GB2486434A - Pixel overdriving host and remote device system using image frame differences - Google Patents

Abstract

A display system comprises a host device (10, Figure 3), e.g. desktop computer, a remote device (30) connected to the host device and a display device (20) connected to the remote device. The host detects 61 difference data between a pair of sequential frames of image data, each frame of image data comprising pixel data values for an array of pixels. The host uses the difference data to identify 62 a pixel, having a first value, which requires overdriving. A pixel data value of the identified pixel is modified 63 from the first value. The difference data and the modified pixel data value is sent 64 to the remote device. Subsequently, the host sends data to restore 65 the first value of the identified pixel, for example after a time period which is less than a frame refresh period. Thus, by providing overdrive processing upstream at a host, overdrive processing at the display is avoided, removing the need for addition frame stores and delay at the display. Identification of pixels requiring overdriving may comprise identifying when difference data is greater than a threshold.

Description

DESCRIPTION

OVERDRIVING PIXELS IN A DISPLAY SYSTEM

This invention relates to a display system which overdrives pixels.

Many displays now use Liquid Crystal Display (LCD) technology. LCD displays have a light source, called a backlight, and an array of liquid crystal pixels are individually controlled so as to pass a desired amount of the backlight to the front face of the display. The liquid crystal pixels are "pulled" into alignment in response to an applied electrical field. The strength of the electrical field determines the opaqueness of the liquid crystal material. In this way, a range of brightness levels of a pixel are achieved.

One criterion by which LCD panels are widely judged is the response is time, which can be defined as the time taken for a pixel to switch from dark to light to dark again. One technique to reduce response time is called overdrive.

Pixels are driven (for a limited time) to a higher value than actually required.

When the display notices that a pixel is making a large jump in value, the pixel is driven for a short time to an even more extreme value. This makes the liquid crystal change orientation faster than it would if it were merely driven to the correct value. For example, if the level of a particular pixel is required to change from 0x40 to OxCO, it could be driven with a field representing a value of OxEO for a short period (possibly one frame) before returning it to its correct level.

Overdrive has some possible side effects. A first side effect is overshoot. If the pixel actually exceeds the desired value during the overdrive operation before settling on it, this can produce an undesirable ringing effect. A second side effect is that, in order to detect large changes, the LCD must add at least one frame of delay in order to difference the images to detect the amplitude change between frames. The side effects of overdrive can be particularly problematic for a display which is required to display fast-moving content, such as fast-moving video during gaming.

It is therefore an object of the invention to improve upon the prior art.

A first aspect of the present invention provides a method of processing display data at a host device in a display system comprising a host device, a remote device connected to the host device and a display device connected to the remote device, the method comprising: detecting difference data between a pair of sequential frames of image data, each frame of image data comprising pixel data values for an array of pixels; using the difference data to identify a pixel, having a first value, which requires overdriving; modifying a pixel data value of the identified pixel from the first value; and sending the difference data and the modified pixel data value to the remote device.

The host performs the processing required to overdrive pixels of a display. This avoids the need to provide overdrive processing at the display, which typically avoids the need for additional framestores, incurring delay, and is processing. This reduces the hardware cost of the display.

Advantageously, the method further comprises, subsequent to the modifying step, sending data to restore the first value of the identified pixel.

The overdriven pixel is restored to the true pixel data value.

Advantageously, the step of sending data to restore the first value of the identified pixel occurs after a time period which is less than a frame refresh period. This is particularly effective where the interface between the remote device and the display allows updating of a selected part of the display, without a conventional raster-based output of an entire frame of pixel data.

A further aspect of the invention provides a host computing device which is connectable to a remote device for processing image data for a display, the host computing device comprising a processor which is arranged to perform any of the described or claimed methods.

The functionality described here can be implemented in hardware, software executed by a processing apparatus, or by a combination of hardware and software. The processing apparatus can comprise a computer, a processor, a state machine, a logic array or any other suitable processing apparatus. The processing apparatus can be a general-purpose processor which executes software to cause the general-purpose processor to perform the required tasks, or the processing apparatus can be dedicated to perform the required functions. Another aspect of the invention provides machine-readable instructions (software) which, when executed by a processor, perform any of the described methods. The machine-readable instructions may be stored on an electronic memory device, hard disk, optical disk or other machine-readable storage medium. The machine-readable instructions can be downloaded to the storage medium via a network connection.

io Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a host connected to a display by a video interface, with overdrive processing at the display; Figure 2 shows a host connected to a display by a general purpose is network/interface and a display control device; Figure 3 shows a display system according to an embodiment of the invention, with a host connected to a display by a general purpose network/interface and a display control device; Figure 4 shows a method of transferring display data between the host, display control device and display; Figure 5 shows a display updated by a raster scanning technique; Figure 6 shows another method of transferring display data between the host, display control device and display; Figure 7 shows a display updated selectively; Figure 8 shows a method performed at a host computing device; Figure 9 shows a display system with a host computing device connected to a display via a general-purpose interface or network; and Figure 10 shows a display system with a server connected to a plurality of client computing devices via a general-purpose network.

Before describing embodiments of the invention, some existing display systems will be described more fully. Figure 1 shows a display system comprising a source, such as a computer, and a display 20 connected by a video interface 11. Display 20 includes a liquid crystal display (LCD) panel 29, a framestore 22 and apparatus 26 for overdriving the liquid crystal display panel 29. Apparatus 26 boosts the value of pixels to a higher value than is actually required so as to increase the response time of the pixels in the LCD panel 29.

Figure 2 shows another type of display system. A host computing device (e.g. desktop computer) 10 is coupled to a display using a general-purpose interface or network 40, such as Universal Serial Bus (USB) or Ethernet. A remote device 30, local to the display 20, acts as a network-to- video converter. The remote device receives image data over the general-purpose interface, stores the data in a frame buffer 33, and outputs the data on a video interface 35 to the display. A system of this type is described in International Patent Application Publication WO 2005/083558. To minimise the is amount of bandwidth used on the general-purpose network, the image data is compressed 15 at the host device 10 before transmission, sent in compressed form over the general purpose network, and decompressed 32 at the remote device 30. To further reduce the amount of data transmitted over the general purpose network, host 10 can compare a current frame and a previous frame and only send any differences between the frames. A framestore 12 stores a current frame and a framestore 13 stores a previous frame. A differencing unit 14 compares the two frames and identifies differences.

A display 20 connected to the remote device 30 can include the overdrive processing functionality 26 previously described.

Figure 3 shows a display system in accordance with an embodiment of the invention. A host computing device (e.g. desktop computer) 10 is coupled to a display using a general-purpose network or interface 40, such as Universal Serial Bus (USB) or Ethernet. A remote device 30, local to the display 20, acts as a network-to-video converter. The remote device 30 has a network interface 31 for receiving image data over the general-purpose network 40, storing the data in a frame buffer 33, and outputting the data on a video interface 35 to the display 20. A framestore 12 stores a current frame and a framestore 13 stores a previous frame. A differencing unit 14 compares the two frames held in framestores 12, 13 and identifies differences. For example, the differencing unit can identify any rectangular areas within the frames which have changed between the frames. The rectangular area could represent a window on a desktop, or it could represent a portion of a window, such as a line of a word processing document which has changed between the frames.

The host 10 in Figure 3 further comprises overdrive processing 18, 19.

The differencing unit 14 provided at the host already identifies differences between two stored frames. Differencing unit 14 supplies an output to a detection unit 18. Detection unit 17 detects individual pixel values which require overdriving. Detection unit 17 can identify pixels requiring overdrive in various ways. For example, detection unit 17 can identify pixels with an amplitude change above a threshold value. Detection unit 17 outputs an is additional amplitude value which is added 19 to the existing pixel value. In this way, the pixels with large changes will be overdriven. The correspondence between amplitude change and overdrive values can be stored as a look-up table in a store 18. As previously described, host 10 can compress the difference data before transmission over the general purpose network or interface 40.

The incorporation of overdrive processing 14, 18, 19 at the host 10 avoids the need for overdrive processing at the display 20. Figure 3 shows a simplified display 20 with just a single framestore 22 and panel 29. The overdrive processing 26, which would typically include an additional framestore) is no longer required, thereby reducing hardware and the associated cost at the display 20. Advantageously, any pixel which is overdriven is restored to a correct pixel value within a short time period, such as the next frame. Detection unit 17 can store 18 the location of the pixel that was overdriven and a correction (negative) value that is required to restore the pixel to the correct pixel value. During the next frame, unit 18 outputs that corrective value to addition unit 19, for transmission to the remote device 30.

There are several different methods of operating the display system shown in Figure 3, as shown in Figures 4 to 7. The methods differ in terms of the type of data that is sent between the entities and the rate at which data is sent.

In Figure 4, remote device 30 is connected to display 20 by a conventional video interface. Conventional video interfaces are raster-based, i.e. they output an entire frame of pixel data after each frame fresh period. The frame refresh rate is typically 60Hz, but can be higher, depending on the type of display and/or source material. For example gaming, can use a higher frame refresh rate. The pixel data is output in a defined scanning pattern, as shown in Figure 5. Pixel data is output line-by-line.

Remote device 30 always stores a complete frame of pixel data, as this is needed to generate the raster-scan output signal over the video interface.

However, there is no need to send an entire frame to the remote device after is each frame refresh period. Instead, the host only sends pixel data for those parts of the new frame that have changed. Host 10 sends difference data and pixel overdrive data to the remote device 30. The difference data is image data which is different between sequential frames. Pixel overdrive data are values of individual pixels which have been boosted to a higher value than the true pixel value in the image to ensure that a display will respond more quickly to the new pixel value. The difference data received from the host 10 updates a selected part, or parts, of the pixel data held at the remote device 30. The host may also periodically refresh the data held at the remote device 30 by sending an entire frame of pixel data.

The video interface 35 operates at a particular frame refresh rate, e.g. 60Hz. Data updates between host 10 and remote device 30 occur at the same rate. For example, with a video interface 35 operating at a frame refresh rate of 60Hz, data updates are sent from the host 10 to remote device 30 at a rate of times per second. After a frame period, the host 10 sends the next set of difference data and pixel overdrive data. Host 10 also sends correction values to restore any pixels which were artificiaUy boosted during the previous frame to their correct pixel values.

At step 100 difference data is sent from host 10 to remote device 30.

Data transferred at step 100 also includes pixel overdrive data for a particular pixel, called pixel X. At step 101 a full frame of pixel data is sent from the remote device 30 to display 20 by the video interface, using a raster scanning technique. The pixel data at step 101 includes the overdrive value for pixel X. At step 102 a new set of difference data is sent from host 10 to remote device 30, representing the difference between the latest frame and the previous frame. Data transferred at step 102 also data includes pixel overdrive data for a particular pixel, called pixel Y and also includes a correction value for pixel X. At step 103 a full frame of pixel data is sent from the remote device 30 to display 20 by the video interface, using a raster scanning technique. The pixel data at step 101 includes the overdrive value for pixel Y and a value for pixel X which will restore pixel X to a correct value.

A disadvantage of this scheme is that a whole frame period (e.g. l6ms is at 60Hz frame refresh rate) elapses before there is an opportunity to restore the pixel to the correct pixel value.

In Figure 6, remote device 30 is connected to display 20 by a non-conventional video interface which is not raster-based. Communication between the host 10 and remote device 30 is similar to Figure 4, but can occur more frequently. The host 10 only sends pixel data for those parts of a new frame that have changed. Host 10 sends difference data and pixel overdrive data to the remote device 30. The difference data is image data which is different between sequential frames. Pixel overdrive data are values of individual pixels which have been boosted to a higher value than the true pixel value in the image to ensure that a display will respond more quickly to the new pixel value. The difference data received from the host 10 updates a selected part, or parts, of the pixel data held at the remote device 30. It is also possible that the host may periodically refresh the data held at the remote device 30 by sending an entire frame of pixel data. Host 10 also sends correction values to restore any pixels which were artificially boosted during the previous frame to their correct pixel values.

Pixel data transferred from the remote device 30 to the display 20 can update only those parts of the image which have changed between frames.

Transfer of pixel data from the remote device 30 to the display 20 is not restricted to the raster-based specification of a conventional video interface and can occur at a higher rate than a conventional frame refresh rate, if needed. An advantage of this scheme is that there is no longer need to wait for an entire frame refresh period (e.g. l6ms) before restoring the pixel to the correct pixel value. The pixel can be restored after a shorter period of, for example, half of a conventional frame refresh period (e.g. 8ms) or any other required time. This reduces the side-effects that would normally be experienced by overdriving pixels.

At step 110 difference data is sent from host 10 to remote device 30.

Data transferred at step 110 also includes pixel overdrive data for a particular pixel, called pixel X. At step 111 pixel data is sent from the remote device 30 to is display 20 by the video interface. The pixel data at step 111 only needs to update parts of the display that have changed. The pixel data at step 111 includes the overdrive value for pixel X. At step 112 a correction value for pixel X is transferred from host 10 to remote device 30. At step 113 the remote device 30 sends data to display 20 which will cause the display to restore pixel X to a correct value (i.e. not overdriven). At step 114 difference data and pixel overdrive data for pixel Y is transferred from host 10 to remote device 30. At step 115 pixel data is sent from the remote device 30 to display 20 by the video interface. Steps 110 and 112 are separated by a time period 116. Steps and 114 are separated by a time period 117. Periods 116, 117 can be much less than the normal frame refresh period. Similarly, steps 111 and 113 are separated by a time period 118. Steps 111 and 115 are separated by a time period 119. Periods 118, 119 can be much less than the normal frame refresh period.

Figure 6 shows an overall frame 200 of pixel data and an area 201 within the frame 200 that is updated by the difference data transmitted at step 111. Area 201 represents an area of the display that has changed between the current frame and a previous frame. Pixel X 202 is located within the area 201.

Pixel X 202 is overdriven at step 111 and restored to a correct value at step 113. Area 201 and pixel 202 can be updated independently of the remainder of the frame 200.

Figure 8 shows a method performed at a host computing device. Step 61 detects difference data between a pair of sequential frames of image data, each frame of image data comprising pixel data values for an array of pixels.

Step 62 uses the difference data to identify a pixel, having a first value, which requires overdriving. Step 63 modifies a pixel data value of the identified pixel from the first value. Step 64 sends the difference data and the modified pixel data value to the remote device. Optionally, step 65 sends data to restore the overd riven pixel to the first value.

In the embodiments described above, a framestore is provided at the display. A framestore is a typical part of most displays but it is not essential to implement embodiments of the invention. The individual pixels of an LCD is effectively act as a framestore, as each pixel latches the voltage it is set to.

In the embodiments described above, the frames of image data can be supplied by any suitable source, such as a broadcast receiver, media player (e.g. DVD player, Blu-Ray player), data network or from an application executed by the processor of the host device (e.g. gaming).

The arrangements described above can be implemented in a system of the type shown in Figures 9 or 10. in Figure 9, a first display 55 is connected to the host computing device 10 in a conventional manner, via a graphics card 53. Second display 20 is connected to the host computing device 10 via a general-purpose network/interface 40, such as Universal Serial Bus (USB). A remote device 30, which can be called a display control device 40, connects between the general-purpose interface 40 and second display device 20. The display control device 30 appears, to the host computing device 10, as a USB-connected device. Any communications between the host computing device and the display control device 30 are carried out under the control of a USB driver specifically for the display control device 30. Such devices allow the connection of the second display device 20 without the need for any hardware changes to the host computing device 10. The display control device 30 connects to the display device 20 via a standard video interface 35, such as VGA or Digital Visual Interface (DVI). Display control device 30 includes processing apparatus 33 which is configured to receive image data over the general-purpose interface 40 and then output the data in a form suitable for the display device 20. The image data is sent over the general-purpose interface 40 in compressed form and decompressed locally at the display control device 30. Advantageously, the display control device 30 is powered by the host computing device 10 via power supply lines of the interface 40.

The display device 20 is a conventional display device which requires no adjustment to operate in the display system shown in Figure 9. As far as the display device 20 is concerned, it could be connected directly to the graphics card of a host computing device and it is unaware that the image data has actually been sent firstly via a USB connection to an intermediate component. Multiple additional display devices 20 can be connected to the is host computing device 10 in this way, as long as suitable USB ports are available on the host computing device 10. The display control device 30 is external to the host computing device 10 and is not a graphics card. It is a dedicated piece of hardware that receives graphical data via the USB connection from the host computing device 10 and transforms that graphics data into a format that will be understood by the display device 20. It will be understood that any general-purpose data network (e.g. Ethernet) can connect the host computing device 10 to the display control device 30 and any display-specific interface is used on the connection from the display control device 30 to the display device 20. Processor 51 at host 10 performs the functionality for detecting differences and overdriving pixels described above.

Figure 10 shows a second type of display system 200 in accordance with an embodiment of the invention. A server 210 connects to multiple client devices 220 by a data pipe 240 over a general-purpose data network 245, such as an Ethernet network. Advantageously, the client devices 220 are thin, ultra-thin or zero client devices. These types of client device have relatively limited processing power. Functionality that would typically be performed by a "fat" or even a normal "thin" client device is performed by the server 210. This type of architecture has the advantages of reliably and securely storing data at the server 210, avoiding the need to install and regularly upgrade software on multiple client devices 220, and requiring low-cost client devices 220. Server 210 maintains an instance of a virtual machine for each connected client device 220. Server 210 performs all of the operating system and application layer processing that would typically be performed at a client device, and generates image data representing the desktop display to be displayed at the client device 220. The client devices 220 are only required to receive the image data from the server 210, display the image data, and return user inputs to the server. Each client device 220 has a terminal unit 221, a display 222 and a user interface, such as a keyboard 224 and mouse 225. Terminal 221 includes the same functionality 230 as the display control device 30 of Figure 9. Terminal 221 receives image data via a forward path over the general purpose network 245 and converts the image data into a form suitable for is output to display 222. The image data is sent over the general-purpose network 245 in compressed form and decompressed locally at the terminal 221. User inputs from the user interface 224, 225, and any control data, are returned to the server via a reverse path over the general purpose network 245. Processor 251 at server 210 performs the functionality for detecting differences and overdriving pixels described above.

Modifications and other embodiments of the disclosed invention will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings.

Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims (7)

1. CLAIMS1. A method of processing display data at a host device in a display system comprising a host device, a remote device connected to the host device and a display device connected to the remote device, the method comprising: detecting difference data between a pair of sequential frames of image data, each frame of image data comprising pixel data values for an array of pixels; io using the difference data to identify a pixel, having a first value, which requires overdriving; modifying a pixel data value of the identified pixel from the first value; and sending the difference data and the modified pixel data value to the is remote device.
2. 2. A method according to claim 1 further comprising, subsequent to the modifying step, sending data to restore the first value of the identified pixel.
3. 3. A method according to claim 2 wherein the step of sending data to restore the first value of the identified pixel occurs after a time period which is less than a frame refresh period.
4. 4. A method according to according to any one of the preceding claims wherein the step of using the difference data to identify a pixel comprises identifying when the difference data for a pixel is greater than a threshold value.
5. 5. A method according to any one of the preceding claims wherein the host device is connected to the remote device by a general purpose interface or network, and the sending step comprises sending the difference data and the modified pixel data value to the remote device over the general purpose interface or network.
6. 6 A host computing device which is connectable to a remote device for processing image data for a display, the host computing device comprising a processor which is arranged to perform the method according to any one of the preceding claims.
7. 7. A computer-readable medium carrying machine-readable io instructions which, when executed by a processor, cause the processor to perform the method according to any one of the preceding claims.