Method and apparatus for displaying an image on an image display device

Abstract

A method of displaying an image on an image display device is provided. The image display device includes a plurality of pixels including a light emitting element. The method includes the steps of comparing a gray level of an image signal of the light emitting element with a given threshold level, driving the light emitting element by a driving signal corresponding to the gray level when the gray level is higher than the threshold level, and driving the light emitting element by a dithering driving signal including the driving signal corresponding to the threshold level when the gray level is lower than the threshold level.

Description

Technical Field

[0001] This invention relates to a method and apparatus for displaying two-dimensional images on a two-dimensional image display device, and more particularly to a method and apparatus for improving the quality of images displayed on an image display device. Background Technology

[0002] Recently, two-dimensional image display devices using light-emitting elements such as Organic Light Emitting Diodes (OLEDs) have been used in smartphones and other devices. In such display devices, multiple pixels, including those of OLEDs, are arranged in a matrix.

[0003] Recent smartphones include cameras, and the images captured by the cameras are displayed on display devices, which need to be of high quality.

[0004] The brightness and color of each pixel may vary. Figure 1 The diagram illustrates the non-uniformity that occurs at different display locations and how to compensate for it. However, compensating for such variations requires advanced technology and is costly.

[0005] In the prior art, a display method is disclosed that changes in response to whether the grayscale value of a pixel of a display device is higher than a threshold. The threshold is determined based on the screen size and peak brightness.

[0006] Prior art discloses a method for driving a display device with both analog and digital driving. The analog driving controls the driving voltage of the display device, while the digital driving controls the light emission cycle. Summary of the Invention

[0007] OLEDs are driven by thin-film transistors (TFTs). Because the threshold voltage Vth driving the TFTs can vary, non-uniformity can appear in the image displayed on the display device. In particular, the non-uniformity problem can be very noticeable when the OLED (pixels) are driven at low brightness levels.

[0008] According to the present invention, the OLED (pixel) is not driven at a brightness level (grayscale level) below a given threshold level th. Instead, a brightness level (grayscale level) below the given threshold level th is achieved through dithering. This dithering includes temporal dithering and spatial dithering.

[0009] The threshold level is determined based on the type of image being displayed and its total brightness (average brightness). Image types include still images and moving images. A high threshold level thH is set for still images, and a low threshold level thL is set for moving images. When the image type is still and the total brightness (average brightness) is lower than a reference brightness, a higher threshold level thHH is set. That is, thL... <thH<thHH。

[0010] According to a first aspect of the present invention, a method for displaying an image on an image display device is provided, the image display device comprising a plurality of pixels including light-emitting elements, the method comprising the following steps:

[0011] Compare the gray level of the image signal used for the luminescent element with a given threshold level;

[0012] When the gray level is higher than the threshold level, the light-emitting element is driven by a driving signal corresponding to the gray level; when the gray level is lower than the threshold level, the light-emitting element is driven by a dithering driving signal including a driving signal corresponding to the threshold level.

[0013] According to a second aspect of the present invention, an apparatus is provided for displaying an image on a display device, the image display device comprising a plurality of pixels including light-emitting elements, the apparatus comprising:

[0014] A comparison unit is used to compare the gray level of the image signal for the luminescent element with a given threshold level;

[0015] The selection unit is used to select a driving signal corresponding to the gray level when the gray level is higher than the threshold level, and to select a dithering driving signal including the driving signal corresponding to the threshold level when the gray level is lower than the threshold level.

[0016] A driving unit is used to drive the light-emitting element by the driving signal or the dithering driving signal. Attached Figure Description

[0017] Figure 1 The non-uniformity at different display positions and the compensation for the non-uniformity are shown.

[0018] Figure 2 The emission mode at a low gray level with a threshold level th=4 is shown.

[0019] Figure 3 The jitter pattern with grayscale level x=1 is shown.

[0020] Figure 4 The jitter pattern with grayscale level x=2 is shown.

[0021] Figure 5 The dithering mode with grayscale level x=3 is shown.

[0022] Figure 6 An example of increasing the threshold level th is shown.

[0023] Figure 7 An example of lowering the threshold level th is shown.

[0024] Figure 8 An exemplary jitter pattern with a threshold level th=6 is shown, where the grayscale level changes in each frame.

[0025] Figure 9 An exemplary jitter pattern using subfield PWM is shown.

[0026] Figure 10 The process of converting source voltage into light pulses is shown.

[0027] Figure 11 A block diagram of an image display device according to the present invention is shown.

[0028] Figure 12 A block diagram of an image quality improvement unit according to the present invention is shown.

[0029] Figure 13 Simulation results showing the characteristics of an image without an image quality enhancement unit are presented.

[0030] Figure 14 Simulation results show the characteristics of the displayed image with the image quality improvement unit. Detailed Implementation

[0031] Figure 2 A method for displaying a two-dimensional image on a two-dimensional image display device is illustrated. On the two-dimensional image display device, multiple pixels are arranged in a matrix. Each pixel includes a light-emitting element, such as an Organic Light Emitting Diode (OLED). However, the light-emitting element in this invention is not limited to OLED. Typically, the light-emitting element emits light with a brightness y proportional to a gray level x. In this invention, a threshold level th is set. When the gray level is higher than the threshold level, the light-emitting element emits light with a brightness y proportional to the gray level x. When the gray level is lower than the threshold level, the light-emitting element uses dithering to emit light. Figure 2The illustration shows a time-jittered emission pattern with a threshold level th = 4. In this time-jitter, each emission element emits light four times across frames 1 through 4. For example, when displaying an image with a grayscale level x = 2, the element emits light corresponding to the brightness at threshold level th = 4 in frame 1, does not emit light in frame 2, emits light corresponding to the brightness at threshold level th = 4 in frame 3, and does not emit light in frame 4. This time-jitter results in an average grayscale level x = 2. This example uses only the threshold level and level 0 (no emission). However, other examples can use levels other than the threshold level and level 0, such as levels 1, 2, or 3.

[0032] Figures 3 to 5 Temporal and spatial jitter are shown using 4x4 elements (pixels) and a threshold level th=4 for frames 1 to 4 at low grayscale levels. Figure 3 The dithering mode used to display an image with grayscale level x=1 is shown. Figure 4 The dithering mode used to display an image with grayscale level x=2 is shown. Figure 5 The dithering mode used to display an image with grayscale level x=3 is shown.

[0033] The threshold level th can be adjusted based on the type of image being displayed (still image or moving image) and the total brightness (average brightness) of the image. Figure 6 The diagram illustrates the case of increasing the threshold level. A high threshold level thH is set when the image type is a still image. A relatively high threshold level thHH is set when the image type is a still image and the total brightness of the image is lower than the reference brightness. That is, thH... <thHH。

[0034] Figure 7 This illustrates the case of lowering the threshold level. When the image type is a moving image, a low threshold level thL is set. That is, thL <thH<thHH。

[0035] In terms of image quality, the threshold level is preferably set to 8 to 10.

[0036] Figure 8 Another dithering pattern with a threshold level th = 6 is shown, where the grayscale level changes every frame. For example, when displaying an image with a grayscale level x = 2, frame 1 displays grayscale level 2, frame 2 displays grayscale level 1, frame 3 displays grayscale level 3, frame 4 displays grayscale level 1, frame 5 displays grayscale level 2, and frame 6 displays grayscale level 3. This dithering results in an average grayscale level x = 2. To prevent flickering, the range of grayscale level variation should be limited. In this example, the average grayscale level x = 2 is limited to the range from grayscale level 1 to grayscale level 3.

[0037] Figure 9An example of subfield PWM with low grayscale level dithering is shown. Typically, displaying low grayscale levels using subfield PWM requires very short emission cycles. In this example, the two emission cycles of the least significant two bits are combined, and the emission is divided into frames 1 through 3. For example, to display an image at grayscale level 1, no emission occurs during the combined cycle of frames 1 and 3, but emission occurs during the combined cycle of frame 2. This dithering makes the average grayscale level x = 1. This example produces the effect of relaxing constraints on the source driver, pixel circuitry, and peripheral circuitry. Extending the subfield cycle simplifies the design of the DAC and amplifier in the driver.

[0038] Figure 10 The process of converting a source voltage into a light-emitting pulse is illustrated. The source voltage and scan voltage are input to a comparator, which outputs a PWM pulse with a pulse width that depends on the source voltage. In this example, gray levels below a threshold are not used. Therefore, the minimum source voltage is maintained at an available level, and the minimum pulse width is maintained at an available width. Furthermore, the luminous performance is improved due to increased element response and reduced noise. In summary, increasing the minimum source voltage simplifies the design.

[0039] Figure 11 A block diagram of an image display device according to the present invention is shown. Image signals and image type information are input into the device. Specifically, the image signal is input to a feature detection unit 1 and an image signal processor (ISP) 2. The ISP 2 adjusts the contrast, color, etc., of the input image signal and sends the adjusted image signal to a panel correction unit 3. The panel correction unit 3 corrects the gamma, uniformity, SPR, DeMura, Burn-In, IRdrop, and other characteristics of the image signal and sends the corrected image signal to an image signal improvement unit 4. The feature detection unit 1 detects the total brightness of the image from the input image signal and sends the detected total brightness information to the image signal improvement unit 4. The image signal improvement unit 4 can be placed between the ISP 2 and the feature detection unit 1.

[0040] Image signal improvement unit 4 takes into account the corrected image signal, total image brightness information, and image type (still image or moving image) information, and outputs a drive signal. The drive signal is input to gate driver controller 5 and source driver controller 6. Gate driver controller 5 controls the gate driver 7 of display panel 9 (image display device), and source driver controller 6 controls the source driver 8 of display panel 9. Gate driver 7 and source driver 8 display the image on display panel 9.

[0041] Figure 12A block diagram of an image quality improvement unit 4 according to the present invention is shown. The image quality improvement unit 4 includes a threshold level determination unit 10, a jitter signal generation unit 11, and a selection unit 13. The threshold level determination unit 10 determines a threshold level th based on the total brightness information and image type information of the image. The jitter signal generation unit 11 compares the gray level of the image signal with the threshold level th, and generates a jitter signal when the gray level of the image signal is lower than the threshold level th. The selection unit 13 selects the jitter signal and outputs a drive signal corresponding to the jitter signal when the gray level of the image signal is lower than the threshold level th. When the gray level of the image signal is higher than the threshold level th, the original image signal is selected, and a drive signal corresponding to the original image signal is output. The output drive signal is sent to the gate driver controller 5 and the source driver controller 6.

[0042] This invention improves the gamma properties and color shift of displayed images. Figure 13 The simulation results of the brightness and color difference of the displayed image are shown without the image quality improvement unit 4. Figure 14 The simulation results of the brightness and color difference of the displayed image are shown with the image quality improvement unit 4 present.

[0043] Without the image quality improvement unit 4, the Just Noticeable Color Difference (JNCD) at the center of the image is 178, while with the image quality improvement unit 4, the JNCD is improved to 3.4. Furthermore, without the image quality improvement unit 4, the JNCD at three points in the image is 1912, while with the image quality improvement unit 4, the JNCD is improved to 7.3.

[0044] Furthermore, this invention improves the brightness and color uniformity of displayed images. For example, when a smartphone includes a camera, images captured by the camera are displayed as high-quality images on the display device.

Claims

1. A method for displaying an image on an image display device, the image display device comprising a plurality of pixels including light-emitting elements, the method comprising the following steps: Compare the gray level of the image signal used for the luminescent element with the threshold level; When the gray level is higher than the threshold level, the light-emitting element is driven by a driving signal corresponding to the gray level; when the gray level is lower than the threshold level, the light-emitting element is driven by a dithering driving signal including a driving signal corresponding to the threshold level. The method further includes: Obtain the type of the displayed image and the total brightness of the image, wherein the type of the displayed image includes still images and moving images; The threshold level is set based on the determined type of the displayed image and the determined total brightness of the image, such that: When the type of the displayed image is determined to be a moving image, a low threshold level thL is set. When the determined type of the displayed image is a still image, a high threshold level thH is set. When the type of the displayed image is a still image, and the total brightness of the image is lower than the reference brightness, a higher threshold level thHH is set, where thL <thH<thHH。 2. The method according to claim 1, characterized in that, The jitter includes temporal jitter and spatial jitter.

3. The method according to claim 1, characterized in that, The light-emitting element is an OLED.

4. An apparatus for displaying an image on an image display device, the image display device comprising a plurality of pixels including light-emitting elements, the apparatus comprising: The comparison unit is used to compare the gray level of the image signal for the luminescent element with the threshold level; The selection unit is used to select a driving signal corresponding to the gray level when the gray level is higher than the threshold level, and to select a dithering driving signal including the driving signal corresponding to the threshold level when the gray level is lower than the threshold level. A driving unit is used to drive the light-emitting element by the driving signal or the dithering driving signal; The device further includes: A threshold level determination unit is configured to acquire the type of the displayed image and the total brightness of the image, wherein the type of the displayed image includes still images and moving images; and to set the threshold level based on the determined type of the displayed image and the determined total brightness of the image, such that: When the type of the displayed image is determined to be a moving image, a low threshold level thL is set. When the determined type of the displayed image is a still image, a high threshold level thH is set. When the type of the displayed image is a still image, and the total brightness of the image is lower than the reference brightness, a higher threshold level thHH is set, where thL <thH<thHH。 5. The apparatus according to claim 4, characterized in that, The jitter includes temporal jitter and spatial jitter.

6. The apparatus according to claim 4, characterized in that, The light-emitting element is an OLED.

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