Abstract:
A scanning projection system includes a scanning mirror module, a controlling circuit, and a laser module. A swinging motion of the scanning mirror module is controlled according to a driving signal, and a combined laser beam reflected by the scanning mirror module is swept across a projection surface to produce plural projection points on a projection surface. The controlling circuit includes a weight mapping unit for converting an image signal into a compensated image signal according to a position-and-weight mapping relationship. The laser module generates the combined laser beam according to the compensated image signal. After plural weights of the corresponding projection points are acquired according to positions of the corresponding projection points and the position-and-weight mapping relationship, the weight mapping unit multiplies the image signal by the corresponding weights according to the positions of the projection points. Consequently, the compensated image signal is generated.

Description:
[0001]    This application claims the benefit of People&#39;s Republic of China Application Serial No. 201410043781.2, filed Jan. 29, 2014, the subject matter of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a projection system, and more particularly to a scanning projection system for producing a frame with uniform brightness. 
       BACKGROUND OF THE INVENTION 
       [0003]    Projectors are widely used in many circumstances. Recently, with increasing development of science and technology, a pico projector has been introduced into the market. The pico projector is designed to have small size and light weightiness. The pico projector may produce projection images by different projecting technologies. For example, in a scanning projection system, a two-dimensional scanning mirror is used to periodically sweep a laser beam across a projection surface in order to produce the projection image on the projection surface. 
         [0004]      FIG. 1A  schematically illustrates the architecture of a conventional scanning projection system. As shown in  FIG. 1A , the scanning projection system  100  comprises a laser module  151 , a scanning mirror module  152 , and a controlling circuit  153 . The laser module  151  comprises plural color laser sources  122 ˜ 124  and plural optical alignment elements  155 . The plural color laser sources  122 ˜ 124  are used for emitting plural color beams, respectively. By the plural optical alignment elements  155 , the plural color beams from the plural color laser sources  122 ˜ 124  are mixed as a combined laser beam  154 . Then, the combined laser beam  154  is reflected by the scanning mirror module  152 , and projected on a projection surface  140 . For example, the plural color laser sources  122 ˜ 124  are used for emitting a red beam, a green beam and a blue beam, respectively. Moreover, the scanning mirror module  152  is a microelectromechanical (MEMS) scanning mirror module. 
         [0005]    Moreover, the controlling circuit  153  is used for outputting an image signal V to the laser module  151  and outputting a driving signal D to the scanning mirror module  152 . 
         [0006]    Please refer to  FIG. 1A  again. The swinging motion of the scanning mirror module  152  is controlled according to the driving signal D. Consequently, the projection points of the combined laser beam  154  are swept across the projection surface  140  by scanning each row of pixels from left to right and then from right to left and scanning rows from top to bottom. Generally, the start point of the scanning cycle is at an upper left corner of the projection surface  140 , and the end point of the scanning cycle is at a lower right corner of the projection surface  140 . According to the image signal V, the combined laser beam  154  with the corresponding image setting is projected on the corresponding scanning position during the swing of the laser module  151 . After one scanning cycle is completed, a frame is displayed on the projection surface  140 . Then, the projection point goes back to the start point (e.g. the upper left corner), and the next scanning cycle is performed to display the next frame. 
         [0007]    Generally, the number of frames to be displayed every second is defined as a frame rate. For example, if the frame rate of the projection surface  140  is 60, it means that 60 scanning cycles are performed per second and 60 frames are continuously displayed on the projection surface  140  per second. 
         [0008]      FIG. 1B  is a schematic timing waveform diagram illustrating associated driving signals of the conventional scanning projection system. The driving signal D contains a fast-axis driving signal and a slow-axis driving signal. According to the fast-axis driving signal, the swinging motion of the scanning mirror module  152  along a fast-axis direction (e.g. a horizontal scanning direction or an x-axis direction) is correspondingly controlled. According to the slow-axis driving signal, the swinging motion of the scanning mirror module  152  along a slow-axis direction (e.g. a vertical scanning direction or a y-axis direction) is correspondingly controlled. 
         [0009]    Please refer to  FIG. 1B . At the time point t 0 , the scanning cycle of a first frame (frame 1 ) is started. The time interval between two troughs of the fast-axis driving signal indicates one back-and-forth swinging motion of the scanning mirror module  152  along the horizontal scanning direction. The time interval between two troughs of the slow-axis driving signal indicates one back-and-forth swinging motion of the scanning mirror module  152  along the vertical scanning direction. Consequently, from the time point t 0  to the time point t 2 , the first frame (frame 1 ) is displayed on the projection surface  140 . At the time point t 1 , the scanning cycle of a first frame (frame 1 ) is ended. The time interval between the time point t 1  and the time period t 2  indicates the time period from the end point of the scanning cycle of the first frame (frame 1 ) to the start point of the scanning cycle of a second frame (frame  2 ). 
         [0010]    Similarly, the second frame (frame 2 ) is displayed on the projection surface  140  from the time point t 2  to the time point t 3 ; and a third frame (frame 3 ) is displayed on the projection surface  140  from the time point t 3  to the time point t 4 . 
         [0011]    As shown in  FIG. 1B , the fast-axis driving signal may drive a fast-axis swinging motion of the scanning mirror module  152  along the horizontal scanning direction at a resonant frequency. Consequently, the scanning mirror module  152  is periodically swung in a sine-like wave form. Due to the sine-like fast-axis swinging motion, the projection points of the combined laser beam  154  are swept across the projection surface  140  at a non-constant velocity. The distance between every two adjacent projection points is not constant under the non-constant velocity, therefore the brightness of the frame is not uniform. 
         [0012]      FIGS. 2 and 3  schematically illustrate the frame displayed on the projection surface of the conventional scanning projection system. As shown in  FIG. 2 , the distribution of the projection points at a left side  242  and a right side  244  of the projection surface  140  is denser, and thus the frame brightness presented at two side of the projection surface  140  is higher. Moreover, the distribution of the projection points at a middle region of the projection surface  140  is sparser, and thus the frame brightness presented at the middle region of the projection surface  140  is lower. In other words, the brightness values presented at the left side and the right side of the whole frame are higher, and the brightness value presented at the middle region of the whole frame is lower. 
         [0013]    As shown in  FIG. 1B , the slow-axis driving signal in the sawtooth wave form may drive the swinging motion of the scanning mirror module  152  in a periodic sawtooth wave form. However, due to the physical properties of the scanning mirror module  152 , some drawbacks may occur. For example, when the slow-axis driving signal in the sawtooth wave form drives the slow-axis swinging motion of the scanning mirror module  152  along the vertical scanning direction, the scanning mirror module  152  may be suffered from jitter. Consequently, the frame brightness presented along the vertical scanning direction is non-uniform. 
         [0014]    In particular, due to the physical properties of the scanning mirror module  152 , the slow-axis driving signal fails to ideally drive the swinging motion of the scanning mirror module  152  at a constant velocity. Under this circumstance, the scanning mirror module  152  may be slightly suffered from jitter. Since the swinging motion of the scanning mirror module  152  is not ideally maintained at the constant velocity, some drawbacks may occur. For example, if the swinging speed is decreased, the distance between two adjacent scan lines is reduced, and thus the scan lines present bright. As shown in  FIG. 3 , if the swinging velocity of the scanning mirror module  152  along the vertical scanning direction and corresponding to a specified region  342  of the projection surface  140  is slower, the distribution of the scan lines at the specified region  342  of the projection surface  140  becomes denser. Consequently, the scan lines at the specified region  342  of the projection surface  140  present brighter than other region. 
         [0015]    From the above discussions about the conventional scanning projection system, the brightness values presented at the left side and the right side of the whole frame are higher, and the scan lines at the middle region of the projection surface present brighter. Consequently, the user&#39;s eyes usually feel uncomfortable with the non-uniform brightness. 
         [0016]    Due to the characteristics of the swinging motion or the characteristics of the driving signal, the projection points of the combined laser beam  154  are swept across the projection surface  140  at the non-constant velocity, and thus the distribution of the projection points are non-uniform. Moreover, if the optical path of the combined laser beam  154  to the projection surface  140  is adversely affected by other optical elements in the optical path, the projecting direction of the combined laser beam  154  is possibly shifted. That is, the positions of the projection points on the projection surface  140  are deviated. Under this circumstance, the distribution of the projection points on the projection surface  140  is not uniform. Consequently, the presented brightness of the frame on the projection surface  140  is not uniform. 
       SUMMARY OF THE INVENTION 
       [0017]    The present invention provides a scanning projection system for producing a frame with uniform brightness. 
         [0018]    An embodiment of the present invention provides a scanning projection system for displaying a frame on a projection surface. The scanning projection system includes a scanning mirror module, a controlling circuit, and a laser module. A swinging motion of the scanning mirror module is controlled according to a driving signal, and a combined laser beam reflected by the scanning mirror module is swept across a projection surface to produce plural projection points on the projection surface. The controlling circuit includes a weight mapping unit for converting an image signal into a compensated image signal according to a position-and-weight mapping relationship. The laser module generates the combined laser beam according to the compensated image signal. After plural weights of the corresponding projection points are acquired according to positions of the corresponding projection points and the position-and-weight mapping relationship, the weight mapping unit multiplies the image signal by the corresponding weights according to the positions of the projection points to generate the compensated image signal. 
         [0019]    Numerous objects, features and advantages of the present invention will be readily apparent upon a reading of the following detailed description of embodiments of the present invention when taken in conjunction with the accompanying drawings. However, the drawings employed herein are for the purpose of descriptions and should not be regarded as limiting. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
           [0021]      FIG. 1A  (prior art) schematically illustrates the architecture of a conventional scanning projection system; 
           [0022]      FIG. 1B  (prior art) is a schematic timing waveform diagram illustrating associated driving signals of the conventional scanning projection system; 
           [0023]      FIGS. 2 and 3  (prior art) schematically illustrate the frame displayed on the projection surface of the conventional scanning projection system; 
           [0024]      FIG. 4  schematically illustrates the architecture of a scanning projection system according to an embodiment of the present invention; 
           [0025]      FIG. 5  schematically illustrates the horizontal position-and-weight mapping relationship and the corresponding frame; 
           [0026]      FIG. 6A  is a schematic timing waveform diagram illustrating a swinging feedback signal corresponding to the swinging motion of the scanning mirror module along the vertical scanning direction; and 
           [0027]      FIG. 6B  schematically illustrates the vertical position-and-weight mapping relationship and the corresponding frame. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0028]      FIG. 4  schematically illustrates the architecture of a scanning projection system according to an embodiment of the present invention. As shown in  FIG. 4 , the scanning projection system  400  comprises a laser module  451 , a scanning mirror module  452 , and a controlling unit  460 . The laser module  451  comprises plural color laser sources  422 ˜ 424  and plural optical alignment elements  455 . The plural color laser sources  422 ˜ 424  are used for emitting plural color beams, respectively. By the plural optical alignment elements  455 , the plural color beams from the plural color laser sources  422 ˜ 424  are mixed as a combined laser beam  454 . Then, the combined laser beam  454  is reflected by the scanning mirror module  452 , and projected on a projection surface  440 . For example, the plural color laser sources  422 ˜ 424  are used for emitting a red beam, a green beam and a blue beam, respectively. Moreover, the scanning mirror module  452  is a microelectromechanical (MEMS) scanning mirror module. 
         [0029]    The controlling circuit  460  may generate a driving signal D to the scanning mirror module  452  in order to drive a swinging motion of the scanning mirror module  452 . Moreover, a swinging feedback signal S corresponding to a swinging status of the scanning mirror module  452  may be received by the controlling circuit  460 . Similarly, the driving signal D contains a fast-axis driving signal and a slow-axis driving signal. Moreover, the controlling circuit  460  comprises a weight mapping unit  462  for deriving the position-and-weight mapping relationship. Moreover, according to the position-and-weight mapping relationship, the weights corresponding to the positions of projection points are acquired by the controlling circuit  460 . After the weights corresponding to the positions of projection points are acquired, the controlling circuit  460  converts an image signal V into a compensated image signal Vc and transmits the compensated image signal Vc to the laser module  451 . According to the compensated image signal Vc, the combined laser beam  454  with the corresponding image setting is projected on the corresponding scanning position during the swing of the laser module  451 . 
         [0030]    In an embodiment, an infrared photographing technology may be used to detect the positions of the projection points on the projection surface  440 . According to the positions of the projection points on the projection surface  440 , the weight mapping unit  462  may simulate the scanning trajectory of the combined laser beam  454  that is reflected by the scanning mirror module  452  and projected on the projection surface  440 . Moreover, according to the simulated scanning trajectory, the weight mapping unit  462  may evaluate the positions of the projection points and the distances between the projection points. Moreover, according to the distribution (also referred as closeness or sparseness) of the projection points, the weight mapping unit  462  may derive the position-and-weight mapping relationship. In another embodiment, according to the swinging feedback signal S, the weight mapping unit  462  may simulate the swinging trajectory of the scanning mirror module  452  and evaluate the positions of the projection points and the distances between the projection points. Consequently, the weight mapping unit  462  may derive the position-and-weight mapping relationship. 
         [0031]    Hereinafter, the position-and-weight mapping relationship along the horizontal scanning direction and the position-and-weight mapping relationship along the vertical scanning direction will be separately illustrated. In some other embodiments, the position-and-weight mapping relationship along both of the horizontal scanning direction and the vertical scanning direction may be taken into consideration. After the weights corresponding to all projection points are determined, the non-uniform presentation resulting from the closeness or sparseness of the projection points along both of the horizontal scanning direction and the vertical scanning direction will be compensated. 
         [0032]      FIG. 5  schematically illustrates the horizontal position-and-weight mapping relationship and the corresponding frame. Generally, in case that the distribution of the projection points is sparser, the weight is higher. Whereas, in case that the distribution of the projection points is denser, the weight is lower. 
         [0033]    As shown in  FIG. 5 , the horizontal weight W 1  is larger than the horizontal weight W 2 ; the horizontal weight W 2  is larger than the horizontal weight W 3 ; and the horizontal weight W 3  is larger than the horizontal weight W 4  (i.e. W 1 &gt;W 2 &gt;W 3 &gt;W 4 ). Similarly, the horizontal weight W 1  is larger than the horizontal weight W 2 ′; the horizontal weight W 2 ′ is larger than the horizontal weight W 3 ′; and the horizontal weight W 3 ′ is larger than the horizontal weight W 4 ′ (i.e. W 1 &gt;W 2 ′&gt;W 3 ′&gt;W 4 ′). In an embodiment, plural weights are assigned to corresponding projection points according to the distribution of the projection points. In some other embodiment, since the swinging velocities along the horizontal scanning direction X are symmetrical, the horizontal weight W 2  is equal to the horizontal weight W 2 ′, the horizontal weight W 3  is equal to the horizontal weight W 3 ′, and the horizontal weight W 4  is equal to the horizontal weight W 4 ′. 
         [0034]    Moreover, after the weight of the corresponding projection point is acquired according to the horizontal position-and-weight mapping relationship, the weight mapping unit  462  may multiply the image signal V by the corresponding weight in order to generate the compensated image signal Vc. In this embodiment, the weight is a bright weight corresponding to the image signal V. As shown in  FIG. 5 , the distribution of the projection points at a left side and a right side of the projection surface  440  is denser, and thus the corresponding weights are lower. After the image signal V is multiplied by the corresponding weight to generate the compensated image signal Vc, the brightness setting value corresponding to the compensated image signal Vc is lower (i.e. darker). Whereas, the distribution of the projection points at the middle region of the projection surface  440  is sparser, and thus the corresponding weights are higher. After the image signal V is multiplied by the corresponding weight to generate the compensated image signal Vc, the brightness setting value corresponding to the compensated image signal Vc is higher (i.e. brighter). Since the brightness setting value corresponding to the image signal V is adjusted according to the weight corresponding to the position of the projection point, the brightness of the whole frame displayed on the projection surface  440  is more uniform. 
         [0035]      FIG. 6A  is a schematic timing waveform diagram illustrating a swinging feedback signal corresponding to the swinging motion of the scanning mirror module along the vertical scanning direction. When the scanning mirror module  452  is swung along the vertical scanning direction Y, the right side of the scanning mirror module  452  is firstly swung from top to bottom and then swung from bottom to top, and the left side of the scanning mirror module  452  is firstly swung from bottom to top and then swung from top to bottom. By detecting the swinging feedback signal S in response to the actual swinging motion of the scanning mirror module  452 , the scanning projection system  400  may realize the swinging status of the scanning mirror module  452 . 
         [0036]    As shown in  FIG. 6A , the swinging feedback signal S contains a Y-direction right-side swinging feedback signal Syr and a Y-direction left-side swinging feedback signal Syl. According to one of the Y-direction right-side swinging feedback signal Syr and the Y-direction left-side swinging feedback signal Syl, the swing status of the scanning mirror module  452  along the vertical scanning direction Y may be realized. While the right side of the scanning mirror module  452  is swung from the position U 0  to the position M 0 , the swinging feedback signal S of the scanning mirror module  452  is suffered from jitter. Similarly, while the left side of the scanning mirror module  452  is swung from the position D 0  to the position M 0 , the swinging feedback signal S of the scanning mirror module  452  is also suffered from jitter. The jitter of the swinging feedback signal S indicates that a non-constant velocity situation occurs during the process of controlling the scanning mirror module  452  to scan a single frame at a constant velocity along the vertical scanning direction. 
         [0037]    Due to the non-constant velocity situation, the distances between the scan lines on the projection surface  440  are not equal, and the scan lines corresponding to the smaller distance present brighter. Consequently, the controlling unit  460  may evaluate the distances between the scan lines along the vertical scanning direction according to the swinging feedback signal S. According to the distribution of the scan lines, the weight mapping unit  462  may derive the position-and-weight mapping relationship. In this embodiment, the distance between adjacent scan lines indicates the distance between adjacent projection points along the vertical scanning direction. 
         [0038]      FIG. 6B  schematically illustrates the vertical position-and-weight mapping relationship and the corresponding frame. Generally, in case that the distribution of the projection points or the scan lines along the vertical scanning direction is sparser, the weight is higher. Whereas, in case that the distribution of the projection points or the scan lines along the vertical scanning direction is denser, the weight is lower. In an embodiment, plural weights are assigned to corresponding projection points according to the distribution of the projection points or the scan lines. As shown in  FIG. 6B , according to the Y-direction right-side swinging feedback signal Syr, the scan lines between the position U 1  and the position U 2  is denser than the scan lines at other positions. Consequently, according to the settings of the weight mapping unit  462 , the vertical weight w 1  is smaller than the vertical weight w 2 , and the vertical weight w 1  is smaller than the vertical weight w 2 ′. 
         [0039]    Moreover, after the weight of the corresponding projection point is acquired according to the vertical position-and-weight mapping relationship, the weight mapping unit  462  may multiply the image signal V by the corresponding weight in order to generate the compensated image signal Vc. In this embodiment, the weight is a bright weight corresponding to the image signal V. As shown in  FIG. 6B , the distribution of the scan lines between the position U 1  and the position U 2  of the projection surface  440  is denser, or the distribution of the projection points between the position U 1  and the position U 2  of the projection surface  440  along the vertical scanning line is denser. Consequently, the corresponding weight w 1  is lower. After the image signal V is multiplied by the corresponding weight W 1  to generate the compensated image signal Vc, the brightness setting value corresponding to the compensated image signal Vc is lower (i.e. darker). Whereas, the distribution of the projection points at other positions of the projection surface  440  is sparser, and thus the corresponding weight w 2  or w 2 ′ is higher. After the image signal V is multiplied by the corresponding weight w 2  or w 2 ′ to generate the compensated image signal Vc, the brightness setting value corresponding to the compensated image signal Vc is higher (i.e. brighter). Since the brightness setting value corresponding to the image signal V is adjusted according to the weight corresponding to the position of the projection point, the brightness of the whole frame displayed on the projection surface is more uniform. 
         [0040]    In the above embodiment, the scanning projection system  400  is capable of adjusting the uniformity of the frame brightness along the fast-axis scanning direction and the slow-axis scanning direction. However, those skilled in the art will readily observe that numerous modifications and alterations may be made while retaining the teachings of the invention. For example, in some other embodiments, the uniformity of the frame brightness along one of the fast-axis scanning direction and the slow-axis scanning direction is adjusted. Moreover, in the above embodiment, the weight mapping unit  462  may derive the position-and-weight mapping relationship and the weights according to the scanning trajectory and/or the swinging feedback signal S, and realize the weight of the corresponding projection point according to this relationship. After the image signal V is multiplied by the corresponding weight to generate the compensated image signal Vc, the uniformity of the frame brightness may be adjusted. 
         [0041]    In case that the uniformity of the frame brightness along both of the fast-axis scanning direction and the slow-axis scanning direction is adjusted, the weight mapping unit  462  may simulate the scanning trajectory of the combined laser beam  454  that is reflected by the scanning mirror module  452  and projected on the projection surface  440 . According to the scanning trajectory, the weight mapping unit  462  may derive the position-and-weight mapping relationship. In addition, the weight of the corresponding projection point is realized according to the relationship. The scanning trajectory of the combined laser beam  454  that is reflected by the scanning mirror module  452  and projected on the projection surface  440  is usually a continuous trajectory varying with time. In other words, the positions of all projection points may be acquired according to the relationship between the scanning trajectory and time, and the position-and-weight mapping relationship is determined according to the closeness or sparseness of each projection point relative to the neighboring projection points. In the above embodiment, each projection point has corresponding weights along the horizontal scanning direction and the vertical scanning direction. According to the corresponding weights, the non-uniform presentation resulting from the closeness or sparseness of the projection points along the horizontal scanning direction and the vertical scanning direction will be compensated. Consequently, the brightness of the whole frame displayed on the projection surface  440  is more uniform. 
         [0042]    Moreover, the position-and-weight mapping relationship may be previously established and calibrated before the scanning projection system leaves the factory, and the position-and-weight mapping relationship is recorded into the weight mapping unit  462 . Alternatively, during operations of the scanning projection system, the position-and-weight mapping relationship is dynamically changed by the weight mapping unit  462  according to the practical operations of the scanning mirror module  452 . 
         [0043]    For example, a first projection point, a second projection point, a third projection point and a fourth projection point are sequentially projected on the projection surface  440  by the scanning mirror module  452 . If the distance between the first projection point and the second projection point is larger than the distance between the third projection point and the fourth projection point, the weight mapping unit  462  may multiply the image signal V corresponding to the first projection point and the second projection point by a first weight, and the weight mapping unit  462  may multiply the image signal V corresponding to the third projection point and the fourth projection point by a second weight. The second weight is smaller than the first weight. Consequently, the brightness setting value corresponding to the first projection point and the second projection point is larger than the brightness setting value corresponding to the third projection point and the fourth projection point. After the non-uniformity of the brightness resulting from the closeness or sparseness of the projection points is compensated, the brightness of the whole frame displayed on the projection surface  440  is more uniform. 
         [0044]    In another embodiment, the closeness or sparseness of the scan lines is taken into consideration. For example, a first scan line, a second scan line, a third scan line and a fourth scan line are sequentially projected on the projection surface  440  by the scanning mirror module  452 . If the distance between the first scan line and the second scan line is larger than the distance between the third scan line and the fourth scan line, the weight mapping unit  462  may multiply the image signal V corresponding to the projection points of the first scan line and the second scan line by a first weight, and the weight mapping unit  462  may multiply the image signal V corresponding to the projection points of the third scan line and the fourth scan line by a second weight. The second weight is smaller than the first weight. Consequently, the brightness setting value corresponding to the first scan line and the second scan line is larger than the brightness setting value corresponding to the third scan line and the fourth scan line. After the non-uniformity of the brightness resulting from the closeness or sparseness of the scan lines is compensated, the brightness of the whole frame displayed on the projection surface  440  is more uniform. 
         [0045]    From the above descriptions, the present invention provides a scanning projection system. The scanning projection system is capable of producing a frame with uniform brightness. 
         [0046]    While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.