the 1st Embodiment of this invention. It is a figure which shows a typical color sampling structure. It is a figure explaining Michelson contrast. It is a figure which shows the contrast correction function used for the contrast correction process in the contrast correction apparatus which concerns on embodiment of this invention. It is a figure which shows the visual spatiotemporal contrast sensitivity characteristic in photopic vision. It is a block diagram which shows the structural example of the contrast correction apparatus which concerns on the 2nd Embodiment of this invention. It is a figure which shows the frequency decomposition image produced | generated by the frequency decomposition part in the contrast correction apparatus which concerns on the 2nd Embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(First embodiment)
The contrast correction apparatus according to the first embodiment of the present invention will be described below. FIG. 1 is a block diagram showing a configuration example of a contrast correction apparatus according to the first embodiment of the present invention. The contrast correction apparatus 1 shown in FIG. 1 includes a frequency decomposition unit 11, a color sampling structure correction coefficient determination unit 12, a contrast correction unit 13, and a frame image reconstruction unit 14.

  The contrast correction apparatus 1 inputs color sampling structure information indicating a color sampling structure (a sampling structure of a red signal, a green signal, and a blue signal) of an image sensor that has captured an image input to the contrast correction apparatus 1. . Typical examples of the color sampling structure include a three-plate type, an F65 type, and a Bayer type. Then, the contrast correction apparatus 1 corrects the contrast of the input video based on the color sampling structure information, generates a contrast-corrected video whose contrast is corrected, and outputs it to the outside.

  The frequency resolving unit 11 performs frequency decomposition on the input video for each color signal to generate a frequency resolved image, and outputs the frequency resolved image to the contrast correcting unit 13. Frequency decomposition is performed by Fourier transform, octave decomposition, wavelet decomposition, or the like. In the present embodiment, description will be made assuming that wavelet decomposition is performed. In addition, it is preferable that the wavelet decomposition is performed so as to satisfy the Parseval equation in order to keep the power level in each decomposition rank uniform. Note that only a specific frequency band may be further subjected to wavelet decomposition.

FIG. 2 is a diagram illustrating a frequency-resolved image wavelet-decomposed by the frequency decomposing unit 11. Here, it is assumed that the decomposition rank n _h = 2 in the horizontal direction and the decomposition rank n _v = 2 in the vertical direction. LH ⁿ , HL ⁿ and HH ⁿ (n = 1, 2) are a horizontal high frequency band image, a vertical high frequency band image, and a diagonal high frequency band image, respectively, and LL ² is a low frequency band image.

  The color sampling structure correction coefficient determination unit 12 grasps the Nyquist frequency of the input video from the color sampling structure, and determines a correction coefficient for each frequency band of the color signal so that the contrast decreases as the Nyquist frequency decreases. And output to the contrast correction unit 13.

  FIG. 3 is a diagram showing a typical color sampling structure. A specific example of the correction coefficient determination method will be described with reference to FIG. FIG. 3A shows a square lattice type sampling structure for sampling green, red, and blue signals in a three-plate sensor camera, and for sampling a green signal in a single-plate sensor camera having an F65 type color sampling structure. Used. FIG. 3B shows a five-eye sampling structure of a single-plate sensor camera having an F65 type color sampling structure and a single-plate sensor camera having a Bayer-type color sampling structure. Used for sampling green signals. FIG. 3C shows a square lattice type sampling structure in which FIG. 3A is thinned 1: 2 pixels in the horizontal and vertical directions, and red and blue signals in a single-plate sensor camera having a Bayer type color sampling structure. Used for sampling.

  For each color signal of the camera, pixels are interpolated and interpolated from the color sampling structure described above, and an image in which the color signal is included in all sample positions is output. Here, when the optical system low-pass filtering of the camera is insufficient, color moire due to aliasing occurs. In a single-plate sensor camera, even when an optical low-pass filter is mounted on the sensor surface, color moiré due to aliasing occurs in the case of a color sampling structure as shown in FIGS. 3B and 3C. there is a possibility. Even in a three-plate sensor camera, the optical low-pass filter characteristics on the sensor surface are not completely cut off at the Nyquist frequency, and color moire due to aliasing cannot be completely suppressed. In addition, when a moving image is taken with an optical low-pass filterless camera or a still camera with a high sampling frequency and the moving image is read by thinning out pixels, a large amount of color moire may occur.

Therefore, in order to suppress color moire, the color sampling structure correction coefficient determination unit 12 sets a correction coefficient for each frequency band. Here, the correction coefficient is determined in accordance with the n-th wavelet decomposition process in the spatial direction in the frequency decomposition unit 11. For example, when the decomposition rank n = 2, the correction coefficients in the horizontal high frequency band LH ⁿ , the vertical high frequency band HL ⁿ , and the diagonal high frequency band HH ⁿ are {M (LH ⁿ ), M (HL ⁿ ), M (HH ⁿ ). } And a correction coefficient is set according to the color sampling structure.

The correction coefficient value is, for example, {M (LH ⁿ ), M (HL ⁿ ), M (HH ⁿ ) | n = 1, 2 when the color sampling structure is a square lattice structure shown in FIG. } = {1,1,1}. That is, in this case, contrast correction is not performed.

In the case of the square lattice structure shown in FIG. 3B, for example, {M (LH ¹ ), M (HL ¹ ), M (HH ¹ )} = {0.75, 0.75, 0.5} And {M (LH ² ), M (HL ² ), M (HH ² )} = {1, 1, 0.75}.

In the case of the square lattice structure shown in FIG. 3C, for example, {M (LH ¹ ), M (HL ¹ ), M (HH ¹ )} = {0.5, 0.5, 0.5} And decide {M (LH ² ), M (HL ² ), M (HH ² )} = {0.75, 0.75, 0.75}.

  The contrast correction unit 13 calculates the contrast for each frequency band of the frequency resolved image. Then, based on the correction coefficient determined by the color sampling structure correction coefficient determination unit 12, the contrast of the frequency-resolved image is corrected for each frequency band, and the frequency-resolved image whose contrast is corrected (contrast-corrected image for each band). Is output to the frame image reconstruction unit 14. The contrast correction unit 13 may use Michelson contrast as the contrast, or may use a ratio between the minimum luminance and the maximum luminance.

The Michelson contrast will be described with reference to FIG. When the maximum brightness of the striped pattern is L _max and the minimum brightness of the striped pattern is L _min , the Michelson contrast C is expressed by Expression (1). In the present embodiment, L _max and L _min used for calculating the contrast are absolute values.

FIG. 5 is a diagram showing the contrast correction function y = f (x) used for the contrast correction processing, where the horizontal axis x represents the absolute value of the element before contrast correction, and the vertical axis y represents the absolute value of the element after contrast correction. Indicates the value. The contrast correction unit 13 converts the contrast into a value multiplied by the correction coefficient M while keeping the average value L _ave = (L _max + L _min ) / 2 of the maximum absolute value and the minimum absolute value constant, for example. In this case, the maximum absolute value L ′ _max of the element after contrast conversion is L _ave + (L _max −L _ave ) × M, and the minimum absolute value L ′ _min of the element after contrast conversion is L _ave − (L _ave − L _min ) × M. The contrast correction unit 13 converts the contrast according to the correction function, and then attaches the same positive / negative sign as the value of the element before the contrast conversion. If the element value after contrast conversion exceeds the lower limit value or the upper limit value, clipping is performed at the lower limit value or the upper limit value.

  The frame image reconstruction unit 14 reversely frequency-decomposes the band-by-band contrast correction image generated by the contrast correction unit 13 into frame images, and outputs the contrast-corrected video (contrast correction video) to the outside of the contrast correction device 1. Output. When the spatial wavelet decomposition is performed in the frequency decomposition unit 11, the spatial wavelet reconstruction is performed in the frame image reconstruction unit 14.

  Note that a computer can be preferably used to cause the above-described contrast correction apparatus 1 to function, and such a computer stores a program describing processing contents for realizing each function of the contrast correction apparatus 1. This program can be realized by reading out and executing this program by the CPU of the computer. This program can be recorded on a computer-readable recording medium.

  As described above, in the invention according to the first embodiment, the Nyquist frequency of the video is grasped from the color sampling structure of the image sensor, and the contrast is reduced as the Nyquist frequency is lower, for each frequency band of the color signal. to correct. With this configuration, according to the present invention, it is possible to suppress color moire caused by the color sampling structure, and it is possible to improve subjective image quality.

(Second Embodiment)
Next, a case where a frame rate converted video is input will be described below as a second embodiment. Here, the frame rate converted video means a video whose frame rate is converted by an arbitrary method.

  As a general method for converting the frame rate, a linear interpolation frame interpolation method and a motion correction frame interpolation method are known. In the linear interpolation frame interpolation method, an interpolation frame is generated by applying linear filter processing to the frame images before and after the interpolation frame position on the time axis. In the motion correction frame interpolation method, motion estimation is performed between frame images before and after the interpolation frame position on the time axis, and a motion correction frame interpolation is performed at the interpolation frame position using the result.

  When a frame rate converted image is sampled, a color multiplex image resulting from the color moire is easily perceived in addition to the color moire. This may be due to the perception of contrast differences that would otherwise be difficult to perceive due to changes in visual contrast sensitivity associated with frame rate conversion. The

  FIG. 6 is a diagram showing visual spatiotemporal contrast sensitivity characteristics in photopic vision. The horizontal axis represents the spatial frequency, and the vertical axis represents the contrast sensitivity. Here, the contrast sensitivity is the reciprocal of the contrast threshold value, and the contrast threshold value is the lowest Michelson contrast with which a stimulus can be detected. FIG. 6 shows an example of the spatio-temporal contrast sensitivity characteristic. The contrast sensitivity is not limited to this value, and may include contrast sensitivity of more time frequencies. For details of spatio-temporal contrast sensitivity characteristics, see, for example, JG Robson, “Spatial and Temporal Contrast-Sensitivity Functions of the Visual System”, Journal of the Optical Society of America, Vol. 56, Issue 8, pp. 1141-1142 ( Mar. 1966).

  FIG. 6 shows the contrast sensitivity when the time frequency is changed. The white circle plot shows the case where the time frequency is 1 cycle / second, the black circle plot shows the case where the time frequency is 6 cycles / second, The white triangle plot shows the case where the time frequency is 16 cycles / second, and the black triangle plot shows the case where the time frequency is 22 cycles / second. In FIG. 6, the time frequency is only shown up to 22 cycles / second (44 frames / second), but if it is more than that, it can be calculated by extrapolating the value. Even if the time frequency is higher than 22 cycles / second, the contrast sensitivity only decreases exponentially.

  As shown in FIG. 6, the visual spatiotemporal contrast sensitivity shows a band-pass characteristic in which the intermediate frequency band is high when the temporal frequency is as low as 1 cycle / second. On the other hand, when the time frequency exceeds 8 cycles / second (that is, 16 frames / second) as in a general image, it exhibits a low-pass characteristic, and as the time frequency increases, it extends over all spatial frequency bands. Contrast sensitivity decreases.

  When the frame rate is reduced by the frame rate conversion, the visual spatio-temporal contrast sensitivity is increased over all the spatial frequency bands as shown in FIG. . For example, when performing frame rate conversion from 120 frames / second super high-definition video to 24 frames / second digital cinema video, the visual spatiotemporal contrast sensitivity increases, so 120 frames / second video before frame rate conversion. When viewing a video, it becomes easier to perceive a contrast difference that is difficult to perceive.

  In addition, when the frame rate increases due to frame rate conversion, the spatio-temporal contrast sensitivity of the vision decreases over the entire spatial frequency band as shown in FIG. 6, so that a contrast difference that should be easily perceived becomes difficult to perceive. Occur. For example, when frame rate conversion is performed from a digital cinema image of 24 frames / second to a super high-definition image of 120 frames / second, the visual spatiotemporal contrast sensitivity decreases, so the image of 24 frames / second before the frame rate conversion. When viewing a video, it becomes difficult to perceive a contrast difference that is easily perceived. In the second embodiment, contrast correction is performed in consideration of the influence of such frame rate conversion.

  FIG. 7 is a block diagram illustrating a configuration example of a contrast correction apparatus according to the second embodiment of the present invention. 7 includes a frequency division unit 11 ′, a color sampling structure correction coefficient determination unit 12, a contrast correction unit 13 ′, a frame image reconstruction unit 14, and a frame rate correction coefficient determination unit 15. With.

  The frequency resolving unit 11 ′ frequency-decomposes the input video in the spatio-temporal direction for each color signal, generates a frequency-resolved image, and outputs it to the contrast correction unit 13 ′. In the present embodiment, description will be made assuming that wavelet decomposition is performed as frequency decomposition. In addition, it is preferable that the wavelet decomposition is performed so as to satisfy the Parseval equation in order to keep the power level in each decomposition rank uniform. Note that only a specific frequency band may be further subjected to wavelet decomposition.

Alternatively, the frequency resolving unit 11 ′ uses the spatio-temporal contrast sensitivity characteristic and the spatial frequency information, and uses the horizontal and vertical decomposition ranks n _h and n _v corresponding to the frequency resolution as a reference for the image based on the spatio-temporal contrast sensitivity characteristic. May be determined from the spatial frequency. In this case, as shown in FIG. 6, since the spatio-temporal contrast sensitivity exhibits a low-pass characteristic at the time frequency of a general image, the space obtained by n-th wavelet decomposition when the image is viewed at the standard viewing distance. The decomposition rank n is determined so that the low frequency band falls within a flat region in the spatio-temporal contrast sensitivity characteristic.

For example, in Super Hi-Vision, since the number of horizontal pixels is 7680 pixels and the viewing angle at a standard viewing distance of 0.75H (H is the screen height) is about 100 degrees, the spatial frequency is 76.8 pixels / degree, and the unit is cycled. When converted to / degree, it becomes about 40 cycles / degree. As shown in FIG. 4, the contrast sensitivity becomes a substantially flat region when the spatial frequency is about 2.5 cycles / degree or less, and the wavelet decomposition is a kind of octave decomposition, so the decomposition rank n _h , N _v only needs to be 4 (40 × (1/2) ⁴ = 2.5).

FIG. 8 is a diagram illustrating a frequency-resolved image wavelet-decomposed by the frequency decomposing unit 11 ′. Here, the decomposition rank n _h = 2 in the horizontal direction, the decomposition rank n _v = 2 in the vertical direction, and the decomposition rank n _t = 1 in the time direction are set.

  Similar to the first embodiment, the color sampling structure correction coefficient determination unit 12 grasps the Nyquist frequency of the input video from the color sampling structure, and the color signal so that the contrast becomes smaller as the Nyquist frequency is lower. The first correction coefficient M1 is determined for each frequency band and is output to the contrast correction unit 13 ′.

  The frame rate correction coefficient determination unit 15 receives the frame rate conversion information, and when the frame rate converted image is an image converted so as to increase the frame rate, the contrast is increased, and the frame rate converted image is converted into the frame rate. In the case of a video converted so as to reduce the contrast, the second correction coefficient M2 is determined for each frequency band of the color signal so as to reduce the contrast, and is output to the contrast correction unit 13 ′. Specific examples will be described later. Here, the frame rate conversion information is information including frame rates before and after conversion.

  The contrast correction unit 13 'calculates the contrast for each frequency band of the frequency-resolved image. Then, based on the first correction coefficient M1 determined by the color sampling structure correction coefficient determination unit 12 and the second correction coefficient M2 determined by the frame rate correction coefficient determination unit 15, each frequency band of the color signal is determined. Then, the contrast of the frequency-resolved image is corrected to generate a frequency-resolved image (contrast-corrected image for each band) in which the contrast is corrected and output to the frame image reconstruction unit 14.

For example, the contrast correction unit 13 ′ may maintain the contrast of the frequency-resolved image for each frequency band while keeping the average value L _ave = (L _max + L _min ) / 2 of the maximum absolute value and the minimum absolute value of luminance constant. It converts into the value which multiplied the 1st correction coefficient M1 and the 2nd correction coefficient M2.

  Similar to the first embodiment, the frame image reconstruction unit 14 performs frequency reverse decomposition of the contrast correction image for each band generated by the contrast correction unit 13 into frame images, and the contrast whose contrast is corrected is corrected. The video (contrast correction video) is output to the outside of the contrast correction device 2. When the space-time wavelet decomposition is performed in the frequency decomposition unit 11 ′, the space-time wavelet reconstruction is performed in the frame image reconstruction unit 14.

The frame rate correction coefficient determination unit 15 may determine the second correction coefficient M2 according to the change rate of the contrast sensitivity using the spatiotemporal contrast sensitivity characteristic. For a specific determination method of the second correction coefficient M2 using the spatio-temporal contrast sensitivity characteristic, an image obtained by converting a frame rate from a 4K digital cinema of 24 frames / second to a 4K television of 60 frames / second is input. An example is described below. In this example, as in FIG. 8, the decomposition rank n _h = 2 in the horizontal direction, the decomposition rank n _v = 2 in the vertical direction, and the decomposition rank n _t = 1 in the time direction.

  4K televisions have a horizontal pixel count of 3840 (or 4096) and a viewing angle of about 60 degrees at a standard viewing distance of 1.5H, so the maximum spatial frequency is 64 pixels / degree, and units are converted to cycles / degrees. Then, it becomes about 30 cycles / degree. In addition, since the number of vertical pixels is 2160 pixels and the viewing angle at a standard viewing distance of 1.5H is about 40 degrees, the maximum spatial frequency is 54 pixels / degree, and when the unit is converted into cycles / degrees, it is about 30 cycles / degrees. Become.

When an image having a maximum spatial frequency of 30 cycles / degree is subjected to second-order wavelet decomposition in the spatial direction, the spatial frequency bands (LH ¹ , HL ¹ , HH ¹ ) of decomposition ranks n _h = 1 and n _v = 1 are 15 to 30 cycles. Therefore, the representative value is 22.5 cycles / degree. In addition, the spatial high frequency band (LH ² , HL ² , HH ² ) of the decomposition ranks n _h = 2 and n _v = 2 is 7.5 to 15 cycles / degree, and the representative value is 11.25. Cycle / degree. Moreover, since the spatial low frequency band (LL ² ) of the decomposition ranks n _h = 2 and n _v = 2 is 0 to 7.5 cycles / degree, the representative value is set to 3.75 cycles / degree.

  Frame rate conversion The frame rate of the video is converted from 24 frames / second to 60 frames / second. When the unit is converted to cycles / second, the maximum time frequency is converted from 12 cycles / second to 30 cycles / second. .

When an image having a maximum time frequency of 12 cycles / second is subjected to first-order wavelet decomposition in the time direction, the time high frequency band (H ¹ ) is 6 to 12 cycles / second, and the time low frequency band (L ¹ ) is 0 to 6 cycles. / Sec. When an image having a maximum time frequency of 30 cycles / second is subjected to first-order wavelet decomposition in the time direction, the time high frequency band (H ¹ ) is 15 to 30 cycles / second, and the time low frequency band (L ¹ ) is 0 to 0. 15 cycles / second. That is, in the frame rate conversion of the present embodiment, the representative value is converted from 9 cycles / second to 22.5 cycles / second for the time high frequency band (H ¹ ), and for the time low frequency band (L ¹ ). The representative value is converted from 3 cycles / second to 7.5 cycles / second.

  The contrast correction unit 13 ′ obtains contrast sensitivity (CS: Contrast Sensitivity) for each frequency band of the frequency-resolved image with respect to the time frequency before and after the frame rate conversion from the spatio-temporal contrast sensitivity characteristic.

The contrast sensitivity of each spatial frequency band in the time high frequency band (H ¹ ) is as follows when the frame rate is converted from 9 cycles / second to 22.5 cycles / second with reference to FIG. In the spatial frequency bands (LH ¹ , HL ¹ , HH ¹ ) of the decomposition ranks n _h = 1 and n _v = 1 in the horizontal and vertical directions, the contrast sensitivity is 6 to 1. The spatial sensitivity in the spatial high frequency band (LH ¹ , HL ¹ , HH ¹ ) of the horizontal and vertical resolution ranks n _h = 2 and n _v = 2 is 26 to 3. The spatial sensitivity in the spatial low frequency band (LL ² ) of the horizontal and vertical decomposition ranks n _h = 2 and n _v = 2 is 90 to 12.

Further, the contrast sensitivity of each spatial frequency band in the temporal low frequency band (L ¹ ) is as follows when the frame rate is converted from 3 cycles / second to 7.5 cycles / second with reference to FIG. . In the spatial frequency bands (LH ¹ , HL ¹ , HH ¹ ) of the decomposition ranks n _h = 1 and n _v = 1 in the horizontal and vertical directions, the contrast sensitivity is 11 to 10. The spatial sensitivity in the spatial high frequency band (LH ¹ , HL ¹ , HH ¹ ) in the horizontal and vertical decomposition ranks n _h = 2 and n _v = 2 is 50 to 40. The contrast sensitivity of the spatial low frequency band (LL ² ) in the horizontal and vertical decomposition ranks n _h = 2 and n _v = 2 is 200 to 180.

The second correction coefficient M2 is obtained as the ratio of the contrast sensitivity at the time frequency before frame rate conversion to the contrast sensitivity at the time frequency after frame rate conversion. These are summarized in Table 1 below. The space-time lowest frequency band (L ¹ and LL ² ) may be regarded as a direct current component for convenience, and the contrast correction may not be performed or may be performed in the same manner.

  The reciprocal of the contrast sensitivity becomes the contrast threshold value, which is the lowest contrast at which stimulation can be detected. The contrast correction unit 13 ′ performs correction according to the change in contrast threshold in each spatiotemporal frequency band. By performing this correction, for example, when the contrast threshold is increased (contrast sensitivity is decreased), the contrast difference is perceived before the frame rate conversion but is not perceived after the conversion. It can be corrected. Also, when the contrast threshold is reduced (contrast sensitivity is increased), the contrast difference that was not perceived before the frame rate conversion but that that became perceived after the conversion is corrected so that it is not perceived. Can do.

  Further, since it is known that the contrast sensitivity in the oblique direction is lowered, the contrast correction unit 13 ′ may further correct the contrast of the frequency-resolved image for each frequency band in consideration of spatial anisotropy. . For example, the spatio-temporal contrast sensitivity characteristic for the oblique direction is prepared separately, or the contrast sensitivity in the oblique direction is estimated by correcting the spatio-temporal contrast sensitivity characteristic.

  In addition, a computer can be suitably used for causing the above-described contrast correction device 2 to function, and such a computer stores a program describing processing contents for realizing each function of the contrast correction device 2. This program can be realized by reading out and executing this program by the CPU of the computer. This program can be recorded on a computer-readable recording medium.

  As described above, in the invention according to the second embodiment, when the frame rate converted image is an image converted so as to increase the frame rate, the contrast increases, and the frame rate converted image decreases the frame rate. In the case of an image converted in such a manner, the contrast is corrected for each frequency band of the color signal so that the contrast becomes small. With this configuration, according to the present invention, artifacts such as color moiré caused by the color sampling structure and color multiplex images caused by frame rate conversion can be suppressed, and subjective image quality can be improved. Become.

  Although the above embodiment has been described as a representative example, it will be apparent to those skilled in the art that many changes and substitutions can be made within the spirit and scope of the invention. Therefore, the present invention should not be construed as being limited by the above-described embodiments, and various modifications and changes can be made without departing from the scope of the claims. For example, it is possible to combine a plurality of constituent blocks described in the configuration diagram of the embodiment into one, or to divide one constituent block.

DESCRIPTION OF SYMBOLS 1, 2 Contrast correction apparatus 11, 11 'Frequency decomposition part 12 Color sampling structure correction coefficient determination part 13, 13' Contrast correction part 14 Frame image reconstruction part 15 Frame rate correction coefficient determination part

Claims (7)

A contrast correction device for correcting the contrast of an image,
A frequency resolving unit that frequency-decomposes the video for each color signal to generate a frequency-resolved image;
A color for determining the first correction coefficient for each frequency band of the color signal so that the Nyquist frequency of the image is grasped from the color sampling structure of the image pickup device that has picked up the image, and the contrast becomes smaller as the Nyquist frequency is lower. A sampling structure correction coefficient determination unit;
Based on the correction coefficient, a contrast correction unit that corrects the contrast of the frequency-resolved image for each frequency band of the color signal and generates a contrast correction image for each band;
A frame image reconstructing unit that outputs a video whose contrast is corrected by performing frequency reverse decomposition on the contrast-corrected image for each band;
A contrast correction apparatus comprising: The video is a frame rate converted video obtained by converting the frame rate,
When the frame rate converted image is an image converted to increase the frame rate, the contrast is increased, and when the frame rate converted image is an image converted to decrease the frame rate, the contrast is increased. A frame rate correction coefficient determination unit that determines a second correction coefficient for each frequency band of the color signal so as to be smaller;
The contrast correction unit corrects a contrast of the frequency-resolved image based on the first correction coefficient and the second correction coefficient to generate the contrast correction image for each band. 2. The contrast correction apparatus according to 1.   The contrast correction unit corrects the contrast of the frequency-resolved image based on a spatio-temporal contrast sensitivity characteristic indicating visual contrast sensitivity with respect to a spatial frequency when the temporal frequency is changed. The contrast correction apparatus described.   The contrast correction apparatus according to claim 3, wherein the frequency resolution unit determines a resolution rank in a horizontal direction and a vertical direction based on the spatio-temporal contrast sensitivity characteristic.   5. The contrast correction apparatus according to claim 1, wherein the contrast correction unit corrects a contrast of the frequency-resolved image in consideration of anisotropy of space. The frequency decomposition unit generates the frequency-resolved image using wavelet decomposition,
The contrast correction apparatus according to claim 1, wherein the frame image reconstruction unit performs inverse transformation of the frequency-resolved image into a frame image using wavelet reconstruction. The program for functioning a computer as a contrast correction apparatus as described in any one of Claims 1-6.

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