Abstract:
An improved solution for generating depth maps using time-of-flight measurements is described, more specifically a time-of-flight imager and a time-of-flight imaging method with an improved accuracy. A depth correction profile is applied to the measured depth maps, which takes into account propagation delays within an array of pixels of a sensor of the time-of-flight imager.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an improved solution for generating depth maps using time-of-flight measurements, and more specifically to a time-of-flight imager and a time-of-flight imaging method with an improved accuracy. 
       BACKGROUND OF THE INVENTION 
       [0002]    For the generation of 3D video content it is important to measure the distance of objects located in a scene that is being captured in addition to the usual video data. For this purpose a so-called depth map is determined. Typically a time-of-flight imager is used for this purpose, i.e. a measurement system that creates distance data with help of the time-of-flight principle. The time-of-flight imager includes a light source and a light sensor, which consists of an array of pixels. To measure a depth map, the light source is triggered with a pulse, and all pixels of the sensor are simultaneously triggered with a Transfer Gate (TG) pulse and a Global Shutter Gate (GSG) pulse. For details see K. Fife et al.: “A 0.5 μm pixel frame-transfer CCD image sensor in 110 nm CMOS”, 2007 IEEE International Electron Devices Meeting, Vol. 35 (2007), pp. 1003-1006. 
         [0003]    Also, WO 2009/135952 discloses methods and sensors for time-of-flight measurements. The sensors are adapted to achieve efficient background radiation suppression at variable background radiation conditions. 
         [0004]    Not only background radiation reduces the accuracy of the depth map measurement. To obtain an accurate depth map it is likewise important that the temporal relations between the light source, the transfer gate pulse and the global shutter gate pulse are constant. 
       SUMMARY OF THE INVENTION 
       [0005]    Accordingly, it is an object of the present invention to propose a solution for time-of-flight generation of depth maps, which achieves an improved accuracy. 
         [0006]    According to the invention, this object is achieved by a time-of-flight imager for measuring a depth map of an object, with a light source for illuminating the object and a sensor with an array of pixels for detecting light reflected by the object to obtain a measured depth map, wherein the time-of-flight imager is adapted to apply a depth correction profile to the measured depth map. 
         [0007]    Similarly, a method for measuring a depth map of an object with a time-of-flight imager has the steps of:
       illuminating the object with light emitted by a light source;   detecting light reflected by the object with a sensor having an array of pixels to obtain a measured depth map; and   applying a depth correction profile to the measured depth map.       
 
         [0011]    Despite a minute control of the temporal relations between the light source, the transfer gate and the global shutter gate pulse, especially for sensors with a large number of pixels the measured depth maps are not satisfactory. It has been found that this is caused by the sensor of the time-of-flight imager, namely by the varying electrical path lengths to the different pixels of the sensor. The transfer gate pulse and the global shutter gate pulse travel along the same path. There is thus no significant shift between these pulses. However, the temporal relation of these pulses with regard to the light source trigger pulse varies across the array of pixels of the sensor, i.e. the pulses are subject to a propagation delay across the array of pixels. A further contribution to the propagation delay stems from the horizontal and vertical drivers of the sensor. The exact propagation delay depends on the location of a pixel within the array of pixels, but can easily be in the range of 10 to 20 ns. In the following this effect is referred to as ‘shading’. 
         [0012]    As light travels at a speed of 3×10 9  m/s, a distance of 1 m will delay the light by 3.3 ns. To measure this distance with a time-of-flight imager, the light has to travel back to the imager, thus a distance of lm results in a delay of 6.6 ns. Consequently, a propagation delay difference across the imager of 20 ns results in an error of 3 m. Apparently the propagation delay of the pulses across the imager has a large impact on the accuracy of the measurement. As the size of the imager becomes larger, the problem of propagation delays increases. A straightforward option to address this problem is to optimize the horizontal and vertical drivers of the sensor for higher speed and lower propagation delay. However, this increases the cost of the sensor and hence of the time-of-flight imager. In addition, a residual delay remains despite the optimized drivers. 
         [0013]    The solution according to the invention solves the problem of propagation delays by correcting the errors in the depth map, which result from the propagation delays, with a depth correction profile. The solution has the advantage that it can be implemented at low cost while ensuring that depth maps with an improved accuracy are generated. In addition, by making the depth correction profile adjustable it becomes possible to cope with effects caused by aging of the sensor or by environmental changes. 
         [0014]    Advantageously, the depth correction profile is a superposition of two n th  order polynomials. A first of the two polynomials has a line number of the sensor as an input value, whereas a second of the two polynomials has a pixel number of a line of the sensor as an input value. Preferably two 2 nd  order polynomials are used, as this is generally sufficient both in line direction and column direction of the sensor. The superposition of the two polynomials results in a 3D depth correction profile, which is subtracted from the measured depth map to obtain a corrected depth map. Of course, it is likewise possible to use different orders for the two polynomials. 
         [0015]    Preferably, the depth correction profile is retrieved from a memory, e.g. from a look-up table. This has the advantage that only a rather limited amount of processing power is needed. Alternatively, the depth correction profile is calculated on the fly based on the polynomials. In this case coefficients of the polynomials are favorably retrieved from a memory. Though the latter solution requires more processing power, the necessary amount of memory is reduced. In order to cope with environmental changes, e.g. changes in temperature or humidity, depth correction profiles for different environmental conditions are favorably available in the memory. In this case the time-of-flight imager includes corresponding sensors, e.g. a temperature sensors and a humidity sensor, to select the appropriate depth correction profile. 
         [0016]    The depth correction profile is preferably determined by measuring a depth map of a known object and comparing the measured depth map with an expected depth map. More specifically, for determining the depth correction profile the following steps are carried out:
       illuminating a flat surface, which is parallel to the sensor of the time-of-flight imager and located at a known distance from the time-of-flight imager, with light emitted by a light source;   detecting light reflected by the flat surface with the sensor to obtain a measured depth map; and   determining the depth correction profile from the measured depth map.       
 
         [0020]    Favorably, the depth correction profile is determined from the measured depth map by:
       performing line averaging on lines of the sensor;   fitting an n th  order polynomial to the averaged line values to determine a first of two polynomials;   subtracting the averaged line values from the measured depth map;   performing column averaging on columns of the sensor; and   fitting an n th  order polynomial to the averaged column values to determine a second of the two polynomials.
 
In this way the depth correction profile is determined in an easy manner.
       
 
         [0026]    Alternatively, the depth correction profile is obtained by subtracting an expected depth map of the known object from the measured depth map. For this purpose advantageously the average of multiple measurements is determined. In this case it is not necessary to fit any functions to the depth correction profile. However, the depth correction profile will be less smooth, as in this case the noise caused by measurement tolerances has more influence on the final depth correction profile. 
         [0027]    In addition, instead of determining the depth correction profile from a depth map measured for a well defined object, i.e. a flat surface, it is likewise possible to directly measure or at least approximately calculate the propagation delays of the transfer gate pulse and the global shutter gate pulse within the array of pixels of the sensor. The determined delays are then transformed into depth values. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    For a better understanding the invention shall now be explained in more detail in the following description with reference to the figures. It is understood that the invention is not limited to this exemplary embodiment and that specified features can also expediently be combined and/or modified without departing from the scope of the present invention as defined in the appended claims. In the figures: 
           [0029]      FIG. 1  illustrates the principle of a time-of-flight imager according to the invention; 
           [0030]      FIG. 2  depicts a depth map of a flat surface obtained with a conventional time-of-flight imager; 
           [0031]      FIG. 3  explains the origin of propagation delays in the sensor of a time-of-flight imager; 
           [0032]      FIG. 4  shows a fit to the vertical shading with a 2 nd  order polynomial; 
           [0033]      FIG. 5  shows a fit to the horizontal shading with a 2 nd  order polynomial, 
           [0034]      FIG. 6  depicts a depth correction map obtained by combining the 2 nd  order polynomials of  FIGS. 4 and 5 , 
           [0035]      FIG. 7  depicts the depth map of  FIG. 3  after correction with the depth correction map of  FIG. 6 , 
           [0036]      FIG. 8  shows an exemplary scene of four objects arranged in different depths, 
           [0037]      FIG. 9  shows a depth map of the scene of  FIG. 8  obtained with a conventional time-of-flight imager, 
           [0038]      FIG. 10  depicts the depth map of  FIG. 9  after correction with the depth correction map of  FIG. 6 , and 
           [0039]      FIG. 11  shows a depth zoom into the corrected depth map of  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0040]      FIG. 1  illustrates the principle of a time-of-flight imager  6  according to the invention. The time-of-flight imager  6  includes a light source  2 , a lens  4 , a sensor  5 , and processing circuitry  7 . An object  1  located at a distance d of 75 cm from the time-of-flight imager  6  is illuminated by the light source  2 , e.g. a 4×4 array of infrared LEDs. The object  1  has a flat surface facing an array of pixels of the sensor  5 . The light  3  reflected by this flat surface is collected by the lens  4  and imaged onto the array of pixels of sensor  5 . This allows to measure a depth map of the object  1 . The processing circuitry  7  enables processing of the measured depth map. 
         [0041]    The depth map of the object  1  measured by the time-of-flight imager  6  is shown in  FIG. 2 . The grey values indicate the measured depth in cm. They are plotted against the pixels of the sensor  5  in x- and y-direction. Though the flat surface of the object  1  has a fixed distance d of 75 cm from the plane of the sensor  5 , the resulting depth map is apparently not flat. Instead, it shows a distance that increases from the expected value in the bottom right corner both for decreasing y-pixels and decreasing x-pixels. 
         [0042]    An explanation for the non-flat depth map of  FIG. 2  are propagation delays within the array of pixels of the sensor  5 , as shall be explained in the following with reference to  FIG. 3 . The sensor  5  includes a plurality of pixels  51 , which are arranged in lines and columns. The transfer gate pulse TG and the global shutter gate pulse GSG need to travel from an input to the respective pixels  51 . As can be seen the distances that the pulses have to cover vary for the different pixels  51 . This leads to an increasing propagation delay Δt h  in the horizontal direction as well as an increasing propagation delay Δt v  in the vertical direction, i.e. the already addressed shading. 
         [0043]    It has been found that a 2 nd  order polynomial allows to approximate the shading effect.  FIG. 4  depicts measured values of the amount of vertical shading in cm against the line number, as well a 2 nd  order polynomial fit to the measured values. The amount of vertical shading is found by line averaging. 
         [0044]    Similarly, as can be seen in  FIG. 5 , also the horizontal shading effect can be approximated by a 2 nd  order polynomial. The amount of horizontal shading is found by subtracting the line averaged image from the original image and then perform column averaging. 
         [0045]    The combination of the two 2 nd  order polynomial obtained for horizontal shading and vertical shading, respectively, results in a 3D polynomial correction image, as illustrated in  FIG. 6 . This correction image is subsequently used by the circuitry  7  to correct any depth map obtained by the sensor  5 . For example,  FIG. 7  depicts the depth map of  FIG. 3  after correction with the depth correction map of  FIG. 6 . Correction is performed by subtracting the correction image from the depth map measured by the time-of-flight imager  6 . As can be seen, the corrected depth map shows the expected distance over the whole sensor area. 
         [0046]    In the following the correction image shall be applied to a depth map measured by the time-of-flight imager  6  for an exemplary scene. The scene is illustrated in  FIG. 8 . Three objects  11 ,  12 ,  13 ,  14  are arranged at different distances from the time-of-flight imager  6 , namely at 45 cm, 75 cm, 105 cm, and 135 cm. The original depth map of this scene obtained by the time-of-flight imager  6  is shown in  FIG. 9 . Again, due to the shading effect the distances measured for the different objects  11 ,  12 ,  13 ,  14  are not constant. However, after subtracting the correction image the corrected depth map of  FIG. 10  is obtained. A zoom into this corrected depth map showing only a reduced depth range from 0 cm to 140 cm is depicted in  FIG. 11 . As can be seen, the distances of the different objects  11 ,  12 ,  13 ,  14  are essentially constant and fit to the expected distances.