JP4292219B2 - Image processing apparatus and image processing program - Google Patents

Description

  The present invention relates to an image processing apparatus and an image processing program having a luminance expansion function function based on linear conversion, in which an image captured by a camera is handled as an input image.

  In a camera image, when the illumination is insufficient, such as when there is no illumination, the screen may be black. On the other hand, the screen may be completely white, such as when a strong light such as a western sun is inserted. In this way, luminance is concentrated in local gradations, the contrast of the entire screen is lost, and there is a case where the video becomes unsuitable for monitoring or image processing.

  Generally, when it is determined that there is a lot of dark luminance for a monochrome input image, many gradations are assigned to the low gradation part, and conversely, when it is determined that there are many white parts, Allocate many gradations to the key part. Conventionally, as this type of gradation conversion technique, there has been an image processing apparatus using “linear conversion” as disclosed in, for example, Patent Document 1 below.

In this type of conventional “linear transformation”, since the expansion is performed based on the maximum and minimum values of the gradation of the input image, it is effective when all the luminances are gathered in the local gradation part. However, when a large amount of luminance is concentrated in a local gradation portion but is distributed over the entire gradation range, there is a problem that luminance extension is not performed.
JP 2006-26183 A

  As described above, in the conventional luminance expansion processing by “linear conversion”, when a large amount of luminance is concentrated in a local gradation portion, but is distributed over the entire gradation range, luminance expansion is performed. There was a problem that was not implemented.

  The present invention has been made in view of the above circumstances, and an image processing apparatus and an image processing that can always perform luminance expansion processing with an appropriate gradation regardless of bias of luminance distribution of an input image in luminance expansion processing by “linear conversion” The purpose is to provide a program.

The present invention is an image processing apparatus that performs luminance expansion processing by linear conversion, and that performs luminance expansion processing by linear conversion, and is an average luminance in an attention area of an input image to be subjected to the luminance expansion processing and average luminance calculating means for calculating said histogram creating means for creating a histogram of the gradation in the area of interest of the input image, the histogram said histogram producing means low gradation portions and high gray portion and the intermediate floor divided into a tone part, the calculates the sum of the luminance values of the sum and the high gradation part of the luminance value in the low gradation part, the luminance of the sum and the high gradation part of the luminance value of the calculated the low gradation portion comparing the sum of the values, the gradation portion and determines luminance expansion determination means for luminance stretching towards sum of luminance values is large, the luminance decompression judging means thus luminance value of the low gradation portion When the sum is determined as the sum is greater than the luminance value of the high gradation part, sequentially calculates the sum of the luminance values of each gradation value in the unit toward the minimum tone value side from the maximum tone value side of the histogram , calculated grayscale value sum of the luminance value reaches said average luminance value and the luminance stretch maximum gradation value of the histogram, as a first luminance expansion range to the minimum tone value from the luminance decompression maximum tone value said first extension range calculation means for, when the sum of the luminance values of Thus the high gradation part on the luminance decompression judging means determines that the sum is greater than the luminance value of the low gradation part, the minimum floor of the histogram successively calculating the sum of brightness values of each gradation value in the unit toward the maximum tone value side from the tone value side, the luminance stretch of said tone value sum reaches said average luminance value of the luminance value histogram the minimum tone value, the maximum gradation from the luminance decompression minimum tone value A second decompression range calculation means for calculating the up as the second luminance expansion range, the first expansion range calculation means and the first luminance expansion ranges calculated, or the second decompression range calculation means has calculated the the linear transformation performed in the second luminance expansion range, is characterized by comprising a luminance expanded image generating means for generating luminance expanded image, the against the input luminance.

Further, the present invention is an image processing program for performing luminance expansion processing by linear conversion, and an image processing apparatus for performing luminance expansion processing by linear conversion has an average luminance in an attention area of an input image to be subjected to the luminance expansion processing. an average luminance calculation function for calculating a histogram creation function to create a histogram of the gradation in the area of interest of the input image, the low gradation portion of the histogram created by the histogram creation function and high gray portion and the intermediate divided into a gradation portion, said calculates the sum of the luminance values of the sum and the high gradation part of the luminance value in the low gradation part, the luminance value of the sum and the high gradation part of the luminance values of the low gradation portion bright sum compares the luminance decompression judging function judges that the gradation portion of the luminance stretch towards sum of luminance values is large, the low gradation portion I by the luminance decompression judging function When the sum of the values is determined as the sum is greater than the luminance value of the high gradation part, the sum of the sequential luminance values of each gradation value in the unit toward the minimum tone value side from the maximum tone value side of the histogram calculated, the tone value sum of the luminance value reaches said average luminance value and the luminance stretch maximum gradation value of the histogram, the minimum tone value to a first luminance extending from the luminance decompression maximum tone value first and decompression range calculation function for calculating a range, when the sum of the luminance values of the high gradation part I by the luminance decompression determining function determines that the sum is greater than the luminance value of the low gradation part, the successively calculating the sum of brightness values of each gradation value toward the maximum tone value side from the minimum tone value side of the histogram in units, the tone values of the sum of the luminance value reaches the average luminance value a histogram luminance stretching minimum tone value, from said luminance decompression minimum tone value most A second decompression range calculation function for calculating the up gradation value as the second luminance expansion range, the first stretch the first luminance decompression range range calculation function is calculated, or the second decompression range calculation function in the calculated second luminance decompression range by performing a linear transformation, characterized in that to realize a luminance expanded image creation function to create a luminance expanded image, the against the input luminance.

  According to the present invention, in the luminance expansion processing by “linear transformation”, the luminance expansion processing can always be performed with an appropriate gradation regardless of the deviation of the luminance distribution of the input image, and a large amount of luminance can be obtained in the local gradation portion. Even when the concentration is concentrated, the luminance expansion processing with an appropriate gradation can be performed even when the gradation is dispersed over the entire range.

Embodiments of the present invention will be described below with reference to the drawings.
The components of the main part of the image processing apparatus according to the embodiment of the present invention are shown in FIG. The luminance expansion processing unit 1 illustrated in FIG. 1 uses an image captured by a monocular camera as an input image, performs luminance expansion processing on the input image by “linear conversion”, and converts the input image into a luminance value distribution state of the input image. To the optimum gradation. Here, for an image of 0 to 255 gradations of QVGA (320 pixels × 240 pixels), the input image is subjected to luminance expansion processing based on the average luminance of the area of interest and the central luminance of all gradations.

In the luminance expansion processing in the luminance expansion processing unit 1, the average luminance of the image of the attention area set in advance for the input image to be subjected to the luminance expansion processing is obtained, and the gradation histogram in the attention area of the input image ( create a luminance value histogram), seeking the sum of the luminance values of the sum and high gradation part of the luminance value in the low gradation part when divided into three gradations respectively, low gray scale portion and the brightness of the high tone part The sum of the values is compared, and the gradation side where the sum of the luminance values is large (the luminance distribution amount is large) is determined as the gradation side where the luminance is expanded. And to luminance stretch more gradation side luminance distribution amount, toward the high gradation side luminance distribution amount less gradation side luminance distribution amount, sequentially continue to calculate the sum of the luminance values, the luminance A gradation whose sum of values reaches the above average luminance is obtained. The entire gradation range spanning the gradation value obtained here and the gradation range on the gradation side with a large amount of luminance distribution is set as a luminance expansion target range, and “linear conversion” is performed on the gradation range.

  As described above, the luminance expansion processing unit 1 uses the linear conversion as a reference for the luminance expansion method itself in the calculation of the luminance expansion range by linear conversion, but the expansion range is determined based on the frequency value distribution of luminance values. , Using the sum of gradations. Even in the case where the luminance expansion processing by the linear conversion in the luminance expansion processing unit 1 as described above is concentrated in a local gradation portion but dispersed over the entire gradation range. A luminance-expanded image with appropriate gradation can be acquired. In addition, with respect to luminance expansion by specifying the area of interest and linear conversion, by setting an area on the screen that should be noted (which should be subject to luminance expansion), an optimal luminance expansion image with linear conversion suitable for that area can be obtained. You can get it.

  The luminance expansion processing operation of the luminance expansion processing unit 1 shown in FIG. 1 will be described with reference to the operation explanatory diagrams shown in FIGS. 2 and 3 and the gradation histograms (luminance value histograms) shown in FIGS. .

  The luminance expansion processing unit 1 executes the following processes (1) to (5) every time an image captured by a camera is input.

  (1). In the attention area 3 of the input image (320 pixels × 240 pixels) 2 captured by the monocular camera, a histogram (luminance value histogram) of gradation (0 to 255) is created, and the average luminance of the attention area 3 is obtained.

(2). In the histogram, the gradation (0 to 255) is divided into three parts, a low gradation part (S1), a high gradation part (S3), and an intermediate gradation part (S2). obtaining a luminance value of the sum and the sum of the luminance values of the high gradation part (S3), respectively. At this time, in the input image (monochrome image) 2, a partial specific gradation region (for example, a gradation region of 0 to 4) including the minimum gradation value “0” of the low gradation portion (S1) and a high gradation portion. Since the gradation may locally gather in a part specific gradation region (for example, the gradation region of 251 to 255 ) including the maximum gradation value “255” in (S3), each of the above-mentioned parts at both ends of the histogram. The specific gradation area is excluded from the gradation for which the sum of the luminance values is obtained.

(3). Comparing the sum of the luminance values of the sum and high gradation part of the luminance value of low gradation portion (S1) (S3), without performing a linear conversion process when equal, the sum of luminance values larger (luminance distribution amount The gradation portion on the (most) side is expanded. As shown in FIG. 3, when the sum of the luminance values of the sum and high gradation part of the luminance value of low gradation portion (S1) (S3) is equal to (S1 = S3) is not performing a linear conversion process, Teikai When the sum of the luminance values of the key portion (S1) is larger than the sum of the luminance values of the high gradation portion (S3) (S1> S3), the maximum gradation value (Max = (c)) of the linear conversion processing range is obtained. When the processing is performed and the sum of the luminance values of the low gradation portion (S1) is smaller than the sum of the luminance values of the high gradation portion (S3) (S1 <S3), the minimum gradation value (Min) of the linear conversion processing range = (B)) is performed.

(4). In the comparison of (3) above, when the sum of the luminance values of the low gradation portion (S1) is larger than the sum of the luminance values of the high gradation portion (S3), that is, compared to the luminance distribution amount of the high gradation portion (S3). when the luminance distribution of the low gradation part (S1) is larger, toward the maximum tone value side of the high gradation part (S3) to the minimum tone value side of the low gradation part (S1), each gradation value The sum of luminance values is calculated in order for each unit. At this time, since the gradation may be locally collected in the specific gradation region (251 to 255), the sum from the gradation value “ 250 ” is calculated. The gradation value at which the sum of the luminance values becomes the average luminance is obtained as the maximum gradation value (Max) of the expansion range. In this case, the minimum gradation value (Min) of the expansion range is the minimum value (gradation value 0) of the histogram (see S1> S3 shown in FIG. 3). FIG. 4 shows a histogram example when the luminance distribution amount of the low gradation portion (S1) is larger than the luminance distribution amount of the high gradation portion (S3) at this time, and shows the maximum gradation value (Max) of the expansion range. calculating the direction of the sum of the brightness values for determining and shows gradation value sum of luminance values reached an average luminance value (Pu) in FIG.

(5). In the comparison of (3) above, when the sum of the luminance values of the high gradation portion (S3) is larger than the sum of the luminance values of the low gradation portion (S1), that is, compared with the luminance distribution amount of the low gradation portion (S1). If you brightness distribution of high gradation portions (S3) it is larger, toward the minimum tone value side of the low gradation part (S1) to the maximum tone value side of the high gradation part (S3), each gradation value The sum of luminance values is calculated in order for each unit. At this time, since the gradation may locally gather in the specific gradation area (0 to 4), the sum from the gradation value “ 5 ” is calculated. The gradation value at which the sum of the luminance values becomes the average luminance is obtained as the minimum gradation value (Min) of the expansion range. In this case, the maximum gradation value (Max) of the expansion range is the maximum value (gradation value 255) of the histogram (see S1 <S3 shown in FIG. 3). FIG. 6 shows a histogram example when the luminance distribution amount of the high gradation portion (S1) is larger than the luminance distribution amount of the low gradation portion (S3) at this time, and the minimum gradation value (Min) of the expansion range is shown. calculating the direction of the sum of the brightness values for determining and shows gradation value sum of luminance values reached an average luminance value (Pu) in FIG.

  (6). In the range obtained by the process (4) or (5), linear conversion is performed on the input image, and a luminance expanded image having the converted luminance with respect to the input luminance is created.

  As described above, the luminance expansion processing unit 1 calculates a linear conversion range based on the frequency distribution of the luminance values of the input image, and creates a luminance expanded image with the converted luminance with respect to the input luminance.

  An example that further embodies the above embodiment is shown in FIGS. FIG. 8 is a block diagram showing the configuration of the image processing apparatus according to the embodiment of the present invention, FIG. 9 is a block diagram showing the configuration of the luminance expansion processing unit provided in the front image processing unit according to the embodiment, and FIG. These are the flowcharts which show the process sequence of the said brightness | luminance expansion process part. Here, the configuration in which the luminance expansion processing function unit as the gist of the present invention is provided in the front image processing unit of the image processing apparatus is illustrated, but the luminance expansion processing function unit is configured as shown in FIG. For example, the present invention can be applied to various image processing apparatuses such as an image processing apparatus that simply performs luminance expansion processing on camera video.

  As shown in FIG. 8, the image processing apparatus according to the embodiment of the present invention includes a camera (monocular camera) 11, a capture unit 12, a front image processing unit 13, an image processing unit 14, and a display unit 15. It is comprised and comprises.

  The camera 11 includes a lens unit and an image pickup device (for example, a CCD solid-state image pickup device or a CMOS image sensor) provided at an image forming position of the lens unit, so that a subject (animal body) that moves outdoors or indoors can be used. A captured image for one screen is output to a target in a predetermined pixel unit (for example, one frame 320 × 240 pixels = QVGA).

  The capture unit 12 has a processing function for capturing an image in frame units captured by the camera 11 and holding it in the image buffer 21 in the front image processing unit 13.

  The pre-image processing unit 13 includes an image buffer 21 that holds an image in frame units captured by the capture unit 12 from the camera 11 as an input image to be subjected to luminance expansion processing, and the functional configuration illustrated in FIGS. 1 to 3 described above. The luminance expansion processing unit 22 has the same processing function as the luminance expansion processing unit 1 of FIG. The internal components of the luminance expansion processing unit 22 provided in the front image processing unit 13 will be described later with reference to FIG.

  The image processing unit 14 takes in an image after luminance expansion processing (linear conversion) held in the image buffer 21 of the front image processing unit 13, for example, difference binarization processing of an image having a time difference, noise removal, and change pixel A series of image processing such as extraction of a rectangular region including the pixel and tracking processing of a changed pixel region are performed. The display unit 15 displays and outputs, for example, an image of a moving object region processed by the image processing unit 14.

  FIG. 9 shows components of the luminance expansion processing unit 22 provided in the front image processing unit 13 shown in FIG. The luminance expansion processing unit 22 illustrated in FIG. 9 includes a histogram generation processing unit 221, a luminance expansion execution determination processing unit 222, a luminance range calculation processing unit 223, and a luminance expansion image generation processing unit 224.

  The histogram creation processing unit 221 acquires a preset image of the attention area from the input image to be subjected to the luminance expansion processing held in the image buffer 21, creates a gradation histogram in the attention area, The average luminance in the attention area is calculated.

The luminance expansion execution determination processing unit 222 uses the histogram created by the histogram creation processing unit 221 as shown in FIGS. 2 to 7 as described above for the low gradation part (S1), the high gradation part (S3), and the intermediate gradation part. (S2) and in 3 divided, and the sum of the luminance values of the low-gradation portion (S1), calculates respective sums of luminance values of the high tone part (S3), the brightness value of low gradation portion (S1) Is compared with the sum of the luminance values (brightness distribution amount) of the high gradation part (S3), and the gradation part on the luminance expansion side is determined based on the frequency distribution of the luminance values. .

As a result of the comparison by the luminance expansion execution determination processing unit 222, the luminance range calculation processing unit 223 determines that the high gradation level when the sum of the luminance values of the low gradation part (S1) is larger than the sum of the luminance values of the high gradation part (S3). It calculates the sum of the minimum gradation value successively the brightness value of each gradation value in the unit toward a side of the maximum tone value side low gradation portion (S1) of the portion (S3), the sum of the luminance values above The gradation value that has reached the average luminance value is calculated as the maximum gradation value (Max) of the luminance expansion range in the histogram. Further, as a result of the comparison by the luminance extension execution determination processing unit 222, when the sum of the luminance values of the high gradation portion (S3) is larger than the sum of the luminance values of the low gradation portion (S1), the low gradation portion (S1). successively calculating the sum of luminance values from the minimum tone value side in units of each tone value toward the maximum tone value side of the high gradation part (S3), the sum of the luminance value reaches the average luminance value The gradation value is calculated as the minimum gradation value (Min) of the luminance extension range in the histogram.

  The luminance extension image creation processing unit 224 performs linear conversion in the luminance extension range calculated by the luminance range calculation processing unit 223, and creates a luminance extension image having the converted luminance with respect to the input luminance.

  FIG. 10 shows a processing procedure of the luminance expansion processing unit 22 described above.

  The luminance expansion processing unit 22 performs the process shown in FIG. 10 every time an input image in frame units captured by the camera 11 is held in the image buffer 21 via the capture unit 12.

  When an input image to be subjected to luminance expansion (linear conversion) processing is held in the image buffer 21, the histogram creation processing unit 221 acquires a target image of a preset attention area from the input image, and the attention area A gradation histogram is created and average brightness in the attention area is calculated (step S11).

When a histogram is created for the target image in the area of interest by the histogram creation processing unit 221 and the average luminance of the image is calculated, the luminance expansion execution determination processing unit 222 displays the histogram created by the histogram creation processing unit 221 as described above. As shown in FIGS. 2 to 7, the low gradation part (S1), the high gradation part (S3), and the intermediate gradation part (S2) are divided into three parts, and the sum of the luminance values of the low gradation part (S1) Then, the sum of the luminance values of the high gradation part (S3) is calculated (step S12). Then, compared with the sum of the luminance values of the low gradation part (S1) (luminance distribution amount), the sum of the luminance values of high gradation portions (S3) (the luminance distribution amount) (step S13), and the frequency of the luminance value The gradation part on the luminance expansion side is determined based on the distribution size.

Here, when the sum of the luminance values of the sum and high gradation part of the luminance value of low gradation portion (S1) (S3) is equal to (S1 = S3) is not performing a linear conversion process, and ends the process ( Step S13 No).

When the sum of the luminance values of the low gradation portion (S1) is larger than the sum of the luminance values of the high gradation portion (S3) (S1> S3) (Yes in step S14), the linear conversion processing range (luminance expansion range) A process for obtaining the maximum value (Max = (c)) is performed (step S15).

When the sum of the luminance values of the low gradation part (S1) is smaller than the sum of the luminance values of the high gradation part (S3) (S1 <S3) (No in step S14), the minimum value (Min) of the linear conversion processing range = (B)) is performed (step S17).

When the maximum gradation value (Max) of the linear conversion processing range is obtained (step S15), the minimum value “0” of the histogram is set as the minimum gradation value (Min) of the linear conversion processing range (step S16). With respect to the linear conversion processing range, the luminance expanded image creation processing unit 224 performs linear conversion to create a luminance expanded image with the converted luminance with respect to the input luminance (step S19).

When the minimum gradation value (Min) of the linear conversion processing range is obtained (step S17), the maximum value “255” of the histogram is set as the maximum gradation value (Max) of the linear conversion processing range (step S18). In this linear conversion processing range, the luminance expanded image creation processing unit 224 performs linear conversion to create a luminance expanded image having the converted luminance with respect to the input luminance (step S19).

As described above, when the luminance expansion processing by the linear expansion by the luminance expansion processing unit 22 is concentrated in a local gradation portion, but is distributed over the entire range of gradation values . In this case, it is possible to obtain a luminance expanded image with appropriate gradation. In addition, with respect to luminance expansion by specifying the area of interest and linear conversion, by setting an area on the screen that should be noted (which should be subject to luminance expansion), an optimal luminance expansion image with linear conversion suitable for that area can be obtained. You can get it.

  In the above-described embodiment, the camera image of 0 to 255 gradations of QVGA (320 pixels × 240 pixels) is used as the input image to be subjected to the luminance expansion processing. However, the present invention is not limited to this, and an image such as VGA is used. The luminance expansion processing function unit according to the present invention can also be applied to an image processing apparatus to be handled. In the above-described embodiment, part or all of the element functions of each component of the luminance expansion processing unit 22 that implements the luminance expansion processing (linear conversion) function can be realized by software processing.

1 is a block diagram showing components of a main part of an image processing apparatus according to an embodiment of the present invention. Operation | movement explanatory drawing for demonstrating the brightness | luminance expansion | extension processing operation of the luminance expansion | extension process part which concerns on the said embodiment. Operation | movement explanatory drawing for demonstrating the luminance expansion | extension range of the luminance expansion process part which concerns on the said embodiment. The figure which shows the luminance value histogram example used as the object of the brightness expansion | extension (linear transformation) process which concerns on the said embodiment. The figure which shows the example which calculated the expansion | extension range of the luminance value histogram used as the object of the brightness | luminance expansion process which concerns on the said embodiment. The figure which shows the luminance value histogram example used as the object of the brightness | luminance expansion | extension (linear transformation) process which concerns on the said embodiment. The figure which shows the example which calculated the expansion | extension range of the luminance value histogram used as the object of the brightness | luminance expansion process which concerns on the said embodiment. The block diagram which shows the structure of the image processing apparatus which concerns on the said embodiment. The block diagram which shows the structure of the brightness expansion process part which concerns on the said embodiment. The flowchart which shows the process sequence of the brightness expansion process part which concerns on the said embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Luminance expansion process part, 2 ... Image (input image) image | photographed with the monocular camera, 3 ... Area of attention, 5 ... Luminance expansion image, 11 ... Camera, 12 ... Capture part, 13 ... Pre-image process part, 14 ... Image processing unit, 15 ... display unit, 21 ... image buffer, 22 ... luminance expansion processing unit, 221 ... histogram creation processing unit, 222 ... luminance expansion execution determination processing unit, 223 ... luminance range calculation processing unit, 224 ... luminance expansion image Creation processing part.

Claims (4)

An image processing apparatus that performs luminance expansion processing by linear conversion,
Average luminance calculating means for calculating an average luminance in an attention area of the input image to be subjected to the luminance expansion processing;
A histogram creation means for creating a histogram of the gradation in the area of interest of the input image,
Dividing the histogram said histogram producing means into a low-gradation portion and high gray portion and a halftone portion, and calculates the sum of the luminance values of the sum and the high gradation part of the luminance values of the low gradation portion with the sum of the luminance value of the calculated the low gradation part to compare the sum of the luminance values of the high gradation part, the tone portion and determines luminance expansion determination means for luminance decompressing the larger sum of luminance values ,
When the sum of the luminance values of Thus the low gradation part on the luminance decompression judging means determines that the sum is greater than the luminance value of the high gradation part, towards the minimum tone value side from the maximum tone value side of the histogram successively calculating the sum of brightness values of each gradation value in units Te, the tone value sum reaches said average luminance value of the luminance value as the luminance stretch maximum gradation value of the histogram, the maximum gray luminance expansion The first extension range calculating means for calculating from the tone value to the minimum gradation value as the first luminance extension range ;
When the sum of the luminance values of Thus the high gradation part on the luminance decompression judging means determines that the sum is greater than the luminance value of the low gradation part, towards the maximum tone value side from the minimum tone value side of the histogram successively calculating the sum of brightness values of each gradation value in units Te, the gradation value sum of the luminance value reaches said average luminance value and the luminance stretch minimum tone value of the histogram, minimum floor luminance expansion Second expansion range calculation means for calculating from the tone value to the maximum gradation value as the second luminance expansion range ;
Said first extension range calculation means and the first luminance expansion ranges calculated, or a linear transformation performed in the second luminance expansion range in which the second decompression range calculation means has calculated, the luminance expanded image against the input luminance Brightness extension image creation means for creating
An image processing apparatus comprising: The luminance stretch determining means, said when calculating the sum of brightness values of the sum and the high gradation part of the luminance value of low gradation portion, the gradation value of a portion including the minimum gradation value of the histogram and The image processing apparatus according to claim 1, wherein a part of gradation values including the maximum gradation value of the histogram is excluded. In an image processing device that performs luminance expansion processing by linear conversion,
An average luminance calculation function for calculating an average luminance in an attention area of the input image to be subjected to the luminance expansion processing;
A histogram creation function to create a histogram of the gradation in the area of interest of the input image,
Dividing the histogram created by said histogram creating function in the low gradation part and high gradation part and a halftone portion, calculates the sum of the luminance values of the sum and the high gradation part of the luminance values of the low gradation portion as well as, with said low the sum of the luminance values of the gradation portion by comparing the sum of the luminance values of the high gradation part, sums determines luminance decompression judging function and gradation portion which luminance decompressing a larger luminance value,
When the sum of the luminance values of the low gradation part I by the luminance decompression determining function determines that the sum is greater than the luminance value of the high gradation part, the minimum tone value side from the maximum tone value side of the histogram successively calculating the sum of the luminance values, the tone values the sum of the luminance value reaches said average luminance value and the luminance stretch maximum gradation value of the histogram in units of each tone value towards, the luminance stretch A first extension range calculation function for calculating from the maximum gradation value to the minimum gradation value as a first luminance extension range ;
When the sum of the luminance values of the high gradation part I by the luminance decompression determining function determines that the sum is greater than the luminance value of the low gradation part, the maximum tone value side from the minimum tone value side of the histogram each tone value sequentially calculated the sum of luminance values in units of tone values the sum of the luminance value reaches said average luminance value and the histogram luminance stretching minimum tone value, the minimum luminance stretch towards A second expansion range calculation function for calculating from the gradation value to the maximum gradation value as a second luminance expansion range ;
The first stretch the first luminance decompression range range calculation function is calculated, or the second decompression range calculation function a linear transformation performed in the second luminance expansion ranges calculated, brightness expanded image against the input luminance Brightness expansion image creation function to create
An image processing program for realizing The luminance stretch determining function, said when calculating the sum of brightness values of the sum and the high gradation part of the luminance value of low gradation portion, the minimum gradation part including value gray level values of said histogram and The image processing program according to claim 3, wherein a part of gradations including the maximum gradation value of the histogram is excluded.

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