Patent Publication Number: US-10317670-B2

Title: MEMS scanning display device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 15/624,477, filed Jun. 15, 2017, which claims priority to U.S. Provisional Patent Application Ser. No. 62/467,086 filed Mar. 3, 2017, the entireties of which are hereby incorporated by reference for all purposes. 
    
    
     BACKGROUND 
     Some display devices employ light source scanning to produce viewable images. In one example, laser light is reflected by a scanning mirror system at different angles to scan the laser across pixels of a projected image. Control of the light color and/or intensity at each pixel allows the image to be projected. 
     SUMMARY 
     Examples are disclosed that related to head-mounted scanning image display systems. In one example, a scanning head-mounted display system includes a light source, a motion sensor, a scanning mirror system configured to scan light from the light source along at least one dimension to form an image, and a controller configured to control the scanning mirror system to scan the light to form the image, receive head motion data from the motion sensor, and adjust one or more of a scan rate and a phase offset between a first frame and a second frame of the image based upon the head motion data. This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of an example MEMS scanning head-mounted display device. 
         FIG. 2  shows an example laser trace diagram. 
         FIG. 3  shows another example laser trace diagram. 
         FIG. 4  shows an example laser die at a first rotational orientation. 
         FIG. 5  shows the example laser die of  FIG. 4  at a second rotational orientation. 
         FIG. 6  shows another example laser trace diagram. 
         FIG. 7  shows another example laser trace diagram. 
         FIG. 8  shows a flow diagram illustrating an example method of dynamically adjusting one or more operating parameters of a scanning head-mounted display device. 
         FIG. 9  shows a block diagram of another example scanning head-mounted display device. 
         FIG. 10  shows a block diagram of another example scanning head-mounted display device. 
         FIG. 11  shows a block diagram of an example computing device. 
     
    
    
     DETAILED DESCRIPTION 
     As described above, some display devices employ light source scanning to produce viewable images. In one example, laser light is reflected by a mirror system at different angles to project reflected laser light throughout a field-of-view (FOV). To achieve a range of reflection angles, a suitable actuator such as a microelectromechanical system (MEMS) actuator may rotate the mirror system. 
     A MEMS actuator may rotate a mirror system in horizontal and vertical directions to produce viewable images in a two-dimensional FOV. To this end, the mirror system may include a single mirror driven in both horizontal and vertical directions, or two mirrors separately driven in horizontal and vertical directions. Different scan rates may be employed in the horizontal and vertical directions. In a two mirror system, for example, a horizontally scanned mirror may be driven at a relatively fast rate (e.g., ˜10 kHz), whereas a vertically scanned mirror may be driven at a relatively slower rate (e.g., ˜60 Hz). The horizontal and vertical scan rates may at least partially determine the resolution of images generated at these rates, along with other factors such as mirror aperture (e.g., diameter) and scan angle. 
     However, current MEMS technology places an upper limit on mirror scan rates, in turn limiting display resolution. As an example, a 27 kHz horizontal scan rate combined with a 60 Hz vertical scan rate may yield a vertical resolution of 720p. Significantly higher vertical resolutions (e.g., 1440p, 2160p) may be desired, particularly for near-eye display implementations, where 720p and similar vertical resolutions may appear blurry and low-resolution. While an increase in the horizontal and/or vertical scan rate would increase display resolution, the former may be technologically infeasible while the latter increases power consumption. Further, high scan rates may at least partially constrain mirror scan angle and aperture, where larger values are also desired. Additionally, supporting higher resolution also may require a larger mirror size due to the diffraction limit associated with smaller “pixel” sizes. The use of such a larger mirror may further increase the difficulties in achieving higher resolutions with scanning displays, as the larger mirror leads to a lower scanning frequency. 
     Examples are thus disclosed for a laser-based MEMS scanning display device configured for high-resolution output. As described below, an interlaced mode of operating multiple lasers may be combined with variable scan rates and/or phase offsets between interlaced frames to achieve desired spacing between laser output, in turn yielding desired image pixel spacing and resolution. The use of multiple lasers allows multiple lines to be scanned per mirror period, thereby allowing higher resolution to be achieved without increasing mirror scan frequencies, and also permits larger mirrors to be used, which may help to avoid issues with pixel size imposed by diffraction limits. Further, examples are disclosed in which output from an eye-tracking sensor is utilized to dynamically alter laser output spacing as a function of user gaze direction. 
       FIG. 1  shows a block diagram of an example display device  100 , shown as a laser-based MEMS scanning display device. Display device  100  comprises a light source  102 , which, as described in further detail below with reference to  FIGS. 4 and 5 , may assume any suitable form (e.g. one or more solid state lasers), and may output light of one or more wavelengths (e.g., light in red, green, and/or blue wavelength ranges). Light source  102  outputs light for reception by a first mirror  104 , which reflects light received from the light source toward a second mirror  106 . First mirror  104  may be configured to scan in a horizontal (e.g., x-axis) direction, while second mirror  106  may be configured to scan in a vertical (e.g., y-axis) direction, so that light is ultimately projected throughout a two-dimensional FOV and forms viewable images. In other examples, the first mirror may scan vertically and the second mirror may scan horizontally. 
       FIG. 1  shows the reflection of light from second mirror  106  toward an output  108  where viewable images are formed. Output  108  may assume any suitable form, such as a display surface, projection optics, waveguide optics, etc. As examples, display device  100  may be configured as a virtual reality head-mounted display (HMD) device with output  108  configured as an opaque surface, or as a mixed reality HMD device with the output configured as a partially transparent surface through which imagery corresponding to the surrounding physical environment can be transmitted and combined with laser light. Display device  100  may assume other suitable forms, such as that of a head-up display, mobile device screen, monitor, television, etc. 
     To enable the generation of desired images, an actuator  110  separately drives first and second mirrors  104  and  106 . In one example, actuator  110  rotates first mirror  104  in the horizontal direction at a first, relatively faster rate (e.g., 27 kHz, 35 kHz), and second mirror  106  in the vertical direction at a second, relatively slower rate (e.g., 60 Hz, 120 Hz). The second rate may be fixed such that second mirror  106  is vertically scanned in a continuous manner, while in other examples the vertical scanning may be carried out stepwise, such that second mirror has a vertical velocity substantially equal to zero prior to completion of a horizontal scan line, and is nonzero upon completion of a horizontal scan line to thereby transition to a subsequent horizontal scan line. Mirrors  104  and  106  may assume any suitable form, such as that of a MEMS actuated mirror (e.g., resonant piezoelectric-actuated). 
     In some implementations, display device  100  may further comprise an eye tracking sensor  112  operable to detect a gaze direction of a user of the display device. The gaze direction may be mapped to a region in display space to determine a location at output  108  where a user&#39;s gaze is directed. As described in further detail below with reference to  FIG. 3 , one or more operating parameters (e.g., vertical scan rate, phase offset) of display device  100  may be changed in response to a determined location of gaze. Sensor  112  may assume any suitable form. As an example, sensor  112  may comprise one or more light sources (e.g., infrared light sources) configured to cause a glint of light to reflect from the cornea of each eye of a user, and one or more image sensors that capture images of the user&#39;s eyes including the glint(s). 
     In some examples, display device  100  may further comprise a motion sensor  113  operable to detect motion of the display device, e.g. arising from movement of a user&#39;s head. As described in further detail below with reference to  FIG. 8 , one or more operating parameters (e.g., vertical scan rate, phase offset) of display device  100  may be changed in response to detected motion. Motion sensor  113  may take any suitable form. As an example, the motion sensor  113  may comprise one or more accelerometers, gyroscopes, and/or magnetometers. 
     Display device  100  further comprises a controller  114  for effecting the approaches described herein. Controller  114  may control operation of lasers  102  (e.g., frequency, intensity, duty cycle) and/or first and/or second mirrors  104  and  106 . Controller  114  may receive output from eye tracking sensor  112  and/or motion sensor  113 , and also may adjust operation of the lasers, first mirror, and/or second mirror based on the eye tracking sensor output and/or the motion sensor output. 
     Display device  100  may comprise alternative or additional elements not shown in  FIG. 1 . For example, one or more optical elements (e.g., collimator, diffuser, combiner, convergent lens, divergent lens, holographic element) may be placed in the optical path along which laser light travels to achieve desired display characteristics. Display device  100  may further include a suitable power source (e.g., battery, power supply) to provide electrical power to the active elements of the display device. Moreover, various modifications to display device  100  are contemplated, such as the inclusion of a single mirror in lieu of first and second mirrors  104  and  106 , where the single mirror is scanned in both the horizontal and vertical directions. 
       FIG. 2  shows an example laser trace diagram  200 . Diagram  200  illustrates optical output that may be produced by display device  100  of  FIG. 1 , for example. Four laser traces corresponding to the output of two lasers in two different frames, which may be successively interlaced, are shown. Each trace corresponds to light produced in display space in an FOV  201 —for example, at output  108  of  FIG. 1 . Thus, the portion of a laser trace between successive turning points, such as turning points  202 A and  202 B, may correspond to a horizontal row of perceived image pixels drawn by that laser trace. In some examples, the turning points may be outside of the image frame. 
     As indicated at a legend  204 , diagram  200  shows a trace of a first laser in a first frame, a trace of a second laser in the first frame, a trace of the first laser in a second frame, and a trace of the second laser in the second frame. In the first frame, the traces of the first and second lasers may be vertically separated in display space by one line. Thus, the traces of the first and second lasers may produce light at vertically adjacent image pixels that are vertically aligned. As an example,  FIG. 2  shows vertically adjacent, vertically aligned image pixels  206 A and  206 B that may be generated by the first and second lasers in the first frame. 
     The example shown in  FIG. 2  may correspond to an approach in which horizontal mirror scanning is achieved with an actuator that harmonically oscillates with substantially sinusoidal motion. Accordingly, the laser traces may exhibit at least partial sinusoidal motion; as shown in  FIG. 2 , each laser trace scans a horizontal row of image pixels in a half period (e.g., pi rad) of a sine wave, such that two horizontal traces are scanned in a full period (e.g., 2*pi rad) of the sine wave. Controlling the vertical mirror allows these two traces to produce two traces of horizontal image pixels. 
     In this formulation, the interlacing of the first and second frames, and other successive, alternate frames, may include applying a phase offset between alternate frames. As an example,  FIG. 2  shows a phase offset of pi radians applied to corresponding sinusoidal waves between the first and second frames. By applying a phase offset between alternate frames in this manner, a more consistent distribution of light throughout FOV  201 , and thus higher resolution imagery and brightness consistency, may be achieved. In view of the high-frequency scanning rate employed by the horizontal mirror in this example, controlling the scanning behavior of the high-frequency horizontal mirror may pose difficulties, as adjusting the scanning rate may interrupt the harmonic oscillation. As such, the phase offset is accomplished by controlling the slower scanning vertical mirror to start vertically scanning either earlier or later in the horizontal scan period, depending upon the adjustment desired. Further, by selecting a sufficiently high alternating frequency between interlaced frames, a stable image without unacceptable flickering may be produced in which light from both frames appears simultaneously to viewers. As an example, each frame may be displayed at a vertical scan frequency of 120 Hz, with a progressive image formed by first and second alternate frames being displayed at a vertical scan frequency of 60 Hz. 
     While a single line (e.g., pixel) spacing may be achieved at certain regions within FOV  201 , less desirable spacing may result in other regions within the FOV. In the example depicted in  FIG. 2 , a high degree of overlap results in output from each laser in the first frame and the same corresponding laser in the second frame within horizontal angles near 0 deg in FOV  201 . In contrast, more desirable single line spacing is achieved toward the edges of FOV  201 —e.g., between horizontal angles of +/−15 and +/−30 deg. Undesirable variance in resolution and brightness may result from such variance in line spacing throughout FOV  201 . To address such variance, the phase offset between alternate frames may be adjusted by adjusting the vertical scanning mirror. 
       FIG. 3  shows an example laser trace diagram  300  produced with a phase offset of pi/2 radians between alternate frames. In contrast to laser trace diagram  200  of  FIG. 2 , produced with a phase offset of pi radians,  FIG. 3  illustrates how the use of a pi/2 radian offset yields single line spacing at other regions within FOV  201 , such as within horizontal angles near 0 deg. Less desirable spacing, and laser output overlap, result in horizontal angles toward the edges of FOV  201 —e.g., between angles of +/−15 and +/−30 deg. 
     The laser trace diagrams shown in  FIGS. 2 and 3  illustrate how adjustment of the phase offset between alternate frames in interlaced, laser-scanned output generates desired line and image pixel spacing at different regions of an FOV in display space. This approach may be extended to the use of any suitable set of phase offsets to achieve desired line spacing at any region of an FOV. Further, phase offset adjustment may be dynamically employed during operating of a display device to achieve desired line spacing in regions where a user&#39;s gaze is directed—e.g., between the end of a frame and beginning of a subsequent during a vertical blank interval. For example with reference to  FIG. 1 , controller  114  may utilize output from eye tracking sensor  112  indicating a user&#39;s gaze direction to determine a region within a FOV of output  108  where the user&#39;s gaze is directed. Controller  114  may then select a phase offset in response to this determination to achieve a desired line spacing in the region where the user&#39;s gaze is directed, thereby optimizing display output perceived by the user throughout operation of display device  100 . Any suitable level of granularity may be employed in the course of dynamically adjusting phase offsets. As an example, an FOV may be divided into quadrants, with a respective phase offset being associated with each quadrant and used to achieve desired line spacing in that quadrant. However, the FOV may be divided into any suitable number regions with any suitable geometry, which may be equal or unequal, and regular or irregular. As another example, a substantially continuous function may be used to map gaze points in the FOV to phase offsets. Monte Carlo testing, for example, may be performed to determine a set of mappings between gaze points and phase offsets. 
     It will be understood that  FIGS. 2 and 3  are provided as examples and are not intended to be limiting in any way. Laser trace diagrams  200  and  300 , for example, may represent laser output prior to processing by one or more optical elements that may be included in display device  100  of  FIG. 1 . Further, any suitable integer number n of lasers may be used, with the vertical resolution of images being proportional to n—e.g., an increase in n number of lasers from 2 to 4 doubles the vertical resolution. Still further, any suitable vertical refresh rate may be used, where increases in the vertical refresh rate reduce the vertical resolution—e.g., a doubling of the vertical refresh rate reduces the vertical resolution by half. As such, the number n of lasers and the vertical refresh rate may be balanced to achieve desired display output. Yet further, a subset of diagrams  200  and  300  may be selected as an FOV in which imagery is provided to viewers. As an example with reference to  FIG. 3 , an example FOV  302  is shown that omits a portion of the laser traces in diagram  300 . The omitted portion may correspond to regions where laser output is asymmetric, vertically misaligned (as in the horizontal extremities of diagram  300 ), or otherwise undesired. The omitted portion may be designated an overscan region, in which laser output is disabled (e.g., by ceasing power supply to the lasers). 
     Various laser configurations may be used to generate single, and other desired, line spacings. As one example,  FIG. 4  shows an example laser die  400  comprising two solid-state lasers  402 A and  402 B. Lasers  402 A and  402 B may be arranged on laser die  400  via suitable lithographic processes, for example. In the orientation shown in  FIG. 4 , laser die  400  is centered about an x-axis  404  and a y-axis  406 , which may respectively correspond to horizontal and vertical axes along which one or more mirrors (e.g., first and second mirrors  104  and  106  of  FIG. 1 ) are scanned. Further, lasers  402 A and  402 B are aligned to, and spaced along, vertical axis  406  by a vertical separation distance  408  measured along the vertical axis from laser center to laser center. Separation distance  408  may be selected to achieve desired laser operation and avoid undesired issues associated with excessively narrow spacing between lasers  402 A and  402 B, such as thermal crosstalk. Separation distance  408  may further accommodate the arrangement of structural and electrical elements required to form laser die  400  and operate lasers  402 A and  402 B, along with mechanical tolerances. As one example, separation distance  408  may be substantially equal to 15 microns. 
     This and other vertical separation distances, however, may manifest in line spacings greater than one line in display space. As described in further detail below, specific vertical refresh rates and/or phase offsets may be selected to compensate and achieve single line spacing. 
     Single line spacing may be achieved with laser die  400  by rotating the laser die relative to the orientation shown in  FIG. 4 , and providing laser light to a mirror system at the rotated orientation. To this end,  FIG. 5  shows laser die  400  in a rotated orientation relative to the orientation shown in  FIG. 4 . The rotated orientation is selected to achieve a reduced vertical separation distance  410  relative to vertical separation distance  408  of the non-rotated orientation. Vertical separation distance  410  may be substantially equal to one micron, for example. In this way, single line spacing may be achieved in display space with laser die  400 , which may have a 15 micron or similar vertical separation distance in the non-rotated orientation. Due to mechanical tolerances, a rotation error may occur in rotation laser die  400 , which may manifest as a line spacing error in display space. To compensate, the vertical refresh rate may be adjusted with known mechanical spacing between lasers  402 A and  402 B, and characterized thermal variance of laser die  400 . As an example, a line spacing in display space greater than one line may be compensated by increasing the vertical refresh rate at the expense of some vertical resolution to achieve single line spacing. Laser trace diagrams  200  and  300  of  FIGS. 2 and 3  may be produced by laser die  400  arranged in the rotated orientation, for example. 
       FIG. 6  shows an example laser trace diagram  600 , including laser traces from two lasers in two alternate frames, as indicated by a legend  602 . Diagram  600  may represent laser output produced by a laser die oriented with a vertical separation between lasers greater than a spacing that results in a single line in display space, such as laser die  400  in the orientation shown in  FIG. 4  with vertical separation distance  408  between lasers  402 A and  402 B. In this example, a sinusoidal profile otherwise assumed by the laser traces is distorted due to the vertical laser separation distance, resulting in a ‘bow’ shape to the laser traces arising from scanning mirror effects. Further, the nature of the distorted sinusoids differs between lasers—the first laser exhibits greater downward curvature during a horizontal line scan, whereas the second laser exhibits a greater upward curvature during a horizontal line scan. However, desired single, and other, line spacings in display space may be achieved by adjusting the vertical refresh rate as described above. For vertical separation distance  408 , for example, the vertical refresh rate may be varied to adjust line spacing and/or resolution while still retaining an integer number of horizontal lines in the image. For example, this may allow scanning to occur faster in non-foveal regions (e.g. in a user&#39;s peripheral view) than in foveal regions (e.g. in the user&#39;s line of gaze) to achieve foveated display using eye tracking data. 
     The phase offset between interlaced frames generated by laser die  400  with vertical separation distance  408  may be adjusted to produce desired line spacings in certain parts of FOV  604 , as described above.  FIG. 7  shows an example laser trace diagram  700  resulting from a change in a phase offset of pi radians used to produce diagram  600  to a phase offset of pi/2 radians. In contrast to diagram  600 , in which undesired line spacing and laser output overlap occurred in horizontal angles proximate 0 deg, the pi/2 radian phase offset represented by diagram  700  results in desired line spacing in the horizontal angles proximate 0 deg. As described above, output from eye tracking sensor  112  of  FIG. 1  may be used to dynamically adjust the phase offset to achieve desired line spacing in response to user gaze. 
     In examples where a scanning display system is implemented as an HMD, a user&#39;s head motion may impact image resolution. For example, vertical head motion (or, more generally, head motion along a lower frequency scan direction) may create aliasing among frames of an interlaced scan pattern, causing lines of the scan pattern to appear to “collapse” upon one another. In such instances, the user may experience a reduction in image resolution due to the head motion along the lower frequency scan direction. Eye motion similarly may lead to aliasing in some instances. 
     To help avoid such visual effects, a motion sensor may be used to sense head motion and/or eye motion for adjusting a scan rate and/or a phase offset based on the motion data.  FIG. 8  shows a flow diagram illustrating an example method  800  of dynamically adjusting one or more operating parameters based upon user motion. Method  800  may be implemented as stored instructions executable by a logic subsystem of a computing system, such as a head-mounted display system. 
     At  802 , method  800  comprises controlling a scanning mirror system to scan light in an interlaced pattern to form an image. This may include controlling the scanning mirror system to scan light from the light source in a first direction at a higher frequency and in a second direction at a lower frequency to form the image, as indicated at  804 . Any suitable scanning mirror system may be used, including but not limited to the examples described herein. 
     Method  800  further includes, at  806 , receiving motion data from one or more sensors. Such motion data may include data from a head motion sensor, as indicated at  808 . Head motion data may include any suitable information regarding a user&#39;s head motion, such as translational and/or rotational velocity and/or acceleration information along three axes. From this data, a head velocity vector along the second direction may be determined. As described in more detail below, motion data also may be acquired from an eye tracking sensor, as indicated at  810 . 
     Continuing, method  800  includes, at  812 , adjusting one or more of a scan rate in the second direction and a phase offset between a first frame and a second frame of the interlaced pattern based upon the motion data to help reduce perceivable aliasing in a projected image. In some examples, a rate of acceleration may be determined and used to compute a likely head velocity at the time of image display, and the likely head velocity may be used to determine the adjustment. 
     Changes in acceleration of a user&#39;s head may affect aliasing across the interlaced pattern of the image even where an adjustment is determined based on a determined acceleration, for example, in instances where a change in acceleration during display of a frame is of sufficient magnitude to render a determined adjustment less effective during that frame. In some examples, changes in acceleration may be predicted to be sufficiently small throughout a duration of a frame (e.g., a 5-10 ms interval, based on an 8 ms frame duration) that a same adjustment is applied for the duration of a frame, and any instances in which acceleration exceeds the predicted level are not explicitly compensated for. In other examples, a real-time acceleration may be determined, and one or more parameters may be adjusted during the display of an image frame to change the scan rate during the frame. 
     As mentioned above, aliasing also may arise from eye motion along the lower frequency scan direction. As such, in some examples, one or more operating parameters may additionally or alternatively be adjusted based upon eye motion data from a gaze tracking system. Thus, method  800  may include, at  810 , receiving data from an eye tracking sensor, and based upon this data, adjusting (at  814 ) one or more of the scan rate in the second direction and the phase offset between frames of the interlaced pattern. 
     Where both head motion and eye motion are taken into account, the phase offset between frames of the interlaced pattern may be adjusted by individually determining a phase offset for head motion and eye motion, and then summing the phase offsets determined, as indicated at  816 . Further, where an offset is determined for foveated display, the foveated display offset also may be summed with the head motion offset and the eye motion offset to determine a total phase offset to apply. 
     In some examples, a display may be configured to use separate scanning display systems for a left-eye image and a right-eye image. For example, the system of  FIG. 1  may be modified to include separate light sources (e.g., separate lasers) and first and second mirrors for each eye, controlled by a common controller. In such an example, the two monocular display systems may overlap to produce stereoscopic display. In such an example, the phase difference between the two monocular display systems may be controllably offset such that the additive binocular display is the summation of the two monocular image scans. As an example, one eye may view odd frames while the other eye views even frames. In such an example, using a sufficiently high frame rate, a viewer viewing the two images will fuse the two images and see the superposition of the odd and even frames simultaneously. In such an example, the phase offset of the odd and even frames may be adjusted based on motion data to preserve a desired line spacing between scans and reduce perceptible aliasing, as described above. 
     Where two independent scanning display systems are used, it may be difficult to achieve a same scan rate in a higher frequency scan direction. For example, manufacturing tolerance issues may make it difficult to ensure a sufficiently consistent (e.g. &lt;100 Hz difference) resonant frequency for both higher frequency scanning mirrors. Such issues may potentially be solved by sorting parts based on resonant frequency. However, this may increase manufacturing costs, and also would not address possible temperature and drive variability issues that also can lead to unequal higher frequency scan rates between the two independent scanning systems. As another possible solution, one or both higher frequency scanning mirrors could be forced to operate off of its resonant frequency so as to match the frequency of the other mirror. However, such a solution may result in significantly higher power consumption than operating the mirror at its resonant frequency, and may impact system stability and field of view. Further, even where the resonant frequencies are close (e.g. &lt;100 Hz) and sufficiently stable, the relative phases still may need to be adjusted for each frame due to the changes in the phase difference arising from the (small) difference in higher frequency scan rates. 
     To avoid such issues with mismatched higher frequency scan rates, a same higher frequency scanning mirror may be used with separate slower frequency scanning mirrors for each eye. An example of such a configuration is shown as scanning display system  900  in  FIG. 9 . Scanning display system  900  includes respective left-eye and right-eye light source  902   a,    902   b.  Each light source directs light (e.g. laser light) towards a same higher frequency scanning mirror  904 . Scanning display system  900  further includes respective left-eye and right-eye lower frequency scanning mirrors  906   a,    906   b  to scan the light for each eye image toward an output  908   a , 908   b . A controller  910  controls the light sources  902   a,    902   b,  the higher-frequency scanning mirror  904 , and each lower-frequency scanning mirror  906   a,    906   b . The controller  910  also receives input from a motion sensor  912 , and may receive input from eye tracking sensors for each eye, shown respectively at  914   a,    914   b.  Based on data from motion sensor  912  and/or eye tracking sensors  914   a,    914   b,  the controller  910  may adjust the phase offset between frames for the first display and/or the second display, which may help to mitigate perceptible aliasing. 
     As another example, a scanning display system may utilize a one-dimensional array of light emitters with a light emitter for each pixel along a first direction, and a scanning mirror to scan light from the one-dimensional array of light emitters along a second direction. An image may be projected by individually controlling light emitted by each light emitter at each pixel row or column in the scan direction. In such a system, sample timing could be offset for left and right-eye images to mitigate aliasing due to head motion. An example of such a system is shown in  FIG. 10 , which depicts a first display  1000   a  and a second display  1000   b  each respectively having a one-dimensional laser array  1002   a,    1002   b  and a scanning mirror  1004   a,    1004   b  to direct a scanned one-dimensional image line toward an output  1006   a,    1006   b.  A controller  1008  controls each display  1000   a,    1000   b . The controller also receives input from a motion sensor  1010 , and also may receive input from eye tracking sensors for each eye, shown respectively at  1012   a,    1012   b.  Based on data from motion sensor  1010  and/or eye tracking sensors  1012   a,    1012   b,  the controller  1008  may adjust the sample timing of images displayed by the first display  1000   a  and/or the second display  1000   b  to mitigate aliasing. 
     In some embodiments, the methods and processes described herein may be tied to a computing system of one or more computing devices. In particular, such methods and processes may be implemented as a computer-application program or service, an application-programming interface (API), a library, and/or other computer-program product. 
       FIG. 11  schematically shows a non-limiting embodiment of a Computing system  1100  that can enact one or more of the methods and processes described above. Computing system  1100  is shown in simplified form. Computing system  1100  may take the form of one or more personal computers, server computers, tablet computers, home-entertainment computers, network computing devices, gaming devices, mobile computing devices, mobile communication devices (e.g., smart phone), and/or other computing devices. 
     Computing system  1100  includes a Logic machine  1102  and a storage machine  1104 . Computing system  1100  may optionally include a display subsystem  1106 , input subsystem  1108 , communication subsystem  1110 , and/or other components not shown in  FIG. 11 . 
     Logic machine  1102  includes one or more physical devices configured to execute instructions. For example, the logic machine may be configured to execute instructions that are part of one or more applications, services, programs, routines, libraries, objects, components, data structures, or other logical constructs. Such instructions may be implemented to perform a task, implement a data type, transform the state of one or more components, achieve a technical effect, or otherwise arrive at a desired result. 
     The logic machine may include one or more processors configured to execute software instructions. Additionally or alternatively, the logic machine may include one or more hardware or firmware logic machines configured to execute hardware or firmware instructions. Processors of the logic machine may be single-core or multi-core, and the instructions executed thereon may be configured for sequential, parallel, and/or distributed processing. Individual components of the logic machine optionally may be distributed among two or more separate devices, which may be remotely located and/or configured for coordinated processing. Aspects of the logic machine may be virtualized and executed by remotely accessible, networked computing devices configured in a cloud-computing configuration. 
     Storage machine  1104  includes one or more physical devices configured to hold instructions executable by the logic machine to implement the methods and processes described herein. When such methods and processes are implemented, the state of storage machine  1104  may be transformed—e.g., to hold different data. 
     Storage machine  1104  may include removable and/or built-in devices. Storage machine  1104  may include optical memory (e.g., CD, DVD, HD-DVD, Blu-Ray Disc, etc.), semiconductor memory (e.g., RAM, EPROM, EEPROM, etc.), and/or magnetic memory (e.g., hard-disk drive, floppy-disk drive, tape drive, MRAM, etc.), among others. Storage machine  1104  may include volatile, nonvolatile, dynamic, static, read/write, read-only, random-access, sequential-access, location-addressable, file-addressable, and/or content-addressable devices. 
     It will be appreciated that storage machine  1104  includes one or more physical devices. However, aspects of the instructions described herein alternatively may be propagated by a communication medium (e.g., an electromagnetic signal, an optical signal, etc.) that is not held by a physical device for a finite duration. 
     Aspects of logic machine  1102  and storage machine  1104  may be integrated together into one or more hardware-logic components. Such hardware-logic components may include field-programmable gate arrays (FPGAs), program- and application-specific integrated circuits (PASIC/ASICs), program- and application-specific standard products (PSSP/ASSPs), system-on-a-chip (SOC), and complex programmable logic devices (CPLDs), for example. 
     The terms “module,” “program,” and “engine” may be used to describe an aspect of Computing system  1100  implemented to perform a particular function. In some cases, a module, program, or engine may be instantiated via Logic machine  1102  executing instructions held by storage machine  1104 . It will be understood that different modules, programs, and/or engines may be instantiated from the same application, service, code block, object, library, routine, API, function, etc. Likewise, the same module, program, and/or engine may be instantiated by different applications, services, code blocks, objects, routines, APIs, functions, etc. The terms “module,” “program,” and “engine” may encompass individual or groups of executable files, data files, libraries, drivers, scripts, database records, etc. 
     It will be appreciated that a “service”, as used herein, is an application program executable across multiple user sessions. A service may be available to one or more system components, programs, and/or other services. In some implementations, a service may run on one or more server-computing devices. 
     When included, display subsystem  1106  may be used to present a visual representation of data held by storage machine  1104 . This visual representation may take the form of a graphical user interface (GUI). As the herein described methods and processes change the data held by the storage machine, and thus transform the state of the storage machine, the state of display subsystem  1106  may likewise be transformed to visually represent changes in the underlying data. Display subsystem  1106  may include one or more display devices utilizing virtually any type of technology. Such display devices may be combined with Logic machine  1102  and/or storage machine  1104  in a shared enclosure, or such display devices may be peripheral display devices. 
     When included, input subsystem  1108  may comprise or interface with one or more user-input devices such as a keyboard, mouse, touch screen, or game controller. In some embodiments, the input subsystem may comprise or interface with selected natural user input (NUI) componentry. Such componentry may be integrated or peripheral, and the transduction and/or processing of input actions may be handled on- or off-board. Example NUI componentry may include a microphone for speech and/or voice recognition; an infrared, color, stereoscopic, and/or depth camera for machine vision and/or gesture recognition; a head tracker, eye tracker, accelerometer, and/or gyroscope for motion detection and/or intent recognition; as well as electric-field sensing componentry for assessing brain activity. 
     When included, communication subsystem  1110  may be configured to communicatively couple with one or more other computing devices. Communication subsystem  1110  may include wired and/or wireless communication devices compatible with one or more different communication protocols. As non-limiting examples, the communication subsystem may be configured for communication via a wireless telephone network, or a wired or wireless local- or wide-area network. In some embodiments, the communication subsystem may allow Computing system  1100  to send and/or receive messages to and/or from other devices via a network such as the Internet. 
     Another example provides a scanning head-mounted display system, comprising a light source, a motion sensor, a scanning mirror system configured to scan light from the light source along at least one dimension to form an image, and a controller configured to control the scanning mirror system to scan the light to form the image, receive head motion data from the motion sensor, and adjust one or more of a scan rate and a phase offset between a first frame and a second frame of the image based upon the head motion data. In such an example, the scanning mirror system may additionally or alternatively comprise a first mirror configured to scan the light in a first direction, and a second mirror configured to scan the light in a second direction. In such an example, the controller additionally or alternatively may be configured to adjust the phase offset between the first frame and the second frame by adjusting a time at which the scanning mirror system initiates the scanning of the light in the second direction relative to the scanning of the light in the first direction. In such an example, the controller additionally or alternatively may be configured to adjust the phase offset between the first frame and the second frame based on a desired line spacing at a region in the image. In such an example, the region may be one of a plurality of regions in the image, and the controller additionally or alternatively may be configured to associate a respective phase offset with each of the plurality of regions. In such an example, the scanning head-mounted display system may further comprise an eye tracking sensor configured to detect a gaze direction of a user, and the controller alternatively or additionally may be further configured to map the gaze direction to a region in the image, and to adjust one or more of the scan rate and the phase offset based on the region in the image. In such an example, the controller alternatively or additionally may be configured to adjust the phase offset in a first manner based on mapping the gaze direction to a first region in the image, and to adjust the phase offset in a second manner based on mapping the gaze direction to a second region in the image. In such an example, the region may be a foveal region of the image, the image may comprise a non-foveal region, and the controller alternatively or additionally may be configured to increase the scan rate in the non-foveal region and to decrease the scan rate in the foveal region. In such an example, the phase offset between the first frame and the second frame may comprise multiple phase offset adjustments, and the controller alternatively or additionally may be configured to determine a first phase offset adjustment based upon the region in the image, determine a second phase offset adjustment based upon the head motion data, and adjust the phase offset between the first frame and the second frame based upon a sum of the first phase offset adjustment determined and the second phase offset adjustment determined. In such an example, the light source may alternatively or additionally comprise a laser light source comprising two or more offset lasers, and the laser light source may be configured to output light of multiple colors. In such an example, the scanning head-mounted display system may comprise an eye tracking sensor configured to detect eye movement, and the controller alternatively or additionally may be configured to map the eye movement, and adjust the phase offset between the first frame and the second frame based upon the eye movement. In such an example, the scanning mirror system may alternatively or additionally be configured to scan a horizontal line of the image in a half period of a microelectromechanical actuator that drives the scanning mirror system. In such an example, the light source may alternatively or additionally comprise a one-dimensional array of light emitters. In such an example, the scanning mirror system may alternatively or additionally comprise a same higher frequency scanning mirror and separate left-eye and right-eye lower frequency scanning mirrors for respectively scanning in a higher frequency scan direction and a lower frequency scan direction. 
     Another example provides a method of displaying an image, the comprising directing light from a light source toward a scanning mirror system, scanning the light from the light source in a first direction at a higher frequency and in a second direction at a lower frequency to thereby scan the light in an interlaced pattern and form the image, receiving head motion data from a motion sensor, and based at least on the head motion data, adjusting one or more of a scan rate in the second direction and a phase offset between a first frame and a second frame of the interlaced pattern. In such an example, the method alternatively or additionally may further comprise mapping a gaze direction determined via an eye tracking sensor to a region in the image, and adjusting one or more of the scan rate in the second direction and the phase offset based on the region in the image. In such an example, the method may alternatively or additionally comprise mapping an eye movement across the image via an eye tracking sensor, and based upon the eye movement, adjusting one or more of the scan rate in the second direction and the phase offset between the first frame and the second frame of the interlaced pattern. In such an example, scanning the light from the light source may alternatively or additionally comprise scanning the light to form the image at a resolution between 1440p and 2160p. 
     Another example provides a scanning head-mounted display system, comprising a light source, a motion sensor, a scanning mirror system configured to scan light from the light source in a first direction at a higher frequency, and in a second direction at a lower frequency to form an image, an eye tracking sensor, and a controller configured to control the scanning mirror system to scan the light in an interlaced pattern to form the image, receive head motion data from the motion sensor, receive data from the eye tracking sensor, and adjust one or more of a scan rate in the second direction and a phase offset between a first frame and a second frame of the interlaced pattern based at least on the head motion data and the data received from the eye tracking sensor. In such an example, the controller alternatively or additionally may be configured to receive the data from the eye tracking sensor by receiving an indication of one or more of a gaze direction and an eye movement, and wherein the controller may alternatively or additionally be configured to determine a first phase offset adjustment based upon the one or more of the gaze direction and the eye movement, determine a second phase offset adjustment based upon the head motion data, and adjust the phase offset between the first frame and the second frame of the interlaced pattern based upon a sum of the first phase offset adjustment determined and the second phase offset adjustment determined. 
     It will be understood that the configurations and/or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The specific routines or methods described herein may represent one or more of any number of processing strategies. As such, various acts illustrated and/or described may be performed in the sequence illustrated and/or described, in other sequences, in parallel, or omitted. Likewise, the order of the above-described processes may be changed. 
     The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various processes, systems and configurations, and other features, functions, acts, and/or properties disclosed herein, as well as any and all equivalents thereof.