JP2006033520A - Image processor, image output device, image processing method, program, and recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that confirmation of contents is difficult, since the reproduction of a delicate part of shade variance becomes weak in a case of an image which has margin in the upper and lower sides of a gradation expression zone. <P>SOLUTION: An image processor is proposed, which comprises (a) a representative value detecting part which obtains a high-luminance representative value and a low-luminance representative value for an input image, (b) a tone width calculation part which calculates the gradation width of the input image, by subtracting the low-luminance representative value from the high-luminance representative value, (c) an amplification rate calculation part which obtains the required amplification rate for amplifying the tone width of the input picture to a permitted tone width, by performing division operation the maximum value of the permitted tone width by the tone width of the input picture, and (d) a tone amplifying part which amplifies the luminance value of each pixel constituting the input image, based on the amplification ratio. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

One embodiment of the present invention relates to an image processing apparatus equipped with a function of amplifying the gradation width of an input image. One embodiment of the present invention relates to an image output apparatus equipped with the image processing apparatus.
Another embodiment of the present invention relates to an image processing method, a program, and a recording medium that realize the same function.

At present, medical information is being digitized in the medical field. Along with this, an environment in which medical diagnostic images photographed by an electronic photographing apparatus can be observed immediately after photographing is being prepared.
Examples of medical diagnostic images include X-ray images and MR images. Examples of the medical diagnostic image output device include a monitor and a printing device. As a printing apparatus, an inkjet method has attracted attention from the viewpoint of running cost.
Advantages of the inkjet method include that development is unnecessary, a short time until a printing result is obtained, and low cost. On the other hand, the fate of discharging liquid ink onto a paper medium is that the limit of the maximum density value is lower than that of a silver salt film. As a result, it has been pointed out that the dynamic range of gradation reproduction is narrowed, and the expressive power with subtle gradation changes is somewhat inferior.
JP 2004-048185 A

By the way, there are various images to be printed, and some images have a low contrast (light / dark ratio) from the stage of shooting. There are also images that do not include low-luminance parts and have a margin above and below the gradation representation area.
Such an image naturally has a weak contrast reproduced as a result of printing, and it tends to be difficult to make a judgment in a portion where the shading slightly changes.
Note that these reproduction problems are not limited to medical diagnostic images. Moreover, it may occur not only during printing but also during screen display.

The inventor pays attention to the above technical problem and proposes the following technical technique.
That is, as an image processing apparatus, (a) a representative value detection unit that obtains a high luminance representative value and a low luminance representative value for an input image, and (b) a low luminance representative value is subtracted from the high luminance representative value, A gradation width calculation unit that calculates a gradation width, and (c) amplification that calculates an amplification factor necessary for amplifying the gradation width of the input image to the allowable gradation width by dividing the allowable gradation width by the gradation width of the input image. The present invention proposes a rate calculation unit and (d) a gradation amplification unit that amplifies the luminance value of each pixel constituting the input image based on the amplification factor.

The image processing apparatus described above preferably has a low-pass filter that removes high-frequency components from the input image, and inputs the input image that has passed through the low-pass filter to the representative value detection unit. That is, it is desirable that the input of the representative value detection unit is obtained by removing a high-frequency component from the input image using a low-pass filter.
By passing through the low-pass filter, noise components and other sudden fluctuation components in the input image are removed. As a result, the representative value detection unit can give a value close to the original maximum luminance value of the image as the high luminance representative value. Further, the representative value detection unit can give a value close to the original minimum luminance value of the image as the low luminance representative value.

Further, the above-described image processing apparatus has a frequency distribution measurement unit that measures the frequency distribution of each luminance value for the entire screen of the input image, and the representative value detection unit of the frequency distribution measured for each luminance value. Of the luminance values exceeding the representative value detection threshold, it is desirable that the largest luminance value be the high luminance representative value and the smallest luminance value be the low luminance representative value.
By using the maximum value and the minimum value of the luminance values in which the frequency distribution above the threshold appears as a representative value, noise components and other sudden fluctuation components in the input image are removed. As a result, the representative value detection unit can give a value close to the original maximum luminance value of the image as the high luminance representative value. Further, the representative value detection unit can give a value close to the original minimum luminance value of the image as the low luminance representative value.
The representative value detection threshold is preferably set based on the number of samples used when measuring the frequency distribution. By making the threshold value depend on the number of samples, the threshold value can be relatively determined. For example, it is desirable to divide the total number of samples by the luminance value to obtain an average value, and set several percent of the average value as the threshold value.

Further, in the above-described image processing apparatus, the amplification factor calculation unit compares the calculated amplification factor with the maximum value of the allowable amplification factor, and when the calculated amplification factor does not exceed the maximum value, the calculated amplification factor is calculated. If the calculated amplification factor exceeds the maximum value, it is desirable to give the maximum value to the gradation amplification unit as the amplification factor.
By setting an upper limit on the calculated amplification factor, it is possible to avoid the meaning of the image after gradation amplification being unclear.
In the above-described image processing apparatus, it is desirable that the gradation amplifying unit amplifies the gradation width of the input image using a gradation value conversion table created according to the amplification factor. By recording the input / output relationship created in advance according to the amplification factor in a table, the load required for the amplification factor conversion process can be reduced.
Note that the image processing apparatus includes an image output apparatus that outputs image information. The image processing apparatus can also be realized as an image processing method, a program, and a recording medium.

  By employing the technical method according to the invention, the contrast of the output image can be amplified with the intensity corresponding to the contrast of the original image. As a result, the original image can be emphasized without impairing the gradation, and the output image can be easily observed.

Embodiments of an image processing apparatus that employs a technical technique according to the invention will be described below.
In addition, the well-known or well-known technique of the said technical field is applied to the part which is not illustrated or described in particular in this specification. The embodiment described below is one embodiment of the present invention and is not limited thereto.

(A) Target image For example, in the case of an image for medical diagnosis, unlike a photographic image, extreme high luminance and low luminance are often not included. In this way, an image that has a margin with respect to the reproducible gradation width is set as a processing target in this embodiment.
If these images are expanded in their own gradation representation range (width from the lowest luminance value to the highest luminance value, also referred to as gradation width), a slight change in gradation is simultaneously amplified. As a result, the contours in the details have the potential to become clearer.

FIG. 1 is an example of an image before gradation amplification. In other words, this is an example of an image in the case where there is a margin with respect to the gradation width that can reproduce the gradation expression area. FIG. 2 is an example of an image after gradation amplification of the same image. It can be seen that, for example, the uneven portions of the glass, the cross-sectional portion of the cake, the napkin folds, and the contours of the flowers are clarified by the extension of the gradation expression area.
However, in order to automate the process of expanding the gradation expression area, it is necessary to optimize the expansion amount according to the characteristics of the image. That is, a function for controlling the adjustment of the gradation gain is required. More specifically, it is essential to quantify the characteristics of the input image as variables.

(B) Control of Gradation Amplification First, quantify how much the input image occupies the entire gradation band. That is, the gradation width t of the input image with respect to the reproducible allowable gradation width Tmax is quantified.
The closer the value of the gradation width t is to Tmax, the more effective the reproduction area is used. That is, it represents an image with high contrast. Conversely, the smaller the value of the gradation width t, the greater the allowable gradation reproduction area. That is, the image has a low contrast.
Therefore, an image whose gradation width t is smaller than the allowable gradation width Tmax means that the gradation can be amplified to the allowable gradation width Tmax under the condition that the density of white and black is not saturated.

An example is shown in FIG. Note that FIG. 3A shows the entire input screen. FIG. 3B shows a change in the image data (luminance value) of line 1 shown in FIG.
In FIG. 3B, two thin lines are drawn. The upper line represents the allowable maximum luminance value (255) that the image data can take, and the lower line represents the allowable minimum luminance value (0). In this example, the allowable gradation width Tmax is “225”.
As shown in FIG. 3B, the gradation width t of the image data relating to the line 1 has a margin with respect to the allowable gradation width Tmax. In this case, the gradation width of the image data can be amplified.
FIG. 3C shows a state in which the gradation width t of line 1 is multiplied by k and expanded to the allowable gradation width Tmax.

It is a technique aimed at in this specification to automatically execute such amplification for the entire screen.
The value of the amplification factor k used for gradation amplification is preferably controlled in an inversely proportional relationship with the difference t (gradation width t) between the high luminance representative value m and the low luminance representative value n included in the input image. .
In this example, this relationship is given by the equation k = A × Tmax / t. However, if the gradation width after amplification exceeds the allowable gradation width Tmax, the gradation of the image is distorted after gradation amplification. Therefore, as each variable, t / Tmax
Those satisfying the conditional expression of ≦ A ≦ 1 are adopted.
Here, A is an adjustment coefficient for the gradation amplification effect. When A = t / Tmax, the gain k becomes 1 and the effect becomes zero. When A = 1.0, the gradation amplification effect is maximized.

If the above conditional expression is satisfied, saturation (beyond the allowable gradation width Tmax) due to amplification of the gradation width t can be eliminated.
However, if only the conditional expression is satisfied, the image may become unnatural due to the gradation amplification. For example, this is the case of an image in which gray clouds are projected on the entire screen.
In such an image, the gradation width t is generally narrow and the amplification factor k is very large.

Therefore, when such an image is amplified with the amplification factor k obtained by the above equation, a slight gradation change appears as an extreme gradation step, and the amplified result may be unnatural. There is.
Therefore, the inventor sets an upper limit value Lgain for the amplification factor k in order to prevent such over-amplification. The upper limit value Lgain is a value greater than 1.0 and is determined empirically using various sample images. It is desirable that the upper limit value Lgain can be changed later by the observer.

(C) Method of detecting high luminance representative value m and low luminance representative value n The basis of gradation amplification is to search image data (luminance value) for the entire input image and detect its maximum and minimum values. is there.
However, the method of detecting the representative luminance value in pixel units may be affected by noise components and other sudden fluctuation values superimposed on the image data. There is a concern that the sudden variation value does not necessarily satisfy the maximum luminance value and the minimum luminance value necessary for the gradation amplification.

Therefore, the present inventor proposes the following two detection methods.
(A) A method for detecting an input image that has passed through a low-pass filter As one of methods for accurately detecting a representative value, a maximum luminance value and a minimum luminance with a high-frequency component included in the input image removed. A method for detecting the value will be described.
Here, as an example of the detection method, the pixel data of each pixel in the pixel matrix of N rows × N columns is weighted, and the average value of the sum of the processing results is calculated as the low-pass output value p ( low).
The maximum value of all the pixels obtained by such calculation is set to the high luminance representative value m, and the minimum value is set to the low luminance representative value n.

FIG. 4 shows a conceptual diagram of this detection process. FIG. 4 shows an example of representative value detection processing using a pixel matrix of 3 rows × 3 columns.
FIG. 4A is an enlarged view of the input screen. A calculation target region 3 used in the filtering calculation is shown by being surrounded by a frame. The calculation result is associated with the central pixel 5 of the calculation target area 3. Incidentally, pixel data corresponding to each pixel position in the pixel matrix is expressed as p1 to p9.

FIG. 4B is a coefficient matrix used in the calculation. The corresponding pixel data of the pixel matrix is multiplied by the coefficients f1 to f9. Incidentally, in this embodiment, 1/16 is set to the coefficients f1, f3, and f7. Moreover, 1/8 is set to the coefficients f2, f4, f6, and f8. Further, 1/4 is set to the coefficient f5 corresponding to the center of the matrix. In other words, the center portion is weighted.
In this case, the low pass output value p5 (low) of the center pixel of the pixel matrix is given by the following equation.
p5 (low) = f1 * p1 + f2 * p2 + f3 * p3 + f4 * p4 + f5 * p5 + f6 * p6 + f7 * p7 + f8 * p8 + f9 * p9

(B) Method for Detecting Representative Value Based on Frequency Distribution Here, a method for determining a representative value based on the frequency distribution will be described.
That is, the pixel data of the input image is classified by luminance value, and frequency distribution data of the number of appearances is obtained for each luminance value. The frequency distribution data is obtained by arranging the luminance values in order from the minimum value to the maximum value and associating the number of appearances with each luminance value.
FIG. 5 shows an example of the frequency distribution data. FIG. 5 is a diagram in which the horizontal axis represents the luminance value and the vertical axis represents the appearance frequency. The minimum value of the luminance value is 0 (zero), and the maximum value is 255.
The determination process is performed by comparing the number of appearances corresponding to each value with the threshold value Lth in order from the minimum value of the distribution data. In FIG. 5, the threshold value Lth is indicated by a broken line extending in parallel with the horizontal axis.

The representative value is given as the minimum value and the maximum value among the luminance values giving the number of appearances exceeding the threshold value Lth.
That is, the number of appearances is compared with the threshold value Lth in order from the smallest luminance value 0 (zero) to the larger one, and the luminance value that first exceeds the threshold value Lth is determined as the low luminance representative value n. Similarly, the number of appearances is compared with the threshold value Lth in order from the maximum luminance value (255) to the smaller value, and the luminance value exceeding the threshold value Lth is first determined as the high luminance representative value m. In the case of FIG. 5, the position of the luminance value exceeding the threshold value Lth is indicated by a broken line extending in parallel with the vertical axis.
Therefore, the luminance value in the range where the number of appearances does not exceed the threshold value Lth is ignored. Of course, this range may include not only the noise component but also the original minimum luminance value and maximum luminance value of the image.
However, by setting the threshold value Lth to an appropriate value, the image reproducibility can be prevented from being affected.

Although the threshold value Lth is given, it is desirable that an appropriate value can be automatically set according to each image. In this embodiment, as an example, a method of using the number of samples that can be counted when creating frequency distribution data will be described.
First, the total number of samples Nsamp constituting the frequency distribution data is obtained by counting all pixels of the input image. When the total number of samples is obtained, this value is divided by the number of luminance values that can appear (in this example, 256), and the average value of the number of appearances is obtained. Here, k% of the average value is set as the threshold value Lth. That is, the threshold value Lth = k * (Nsamp / 256) is calculated.
The coefficient k is determined empirically using various sample images. However, it is desirable that the coefficient k can be changed later by the observer.

6 and 7 show changes in luminance distribution after gradation amplification. 6 corresponds to the original image, and FIG. 7 corresponds to the image after gradation amplification.
As can be seen by comparing FIG. 6A and FIG. 7A, the tiger stripe pattern as the subject is emphasized.
Further, the luminance distribution of the original image has a margin between the lower limit and the upper limit as shown in FIG. 6B, but the luminance distribution of the image after gradation amplification is within the allowable range as shown in FIG. 7B. It has been expanded.

(D) Method of Gradation Amplification Processing Hereinafter, a method of gradation amplification processing will be shown. Here, two types of processing methods are shown as an example.
(A) Method realized by multiplication operation of amplification factor The gradation amplification process can be realized by multiplying each pixel data by the amplification factor k (= A × Tmax / t) calculated using the gradation t.
For example, when the low luminance representative value n is 0 (zero) and the high luminance representative value m is 230, each pixel data P is multiplied by 255/230.
If the low luminance representative value n is not 0 (zero), the value obtained by subtracting n from each pixel data P is multiplied by 255/230. By executing the offset correction, the allowable gradation width Tmax can be used to the maximum.

The principle of offset correction will be described with reference to FIG. FIG. 8A shows the entire input screen. FIG. 8B shows a change in the image data (luminance value) of the line 7 shown in FIG.
In the case of FIG. 8B, there is a margin on both the high luminance side and the low luminance side. That is, there is a margin between the allowable maximum luminance level (255) and the high luminance representative value m, and at the same time, there is a margin between the allowable minimum luminance level (0) and the low luminance representative value n.

Here, the gradation width t is t = mn. Therefore, in order to amplify the gradation width t to the allowable gradation width Tmax, each pixel data may be multiplied by the amplification factor k (= Tmax / (mn)).
However, offset correction is necessary. The calculation formula considering offset correction is given by P = k * (pn) where P is pixel data after gradation amplification and p is pixel data before gradation amplification.
FIG. 8C shows a gradation distribution after gradation amplification. As shown in FIG. 8C, it can be seen that the luminance value of the pixel position 9 corresponding to the low luminance representative value n is zero. It can also be seen that the luminance value of the pixel position 11 corresponding to the high luminance representative value m is 255.

(B) Method using a lookup table The gradation amplification process can be realized by reading a lookup table using pixel data as an input value.
The look-up table is a gradation conversion table in which pixel data before gradation amplification is an input value and pixel data after gradation amplification is an output value. Here, the input / output relationship is linear. That is, linear amplification. Therefore, the same processing result as that obtained by multiplying the amplification factor k can be obtained.

9 and 10 show examples of input / output characteristics of the lookup table.
Here, FIG. 9 is an example of input / output characteristics when there is a margin only between the allowable maximum luminance level (255) and the high luminance representative value m.
On the other hand, FIG. 10 shows an example in which there is a margin not only between the allowable maximum luminance level (255) and the high luminance representative value m but also between the allowable minimum luminance level (0) and the low luminance representative value n. .
In any case, the gradation width of the output value is from the allowable minimum luminance level (0) to the allowable maximum luminance level (255).

(E) Application Example of Image Processing Device (Image Output Device) An application example of an image processing device that employs the above-described gradation amplification technique will be described below. Here, an image output apparatus equipped with an image processing apparatus will be described. The image output device includes a display device and a printing device.
Here, an ink jet printing apparatus will be described.

(1) Configuration example 1 of printing apparatus
FIG. 11 shows a configuration example of the printing apparatus. In this embodiment, a method for realizing gradation amplification processing by multiplication processing of amplification factor k will be described.
This printing apparatus includes an image buffer memory 21, a luminance representative value detection unit 23, an amplification factor calculation unit 25, an adjustment coefficient memory 27, an upper limit value memory 29, a gradation amplification unit 31, a luminance / density conversion unit 33, and a gamma conversion unit 35. The halftoning unit 37 and the print head driving unit 39 are included as main components.

The image buffer memory 21 is a storage device that can hold an input image for at least one frame. As the storage device, for example, a semiconductor storage device, a hard disk or other magnetic storage device, or an optical storage device is used.
The input image is held in the image buffer memory 21 until at least the low luminance representative value n and the amplification factor k are determined.
The representative value detection unit 23 is a processing device that detects a high luminance representative value m and a low luminance representative value n related to an input image. For the detection of the representative value, a method using a low-pass filter or a method using a frequency distribution is applied.

The amplification factor calculator 25 is a processing device that determines an amplification factor k to be multiplied with the input image. The amplification factor calculator 25 corresponds to a function of calculating the gradation width t (= mn) and a function of calculating the amplification factor k by dividing the allowable gradation width Tmax by the calculated gradation width t. That is, it corresponds to functions as a gradation width calculation unit and an amplification factor calculation unit.
The adjustment coefficient A necessary for calculating the amplification factor k is held in the adjustment coefficient memory 27. The effect adjustment coefficient is stored in the adjustment coefficient memory 27 through an input device or a communication port provided in the printing apparatus.
The upper limit value Lgain that limits the amplification factor k is held in the upper limit value memory 29. The upper limit value Lgain is also stored in the upper limit value memory 29 through an input device or a communication port provided in the printing apparatus.

The gradation amplifying unit 31 is a processing device that amplifies an input image by gradation. As shown in FIG. 11, the gradation amplifying unit 31 includes a subtractor 31A and a multiplier 31B.
Here, the subtractor 31A is an operator that subtracts the low luminance representative value n from the pixel data of each pixel. The subtractor 31A functions as an offset correction circuit. The multiplier 31B is an operator that multiplies pixel data after offset correction by an amplification factor k.
By the multiplication process by the multiplier 31B, the gradation width t of the input image is amplified to the allowable gradation width Tmax. Of course, the white component is not saturated. In addition, since the amplification is linear, the reproducibility of gradation is maintained.

The luminance / density conversion unit 33 is a processing device that converts the luminance value into, for example, 256 levels of density data.
The gamma conversion unit 35 is a processing device that corrects density data according to a preset gamma characteristic. An appropriate gamma characteristic is selectively used according to a printing device or a printing medium.
The halftoning unit 37 is a processing device that performs a quantization process on density data after gamma correction and an error diffusion process that disperses a quantization error that appears at that time to surrounding pixels. In the halftoning unit 37, the density data is converted into quantized data representing the number of ink droplets.

The print head drive unit 39 is a processing device that drives and controls the print head based on the quantized data input from the halftoning unit 37. For the contents of the control, a well-known technique is applied according to the structure of the print head and the driving method.
As described above, by installing the gradation amplification function in the printing device, even when an input image whose gradation width is narrower than the allowable gradation width is input, the input image is automatically displayed without impairing the gradation of the output image. Can be emphasized.

(2) Configuration example 2 of printing apparatus
FIG. 12 shows a configuration example of the printing apparatus. In this embodiment, a method for realizing gradation amplification processing using a lookup table will be described.
The printing apparatus includes an image buffer memory 41, a luminance representative value detection unit 43, a table data generation unit 45, an adjustment coefficient memory 47, an upper limit value memory 49, a gradation amplification unit 51, a luminance / density conversion unit 53, and a gamma conversion unit 55. The halftoning unit 57 and the print head driving unit 59 are included as main components.

Among these, the configuration unique to the present embodiment is a table data generation unit 45 and a gradation amplification unit 51. The other processing devices are the same as the processing devices of the printing apparatus shown in FIG.
The table data generation unit 45 is a processing device that generates a unique lookup table for each input image according to the characteristics of the input image. The table data generation unit 45 corresponds to a function of calculating the gradation width t (= mn) and a function of calculating the amplification factor k by dividing the allowable gradation width Tmax by the calculated gradation width t. That is, it corresponds to functions as a gradation width calculation unit and an amplification factor calculation unit.

Of course, the table data generation unit 45 also compares the calculated amplification factor k with the upper limit value Lgain in order to limit the amplification factor k from becoming larger than necessary. If the upper limit value Lgain is not exceeded, the output value Tout with respect to the input value Tin is sequentially determined using the following equation.
Tout
= A * Tmax * (Tin-n) / t
Here, A is an adjustment coefficient. This calculation process is calculated for all gradation values that can be taken by the input image. That is, each gradation value from the minimum luminance value n to the maximum luminance value m is individually input as the input value Tin, and the corresponding output value Tout is calculated.

Of course, also in this case, the input / output relationship is calculated in consideration of the offset correction. When the calculated amplification factor k exceeds the upper limit value Lgain, the portion of Tmax / t in the previous equation is replaced with Lgain. That is, the following equation is used.
Tout
= A * Lgain * (Tin-n)
The table data generation unit 45 gives the input / output relation data generated using these equations to the gradation amplification unit 51 and stores it in the lookup table 51A.

On the other hand, the gradation amplifying unit 51 gives each pixel data of the input image to the lookup table 51A, and reads out the value after gradation amplification. That is, the gradation amplifying unit 51 does not execute the calculation process, but only executes the read process of the output value Tout using the pixel data of the input image as the read address.
For this reason, the gradation amplification processing in the gradation converting unit 51 is speeded up as compared with the above-described embodiment.
As described above, even when the configuration according to the present embodiment is adopted, the input image is automatically enhanced without degrading the gradation of the output image with respect to the input image whose gradation width is narrower than the allowable gradation width. can do.

(F) Other Embodiments (a) In the above-described embodiments, the case where an inkjet print head is applied as a printing device has been described. it can.
(B) The printing apparatus in the above-described embodiment may be a dedicated printing machine or a multifunction machine having other functions. In addition, the usage of the printing apparatus is not limited to the premise for use in an office or home, but includes medical usage. For example, the present invention can be applied to printing of an appearance image of an affected area, an X-ray image, an echo image, and other medical images.

(C) Although the image output apparatus has been described in the above-described embodiments, the image output apparatus can be realized as an image processing apparatus that provides a gradation amplification function.
Examples of the image processing apparatus include a computer, a video camera, a digital camera, a game machine, a scanner, a portable information terminal (a portable computer, a cellular phone, a portable game machine, an electronic book, etc.), a clock, and an image reproduction apparatus (for example, An optical disk device, a home server), a television receiver, and a processing board and a processing card equipped with the functions according to the invention.

(D) In the above embodiment, the signal processing of the gradation amplification function has been described in hardware. However, when the signal processing of the gradation amplification function is defined by firmware or an execution program, each signal processing is performed by software. May be realized.
The execution program is preferably stored in a semiconductor memory, hard disk, optical storage medium, or other storage medium.

(E) In the above-described embodiment, the low-pass filter is realized by using a coefficient matrix of 3 rows × 3 columns, but may be realized by using a coefficient matrix having a larger size. In general, the higher the size, the higher frequency components can be removed.
(F) Various modifications can be considered in the above-described embodiment within the scope of the gist of the invention. Various modifications and application examples created based on the description of the present specification are also conceivable.

It is a figure which shows the example of an image before gradation amplification. It is a figure which shows the example of an image after gradation amplification. It is a figure which shows the amplification principle of a gradation width | variety. It is a figure which shows the relationship between a pixel matrix and a coefficient matrix. It is a figure which shows the example of distribution of the luminance value measured about the input image. It is a figure which shows the example of an image before gradation amplification, and its luminance distribution. It is a figure which shows the example of an image after gradation amplification, and its luminance distribution. It is a principle explanatory view of offset correction. It is a figure which shows the input / output characteristic of the look-up table applied to the input image which has the margin of a gradation on the high-intensity side. It is a figure which shows the input / output characteristic of the look-up table applied to the input image which has the margin of a gradation on both the low-intensity side and the high-intensity side. It is a figure which shows the Example of the printing apparatus carrying a gradation amplification function. It is a figure which shows the Example of the printing apparatus carrying a gradation amplification function.

Explanation of symbols

21, 41 Image buffer memory 23, 43 Luminance representative value detection unit 25, Amplification factor calculation unit 27, 47 Adjustment coefficient memory 29, 49 Upper limit value memory 31, 51 Gradation amplification unit 33, 53 Luminance / density conversion unit 35, 55 Gamma conversion unit 37, 57 Halftoning unit 39, 59 Print head drive unit 45 Table data generation unit 51A Look-up table

Claims (12)

For an input image, a representative value detection unit for obtaining a high luminance representative value and a low luminance representative value;
A gradation width calculator that subtracts the low brightness representative value from the high brightness representative value to calculate a gradation width of the input image;
An amplification factor calculation unit for dividing an allowable gradation width by the gradation width of the input image and obtaining an amplification factor necessary for amplifying the gradation width of the input image to the allowable gradation width;
An image processing apparatus comprising: a gradation amplification unit that amplifies the luminance value of each pixel constituting the input image based on the amplification factor. The image processing apparatus according to claim 1,
It has a low-pass filter that removes high-frequency components from the input image,
An image processing apparatus, wherein an input image that has passed through the low-pass filter is supplied to the representative value detection unit. The image processing apparatus according to claim 1,
A frequency distribution measurement unit that measures the frequency distribution of each luminance value for the entire input image screen,
The representative value detection unit sets the largest luminance value among the luminance values exceeding the representative value detection threshold in the frequency distribution measured for each luminance value as a high luminance representative value, and sets the smallest luminance value as a low luminance. An image processing apparatus characterized by having a representative value. The image processing apparatus according to claim 3.
The threshold value for representative value detection is set based on the number of samples used when measuring the frequency distribution. The image processing apparatus according to claim 1,
The amplification factor calculation unit compares the calculated amplification factor with the maximum value of the allowable amplification factor,
If the calculated amplification factor does not exceed the maximum value, the calculated amplification factor is provided to the gradation amplification unit,
When the calculated amplification factor exceeds the maximum value, the maximum value is given as an amplification factor to the gradation amplification unit. The image processing apparatus according to claim 1.
The gradation processing unit amplifies a gradation width of the input image by using a gradation value conversion table created according to the amplification factor. A frame memory for holding an input image for at least one frame;
For an input image, a representative value detection unit for obtaining a high luminance representative value and a low luminance representative value;
A gradation width calculator that subtracts the low brightness representative value from the high brightness representative value to calculate a gradation width of the input image;
An amplification factor calculation unit for dividing an allowable gradation width by the gradation width of the input image and obtaining an amplification factor necessary for amplifying the gradation width of the input image to the allowable gradation width;
A gradation amplifying unit that amplifies the luminance value of each pixel constituting the input image based on the amplification factor;
An image output apparatus comprising: a gamma conversion unit that gives a gamma characteristic corresponding to an output device to an input image after gradation amplification. The image output device according to claim 7.
The image output apparatus, wherein the output device is a printing device. The image output device according to claim 7.
The image output apparatus, wherein the output device is a display device. Representative value detection processing for obtaining a high luminance representative value and a low luminance representative value for an input image;
A gradation width calculation process for subtracting the low brightness representative value from the high brightness representative value to calculate a gradation width of the input image;
An amplification factor calculation process for dividing an allowable gradation width by the gradation width of the input image and obtaining an amplification factor necessary for amplifying the gradation width of the input image to the allowable gradation width;
And a gradation amplification process for amplifying the luminance value of each pixel constituting the input image based on the amplification factor. In the computer that controls the image processing apparatus,
Representative value detection processing for obtaining a high luminance representative value and a low luminance representative value for an input image;
A gradation width calculation process for subtracting the low brightness representative value from the high brightness representative value to calculate a gradation width of the input image;
An amplification factor calculation process for dividing an allowable gradation width by the gradation width of the input image and obtaining an amplification factor necessary for amplifying the gradation width of the input image to the allowable gradation width;
And a gradation amplification process for amplifying the luminance value of each pixel constituting the input image based on the amplification factor. A recording medium storing the program according to claim 11.