Patent Publication Number: US-11663938-B2

Title: Viewing device and method for providing virtual content overlapping visual objects

Description:
This application is a 35 U.S.C. § 371 national phase filing of International Application No. PCT/EP2018/077350, filed Oct. 8, 2018, the disclosure of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a viewing device and a method, and in particular to a viewing device and a method for providing virtual content overlapping visual objects that is easy to perceive. 
     BACKGROUND 
     Virtual Reality (VR) systems are growing in popularity. Similarly so-called mixed or augmented reality systems are also growing in popularity. An augmented reality system will, herein, be defined as a system where virtual content is displayed as overlapping (physical) real-life visual objects. There will thus not be made any difference between augmented reality, mixed reality or enhanced reality for the context of this application and the problems and their solutions discussed herein applies to all Optical See-Through (OST) devices. 
     As the virtual content of augmented reality (AR) systems is displayed as overlapping visual content, there may be circumstances where the virtual content is difficult to see or perceive.  FIG.  1    shows an example of such a situation. In  FIG.  1   , there are two areas where virtual content (DVC) is to be displayed. As can be seen in  FIG.  1   , it is difficult to clearly see the virtual content when it overlaps some visual objects. In  FIG.  1   , as well as in  FIGS.  4  and  5   , white (or light colored) text is illustrated as outlined text in order to avoid having to use greyscale objects in the drawings. 
     Proposed prior art solutions have been focused on analyzing the background and blocking out disturbing objects in the background to render the virtual content easy to perceive. 
     However, such solutions require a lot of processing power, and usually also costly peripherals to be used. Furthermore, as the background changes, the manipulation of the background must also change which is distracting to a user. 
     There is thus a need for a viewing device and a method for providing virtual content that is easy to perceive. 
     SUMMARY 
     An object of the present teachings is to overcome or at least reduce or mitigate the problems discussed in the background section. 
     According to one aspect a viewing device is provided, the viewing device comprising a controller, a transparent display and a visual shutter, wherein the controller is configured to: determine a color of virtual content to be displayed; determine a background color contrasting the virtual content displayed; determine a display area corresponding to where the virtual content is to be displayed; cause the visual shutter to enable a background to the virtual content to be displayed, the background having the contrasting background color thereby providing a contrasting background to the displayed virtual content; and to cause the transparent display to display virtual content for overlapping at least one visual object perceivable through the transparent display at least partially onto the background, wherein the visual shutter is configured to enable the background by operating in a mode wherein at least a portion of the visual shutter is blocked so as to obstruct the visual object in the blocked portion of the visual shutter. 
     This has the benefit that by selecting the color of the virtual content and the color of the background enabled by the visual shutter, the visual shutter blocking out the background view of visual objects, as opposed to selecting what portions of the background provided by the visual objects seen through the viewing device that should be blocked, a contrast for the virtual content can be achieved without substantive processing, and without having the blocked-out portions change according to changes in the background view, giving a more stable view to observe for the user. The color of the virtual content, the area where the virtual content is to be displayed, and the color possible to achieve by the blocked-out portion are all very easy to determine for the controller, and thus requires a minimum of processing utilizing an ingeniously simple manner of providing a contrast for the virtual content. 
     In one embodiment the color of the virtual content to be displayed is determined by the controller being further configured to select it based on a blocked color of the visual shutter. 
     This has the benefit that the virtual content is ensured to be perceived by selecting a color to display it in, which color contrasts the background of the blocked-out section, thereby providing an improved readability (as in perceivability) of the virtual content without substantive processing in a ingeniously simple manner. 
     In one embodiment the color of the background is determined by the controller being further configured to select it based on the color of the virtual content to be displayed. 
     This has the benefit that the virtual content is ensured to be perceived by selecting a color for the background to display the virtual content on, i.e. the blocked-out section, which color contrasts the virtual content, thereby providing an improved readability (as in perceivability) of the virtual content without substantive processing in a ingeniously simple manner. 
     In one embodiment the controller is further configured to provide said contrasting color of said background by causing the transparent display to display the background overlapping the blocked portion of the visual shutter along with the displayed virtual content. 
     This has the benefit that the selection of background color is not limited to the color of the visual shutter. 
     In one embodiment the visual shutter comprises a PN-LC display comprising at least one PN-LC cell, wherein the visual shutter is configured to provide a blocked-out portion perceivable as white onto which the virtual content is to be displayed. This provides a blocked-out portion of a white color, thereby facilitating other colors to be used easily as background colors. 
     In one embodiment the visual shutter comprises a color liquid crystal display, wherein the visual shutter is configured to provide a blocked-out portion perceivable as a selected color onto which the virtual content is to be displayed. This can provide a filtering effect and/or for providing a background of a selected color. 
     In one embodiment the visual shutter comprises a color filter, wherein the visual shutter is configured to provide a blocked-out portion perceivable as a color of the color filter onto which the virtual content is to be displayed. 
     In one embodiment the transparent display is a self-illuminating transparent display configured to display visual content through the use of an internal light source. 
     This enables for use of visual shutters that has a color perceivable as dark, which greatly absorbs the incoming illumination, while still providing backgrounds of other colors and for being able to see the virtual content being displayed. 
     In one embodiment the visual shutter is a Liquid Crystal Display, wherein the visual shutter is configured to provide a blocked-out portion perceivable as black onto which the virtual content is to be displayed. 
     In one embodiment the visual shutter comprises at least one PN-LC cell and at least one LCD, wherein the visual shutter is configured to provide a blocked-out portion perceivable as partially white, partially black and/or in a gray scale onto which the virtual content is to be displayed. 
     In one embodiment the transparent display comprises a projector device and a partially transparent mirror, wherein the projector device is arranged to project the content to be displayed onto the partially transparent mirror and the partially transparent mirror is arranged to reflect the displayed virtual content and to enable visual objects to be perceivable through the partially transparent mirror, the displayed virtual content thereby being perceived as overlapping the visual objects. 
     In one embodiment the transparent display comprises a plurality of wave guides. 
     According to one aspect a method for use in a viewing device is provided, the method comprises determining a color of virtual content to be displayed; determining a background color contrasting the virtual content displayed; determining a display area corresponding to where the virtual content is to be displayed; causing the visual shutter to enable a background to the virtual content to be displayed, the background having the contrasting background color thereby providing a contrasting background to the displayed virtual content; and causing the transparent display to display virtual content for overlapping at least one visual object perceivable through the transparent display at least partially onto the background, wherein the method comprises enabling the background by operating the visual shutter in a mode wherein at least a portion of the visual shutter is blocked so as to obstruct the visual object in the blocked portion of the visual shutter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will be described in the following, reference being made to the appended drawings which illustrate non-limiting examples of how the inventive concept can be reduced into practice. 
         FIG.  1    shows a schematic view of the problem of how virtual content being displayed overlapping a visual background of low contrast or being cluttered; 
         FIG.  2    shows a schematic view of a viewing device according to an embodiment of the present invention; 
         FIG.  3    shows a schematic view of an exemplary view seen through a viewing device according to  FIG.  2    according to one embodiment of the teachings herein; 
         FIG.  4    shows a schematic view of a viewing device according to an embodiment of the present invention; 
         FIG.  5    shows a schematic view of an exemplary view seen through a viewing device according to  FIG.  4    according to one embodiment of the teachings herein; 
         FIG.  6    shows an example of how the displayed virtual content of  FIG.  1    may be rendered perceivable according to one embodiment of the teachings herein; 
         FIG.  7    shows a schematic view of a viewing device  100  according to one embodiment of the teachings herein; 
         FIG.  8    shows a schematic view of a viewing device  100  according to one embodiment of the teachings herein; and 
         FIG.  9    shows a flowchart of a general method according to an embodiment of the teachings herein. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  2    shows a schematic view of a viewing device  100  according to an embodiment of the present invention. The viewing device  100  is a see-through device, where a user looks in through one end, and sees the visual objects present at the other end or in the line of sight at the other end of the viewing device  100 . In one embodiment the viewing device  100  is a head mounted viewing device  100  to be worn by a user (not shown explicitly but represented by the eye E in  FIG.  2   ) for looking through the viewing device  100 . In one such embodiment the viewing device  100  is arranged as glasses to be worn by a user. The viewing device  100  comprises a housing  105  and transparent display arrangement  110 . The transparent display arrangement  110  is configured to be self-illuminating and will be referred to herein as an ITD (Illuminated Transparent Display). An ITD  110 , being self-illuminating, is capable of displaying content using an internal light source  111 , thereby not relying on other light sources to render the displayed content perceivable. The ITD  110  is arranged in the housing  105  so that a visual object VO may be perceived along the line of sight, indicated LS in the figure, or field of view of an eye E looking through the viewing device  100 . 
     A visual shutter  120  VS is also arranged in the housing  105  of the viewing device  100 . The visual shutter  120  is arranged after the ITD  110  in the direction of the line of sight, LS, so that as a real life visual object VO is viewed, any illumination emanating (emitted or reflected) from the visual object VO first travels through the visual shutter  120  and then the ITD  110  before being perceived by the eye E. 
     The ITD  110  is configured to display virtual content DVC to be viewed by the user. As the ITD is transparent and as the ITD  110  is located before the visual shutter  120  in the line of sight, the displayed virtual content DVC will be perceived by a user as overlapping any visual objects VO viewed through the viewing device  100 . The viewing device  100  is thus suitable for use with augmented reality, mixed reality and extended reality systems. For the purpose of this application, augmented reality AR will be considered to be the equivalent and interchangeable with the terminologies of mixed reality and extended reality. Further references will simply be to Augmented Reality (AR), but applies equally to mixed or extended reality. 
     The viewing device  100  further comprises a memory  102  and a controller  101 . The controller is configured to control the ITD to display the virtual content. 
     The memory  102  is configured to store graphics data and computer-readable instructions that when loaded into the controller indicates how the viewing device  100  is to be controlled. The controller  101  is configured to control the overall operation of the viewing device  100 , including the ITD  110  and the visual shutter  120 . In one embodiment, the controller  101  is a graphics controller. In one embodiment, the controller  101  is a general purpose controller. In one embodiment, the controller  101  is a combination of a graphics controller and a general purpose controller. In one embodiment, the controller  101  is configured to receive the virtual content to be displayed. In one embodiment, the controller  101  is configured to generate the virtual content to be displayed. In one embodiment, the controller  101  is configured to receive the virtual content to be displayed. In one embodiment, the controller  101  is configured to receive parts of the virtual content to be displayed and to generate parts of the virtual content to be displayed. 
     In one embodiment the viewing device  100  further comprises a communication interface  103 . The communication interface may be wired and/or wireless. The communication interface may comprise several interfaces. 
     In one embodiment the communication interface comprises a USB (Universal Serial Bus) interface. In one embodiment the communication interface comprises a HDMI (High Definition Multimedia Interface) interface. In one embodiment the communication interface comprises a Display Port interface. In one embodiment the communication interface comprises an Ethernet interface. In one embodiment the communication interface comprises a MIPI (Mobile Industry Processor Interface) interface. 
     In one embodiment the communication interface comprises a radio frequency (RF) communications interface. In one such embodiment the communication interface comprises a Bluetooth™ interface, a WiFi™ interface, a ZigBee™ interface, a RFID™ (Radio frequency IDeintifier) interface, Wireless Display (WiDi) interface, Miracast interface, and/or other RF interface commonly used for short range RF communication. In an alternative or supplemental such embodiment the communication interface comprises a cellular communications interface such as a fifth generation (5G) cellular communication interface, an LTE (Long Term Evolution) interface, a GSM (Global Systéme Mobile) interface and/or other interface commonly used for cellular communication. In one embodiment the communications interface is configured to communicate using the UPnP (Universal Plug n Play) protocol. In one embodiment the communications interface is configured to communicate using the DLNA (Digital Living Network Appliance) protocol. The communications interface  103  is configured to enable the viewing device  100  to receive and/or transmit data and/or instructions regarding the virtual content to be displayed. The communications interface  103  may also be configured to enable the viewing device  100  to communicate with other devices, such as other viewing devices  100 , smartphones (not shown), internet tablets, computer tablets or other computers, media devices, such as television sets, gaming consoles, video viewer or projectors. 
       FIG.  3    shows a schematic view of an exemplary view seen through the viewing device  100 . On the left of the figure, there is shown the ITD  110  and the displayed virtual content DVC being displayed on the ITD  110  and also the current field of view (dashed rectangle) with the visual objects VO currently viewable. In this example view, the visual objects (VO) are exemplified as a box showing a cat, a box showing a car, and a box showing a house. On the right of the figure, the composite of the two is shown, i.e. the view actually seen through the viewing device  100 . As can be seen, the displayed virtual content DVC is perceived as overlapping the real life visual objects VO thereby providing an augmented reality. As can also be seen, when there is little contrast between the visual objects and the virtual content to be displayed, it may be difficult to discern the displayed virtual content. 
     The visual shutter  120  VS is configured to operate in different modes. A first mode is an open mode where light is permitted to travel through the visual shutter  120 . A second mode is a blocking mode where light is blocked and not permitted to travel through the visual shutter  120 . 
     The modes may also be combined so as for example, the visual shutter  120  may be operating to permit light unaffected in one or more portions of the visual shutter  120  and block light entirely in one or more portions. 
     In one embodiment the visual shutter  120  comprises a Liquid Crystal display, commonly referred to as a LCD. Even though the figures only show one LCD  121 , the visual shutter  120  may comprise a plurality of displays  121 . Examples of LCD technologies s are Twisted Neumatic, Different Vertical Alignment (VA), Different In Plane Switching (IPS), different Fringe Field Switching (FFS) or Advanced Fringe Field Switching (AFFS). 
     As is commonly known to a skilled person, an LCD  121  has a resolution determined by the number of and arrangement of cells. 
     In one embodiment, the LCD  121  has a single cell corresponding to a main portion of the display surface. In one embodiment, the LCD  121  has two cells, each cell corresponding to half (upper/lower, or right/left) of a main portion of the display surface. 
     In one embodiment, the LCD  121  has a plurality of cells, each cell corresponding to a section of a main portion of the display surface. 
     In one embodiment the LCD  121  has a resolution corresponding to the resolution of the ITD  110 . 
     In one embodiment the LCD  121  has a resolution corresponding to the ratio of the ITD  110 . In one embodiment the LCD  121  has a resolution corresponding to a multiple of the ratio of the ITD  110 . In such embodiments, if the ITD  110  has a ratio of for example 16:9, the LCD  121  will have cells arranged in 16 columns and 9 rows, or a multiple thereof up to and including the resolution of the ITD  110 . In embodiments where the cell arrangement is based on the ratio or resolution of the ITD  110 , the cells may have a uniform shape. 
     The cells may be activated by the controller  101  or by a controller of the visual shutter (which is assumed to be part of the controller  101  for the purpose of this application), in group or one-by-one, to provide a blocked-out section  125 . In the simplest arrangement, each cell of the LCD  121  corresponds to one blocked-out section  125 . 
     In one embodiment, the LCD  121  is arranged to provide segments of a dark or black color. The visual shutter  120  is thus configured to provide a blocked-out portion  125  perceivable as black onto which the virtual content is to be displayed. Examples of such LCDs  121  are TN, VA, IPS or AFFS LCDs. 
     In one embodiment, the visual shutter  120  comprises a Polymer Network Liquid Crystal PNLC layer, herein referred to as a PNLC Display  121 . In such an embodiment the LCD  121  may be arranged to provide one or several, depending on the cell arrangement, blocked-out sections  125  of a light or (milky) white color. The visual shutter  120  is thus configured to provide a blocked-out portion  125  perceivable as white onto which the virtual content is to be displayed. 
     In one embodiment the visual shutter  120  comprises a color LCD  121 . The color LCD  121  may be utilized to provide a color filtering of the visual content perceived through the viewing device  100 . 
     In an embodiment where the visual shutter comprises a PNLCD  121  and a color LCD  121 , the visual shutter may be arranged to provide a blocked-out portion  125  of any color selected (within the capabilities of the color LCD  121 ). The visual shutter  120  is thus configured to provide a blocked-out portion  125  perceivable as any selected color onto which the virtual content is to be displayed. 
     In one embodiment, the visual shutter comprises a color filter having a color, whereby the visual shutter is configured to provide a blocked-out portion in the color of the filter. 
       FIG.  4    shows a schematic view of a viewing device  100  according to an embodiment of the present invention. In  FIG.  4   , the visual shutter  120  is operating to partially block light in a portion of the visual shutter  120 . This may be done by operating the visual shutter in an open mode in some portions and in a blocked mode in other portions, i.e. the portion(s) to be blocked. In the example of  FIG.  4   , the visual shutter  120  is operating to block light in a lower part of the visual shutter  120 , whilst permitting light to travel through in an upper part of the visual shutter  120 . It should be noted that the visual shutter  120  may be configured to operate in different portions than the portions shown as examples in  FIG.  4   , and as a skilled person would understand, any portion would be possible to open, block or partially block (as in decreasing the intensity). 
       FIG.  5    shows a schematic view of an exemplary view seen through the viewing device  100 . On the left of the figure, there is shown the ITD  110  and the displayed virtual content DVC being displayed on the ITD  110  along with the visual shutter  120  with a portion  125  being operated in a blocked mode, as indicated by that portion or section being shown as blacked out. As can be seen the displayed virtual content is in different colors making some of the displayed virtual content difficult to discern. On the right of the figure, the composite of the two is shown, i.e. the see-through view provided by the combination of the ITD  110  and the visual shutter  120 . As can be seen, the displayed virtual content DVC is seen more clearly as the contrast to the background is dramatically changed through the operation of the visual shutter  120 . All displayed virtual content can now be clearly seen by the viewer. 
     Returning to  FIG.  1    showing the problem of how virtual content being displayed overlapping a visual background of low contrast or being cluttered, it will now be disclosed how a viewing device  100  according to the teachings herein is configured to at least partially overcome this problem with simultaneous reference to  FIGS.  1 ,  6  and  9   ,  FIG.  9    showing a schematic flowchart of a general method according to the teachings herein. 
     As can be seen in  FIG.  1   , if virtual content of a certain color is to be displayed onto a background of the same or similar color, it may be difficult for a user to perceive the content. The same applies if the background is cluttered or busy. Likewise, backgrounds having moving objects may also be distracting and make it more difficult to perceive the virtual content to be displayed. 
     The inventors have realized through inventive thinking and reasoning that it is possible to display content that will always be easily perceivable while saving on processing power not having to analyze any backgrounds as regards color, cluttering, movable objects and so on, by instead focusing on the properties of the content to be displayed. 
     As the content to be displayed is already known to the controller  101  of the viewing device  100 , as it is received or even generated by the viewing device  100 , the analysis becomes easy and uncomplicated not requiring much processing power. In an embodiment where the content to be displayed is received by the viewing device, the results of the analysis may also be received along with the content, or data indicating the content. 
     One main aspect of virtual content to be displayed is the color. The viewing device  100  therefore determines what color(s) the virtual content is to be displayed in. The determination may be performed as retrieving color information from the virtual content, or by selecting what color the virtual content should be displayed in. 
     The selected color for the virtual content may be selected based on the type of visual shutter being used. In the case of a visual shutter  120  providing backgrounds that are perceived to be black, such as utilizing a TN LCD (or other LCD technology) based visual shutter, the selected color for the virtual content may be chosen to be a light color, such as white, yellow, light blue or light green, thereby contrasting the background and rendering the virtual content to be displayed easily perceivable. In the case of a visual shutter  120  providing backgrounds that are perceived to be black, such as a TN LCD based visual shutter, the selected color may be chosen to be a dark color, such as black, dark blue, dark green or red, thereby contrasting the background and rendering the virtual content to be displayed easily perceivable. In such embodiments the background color is implicitly determined by the shutter, or rather the perceived color of a blocked-out portion. It should be noted that even though all examples herein are focused on text of a uniform color, a skilled person would understand that the teachings herein also apply to other forms of virtual content and to a mixture of colors. 
     In an embodiment where the selected color is the color received for the virtual content to be displayed and where the visual shutter comprises a color LCD  121 , the controller  121  may be configured to select the background color based on the virtual content color and adapt the color LCD  121  accordingly. 
     Examples of contrasting colors are red contrasting green, black contrasting white, blue contrasting yellow and orange contrasting purple. As would be understood, any colors not adjacent on a color wheel could be used as contrasting colors. 
     In embodiments where the ITD  110  is self-illuminating, there is an added benefit in that the ITD  110  is capable of providing a background even when the light originating from the visual objects is blocked, the blocked sections appearing to be black. If the ITD  110  is not self-illuminating any blocked portions would also not allow light to permeate whereby any content displayed in those blocked portions would not be illuminated and thus also not being perceivable to a user. In some embodiments, the ITD  110  is therefore configured to provide a background of a color different from that of the visual shutter. In such embodiments the ITD  110  provides the background color and it may be determined by the controller. As the visual shutter, blocks the light coming from behind (i.e from the visual objects in the viewing direction), the visual shutter  120  has enabled the background to be displayed in the determined color in the blocked portions. 
       FIG.  6    shows an example of how the displayed virtual content of  FIG.  1    may be rendered perceivable according to the teachings herein.  FIG.  6    shows a schematic view of what is perceived through a viewing device  100 , i.e. the composite of the displayed virtual content DVC being displayed by the ITD  110 , the blocked portions of the visual shutter  120  and the visual objects VO. As in  FIG.  1   , there is a dark-colored visual object VO 1  on the left side of the figure and a light-colored visual object VO 2  on the right side of the figure. The colors of the visual objects are schematically indicated by dashed lines of different lengths in the two visual objects. 
     The viewing device  100  determines which color(s) the virtual content is to be displayed in. For the displayed virtual content on the left side, the color is a dark color, in this example black. For the displayed virtual content on the right side, the color is a dark color, in this example white or light grey. 
     The viewing device  100  may also determine a display area which the virtual content will be displayed in. The viewing device  100  causes the visual shutter  120  to enter a blocked mode corresponding to the determined display area. 
     The viewing device  100  also determines a contrasting color for the background. The determination may be done by default to be a color contrasting the perceived color of the visual shutter when operated in a blocked mode. In one such embodiment where the visual shutter blocks light in a certain color, the viewing device  100  is configured to determine that the displayed virtual content is to be displayed in a contrasting color to that perceived for the visual shutter  120  when in a blocked mode. In the example of an TN LCD visual shutter, the perceived color would be black, so white may be chosen as the contrasting color for the background. In the example of a PN-LCD visual shutter, the perceived color would be white, so black may be chosen as the contrasting color for the background. 
     In embodiments where the visual shutter comprises a color LCD  121 , the color LCD  121  may be utilized as a filter for providing a filtered light in the blocked-out section  125 , only blocking out a portion of the light according to the color setting of the color LCD  121 . 
     As mentioned above, the visual shutter  120  comprises a combination of arrays in some embodiments. In one such example, the visual shutter comprises one TN LCD (being an example of a LCD  121  providing a perceived black background) and one PN-LCD (being an example of a LCD  121  providing a perceived white background). This enables the perceived background color to be provided in a gray scale, depending on the settings of the TN LCD array and the PN-LCD array. 
     The viewing device  100  causes the visual shutter  120  to block the display areas, providing the background in the determined background color(s) while displaying the virtual content in the determined content color(s). The ITD  120  may additionally display a supplemental background, supplementing the background provided by the visual shutter  120 . If the visual shutter  120  is only able to provide a blocked background in one color, the ITD  110  is thus able to adapt the background by displaying a specific background. As the ITD  110  is self-illuminating it is not dependent on the light permeating the visual shutter, and can thus display its own image on the blocked areas. Any color of the background may thus be displayed regardless what perceived color the visual shutter  120  provides. It should be noted that a visual shutter blocking light illuminating the visual objects may only be perceived to be black (as no or little light permeates the visual shutter), whereas its true color may in fact be another, such as white, which can be perceived if the visual shutter is illuminated, as may be done by an ITD projecting light. 
     Returning to  FIG.  6   , a white background has been determined for the virtual content to be displayed on the left side, and a black background has been determined for the virtual content to be displayed on the right side. As can be seen, the displayed virtual content DVC now clearly contrasts the background and are easily perceivable over the visual objects. 
     In one embodiment, the viewing device  100  is further configured to display also the background, the visual shutter&#39;s  120  blocked areas  125  then serving to clear up the background from disturbing clutter. In such embodiments, where the ITD  110  is self-illuminating, the displayed background will be clearly visible to a user, regardless of the visual objects as they are being blocked. 
     It should be noted that the number of blocked areas may differ depending on the design of the viewing device  100  and the virtual content to be displayed and is not limited to the examples given herein. The shapes of the blocked areas may also change and vary depending on the design of the viewing device  100  the virtual content to be displayed. 
       FIG.  7    shows a schematic view of a viewing device  100  according to one embodiment of the teachings herein. The viewing device  100  comprises an ITD comprising a projector device  111 , operating as the internal light source, and a partially transparent mirror  112 . Examples of projector devices  111  are, but not restricted to, a DLP (Digital Light Processing) projector or an LED (Light Emitting Diode) or OLED (Organic Light Emitting Diode) micro display. The projector device  111  is arranged to project the content to be displayed onto the partially transparent mirror  112 . The partially transparent mirror  112  is arranged to reflect the displayed virtual content DVC to a user and to enable visual objects VO to be perceivable through the partially transparent mirror. The user will thereby perceive the displayed virtual content DVC as overlapping the visual objects VO. A visual shutter  120  is arranged subsequent the partially transparent mirror  112  in the direction of the line of sight LS, in order to block visual objects to be perceivable through the viewing device. 
       FIG.  8    shows a schematic view of a viewing device  100  according to one embodiment of the teachings herein. The viewing device  100  comprises an ITD  110  comprising a wave guide display  111 , operating as the internal light source. In an embodiment where the waveguide display  111  comprises a single waveguide the wave guide display  111  may be used to display the color of the waveguide. In an embodiment where the waveguide display  111  comprises a plurality of waveguides  111  the wave guide display may be used to display different colors. In the illustrated example the waveguide display  111  comprises three waveguides that are indicated by RGB (Red Green Blue, being a common color denomination for color displays in the field of displays) in the example of  FIG.  6   . The waveguide display  111  is configured to display the displayed virtual content DVC to a user and to enable visual objects VO to be perceivable through the partially transparent mirror. The user will thereby perceive the displayed virtual content DVC as overlapping the visual objects VO. A visual shutter  120  is arranged subsequent the wave guides in the direction of the line of sight LS, in order to block visual objects to be perceivable through the viewing device. 
     In one embodiment the ITD  110  comprises a plurality of waveguide displays  111  or rather a multiple of three waveguides. For the purpose of the teachings herein and for simplicity a waveguide display  111  is considered to comprise three waveguides. In such an embodiment, each waveguide display  111  may be utilized by the controller to provide one focal plane for the virtual content to be displayed. Utilizing more than one waveguide may then provide a three dimensional effect. The ITD  110  may be configured to shift the depth plane of the virtual content being displayed thereby providing the three dimensional effect. 
       FIG.  9    shows a flowchart of a general method according to an embodiment of the teachings herein. The method utilizes a viewing device  100  as taught herein for enhancing the readability of virtual content in the viewing device  100 . The method comprises determining  910  a color of virtual content to be displayed, determining  920  a background color contrasting the virtual content displayed and determining  930  a display area corresponding to where the virtual content is to be displayed. Thereafter the visual shutter is operated  940  to enable a background to the virtual content to be displayed. The background has the contrasting background color, as explained above, and thereby provides a contrasting background to the displayed virtual content DVC. The transparent display  110  is also caused to display virtual content DVC for overlapping at least one visual object VO perceivable through the transparent display  110  at least partially onto the background. Wherein the method comprises enabling the background by operating the visual shutter  120  in a mode wherein at least a portion of the visual shutter  120  is blocked so as to obstruct the visual object VO in the blocked portion  125  of the visual shutter  120 . The readability of the virtual content has thus been improved. Even though the examples herein have been focused on displaying text as virtual content, the teachings herein also apply to displaying other graphics (such as shapes, images and so on) as the virtual content.