Patent Publication Number: US-10318226-B2

Title: Global command interface for a hybrid display

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to display systems and more particularly to interfaces to display systems. 
     BACKGROUND 
     Immersive virtual reality (VR) and augmented reality (AR) systems typically utilize a head mounted display (HMD) device that presents stereoscopic imagery to the user so as to give a sense of presence in a three-dimensional (3D) scene. For example, a typical HMD device is designed to produce a stereoscopic image over a field-of-view that approaches or is equal to the field-of-view of a human eye, which is approximately 180°. Conventional HMD devices implement either a single flat display that is separated into two independent display regions, one for the left eye and one for the right eye of the user, or a pair of independent flat displays, one for each eye of the user. The conventional HMD device further includes a circular lens for each eye so as to focus the entire image of the display into the user&#39;s eye. 
     The human eye is able to differentiate between adjacent features of an image to an eye-limiting resolution of approximately 1 arcmin/pixel, which corresponds to a pixel density of approximately 60 pixels/degree. However, the human eye is only able to perceive features at eye-limiting resolution within an area of focus of approximately 60° from a center of focus. Features outside of the area of focus (e.g., at angles between 60° and 180°) are perceived with peripheral vision at a much lower resolution. The cost and complexity of an HMD is significantly and unnecessarily increased if the display is implemented with a sufficient pixel density to provide eye-limiting resolution of over the entire 180° field-of-view. Conventional HMD devices are therefore designed to balance the competing demands for high resolution and low cost/complexity by using displays that have a pixel density of 10-20 pixels/degree. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may be better understood by, and its numerous features and advantages made apparent to, those skilled in the art by referencing the accompanying drawings. The use of the same reference symbols in different drawings indicates similar or identical items. 
         FIG. 1  illustrates a display system that includes an electronic device configured to provide AR or VR functionality via a hybrid display in accordance with at least one embodiment of the present disclosure. 
         FIG. 2  illustrates an example cross-section view of the electronic device as mounted on the head of a user in accordance with at least one embodiment of the present disclosure. 
         FIG. 3  is a block diagram of a processing system that is used to generate a display imagery using a hybrid display that includes high resolution and low-resolution displays in accordance with at least one embodiment of the present disclosure. 
         FIG. 4  is a block diagram of a display system including a global interface that implements a finite state machine in accordance with at least one embodiment of the present disclosure. 
         FIG. 5  illustrates VSYNC signals utilized by a high-resolution display and a low-resolution display that form a hybrid display in accordance with at least one embodiment of the present disclosure. 
         FIG. 6  is a block diagram of a hybrid display including a low-resolution display and a high-resolution display in accordance with at least one embodiment of the present disclosure. 
         FIG. 7  is an XY chromaticity diagram that indicates a color gamut of a high-resolution display and a color gamut of a low-resolution display in accordance with at least one embodiment of the present disclosure. 
         FIG. 8  is a flow diagram of a method of providing global commands to interfaces of a high-resolution display and a low-resolution display in a hybrid display in accordance with at least one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is intended to convey a thorough understanding of the present disclosure by providing a number of specific embodiments and details involving imagery capture and display systems. It is understood, however, that the present disclosure is not limited to these specific embodiments and details, which are examples only, and the scope of the disclosure is accordingly intended to be limited only by the following claims and equivalents thereof. It is further understood that one possessing ordinary skill in the art, in light of known systems and methods, would appreciate the use of the disclosure for its intended purposes and benefits in any number of alternative embodiments, depending upon specific design and other needs. 
       FIGS. 1-8  illustrate embodiments of a hybrid display that combines a low-resolution display (e.g., a display constructed of pixels that have a pixel size of approximately 60 μm) with a high-resolution display (e.g. a semiconductor micro display constructed of pixels that have a pixel size of approximately 2 μm or less). For example, an HMD may include two high-resolution displays for displaying a portion of an image in the areas of focus of the left and right eyes of the user of the HMD. The hybrid display also includes two low-resolution displays for displaying a portion of the image that falls outside the areas of focus of the left and right eyes of the user. The high and low-resolution displays may be overlaid with each other or the low-resolution display may be formed in a region that wraps around or encompasses the outer boundary of the high-resolution display. Additional optical elements may be used to combine and focus light emitted by the pixels in the high-resolution and low-resolution displays on to the user&#39;s eyes. 
     Conventional high-resolution displays and low-resolution displays are controlled using different interfaces. For example, the interface to a conventional high-resolution display may be implemented as an inter-integrated circuit (I 2 C) and the interface to a conventional low-resolution display may be implemented as a Mobile Industry Processor Interface (MIPI), an embedded display port (eDP) interface, or a mobile display digital interface (MDDI). The control path speed for an I 2 C interface is on the order of hundreds of kilohertz (kHz) and the control path speed for a MIPI (or other low-resolution display) interface is on the order of tens of megahertz (MHz). Furthermore, the packet overhead in data packets transmitted by an I 2 C interface is larger than the packet overhead in data packets transmitted by a MIPI interface. The different control path speeds and packet overheads introduce timing differences between the two interfaces that can generate noticeable visual artifacts such as flicker in response to global commands that affect pixels in both the high-resolution display and the low-resolution display. For example, a dimming algorithm can modify the brightness of the image produced by the display by transmitting (via the respective interfaces) global dimming commands to the high-resolution and low-resolution displays. The different control path speeds and packet overheads result in different values of the lag between transmission of the global command to the displays via the corresponding interfaces and the resulting change in the pixel values. The relative timing delays produced by the different values of the lag may generate flicker in the combined image produced by the high-resolution display and the low-resolution display. The flicker may be exacerbated by different color matrices that map the brightness indicated by the global command to different values of the brightness produced by the high resolution and low-resolution displays. 
     The visual artifacts produced by a hybrid display in response to receiving a global command may be reduced by translating the global command to interface-specific commands that are understood by corresponding interfaces to the high-resolution display and the low-resolution display. The interface-specific commands include values that represent one or more characteristics of the images. The different values in the interface-specific commands are generated from a corresponding value in the global command based on different physical properties of the high-resolution display and the low-resolution display. For example, a global command may be issued to reduce the brightness of an image produced by the hybrid display by a predetermined amount that is represented by a change in a value of the luma for each pixel. The luma values are stored in one or more registers associated with the high-resolution display and the low-resolution display. The stored values are used to control a bias current or voltage that is applied to pixels in response to input signals provided by the interfaces to the high-resolution display and the low-resolution display. The bias currents or voltages applied to the pixels in the different displays, as well as the colors produced by the pixels in response to the applied bias current or voltage, are typically different due to different physical properties of the high-resolution display and the low-resolution display. The values of the luma indicated in the global command may therefore be translated to interface-specific values based on the physical properties of the displays. For example, a finite state machine may receive the global command and selectively modify the luma value (or change in the luma value) indicated in the global command based on different color matrices that represent the relationship between a value of luma and corresponding RGB values that define the colors displayed by the high-resolution display and the low-resolution display. The finite state machine modifies the luma values so that the change in brightness perceived by the user is substantially the same for the high-resolution display and the low-resolution display. The finite state machine may also translate other global command values such as values that indicate a chroma that controls the bias currents or voltages applied to pixels in response to input signals representative of chroma, color matrices that are used to transform luma and chroma to RGB values used by the displays, gamma values that determine a power law relationship between an input value of luma and a gamma-corrected value of luma used for encoding, color coordinates, a color gamut, and the like. 
     The global command interface may then provide the interface-specific commands (including the translated global command values) to the high-resolution display interface and the low-resolution display interface. To avoid flickering, the finite state machine time synchronizes the interface-specific commands so that they are provided to the different interfaces at substantially the same time. For example, the finite state machine may provide the interface-specific commands to first-in-first out (FIFO) buffers at times that are offset by values of relative delays between signals indicating a refresh interval and a blanking interval for the high-resolution display and the low-resolution display. The values in the FIFO buffers are then shifted into the appropriate registers during the synchronized blanking intervals. Some embodiments of the offset may be determined based on the amount of overhead or data in the packets transmitted over the different interfaces. The finite state machine may also configure timing parameters of the high-resolution display interface and the low-resolution display interface using interface-specific commands. Examples of timing parameters include parameters that specify timing of a horizontal synchronization signal that separates scan lines in the displays, a vertical synchronization signal that separates fields in the displays, pixel scan directions, a blanking interval during which no data is transmitted, and the like. 
       FIG. 1  illustrates a display system  100  that includes an electronic device  105  configured to provide AR or VR functionality via a hybrid display in accordance with at least one embodiment of the present disclosure. The illustrated embodiment of the electronic device  105  can include a portable user device, such as head mounted display (HMD), a tablet computer, computing-enabled cellular phone (e.g., a “smartphone”), a notebook computer, a personal digital assistant (PDA), a gaming console system, and the like. In other embodiments, the electronic device  105  can include a fixture device, such as medical imaging equipment, a security imaging sensor system, an industrial robot control system, a drone control system, and the like. For ease of illustration, the electronic device  105  is generally described herein in the example context of an HMD system; however, the electronic device  105  is not limited to these example implementations. 
     A back plan view of an example implementation of the electronic device  105  in an HMD form factor in accordance with at least one embodiment of the present disclosure is shown in  FIG. 1 . The electronic device  105  may be implemented in other form factors, such as a smart phone form factor, tablet form factor, a medical imaging device form factor, and the like, which implement configurations analogous to those illustrated. As illustrated by the back plan view, the electronic device  105  can include a face gasket  110  mounted on a surface  115  for securing the electronic device  105  to the face of the user (along with the use of straps or a harness). 
     The electronic device  105  includes a hybrid display  120  that is used to generate images such as VR images or AR images that are provided to the user. The hybrid display  120  is divided into two substantially identical portions, a right portion to provide images to the right eye of the user and a left portion to provide images to the left eye of the user. The left portion includes a relatively high resolution display  125  that is positioned at the center of the field-of-view of the user and a relatively low-resolution display  130  that encompasses the high-resolution display  125  and provides images at larger angles with respect to the center of the field-of-view that correspond to the regions of peripheral vision of the user. The right portion includes a high-resolution display  135  and a low-resolution display  140  that are substantially identical to the corresponding high resolution display  125  and low-resolution display  130 . The high-resolution displays  125 ,  135  have a larger pixel density than the low-resolution displays  130 ,  140 . Pixel sizes in the high-resolution displays  125 ,  135  are smaller than pixel sizes in the low-resolution displays  130 ,  140 . For example, the high-resolution displays  125 ,  135  may have a pixel size of less than 5 μm (such as a pixel size of 2 μm) and the low-resolution displays  130 ,  140  may have a pixel size in the range of 10-100 μm, such as a pixel size of 60 μm. 
     The high-resolution displays  125 ,  135  are controlled using signals provided by one or more high-resolution interfaces  145 . Some embodiments of the high-resolution interfaces  145  are implemented as an inter-integrated circuit (I 2 C) interface that has a control path speed on the order of hundreds of kilohertz (kHz). The low-resolution displays  130 ,  140  are controlled using signals provided by one or more low-resolution interfaces  150 . The low-resolution interfaces  150  may be implemented as a Mobile Industry Processor Interface (MIPI), an embedded display port (eDP) interface, or a mobile display digital interface (MDDI). The control path speed for a MIPI (or other low-resolution) interface is on the order of tens of megahertz (MHz). In some embodiments, the packet overhead in data packets transmitted by an I 2 C interface such as the high-resolution displays  125 ,  135  is larger than the packet overhead in data packets transmitted by a MIPI, eDP, or MDDI interface such as the low-resolution displays  130 ,  140 . 
     The display system  100  includes a global interface  155  for receiving global commands to control or modify images displayed by the high-resolution displays  125 ,  135  and the low-resolution displays  130 ,  140  in the hybrid display  120 . As used herein, the term “global command” refers to a command that is used to control or modify characteristics of the light generated by one or more pixels in the high-resolution displays  125 ,  135  and one or more pixels in the low-resolution displays  130 ,  140 . For example, the global interface  155  may receive a global command to dim the light generated by the high-resolution displays  125 ,  135  and the low-resolution displays  130 ,  140  by changing the brightness of the pixels in the hybrid display  120 . Global commands indicate values (or modifications to values) of characteristics of the light generated by the pixels that are intended to cause predetermined modifications in observed images produced by the pixels in the hybrid display  120 . However, the different physical characteristics of the high-resolution displays  125 ,  135  and the low-resolution displays  130 ,  140  may cause the pixels in these displays to produce light having different characteristics in response to the same global command. Moreover, timing differences between the high-resolution interface  145  and the low-resolution interface  150  may cause the modifications to occur at different times in the high-resolution displays  125 ,  135  and the low-resolution displays  130 ,  140 . 
     In order to reduce the flicker caused by timing differences or different physical characteristics, some embodiments of the global interface  155  perform translation and synchronization operations on the global command to produce interface-specific commands that are transmitted to the high-resolution interface  145  and the low-resolution interface  150 . For example, the global interface  155  may receive a global command that includes one or more values indicating a modification of signals provided to pixels in the hybrid display  120 . The global interface  155  may then translate the one or more values into corresponding values indicating modifications of the signals provided to the pixels in the high-resolution displays  125 ,  135  and the low-resolution displays  130 ,  140  based on the different physical properties or characteristics of the displays  125 ,  130 ,  135 ,  140 . The global interface  155  can access information indicating timing delays (or a relative timing delay) between control signals used by the high-resolution displays  125 ,  135  and the low-resolution displays  130 ,  140 . The global interface  155  uses the timing delays (or relative timing delay) to synchronize the interface-specific commands, e.g., by transmitting commands to the high-resolution interface  145  and the low-resolution interface  150  at times determined by the timing delays (or relative timing delay) so that interface-specific commands including the translated values are provided at substantially the same time. Some embodiments of the global interface  155  modify timing of a vertical synchronization signal (VSYNC) of one or more of the interfaces  145 ,  150  to align the vertical synchronization signals of the interfaces  145 ,  150 , as discussed herein. 
       FIG. 2  illustrates an example cross-section view  200  of the electronic device  105  as mounted on the head  202  of a user in accordance with at least one embodiment of the present disclosure. As illustrated, the electronic device  105  includes a housing  204  that includes the high-resolution displays  125 ,  135  and the low-resolution displays  130 ,  140  that form the hybrid display  120 . The electronic device  105  also includes eyepiece lenses  206  and  208  disposed in corresponding apertures or other openings in the user-facing surface  115  of the housing  204 . The electronic device  105  further includes the hybrid display  120  disposed distal to the eyepiece lenses  206  and  208  within the housing  204 . The eyepiece lens  206  is aligned with the high-resolution display  125  and the low-resolution display  130  shown in  FIG. 1 , while the eyepiece lens  208  is aligned with the high-resolution display  135  and the low-resolution display  140  shown in  FIG. 1 . 
     In a stereoscopic display mode, imagery may be displayed by the combination of the high-resolution display  125  and the low-resolution display  130  and viewed by the user&#39;s left eye via the eyepiece lens  206 . Imagery may be concurrently displayed by the combination of the high-resolution display  135  and the low-resolution display  140  and viewed by the user&#39;s right eye via the eyepiece lens  208 . Some embodiments of the high-resolution displays  125 ,  135  may be fabricated to include a bezel (not shown in  FIG. 2 ) that encompasses an outer edge of the high-resolution displays  125 ,  135 . The lenses  206 ,  208  or other optical devices may therefore be used to combine the images produced by the displays  125 ,  130 ,  135 ,  140  so that bezels around the high-resolution displays  125 ,  135  are not seen by the user. Instead, lenses  206 ,  208  merge the images to appear continuous across boundaries between the displays  125 ,  130 ,  135 ,  140 . 
     In some embodiments, some or all of the electronic components that control and support the operation of the hybrid display  120  and other components of the electronic device  105  may be implemented within the housing  204 . For example, the housing  204  may incorporate a global interface  210 , high-resolution interfaces  212  and  214 , and low-resolution interfaces  216  and  218 . As discussed herein, the global interface  210  receives global commands  220  and translates or synchronizes the commands to form interface-specific commands that are provided to the interfaces  212 ,  214 ,  216 ,  218 . Although the components  210 ,  212 ,  214 ,  216 ,  218  are depicted as monolithic blocks for ease of illustration, it will be appreciated that these electronic components may be implemented either as a single package or component, or as a set of discrete, interconnected electronic components. Moreover, in some embodiments, some or all of these electronic components may be implemented remote to the housing  204 . To illustrate, the processing components of the display system may be implemented in a separate device, such as a tablet computer, notebook computer, desktop computer, compute-enabled cellphone, and which is connected to a HMD incorporating the hybrid display  120  via one or more wireless or wired connections. 
       FIG. 3  is a block diagram of a processing system  300  that is used to generate display imagery using a hybrid display that includes high resolution and low-resolution displays in accordance with at least one embodiment of the present disclosure. The processing system  300  includes at least one processing unit  305  that generates signals representative of images for display, such as stereoscopic images for display to the left eye and the right eye of a user. The processing unit  305  may be implemented as a central processing unit (CPU), a graphics processing unit (GPU), one or more processor cores implemented in the CPU or GPU, or other processing device. In the illustrated embodiment, the processing unit  305  generates signals representative of images that are to be displayed using a hybrid display formed of four separate displays: a high-resolution display  310  and a low-resolution display  315  that are combined to provide images to a left eye of a user and a high-resolution display  320  and a low-resolution display  325  that are combined to provide images to a right eye of the user. Thus, the processing unit  305  includes four ports  330 ,  331 ,  332 ,  333  (referred to collectively as “the ports  330 - 333 ”) for providing signals associated with the corresponding displays  310 ,  315 ,  320 ,  325 . The ports  330 - 333  may be physical ports or logical ports. 
     The processing system  300  includes an interface  335  that receives signals from the processing unit  305  at corresponding ports  340 ,  341 ,  342 ,  343  (referred to collectively as “the ports  340 - 343 ”). The ports  340 - 343  may be physical ports or logical ports. The interface  335  combines the signals received at the ports  340 - 343  so that they may be transmitted over a single line such as a fiber-optic cable  345  that is coupled to a port  346 . Some embodiments of the interface  335  include a processing unit and a transceiver that are configured to perform a multiplexing operation to combine the signals received at the ports  340 - 343  and transmit the multiplexed signal over the fiber-optic cable  345 . 
     The fiber-optic cable  345  is also connected to a port  347  of an interface  350 . Some embodiments of the interface  350  include a global interface such as the global interface  155  shown in  FIG. 1 , one or more high-resolution interfaces such as the high-resolution interface  145  shown in  FIG. 1 , and one or more low-resolution interfaces such as the low-resolution interface  150  shown in  FIG. 1 . The interface  350  may therefore receive signals including global commands generated by the processing unit  305 . As discussed herein, the global commands may be translated into interface-specific commands, which may be synchronized to reduce visual artifacts in the images displayed by the displays  310 ,  315 ,  320 ,  325 . 
       FIG. 4  is a block diagram of a display system  400  including a global command interface  405  that implements a finite state machine  410  in accordance with at least one embodiment of the present disclosure. Although the finite state machine  410  is depicted as an integral component of the global command interface  405 , some embodiments may implement the finite state machine  410  external to the global command interface  405 . The global command interface  405  is configured to receive signals  415  including global commands that are used to control the output of the displays  420 ,  421 ,  422 ,  423  (referred to collectively as “the displays  420 - 423 ”), which include high-resolution displays  420 ,  422  and low-resolution displays  421 ,  423 . In the interest of clarity, the displays  420 - 423  are depicted side-by-side in  FIG. 4 . However, the displays  420 - 423  are intended to implement a hybrid display such as the hybrid display  120  shown in  FIG. 1 . For example, the high-resolution display  420  may be combined or overlaid with the low-resolution display  421  to provide images to the left eye of a user. The high-resolution display  422  may be combined or overlaid with the low-resolution display  423  to provide images to the right eye of a user. 
     Each of the displays  420 - 423  receives data and control signals via a corresponding one of interfaces  425 ,  426 ,  427 ,  428  (collectively referred to as “the interfaces  425 - 428 ”). In the illustrated embodiment, the high-resolution displays  420 ,  422  are controlled by signals received from the high-resolution interfaces  425 ,  427  and the low-resolution displays  421 ,  423  are controlled by signals received from the low-resolution interfaces  426 ,  428 . The interfaces  425 - 428  are connected to corresponding sets of registers  430 ,  431 ,  432 ,  433  (collectively referred to as “the register sets  430 - 433 ”) that store values that are used to determine the signals provided to the displays  420 - 423 . For example, the register sets  430 - 433  may be used to store luma values that are used to determine a bias current or voltage that is applied to pixels in the displays  420 - 423  in response to input signals representative of the luma, e.g., input signals received from the global command interface  405 . The register sets  430 - 433  may also be used to store values of chroma that control the bias currents or voltages applied to pixels in the displays  420 - 423  in response to input signals representative of chroma, values of color matrices that are used to transform luma and chroma to RGB values used by the displays  420 - 423 , values of gamma that determine a power law relationship between an input value of luma and a gamma-corrected value of luma used for encoding, values of color coordinates, or values of a color gamut. Some embodiments of the register sets  430 - 433  store values of timing parameters that are used to determine timing of the signals provided to the interfaces  425 - 428  or transmitted by the interfaces  425 - 428 . For example, the register sets  430 - 433  may store values that specify timing of a horizontal synchronization signal that separates scan lines in the displays  420 - 423 , a vertical synchronization signal that separates fields in the displays  420 - 423 , pixel scan directions, a blanking interval during which no data is transmitted from the interfaces  425 - 428  to the corresponding displays  420 - 423 , and the like. 
     The displays  420 - 423  are composed of pixels  435  formed of display elements such as light emitting diodes (LEDs) or organic light emitting diodes (OLEDs). In the interest of clarity only one of the pixels  435  is indicated by a reference numeral. The pixels  435  in the displays  420 - 423  are selectively activated though respective column drivers  440 ,  441 ,  442 ,  443  (collectively referred to as “the column drivers  440 - 443 ”) and row drivers  445 ,  446 ,  447 ,  448  (collectively referred to as “the row drivers  445 - 448 ”). The interfaces  425 - 428  provide control signals to the corresponding column drivers  440 - 443  and row drivers  445 - 448 . For example, the interface  425  may provide a row select indicator to the row driver  445  indicating a row of the display  421  based on a row position of a received pixel row and provide pixel row data to the column driver  440  representing the pixel values of the pixels in the received pixel row. The row driver  445  and the column driver  440  then control their respective outputs to the display  421  based on these inputs so as to selectively activate pixels in the corresponding row of the display  421  so as to display a representation of the pixel row at that row of display  421 . 
     Global commands include values (or modifications to values) of image parameters that are intended to produce a predetermined modifications in observed images produced by the pixels in the displays  420 - 423 . Conventional global commands assume that providing the same value (or the same modification to the value) of the image parameters will produce the same modification in the observed images produced by the pixels in all of the displays  420 - 423 . This assumption does not necessarily hold true in a hybrid display that includes both high-resolution displays  420 ,  422  and low-resolution displays  421 ,  423  because of the different physical characteristics of these displays. For example, configuring the interfaces  425 - 428  with a single value of luma included in a global command may result in different bias currents or voltages being applied to the pixels in the high-resolution displays  420 ,  422  and low-resolution displays  421 ,  423 , which may result in a different observed brightness or change in the observed brightness in the different displays  420 - 423 . 
     The finite state machine  410  may therefore translate the parameters in global commands based on the different physical characteristics of the displays  420 - 423 . For example, gamma correction (or gamma encoding and decoding) may be used to compress the number of bits needed to encode an image by taking advantage of the non-linear response of the human eye to light in color. A power law relation between an input value (such as luminance) and an output value is set by the value of γ:
 
 V   out   =AV   in   γ 
 
The input value a parameter is provided by the global command and the output value is used to encode the parameter. However, the high-resolution displays  420 ,  422  may have a value of γ hi  that differs from the value of γ lo  associate with the low-resolution displays  421 ,  423 . The finite state machine  410  may therefore translate the global command to interface-specific commands to account for the differences between the gamma values used by the different displays  420 - 423 . For example, the finite state machine  410  may modify values of γ hi  or γ lo  so that the interface-specific commands generated based on a global command produce a predetermined modification in the observed images produced by the pixels in all of the displays  420 - 423 . The finite state machine  410  may also translate the global command to correspond to the format used by the corresponding interfaces  425 - 428 . For example, the finite state machine  410  may transmit the format of the global command to formats used by an inter-integrated circuit (I 2 C), a Mobile Industry Processor Interface (MIPI), an embedded display port (eDP) interface, or a mobile display digital interface (MDDI), depending on the type of interface.
 
     The global command interface  405  provides the interface-specific commands to the interfaces  425 - 428  during blanking intervals that occur between the refresh periods during which the corresponding interfaces  425 - 428  refresh the signal applied to the pixels  435  to reflect any changes in the image. The blanking intervals are defined by a vertical synchronization signal (VSYNC). For example, a blanking interval may begin with the rising edge of the VSYNC signal and may end with the falling edge of the VSYNC signal. Alternatively, the blanking interval may begin with the falling edge of the VSYNC signal and end with the rising edge of the VSYNC signal, depending on the state of a polarity signal. However, differences in the control path speeds of the interfaces  425 - 428  and different amounts of overhead in the data packets that include the interface-specific commands transmitted to the interfaces  425 - 428  can introduce timing delays between the VSYNC signals so that the blanking intervals for the different interfaces  425 - 428  are out of synchronization. 
     A register  450  includes information indicating the relative timing delays between the high-resolution interfaces  425 ,  427  and the low-resolution interfaces  426 ,  428 . The finite state machine  410  may access the information in the register  450  and use this information to account for the timing delays when scheduling transmission of the interface-specific commands to the interfaces  425 - 428 . For example, the finite state machine  410  may provide the interface-specific commands to first-in-first-out (FIFO) buffers  451 ,  452 ,  453 ,  454  (collectively referred to herein as “the FIFO buffers  451 - 454 ”) at times that are determined based on the relative timing delay so that this information is available to configure the interfaces  425 - 428  during the corresponding blanking intervals. Configuring the interfaces  425 - 428  may include updating or modifying timing information in the register sets  430 - 433  using information included in the interface-specific commands. The finite state machine  410  may also use the relative timing delays indicated by the value of the register  450  to synchronize the blanking intervals by selectively delaying one or more VSYNC signals utilized by one or more of the interfaces  425 - 428 . 
       FIG. 5  illustrates VSYNC signals utilized by a high-resolution display and a low-resolution display that form a hybrid display in accordance with at least one embodiment of the present disclosure. The VSYNC signal  505  is provided to the high-resolution display and the VSYNC signal  510  is provided to the low-resolution display. In the illustrated embodiment, a falling edge of the VSYNC signals  505 ,  510  indicates the beginning of a refresh interval such as the refresh interval  515 . The pixels in the displays are refreshed during the refresh interval  515  based on signals provided by the corresponding interfaces and values of configuration registers such as the register sets  430 - 433  shown in  FIG. 4 . A rising edge of the VSYNC signals  505 ,  510  indicates the beginning of a blanking interval such as the blanking interval  520 . No data is transmitted from the interfaces to the corresponding displays during the blanking interval  520 . The blanking interval  520  may therefore be used to update the configuration of the interfaces to the displays. For example, configuration information provided to the interfaces  425 - 428  by the global command interface  405  shown in  FIG. 4  may be used to reconfigure the interfaces  425 - 428  during the blanking interval  520 , e.g., by storing updated or modified information in the register sets  430 - 433 . 
     The VSYNC signals  505 ,  510  are out of synchronization. In the illustrated embodiment, the VSYNC signal  505  is delayed relative to the VSYNC signal  510  by a relative delay  525 . A finite state machine (such as the finite state machine  410  shown in  FIG. 4 ) may therefore synchronize operation of the high-resolution and low-resolution displays by modifying one or more of the VSYNC signals  505 ,  510 . In the illustrated embodiment, the finite state machine determines a value of the relative delay  525  (e.g. by accessing the register  450  shown in  FIG. 4 ) and then delays the VSYNC signal  510  by a delay interval  535  to form the delayed VSYNC signal  515 . The interface-specific commands that are generated based on a global command may be used to configure the corresponding interfaces of the high-resolution display and the low-resolution display during the synchronized blanking intervals  540 ,  545 . Synchronizing the interface-specific commands may reduce flicker caused by applying a global command to both the high-resolution display and the low-resolution display in a hybrid display. 
       FIG. 6  is a block diagram of a hybrid display  600  including a low-resolution display  605  and a high-resolution display  610  in accordance with at least one embodiment of the present disclosure. The low-resolution display  605  may be used to implement some embodiments of the low-resolution displays  130 ,  140  shown in  FIG. 1  and the high-resolution display  610  may be used to implement some embodiments of the high-resolution displays  125 ,  135  shown in  FIG. 1 . Pixels in the low-resolution display  605  and the high-resolution display  610  are refreshed in a sequence indicated by scan lines such as the scan line  615  at the top of the low-resolution display  605 . The arrow in the scan line  615  indicates that the pixels are refreshed from left to right. Separate scan lines in the illustrated embodiment are refreshed from top to bottom, as indicated by the dotted arrow  620 . However, some embodiments of the hybrid display  600  may use other scan patterns including refreshing the scan lines from right to left or bottom to top. 
     The scan lines in the low-resolution display  605  are interrupted by scan lines in the high-resolution display  610 . For example, the scan line  625  in the low-resolution display  605  is interrupted by the scan line  630  in the high-resolution display  610 , before resuming as the scan line  635  and the low-resolution display  605 . Furthermore, the low-resolution display  605  has a lower pixel density than the high-resolution display  610  and, consequently, the density of scan lines is lower in the low-resolution display  605  than the high-resolution display  610 . For example, there are three scan lines in the high-resolution display  610  for each scan line in the low-resolution display  605 . 
     Absence of coordination between the timing used to refresh the scan lines in the low-resolution display  605  and the high-resolution display  610  may generate flicker in the resulting image, particularly when executing global commands. A finite state machine associated with a global command interface (such as finite state machine  410  associated with the global command interface  405  shown in  FIG. 4 ) may therefore generate, in response to receiving a global command, interface-specific commands to configure the high and low-resolution interfaces to coordinate the timing used to refresh the scan lines in the two displays  605 ,  610 . Some embodiments of the interface-specific commands include information to configure or modify values indicating timing parameters such as a timing of a horizontal synchronization signal that separates scan lines in the displays, pixel scan directions, and the like. For example, the interface-specific commands may include values that configure the high-resolution display  610  to begin refreshing the scan line  630  at substantially the same time that the low-resolution display  605  finishes refreshing the scan line  625 . The low-resolution display  605  may also be configured to begin refreshing the scan line  635  at substantially the same time that the high-resolution display  610  finishes refreshing the scan line  630 . Timing parameters may also be configured or modified so that scan lines in the high-resolution display  610  are refreshed at a higher frequency than the scan lines in the low-resolution display  605  to account for the different pixel densities or scan line densities. The values may be stored in registers such as the register sets  430 - 433  shown in  FIG. 4  during a blanking interval, as discussed herein. 
       FIG. 7  is an XY chromaticity diagram  700  that indicates a color gamut  705  of a high-resolution display and a color gamut  710  of a low-resolution display in accordance with at least one embodiment of the present disclosure. The vertical axis indicates the y-coordinate in the XYZ color system established by the International Commission on Illumination (CIE) and the horizontal axis indicates the x-coordinate in the XYZ color system. The values of these coordinates determine the RGB values of the colors that can be displayed by the corresponding displays. For example, the color gamut  705  may correspond to 80% of the standard RGB (sRGB) color range and 75% of the National Television System Committee (NTSC) standard color range. Although the XY chromaticity diagram  700  shown in  FIG. 7  is depicted in black and white, persons of ordinary skill in the art should appreciate that each point in the XY chromaticity diagram  700  corresponds to a different color with greener colors being found in the upper left, bluer colors in the lower left, and redder colors to the right of the XY chromaticity diagram  700 . 
     The different shapes of the color gamut  705  and the color gamut  710  indicate that the high-resolution display and the low-resolution display are capable of displaying different ranges (or gamuts) of colors. Consequently, a global command that indicates one or more colors (or one or more modifications to colors) that are to be displayed by the high-resolution display and the low-resolution display may result in different colors in the observed images produced by the high-resolution display and the low-resolution display. A finite state machine associated with a global command interface (such as finite state machine  410  associated with the global command interface  405  shown in  FIG. 4 ) may therefore translate the values in the global command to interface-specific commands based on the color gamut  705  and the color gamut  710 . For example, the finite state machine may apply a filter to the values in the global command to generate interface-specific commands that control the high-resolution display to account for the larger color gamut  705  reproducible by the high-resolution display. Applying the filter may reduce differences in the colors in the observed image produced by the high-resolution display and the low-resolution display in response to the global command. 
       FIG. 8  is a flow diagram of a method  800  of providing global commands to interfaces of a high-resolution display and a low-resolution display in a hybrid display in accordance with at least one embodiment of the present disclosure. The method  800  may be implemented in some embodiments of the display system  100  shown in  FIG. 1 , the display system as shown via cross-section view  200  in  FIG. 2 , the processing system  300  shown in  FIG. 3 , or the display system  400  shown in  FIG. 4 . 
     At block  805 , a global command interface receives a global command that indicates values (or modifications to values) of pixels in one or more high-resolution displays and one or more low-resolution displays of a hybrid display. At block  810 , a finite state machine associated with the global command interface translates the global command to interface-specific commands. Translation of the global command to the interface-specific commands may include modifying values of parameters that control the displays based on the characteristics of the interfaces to the high-resolution displays and the low-resolution displays, modifying the format of the global command to correspond to the formats used by the interfaces to the high-resolution displays and the low-resolution displays, and the like. 
     At block  815 , the finite state machine accesses delay information that indicates relative timing delays between the high-resolution displays and the low-resolution displays. For example, the delay information may indicate a relative delay between VSYNC signals that define refresh intervals and blanking intervals for the displays. Some embodiments of the finite state machine access the delay information from an associated register. At block  820 , the global command interface provides the interface-specific commands to the interfaces for the high-resolution displays and the low-resolution displays at times determined by the delay information. For example, the global command interface may delay (or may instruct the high-resolution display interface or the low-resolution display interface to delay) one or more of the VSYNC signals to synchronize the blanking intervals for the different interfaces. The global command interface may then provide the interface-specific commands to FIFO buffers associated with the different interfaces so that they are available to configure the interfaces during the blanking interval. 
     At block  825 , register sets associated with the different interfaces are updated based on the interface-specific commands. For example, values of the registers (or modifications to the values of the registers) may be written from the FIFO buffers to the corresponding register sets to reconfigure the different interfaces. Once the blanking interval ends, the updated or modified values in the register sets are used to control the image that is produced during the subsequent refresh interval. 
     Much of the inventive functionality and many of the inventive principles described above are well suited for implementation with or in integrated circuits (ICs) such as application specific ICs (ASICs). It is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such ICs with minimal experimentation. Therefore, in the interest of brevity and minimization of any risk of obscuring the principles and concepts according to the present disclosure, further discussion of such software and ICs, if any, will be limited to the essentials with respect to the principles and concepts within the preferred embodiments. 
     In this document, relational terms such as first and second, and the like, may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element preceded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. The term “another”, as used herein, is defined as at least a second or more. The terms “including” and/or “having”, as used herein, are defined as comprising. The term “coupled”, as used herein with reference to electro-optical technology, is defined as connected, although not necessarily directly, and not necessarily mechanically. The term “program”, as used herein, is defined as a sequence of instructions designed for execution on a computer system. A “program”, or “computer program”, may include a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a computer system. 
     The specification and drawings should be considered as examples only, and the scope of the disclosure is accordingly intended to be limited only by the following claims and equivalents thereof. Note that not all of the activities or elements described above in the general description are required, that a portion of a specific activity or device may not be required, and that one or more further activities may be performed, or elements included, in addition to those described. Still further, the order in which activities are listed are not necessarily the order in which they are performed. The steps of the flowcharts depicted above can be in any order unless specified otherwise, and steps may be eliminated, repeated, and/or added, depending on the implementation. Also, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present disclosure as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present disclosure. 
     Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims.