Patent Description:
Recently, a two-dimensional image display device using light emitting elements such as Organic Light Emitting Diodes (OLEDs) has been used for smart phones, etc. On such a display device, a plurality of pixels including the light emitting elements such as OLEDs are arranged in the form of a matrix.

A recent smart phone comprises a camera, and an image taken by the camera is displayed on the display device. High quality is required for the displayed image.

Variation of luminance and color for each pixel may occur. <FIG> shows appearance of unevenness at different display positions and compensations of unevenness. Compensating for this variation requires high technology and high cost.

In the prior art, a display method changing in response to whether a gray scale value for a pixel of a display device is higher than a threshold value or not was disclosed. The threshold value is determined in accordance with screen size and peak luminance.

A method for driving a display device with an analog and digital hybrid driving was disclosed in the prior art. The analog driving controls driving voltage of the display device, and the digital driving controls light emitting period.

<CIT> discloses a method that, when a pixel is to be dictated by a digital pixel signal to operate within an unstable brightness region, employs a control mechanism to control the DAC circuit module to operate the pixel in the block at a DAC level below the unstable brightness region or at a different DAC level above the respective unstable brightness region, to achieve a perceived brightness level within the respective unstable brightness region.

<CIT> discloses a display device and a method for controlling luminance which are capable of possibly improving image quality even at low luminance. According to an embodiment of the present invention, provided are the display device and the method for controlling luminance which are capable of possibly improving image quality by applying a grayscale division control method to a low grayscale area.

An OLED is driven by a driving TFT (Thin Film Transistor). Threshold voltage Vth of the driving TFT may include variation. Thereby, unevenness may appear in an image displayed on a display device. In particular, the unevenness may be noticeable when an OLED (a pixel) is driven on a low luminance level.

Embodiments of this application provide a method for displaying an image on an image displaying device of claim <NUM> and a related image displaying device of claim <NUM>.

According to the present invention, an OLED (a pixel) is not driven on a luminance level (a gray scale level) lower than a given threshold level th. A luminance level (a gray scale level) lower than the given threshold level th is implemented by dithering. This dithering includes temporal dithering and spatial dithering.

The threshold level is determined in accordance with the type of a displayed image and total luminance (average luminance) of the displayed image. The type of an image includes a still image or moving images. High threshold level thH is set for still images, and low threshold level thL is set for moving images. When the type of an image is a still image and total luminance (average luminance) of the image is lower than the reference luminance, higher threshold level thHH is set. That is, thL<thH<thHH.

<FIG> shows a method for displaying a two-dimensional image on a two-dimensional image display device. On the two-dimensional image display device, a plurality of pixels are arranged in the form of a matrix. Each pixel includes a light emitting element such as an Organic Light Emitting Diode (OLED). However, a light emitting element in the present invention is not limited to an OLED. Conventionally, the light emitting element emits light of luminance y in proportion to gray scale level x. In the present invention, a threshold level th is set. When a gray scale level is higher than the threshold level, the light emitting element emits light of luminance y in proportion to gray scale level x. When a gray scale level is lower than the threshold level, the light emitting element emits light using dithering. <FIG> shows light emitting patterns of temporal dithering with threshold level th=<NUM>. In this temporal dithering, each light emitting element emits light <NUM> times of Frames <NUM> to <NUM>. For example, when an image of gray scale level x=<NUM> is displayed, the element emits light of luminance corresponding to threshold level th=<NUM> for Frame <NUM>, the element does not emit light for Frame <NUM>, the element emits light of luminance corresponding to threshold level th=<NUM> for Frame <NUM>, and the element does not emit light for Frame <NUM>. Such a temporal dithering results in average gray scale level x=<NUM>. This example uses threshold level and level <NUM> (no light emitting) only. However, another example can use threshold level and a level other than level <NUM>, such as level <NUM>, <NUM>, or <NUM>.

<FIG> show temporal and spatial dithering for low gray scale levels with threshold level th=<NUM> using 4x4 elements (pixels) and Frames <NUM> to <NUM>. <FIG> shows a dithering pattern in order to display an image of gray scale level x=<NUM>, <FIG> shows a dithering pattern in order to display an image of gray scale level x=<NUM>, and <FIG> shows a dithering pattern in order to display an image of gray scale level x=<NUM>.

The threshold level th is adjusted depending on a type of a displayed image (a still image or moving images) and total luminance (average luminance) of the image. <FIG> shows a case of increasing the threshold level. When a type of the image is a still image, a high threshold level thH is set. When the type of the image is a still image and total luminance of the image is lower than the reference luminance, a higher threshold level thHH is set. That is, thH< thHH.

<FIG> shows a case of decreasing the threshold level. When the type of the image is moving images, a low threshold level thL is set. That is, thL<thH<thHH.

In terms of image quality, it is preferable that the threshold level is set to <NUM> to <NUM>.

<FIG> shows another dithering pattern with threshold level th=<NUM>, in which gray scale levels are changed for every Frame. For example, when an image of gray scale level x=<NUM> is displayed, gray scale level <NUM> is displayed for Frame <NUM>, gray scale level <NUM> is displayed for Frame <NUM>, gray scale level <NUM> is displayed for Frame <NUM>, gray scale level <NUM> is displayed for Frame <NUM>, gray scale level <NUM> is displayed for Frame <NUM>, and gray scale level <NUM> is displayed for Frame <NUM>. Such dithering results in average gray scale level x=<NUM>. In order to prevent a flicker, a changing range of gray scale levels should be limited. In this example, the changing range for average gray scale level x=<NUM> is limited within the range from gray scale level <NUM> to gray scale level <NUM>.

<FIG> shows an example of sub-field PWM in which dithering of low gray scale level is applied. Conventionally, very short light-emitting period is needed for displaying an image of low gray scale level using sub-field PWM. In this example, two light-emitting periods for least significant <NUM> bits are combined, and the light emitting is divided into Frames <NUM> to <NUM>. For example, in order to display an image of gray scale level <NUM>, light is not emitted during the combined period for Frame land for Frame <NUM> but is emitted during the combined period for Frame <NUM>. Such dithering results in average gray scale level x=<NUM>. This example produces the effect of easing of constraints for a source driver, pixel circuits, and peripheral circuits. Lengthening a sub-field period makes design of a DAC and an amplifier in the driver easy.

<FIG> shows a process for converting source voltage into a light-emitting pulse. Source voltage and sweep voltage are inputted into a comparator, and the comparator outputs a PWM pulse having pulse width depending on source voltage. In this example, a gray scale level lower than the threshold level is not used. Thus, minimum source voltage is kept at a usable level, and minimum pulse width is kept at a usable width. Furthermore, light-emitting performance is improved since a response of an element is improved and noise is reduced. Hightening the minimum source voltage makes design easy.

<FIG> shows a block diagram of an image display apparatus according to the present invention. An image signal and information of a type of the image are inputted into the apparatus. The image signal is inputted into a characteristic detection unit <NUM> and ISP (Image Signal Processor) <NUM>. The ISP <NUM> adjusts contrast, color, etc. of the inputted image signal, and sends the adjusted image signal to a panel correction unit <NUM>. The panel correction unit <NUM> corrects characteristics of gamma, uniformity, SPR, DeMura, Burn-In, IRdrop, etc. of the image signal, and sends the corrected image signal to an image signal improving unit <NUM>. The characteristic detection unit <NUM> detects total luminance of the image from the inputted image signal, and sends information of the detected total luminance of the image to the image signal improving unit <NUM>. It is possible to place the image signal improving unit <NUM> between the ISP <NUM> and the characteristic detection unit <NUM>.

The image signal improving unit <NUM> inputs the corrected image signal, information of the total luminance of the image, and information of a type of the image (a still image or moving images), and outputs a driving signal. The driving signal is inputted into a gate driver controller <NUM> and a source driver controller <NUM>, the gate driver controller <NUM> controls a gate driver <NUM> of a display panel <NUM> (an image displaying device), and the source driver controller <NUM> controls a source driver <NUM> of the display panel <NUM>. The gate driver <NUM> and the source driver <NUM> displays an image on the display panel <NUM>.

<FIG> shows a block diagram of the image quality improving unit <NUM> according to the present invention. The image quality improving unit <NUM> comprises a threshold level determining unit <NUM>, a dithering signal generating unit <NUM>, and a selecting unit <NUM>. The threshold level determining unit <NUM> determines the threshold level th in accordance with information of total luminance of the image and information of a type of the image. The dithering signal generating unit <NUM> compares the gray scale level of the image signal with the threshold level th, and generates a dithering signal when the gray scale level of the image signal is lower than the threshold level th. The selecting unit <NUM> selects the dithering signal and outputs a driving signal corresponding to the dithering signal when the gray scale level of the image signal is lower than the threshold level th. When the gray scale level of the image signal is higher than the threshold level th, the original image signal is selected and a driving signal corresponding to the original image signal is outputted. The outputted driving signal is sent to the gate driver controller <NUM> and the source driver controller <NUM>.

The present invention improves gamma characteristics and color drift of the displayed image. <FIG> shows simulation results of luminance and color difference of the displayed image without the image quality improving unit <NUM> and <FIG> shows simulation results of luminance and color difference of the displayed image with the image quality improving unit <NUM>.

JNCD (Just Noticeable Color Difference) at the center position of the image is <NUM> without the image quality improving unit <NUM>, whereas the JNCD is improved to be <NUM> with the image quality improving unit <NUM>. Furthermore, JNCD at three points in the image is <NUM> without the image quality improving unit <NUM>, whereas the JNCD is improved to be <NUM> with the image quality improving unit <NUM>.

Claim 1:
A method for displaying an image on an image displaying device, the image displaying device comprising a plurality of pixels including light emitting elements, the method including steps of:
comparing gray scale level of an image signal for the light emitting element with a threshold level; and
when the gray scale level is higher than the threshold level, driving the light emitting element by a driving signal corresponding to the gray scale level, and when the gray scale level is lower than the threshold level, driving the light emitting element by a dithering driving signal including a driving signal corresponding to the threshold level;
characterized in that the method further comprising:
obtaining a type of a displayed image and a total luminance of the image, the type of displayed image including still image and moving images; and
setting the threshold level depending on the determined type of a displayed image and the determined total luminance of the image such that:
when the determined type of the image is moving images, a low threshold thL level is set,
when the determine type of the image is still image, a high threshold level thH is set,
when the determined type of the image is a still image and the determined total luminance of the image is lower than a reference luminance, a higher threshold level thHH is set, wherein thL < thH < thHH.