JP2005257761A - Image display device and image display method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image display device and image display method which prevent lost highlight details without changing the impression of a display image as a whole. <P>SOLUTION: In the image display device, the display image is regulated by a dimmer function processing of illumination light and an expansion processing of the number of gradations of an image signal. The image display device is equipped with an image parameter extracting means 82 for extracting the image parameter characterizing the brightness of the display image from the image signal by each of unit time, a dimmer function control means 83 for having the dimmer function processing executed based on the image parameter extracted by the image parameter extracting means 82, and an expansion processing means 86 for performing the expansion processing based on the image parameter extracted by the image parameter extracting means 82. The expansion processing means 86 performs the expansion processing by determining an expansion control parameter in such a manner that the maximum gradation of the inputted image signal attains the gradation lower than the maximum gradation that can be outputted. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an image display device and an image display method.

In recent years, the development of information devices has been remarkable. For example, image display devices have been researched and developed in accordance with demands for high resolution, low power consumption, and thinness. Among them, the liquid crystal display device is expected as a display device that can electrically control the arrangement of liquid crystal molecules to change the optical characteristics and can meet the above-mentioned needs. As one form of such a liquid crystal display device, a liquid crystal projector that enlarges and projects a display image emitted from an optical system using a liquid crystal light valve onto a screen through a projection lens is known.
A liquid crystal projector uses a liquid crystal light valve as an optical modulator, but a projector using a digital mirror device as an optical modulator in addition to the liquid crystal light valve has been put into practical use. However, this type of conventional projector has the following problems.

(1) A sufficient contrast cannot be obtained due to light leakage and stray light generated by various optical elements constituting the optical system. For this reason, the displayable gradation range (dynamic range) is narrow, and the quality and power of the displayed image are inferior to those of an existing television receiver using a cathode ray tube (CRT).
(2) Even if an attempt is made to improve the quality of a display image by various video signal (image signal) processes, the dynamic range is fixed, so that a sufficient effect cannot be exhibited.
As a solution to such a projector problem, that is, a method of extending the dynamic range, it is conceivable to change the amount of light incident on the light modulator (light valve) in accordance with the image signal. The simplest way to achieve this is to change the light output intensity of the lamp. A method for controlling output light of a metal halide lamp in a projector has been proposed (see, for example, Patent Document 1).
JP-A-5-66501

As a lamp used in a projector, a high-pressure mercury lamp is currently mainstream, but it is extremely difficult to control the light output intensity with the high-pressure mercury lamp. For this reason, there is a need for a method that can change the amount of light incident on the light modulator according to the image signal without changing the light output intensity itself of the lamp.
Furthermore, in addition to the above-mentioned problems, the brightness of the light source is fixed in the current projector, so that the display image becomes too bright in a dark viewing environment, for example, or the projection screen or zooming of the projection lens causes a projection screen. When the size is changed, there is a problem that the brightness of the display image changes accordingly.

  In order to solve such problems, a projector having a structure in which a dimming louver (light-shielding plate) is combined with a light source such as the above-described metal halide lamp or high-pressure mercury lamp has been proposed in recent years. Specifically, a light shielding plate is arranged on the optical axis of the light emitted from the light source, and the light is partially shielded by rotating it around a rotation axis parallel to the main surface. Dimming is done.

  According to such a configuration, the light amount of the light emitted from the light source can be arbitrarily adjusted by the light control means provided separately from the light source. For this reason, by adjusting the illumination light based on, for example, an image signal, it is possible to obtain light with brightness according to the display image in the illuminated area (light modulator) even when the light output intensity of the light source remains constant. This can contribute to the expansion of the dynamic range of the projector. Similarly, it is possible to obtain light having a brightness according to the projection magnification ratio, the brightness condition in the use environment, or the user's preference.

However, on the principle that the black level is changed by such dimming processing and expansion processing performed separately from this, it is inadvertent to perform dimming processing and expansion processing in synchronization with the image signal. Will cause a change in brightness. Therefore, such a brightness change is recognized by the viewer as flickering on the display image. For this reason, in the conventional projector, the flickering of the display image is prevented by performing the dimming process and the expansion process gradually, that is, at a relatively low speed without matching the switching of the image signal. .
However, the delay in the dimming process and the expansion process for the image signal causes a new problem such as overexposure in the display image when the display image is switched from a dark scene to a bright scene.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide an image display device and an image display method that prevent overexposure without changing the impression of the display image as a whole.

In order to achieve the above object, an image display device of the present invention is an image display device in which a display image is adjusted by dimming processing of illumination light and expansion processing of the number of gradations of an image signal,
Image parameter extracting means for extracting an image parameter characterizing the brightness of the display image from the image signal every unit time;
A dimming control unit for determining a dimming control parameter related to the dimming process based on the image parameter extracted by the image parameter extracting unit, and for performing the dimming process based on the dimming control parameter;
A decompression processing unit that determines a decompression control parameter related to the decompression process based on the image parameter extracted by the image parameter extraction unit, and performs the decompression process based on the decompression control parameter;
The expansion processing means determines the expansion control parameter so that the maximum gradation of the input image signal is lower than the maximum output gradation after the expansion processing, and performs the expansion processing. It is a feature.

The image display method of the present invention is an image display method in which a display image is adjusted by dimming processing of illumination light and expansion processing of the number of gradations of an image signal,
A first step of extracting an image parameter characterizing the brightness of the display image from an image signal every unit time;
A second step of determining a dimming control parameter related to the dimming process based on the image parameter extracted in the first step, and performing the dimming process based on the dimming control parameter;
A third step of determining a decompression control parameter related to the decompression process based on the image parameter extracted in the first step, and performing the decompression process based on the decompression control parameter,
In the third step, the expansion control parameter is determined so that the maximum gradation of the input image signal is lower than the maximum gradation that can be output after the expansion process, and the expansion process is performed. .

For example, when the pixel parameter is not the maximum value of the input gradation but the number of gradations lower than this, the expansion control parameter is determined based on the number of gradations (pixel parameter) and the expansion process is performed as in the past. Then, there is a possibility that overexposure occurs due to gradation collapse on the high gradation side.
However, according to the image display device and the image display method, the expansion processing means (third step) causes the maximum gradation of the input image signal to be lower than the maximum output gradation after the expansion processing. As described above, since the expansion control parameter is determined and the expansion process is performed, the occurrence of overexposure due to gradation collapse on the high gradation side as described above is prevented.

In the image display device, it is preferable that the expansion control parameter is determined so as to be smaller than a value obtained by dividing the maximum gradation that can be output after the expansion processing by the image parameter at least on the high gradation side.
For example, when the image parameter is the maximum value (lum) of the input gradation and the maximum gradation that can be output after the expansion process is 255, the expansion control parameter (C2) is
It is assumed that C2 <(255 / lum).
By determining the expansion control parameter (C2) in this way, when the newly input image signal is expanded, if the newly input image signal has a number of gradations equal to or less than (lum), it is expanded. The processed output gradation is always lower than the maximum gradation that can be output, and gradation collapse does not occur.

In the image display device, it is preferable that the extension control parameter is determined to be smaller than the reciprocal of the dimming rate by the dimming process at least on the high gradation side.
For example, when the dimming rate (1 / m) by dimming processing is (1 / m) = (lum / 255), the reciprocal m is m = (255 / lum) (however, , (Lum) is the number of gradations when the image parameter is the maximum value of the input gradation, and 255 is the maximum gradation that can be output after the decompression process).
Accordingly, by setting the expansion control parameter (C2) to C2 <m = (255 / lum), when the newly input image signal is expanded as described above, the newly input image signal is (lum When the number of gradations is as follows, the output gradation after the expansion process is always lower than the maximum outputable gradation, and gradation collapse does not occur.

In the image display device, the expansion processing may be performed by multiplication processing or a combination of multiplication processing and addition processing.
In this way, for example, one expansion process is performed by a conventional simple multiplication process, and the other expansion process is performed by a combination of a multiplication process and an addition process, for example, low gradation on the input gradation side. The low gradation side and the high gradation side can be made continuous while different expansion coefficients are used on the side and the high gradation side.

Further, in the image display device, the expansion processing may be performed based on a look-up table.
This makes it possible to perform more complicated decompression processing.

In the image display device, the expansion processing unit may perform different expansion processing in accordance with the input gradation within the same screen.
In this way, for example, when the display image changes from a dark state to a bright state, a weak expansion process (an expansion process with a small expansion coefficient) is performed particularly on the high input gradation side, and the obtained output gradation is converted. By adopting and adjusting the display image, it is possible to prevent overexposure due to gradation collapse.

The expansion processing means preferably has a large expansion coefficient on the low gradation side and a small expansion coefficient on the high gradation side.
In this way, as described above, for example, when the display image is changed from a dark state to a bright state, the expansion coefficient is reduced particularly on the higher input gradation side (high gradation side), and thus obtained. By adjusting the display image using the output gradation, it is possible to prevent overexposure due to gradation collapse.

In the image display device, different expansion processes according to the input gradation may be controlled with the same time constant.
By performing the decompression process at a relatively low speed without matching the switching of the image signal according to the time constant, it is possible to prevent the flickering of the display image in all display gradations.

Hereinafter, an image display device and an image display method of the present invention will be described in detail with reference to the drawings.
First, as an image display apparatus using the image display method of the present invention, a three-plate projector having liquid crystal light valves for different colors of R (red), G (green), and B (blue) will be described as an example. To do.

  FIG. 1 is a diagram showing a schematic configuration of an example of such a projector. The projector shown in FIG. 1 includes a light source 510, a light control element 26, dichroic mirrors 513, 514, reflection mirrors 515, 516, 517, relay lenses 518, 519, 520, a liquid crystal light valve 522 for red light, and a liquid crystal for green light. A light valve 523, a blue light liquid crystal light valve 524, a cross dichroic prism 525, and a projection lens system 526 are provided.

  As the light control element 26, for example, a liquid crystal panel whose transmittance is variable may be used, or a movable light shielding plate or the like may be used. A light control element using a liquid crystal panel has a relatively fast response speed, while a mechanical light control element using a movable light shielding plate has a relatively low response speed. In any case, the response speeds of the light control device and the liquid crystal light valve are different.

The light source 510 includes a lamp 511 such as an ultra-high pressure mercury lamp and a reflector 512 that reflects light from the lamp 511. Between the light source 510 and the dichroic mirror 513, a light control element 26 that adjusts the amount of light from the light source 510 is disposed.
The blue / green light reflecting dichroic mirror 513 transmits the red light LR of the white light from the light source 510 and reflects the blue light LB and the green light LG. The transmitted red light LR is reflected by the reflection mirror 517 and is incident on the red light liquid crystal light valve 522.

  On the other hand, the green light LG reflected by the dichroic mirror 513 is reflected by the dichroic mirror 514 for reflecting green light and enters the liquid crystal light valve 523 for green light. The blue light LB reflected by the dichroic mirror 513 also passes through the dichroic mirror 514, and passes through a relay system 521 including a relay lens 518, a reflection mirror 515, a relay lens 519, a reflection mirror 516, and a relay lens 520, and then blue light. The liquid crystal light valve 524 is incident.

  The three color lights modulated by the liquid crystal light valves 522, 523, and 524 are incident on the cross dichroic prism 525. In this prism, four right-angle prisms are bonded together, and a dielectric multilayer film that reflects red light and a dielectric multilayer film that reflects blue light are formed in a cross shape on the inner surface thereof. These dielectric multilayer films combine the three color lights to form light representing a color image. The synthesized light is projected on the screen 527 by the projection lens system 526 which is a projection optical system, and the image is enlarged and displayed. Each of the liquid crystal light valves 522, 523, and 524 is connected to an image processing unit (not shown in FIG. 1) that performs predetermined image processing on each color light based on an image signal.

Next, an image display apparatus and an image display method of the present invention will be described based on such a projector driving method.
FIG. 2 is a block diagram showing a configuration of a driving circuit of the projector using the image display device of the present invention. In the case of a projector that does not have a dimming function, an input image signal (video signal) undergoes appropriate correction processing and is supplied to the liquid crystal panel driver as it is. However, as in the image display device of the present invention, the dimming function In the case of a method of controlling the image signal based on the image signal, the basic configuration includes circuits such as DSP (1) to DSP (3) which are digital signal processing blocks as described below. ing.

  That is, in this embodiment, the image signal input as an analog signal is input to the DSP (1) 82 (image parameter extraction means) which is the first digital signal processing circuit via the AD converter 81. The DSP (1) 82 extracts a control signal (image parameter) characterizing the brightness of the display image from the input image signal for each unit time (first step), and uses the extracted control signal as a second digital signal. The data is sent (output) to both the DSP (2) 83 which is a processing circuit and the DSP (3) 86 which is a third digital signal processing circuit.

  The DSP (2) 83 (dimming control means) determines a dimming control parameter based on the control signal (image parameter), and controls the dimming element driver 84 based on the dimming control parameter (second). Step). Finally, the dimming element driver 84 actually drives the dimming element 26, so that the DSP (2) 83 (dimming control means) drives the dimming element 26 to perform dimming processing. It will be a thing.

In addition, the control signal (image parameter) extracted by the DSP (1) 82 is input to the DSP (3) 86 (expansion processing means) together with the image signal sent through the AD converter 81. The DSP (3) 86 determines an expansion control parameter based on the control signal, and expands the image signal to an appropriate gradation range based on the expansion control parameter as will be described later (third step). .
The decompressed image signal is converted again to an analog signal by the DA converter 87 and then input to the panel driver 88. The panel driver 88 further outputs a red light liquid crystal light valve 522 and a green light liquid crystal light valve. 523 and the liquid crystal light valve 524 for blue light are respectively supplied.

Here, control and driving of the light control element 26 by the DSP (2) 83 (light control device) will be described. In the present embodiment, a display video adaptive control, that is, brightness control adapted to a display video in which the light amount increases in a bright video scene and the light amount decreases in a dark scene will be described.
As described above, the control signal (image parameter) is extracted and determined by the DSP (1) 82 every unit time based on the image signal (step 1). Specifically, as the method, the following (a) ) To (c).

(A) A method in which the number of gradations having the maximum brightness among the pixel data included in the frame of interest is used as a control signal (image parameter).
For example, an image signal composed of 256 steps of gradations from 0 to 255 is assumed. When attention is paid to an arbitrary frame constituting a continuous video, it is assumed that the appearance number distribution (histogram) for each number of gradations of pixel data included in the frame is as shown in FIG. In the case of FIG. 3, since the brightest number of gradations included in the histogram is 150, this number of gradations 150 is used as a control signal (image parameter). This method is a method that can express the brightness most faithfully to the input image signal.

(B) From the appearance number distribution (histogram) for each number of gradations included in the frame of interest, the control signal is used to control the number of gradations that is a certain ratio (for example, 10%) from the maximum brightness to the number of appearances. (Image parameter).
For example, when the appearance number distribution of the image signal is as shown in FIG. 4, a region of 10% is taken from the brighter side than the histogram. If the number of gradations corresponding to 10% is 230, this number of gradations 230 is used as a control signal (image parameter). If there is a sudden peak in the vicinity of the number of gradations 255 as in the histogram shown in FIG. 4, the number of gradations 255 becomes the brightness control signal if the method (a) is adopted. However, this sudden peak portion does not make much sense as information in the entire display image. On the other hand, the present method using the number of gradations 230 as the brightness control signal can be said to be a method of determining by a region having meaning as information in the entire display image. In addition, you may change the said ratio in the range of about 2 to 50%.

  However, when the value obtained in this way is used as a control signal (image parameter), if the expansion control parameter is determined based on the control signal and the expansion process is performed as in the prior art, the gradation is increased on the high gradation side. Overexposure due to crushing may occur. That is, the expansion coefficient (expansion control parameter) is determined so that the control signal has the maximum gradation that can be output after the expansion process, and the input gradation of the newly input image signal is multiplied by this expansion coefficient. For example, when a pixel signal having a number of gradations in a range corresponding to 10% of the entire range indicated by hatching in FIG. 4 is newly input, the pixel signal is expanded. It becomes higher than the maximum gradation that can be output later, resulting in overexposure due to gradation collapse.

(C) A method in which a display image is divided into a plurality of blocks, an average value of the number of gradations of pixels included in each block is obtained, and the maximum one is used as a control signal (image parameter).
For example, as shown in FIG. 5, the display image is divided into m × n blocks, and the average value of the brightness (the number of gradations) for each block A11,..., Amn is calculated. The control signal (image parameter) is used. Note that the number of divisions of the display image is desirably about 6 to 200. In this method, the brightness can be controlled without impairing the atmosphere of the entire display image.

  In the methods (a) to (c), the control signal (image parameter) is usually extracted and determined for the entire display area. It is also possible to apply the method only to the part. In that case, it is possible to perform control such that the brightness is determined from the portion that the viewer is paying attention to.

  Next, the DSP (2) 83 determines a dimming control parameter for controlling the dimming element driver 84 based on the control signal extracted and determined by the above method, and based on this dimming control parameter. The dimming element driver 84 is controlled. Even in this method, for example, the following two methods can be considered.

(A) A dimming control parameter is determined so that a change in brightness per unit time (for example, 1/30 second) by the dimming element 26 does not exceed a predetermined value with respect to the output control signal (image parameter). How to control.
In this case, a dimming control parameter is determined by comparing a control signal (image parameter) with a LUT (Look Up Table) prepared in advance, and element control according to brightness is performed. If the dimming element 26 is operated at a relatively low speed, the element does not follow a minute change in time, so that a change in light and darkness in a short cycle can be avoided.

(B) A method of performing control by applying an LPF (low-pass filter) to the output control signal (image parameter) and using the obtained output as a dimming control parameter.
For example, a change in control signal (image parameter) of 1 to 30 seconds or less is cut by the LPF, and the output is controlled as a dimming control parameter. According to this method, since the minute change in time is cut, it is possible to avoid the change in brightness in the short cycle as described above.

  In this way, for example, when the number of gradations 190 is determined as the control signal (image parameter), assuming that the light amount of the maximum brightness (the number of gradations 255) is 100%, a light amount of 190/255 = 75% is obtained. The dimming control parameter is determined so that the dimming element 26 is driven. When the light control element 26 is formed of, for example, a light shielding plate, the light shielding plate is rotated so that the transmittance (m) is 75% (the light shielding rate [1 / m] is 25%). Similarly, when the number of gradations 230 is a control signal (image parameter), the light control element 26 is driven (turned) so that a light quantity of 230/255 = 90% is obtained.

On the other hand, the DSP (3) 86 determines an expansion control parameter based on the control signal (image parameter) extracted and determined by the DSP (1) 82 as described above, and the image signal based on the expansion control parameter. Is expanded to an appropriate gradation range, for example, a gradation range of 0 to 255 (third step).
Here, in the present invention, the DSP (3) 86 (expansion processing means) is configured to output the maximum gradation in which the maximum gradation of the input image signal, that is, the image signal sent through the AD converter 81 can be output after the expansion processing. The expansion control parameter is determined so that the gradation becomes lower than the gradation, and the expansion process is performed.

[First example of decompression processing]
FIG. 6 is a block diagram for explaining a first example of decompression processing by the DSP (3) 86 (decompression processing means), and FIG. 7 is a graph for explaining the decompression processing of this first example. is there.
In this first example, as a method for determining the expansion control parameter, in particular, a plurality of different expansion processes are performed in the same screen, and the lower one of the output gradations obtained after the expansion process is selected. I am trying to select it. That is, the DSP (3) 86 (expansion processing means) of this example performs two different expansion processes in the same screen, for example. Therefore, as shown in FIG. And an arithmetic unit that compares the levels (large and small) of the output gradations after being expanded by these processing units 31 and 32, and selects the lower (smaller) number of gradations. 33 is provided.

  The first processing unit 31 calculates an output gradation with a maximum gradation of 255 by simply multiplying the input gradation in the input image signal by the expansion coefficient, as in the conventional case. . That is, the first processing unit 31, as indicated by the solid line A in FIG. 7, has an expansion coefficient (expressed as the slope of the solid line A in FIG. 7) C1 with respect to the input gradation (x) indicated on the horizontal axis. For example, the maximum gradation (max1) at the input gradation is set to be the maximum gradation (255) at the output gradation (y) shown on the vertical axis.

  Here, the first processing unit 31 determines an expansion coefficient as an expansion control parameter based on the control signal (image parameter), and multiplies the obtained expansion coefficient by the input gradation as described above. As a result, the output gradation is calculated. That is, the expansion coefficient C1 expressed as the slope of the solid line A in FIG. 7 is determined based on the control signal (image parameter). However, in this first example, the expansion processing by the first processing unit 31 is performed based on a control signal having a preset time constant. In other words, by using a control signal of a predetermined time before set by a time constant, this expansion processing is performed at a relatively low speed without matching with the switching of the image signal, thereby displaying in all display gradations. The image flicker is prevented.

As a method of determining the expansion coefficient C1 based on the control signal (image parameter), for example, it can be obtained by the following equation.
C1 = 255 / max1
However, in the above expression, max1 is the maximum value in the control signal of a predetermined time before set by the time constant (the maximum gradation in the input gradation, that is, the control signal based on the image signal in the DSP (1) 82) Image parameters) are extracted every unit time (step 1), and are determined as image parameters determined by the method shown in (a) above.

In addition to the calculation based on such an expression, for example, an LUT (Look Up Table) as shown in FIG. 8 is prepared in advance, and the expansion coefficient C1 is obtained from the control signal based on the time constant. It may be.
Note that, as will be described later, the first processing unit 31 actually performs an expansion process on the low gradation side of the newly input pixel signal.

  Further, the second processing unit 32 multiplies a newly input image signal (input gradation) by an expansion coefficient and combines an addition process to calculate an output gradation with a maximum gradation of 255. In particular, the expansion processing is actually performed on the high gradation side of the input gradation. That is, the second processing unit 32 multiplies the input gradation (x) by an expansion coefficient C2 (expressed as the slope of the broken line B in FIG. 7) as indicated by the broken line B in FIG. Then, a preset constant a (expressed as y-intercept in FIG. 7) is added to the obtained product.

  Here, in the second processing unit 32 as well, as in the first processing unit 31, an expansion coefficient and a constant (y intercept) serving as an expansion control parameter are determined based on the control signal (image parameter), as described above. The output gradation is calculated by multiplying the obtained expansion coefficient by the input gradation and adding a constant. That is, the expansion coefficient C2 expressed as the slope of the broken line B in FIG. 7 and the y-intercept a of the broken line B are based on the control signal (the image parameter obtained by the method shown in (a)). It is to be decided. In the first example, the expansion processing by the second processing unit 32 is also performed based on a control signal having the same time constant as the time constant in the first processing unit 31. In other words, by using the control signal of a predetermined time before set by the time constant, this expansion processing is performed at a relatively low speed without matching with the switching of the image signal, and thereby display on the high gradation side in particular. The image flicker is prevented.

As a method for determining the expansion coefficient C2 and the y-intercept a based on the control signal (image parameter), an LUT (look-up table) as shown in FIG. 8 is prepared in advance and is based on the time constant. The expansion coefficient C2 and the y-intercept a can be obtained from the control signal. In addition to the LUT method, the calculation may be performed using a preset equation.
However, with respect to the expansion control parameter composed of the expansion coefficient C2 and the y-intercept a, the maximum gradation maxh of the newly input image signal is the maximum level that can be output after the expansion processing, as indicated by the broken line B in FIG. The gradation is determined to be lower than the tone (255). That is, as indicated by a broken line in FIG. 7, the output gradation corresponding to the maximum gradation (max1) in the input gradation is determined to be lower than the maximum gradation (255).

Specifically, a value smaller than the expansion coefficient C1 in the first processing unit 31 is selected as the expansion coefficient C2. That is, the expansion coefficient C1 in the first processing unit 31 is obtained by dividing, for example, the maximum gradation (255) that can be output after the expansion processing by the image parameter (maximum value in the control signal before a predetermined time; max1) as described above. That is, the values shown below.
C1 = 255 / max1
On the other hand, C2 is preferably determined to be smaller than the value obtained by dividing the maximum gradation that can be output after the decompression process by the image parameter. For example, when the image parameter is the maximum value (lum) of the input gradation and the maximum gradation that can be output after the decompression process is 255,
C2 <(255 / lum).

Here, the maximum value (lum) of the input gradation as an image parameter is extracted per unit time based on the image signal by the DSP (1) 82 in this example (step 1). Since it is equal to the image parameter (max1) obtained by the method shown in (a) above when determining,
C2 <(255 / lum) = 255 / max1 = C1,
That is, C2 <C1.
Then, by determining the expansion control parameter (C2) as described above, when the newly input image signal is expanded, the newly input image signal is input to the broken line B corresponding to the second processing unit 32. If the number of gradations (for example, maxh) is smaller than the maximum gradation (max2), the output gradation after decompression processing will always be a gradation (for example, 230) that is lower than the maximum gradation that can be output, and the gradation collapses. Will not occur.

Similarly, the expansion coefficient C2 is determined so as to be smaller than the reciprocal of the dimming rate by the dimming process.
For example, when the dimming rate (1 / m) by dimming processing is (1 / m) = (lum / 255), the reciprocal m is m = (255 / lum) (however, , (Lum) is the maximum value of the input gradation, and 255 is the maximum gradation that can be output after the decompression process).
Therefore, by setting C2 <m = (255 / lum), when the newly input image signal is expanded as described above, the newly input image signal corresponds to the broken line B corresponding to the second processing unit 32. If the number of gradations (for example, maxh) is smaller than the input-side maximum gradation (max2), the output gradation after the expansion process is always lower than the maximum gradation that can be output (for example, 230), The gradation collapse does not occur.

In this way, when the first processing unit 31 and the second processing unit 32 respectively perform the expansion processing, that is, for the input gradation of the image signal, the expansion processing indicated by the solid line A in FIG. When the expansion processing is performed, the output gradation obtained after the expansion processing is input to the calculation unit 33 as shown in FIG. Then, the computing unit 33 selects the lower one of the input output gradations and sets it as the image signal after the decompression process. That is, in the example illustrated in FIG. 7, the solid line A (A ₀ ) is located on the lower gradation side than the intersection (x ₀ ) between the solid line A corresponding to the first processing unit 31 and the broken line B corresponding to the second processing unit 32. ) Based on the broken line B (B ₀ ) on the high gradation side.

Thereafter, the image signal after the expansion processing is converted again to an analog signal by the DA converter 87 as described above, and then input to the panel driver 88. Further, the panel driver 88 further outputs the R panel 51 (red light liquid crystal light). Valve 522), G panel 52 (green light liquid crystal light valve 523), and B panel 53 (blue light liquid crystal light valve 524).

Next, the operation of the DSP (3) 86 when the display image scene is switched from the dark scene to the bright scene, that is, the operation of the decompression process of the first example will be described. In the following description, it is assumed that the DSP (3) 86 extends the number of gradations of the input image signal to the maximum gradation range (0 to 255).
The first processing unit 31 in the DSP (3) 86 basically performs the same expansion process as in the conventional case, and gradually expands the image signal in synchronization with the operation of the light control element 26. Therefore, for example, when the image signal shown in FIG. 3 is changed to the image signal shown in FIG. 9, if the DSP (3) 86 is configured only from the first processing unit 31 as in the prior art, the high gradation side Overexposure (gradation loss) occurs.

  That is, since the DSP (3) 86 (the first processing unit 31) extends the gradation number of the image signal having the brightness as shown in the histogram of FIG. 3 from 150 to 255, in this state. When an image signal having the brightness as shown in the histogram of FIG. 9 is input, the number of gradations exceeding 150 is expanded so that it exceeds 255. Therefore, as shown in FIG. Overexposure occurs due to crushing. The DSP (3) 86 (first processing unit 31) gradually changes the number of gradations 0 to 200 of the image signal shown in FIG. 9 based on the set time constant as shown in FIG. Extends to 255 (target gradation). Therefore, overexposure occurs in the display image while the image signal input to the DSP (3) 86 (first processing unit 31) extends to the target gradation.

  However, in the decompression process of the first example according to the present embodiment, the second processing unit 32 in the DSP (3) 86 performs the decompression process in parallel with the first processing unit 31, and the output gradations obtained respectively. Of these, the one with the lower number of gradations is determined as a normal output gradation by the calculation unit 33 and is adopted as the image signal after the decompression process. At this time, the second processing unit 32 determines the expansion control parameter so that the maximum gradation (maxh) of the input image signal is lower than the maximum gradation (255) that can be output after the expansion processing. Has been.

  Therefore, even if the maximum gradation (maxh) of the number of gradations of the newly input image signal is larger than the input side maximum gradation (max1) of the solid line A corresponding to the first processing unit 31 as shown in FIG. Since the input-side maximum gradation (max2) of the broken line B corresponding to the second processing unit 32 is smaller than the maximum gradation (255) as described above by the extension processing by the broken line B in the second processing unit 32. It is output at a low gradation (for example, 230), and therefore, no overexposure occurs due to the loss of gradation.

That is, in the first example, the extension processing based on the solid line A (A ₀ ) is adopted on the lower gradation side than the intersection (x ₀ ) of the solid line A and the broken line B as described above, and the broken line on the higher gradation side. Since the decompression process based on B (B ₀ ) is adopted, the decompression process can be performed in a continuous state without causing gradation collapse over all gradations of the input gradation. That is, since the solid line A ₀ and the broken line B ₀ that are employed as regular expansion processing are continuous from the low gradation side to the high gradation side through the intersection (x ₀ ), the obtained display image has a gradation level. It does not fly unnaturally and is visually recognized as a continuous natural gradation.

  Therefore, in the image display apparatus and the image display method of the present example, different expansion processing is performed in accordance with the input gradation within the same screen. For example, when the display image changes from a dark state to a bright state, By performing weak extension processing (extension processing with a low extension coefficient) on the higher input gradation side and adjusting the display image using the obtained output gradation, it is possible to prevent overexposure due to gradation loss it can. Therefore, in the image display apparatus and the image display method of this example, overexposure can be prevented without changing the impression of the display image as a whole with respect to a projector to which the image display apparatus and the image display method are applied, for example.

Further, as the image parameter, for example, the DSP (1) 82 extracts a control signal (image parameter) based on the image signal for each unit time (step 1) and determines the image parameter by the method shown in (b) above. When the obtained image parameter is adopted, as described above, there is a case where the conventional whiteout occurs due to the loss of gradation.
However, in the image display apparatus and the image display method of this example, the DSP (3) 86 includes the second processing unit 32, and the second processing unit 32 uses the maximum gradation of the input image signal. After the decompression process, the decompression control parameters are determined so that the gradation is lower than the maximum output gradation, and the decompression process is performed. Therefore, the occurrence of overexposure due to the gradation collapse on the high gradation side as described above Is reliably prevented.

[Second example of decompression processing]
Next, a second example of expansion processing by the DSP (3) 86 (expansion processing means) will be described.
FIG. 12 is a graph for explaining the decompression process of the second example.
In this second example, the DSP (3) 86 (decompression processing means) is different from the first example in that it has only one processing unit for performing the decompression processing. Similar to the first processing unit 31 in the first example, this processing unit simply multiplies the input gray level in the input image signal by the expansion coefficient, so that the maximum gray level is 255. Is to be calculated. That is, as indicated by a solid line B in FIG. 12, the input gradation (x) indicated on the horizontal axis is multiplied by an expansion coefficient C2 (expressed as the slope of the solid line B in FIG. The maximum gradation (max1) at the gradation is set to be smaller than the maximum gradation (255) at the output gradation (y) shown on the vertical axis. That is, C2 is determined so that the output gradation corresponding to the maximum gradation (max1) in the input gradation is lower than the maximum gradation (255).
Therefore, the processing unit in the second example is smaller than the expansion coefficient C1 of the first processing unit 31 in the first example. On the graph of FIG. 12, the straight line A indicated by the two-dot chain line in FIG. Compared to (the same straight line as the solid line A in FIG. 7), the inclination is small.
Therefore, for example, even if the maximum gradation (maxh) of the number of gradations of the newly input image signal is larger than the input-side maximum gradation (max1) of the two-dot chain line A as shown in FIG. Since it is smaller than the side maximum gradation (max2), the expansion processing by the solid line B in this processing unit outputs to a gradation (for example, 230) lower than the maximum gradation (255) as described above. Overexposure does not occur.
The decompression process by this processing unit is also performed based on a control signal having a preset time constant.

As a method for determining the expansion control parameter (expansion coefficient C2) in this processing unit, as in the first example, an LUT (Look Up Table) as shown in FIG. For example, the DSP (1) 82 extracts a control signal (image parameter) every unit time based on the image signal (step 1), and the image parameter obtained by the method shown in the above (a) at the time of determination. ) To obtain the expansion coefficient C2.
Note that the expansion coefficient C2 may be calculated by a preset equation, separately from the LUT method.

By performing the expansion process indicated by the solid line B in FIG. 12 on the input gradation of the image signal in such a processing unit, the output gradation, that is, the image signal after the expansion process is obtained.
Thereafter, the image signal after the expansion processing is converted again into an analog signal by the DA converter 87 as described above, and then input to the panel driver 88. Further, the panel driver 88 further outputs the liquid crystal light valve 522 for red light, green The light is supplied to the light liquid crystal light valve 523 and the blue liquid crystal light valve 524, respectively.

Next, the operation of the DSP (3) 86 when the display image scene is switched from the dark scene to the bright scene, that is, the operation of the decompression process of the second example will be described. In the following description, it is assumed that the DSP (3) 86 extends the number of gradations of the input image signal to the maximum gradation range (0 to 255).
The processing unit in the DSP (3) 86 basically performs expansion processing by multiplication as in the conventional case, and gradually expands the image signal in synchronization with the operation of the dimmer 26. However, since the expansion coefficient C2 is smaller than in the conventional case indicated by the two-dot chain line in FIG. 12, the output gradation corresponding to the input maximum gradation maxh of the newly input image signal is the maximum scale. The gradation (for example, 230) is lower than the gradation (255).

Therefore, even in the image display device and the image display method of this example, it is possible to prevent overexposure due to gradation collapse. Therefore, in the image display apparatus and the image display method of this example, overexposure can be prevented without changing the impression of the display image as a whole with respect to a projector to which the image display apparatus and the image display method are applied, for example.
Further, as compared with the first example, the processing in the DSP (3) 86 is not a plurality of processing but a single processing only by the processing unit, so the load on the DSP (3) 86 can be reduced. .

Further, as the image parameter, for example, the DSP (1) 82 extracts a control signal (image parameter) based on the image signal for each unit time (step 1) and determines the image parameter by the method shown in (b) above. When the obtained image parameter is adopted, as described above, there is a case where the conventional whiteout occurs due to the loss of gradation.
However, in the image display apparatus and the image display method of this example, in particular, the processing unit of the DSP (3) 86 has a step in which the maximum gradation of the input image signal is lower than the maximum gradation that can be output after the expansion process. Since the expansion control parameter is determined so as to be a key and the expansion process is performed, the occurrence of overexposure due to gradation collapse on the high gradation side as described above is reliably prevented.

  The preferred embodiments of the image display apparatus and the image display method of the present invention have been described above, but it goes without saying that the present invention is not limited to the above-described embodiments. Various shapes, combinations, and the like of the constituent members described in the above-described embodiments are examples, and various modifications can be made based on design requirements and the like without departing from the gist of the present invention.

  For example, in the first example of the decompression process, two decompression processes are performed as different decompression processes. However, the present invention is not limited to this, and three or more decompression processes are performed. It may be. In this case, for example, as a stretch process line combined on the graph by each decompression process as shown in FIG. 7, the slope is small on the low gradation side, the slope is large in the middle gradation part, and on the high gradation side. It is preferable to determine the expansion coefficient so as to have an S-curve shape so that the inclination becomes smaller again.

Moreover, in the said embodiment, what used the light control element 26 as a light control means was illustrated. As the light control element 26, for example, a mechanical shutter, a liquid crystal shutter, or the like can be considered.
Further, the technical scope of the present invention is not limited to such a light control means. For example, for a projector using a light control means that directly changes the output intensity of a light source such as a high-pressure mercury lamp or LED (light emitting diode). The present invention can also be applied.

  In the above embodiment, a projector using a transmissive liquid crystal device as an optical modulation unit is exemplified. However, for example, a reflective projector such as LCOS, or a mirror-type spatial light modulator based on MEMS technology is used. The present invention can also be applied to the projector used.

It is a schematic block diagram of the projector which concerns on this invention. It is a block diagram concerning the image display device of the present invention. It is the histogram which showed the gradation number distribution of the image signal. It is the histogram which showed the gradation number distribution of the image signal. It is the figure which showed a mode that the display image was divided | segmented into the several area | region. It is a block diagram for demonstrating the 1st example of an expansion | extension process. It is a graph for demonstrating the 1st example of an expansion | extension process. It is a figure which shows an example of a look-up table. It is the histogram which showed the gradation number distribution of the image signal. . It is the figure which showed the gradation characteristic of the image signal. It is the figure which showed the gradation characteristic of the image signal. It is a graph for demonstrating the 2nd example of an expansion | extension process.

Explanation of symbols

26: Light control element, 31: First processing unit, 32: Second processing unit, 33: Calculation unit,
82 ... DSP (1) (image parameter extracting means),
83 ... DSP (2) (dimming control means),
86: DSP (3) (extension processing means), 510: Light source

Claims (9)

An image display device in which a display image is adjusted by dimming processing of illumination light and expansion processing of the number of gradations of an image signal,
Image parameter extracting means for extracting an image parameter characterizing the brightness of the display image from the image signal every unit time;
A dimming control unit for determining a dimming control parameter related to the dimming process based on the image parameter extracted by the image parameter extracting unit, and for performing the dimming process based on the dimming control parameter;
A decompression processing unit that determines a decompression control parameter related to the decompression process based on the image parameter extracted by the image parameter extraction unit, and performs the decompression process based on the decompression control parameter;
The expansion processing means determines the expansion control parameter so that the maximum gradation of the input image signal is lower than the maximum output gradation after the expansion processing, and performs the expansion processing. A characteristic image display device.   2. The image display apparatus according to claim 1, wherein the expansion control parameter is determined so as to be smaller than a value obtained by dividing a maximum gradation that can be output after expansion processing by the image parameter at least on a high gradation side.   The image display apparatus according to claim 1, wherein the extension control parameter is determined to be smaller than an inverse number of a dimming rate by the dimming process at least on a high gradation side.   The image display device according to claim 1, wherein the expansion processing is performed by multiplication processing or a combination of multiplication processing and addition processing.   The image display device according to claim 1, wherein the decompression process is performed based on a look-up table.   The image display apparatus according to claim 1, wherein the expansion processing unit performs different expansion processing in accordance with an input gradation within the same screen.   7. The image display device according to claim 6, wherein the expansion processing means has a large expansion coefficient on the low gradation side and a small expansion coefficient on the high gradation side.   8. The image display apparatus according to claim 6, wherein the different expansion processes are controlled with the same time constant. An image display method in which a display image is adjusted by dimming processing of illumination light and expansion processing of the number of gradations of an image signal,
A first step of extracting an image parameter characterizing the brightness of the display image from an image signal every unit time;
A second step of determining a dimming control parameter related to the dimming process based on the image parameter extracted in the first step, and performing the dimming process based on the dimming control parameter;
A third step of determining a decompression control parameter related to the decompression process based on the image parameter extracted in the first step, and performing the decompression process based on the decompression control parameter,
In the third step, the expansion control parameter is determined so that the maximum gradation of the input image signal is lower than the maximum gradation that can be output after the expansion process, and the expansion process is performed. Image display method.

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