Patent Publication Number: US-2023142214-A1

Title: Stereo imaging system with automatic disparity adjustment for displaying close range objects

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Application Serial No. 16/202,211, filed Nov. 28, 2018, which is a continuation of U.S. Application Serial No. 14/053,021 filed Oct. 14, 2013 (now U.S. Pat. No. 10,178,368), which claims benefit of provisional U.S. Application No. 61/717,443, filed Oct. 23, 2012 (now expired), each of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to stereo imaging systems. In particular, it relates to a stereo imaging system, and a method implemented therein, for providing automatic disparity adjustment for displaying close range objects. 
     BACKGROUND OF THE INVENTION 
     A stereo imaging system includes a stereoscopic camera which has two image capturing elements for capturing left and right stereo images. Details for such a stereo imaging system may be found, for example, in U.S. Pat. No. 6,720,988 entitled “Stereo Imaging System and Method for Use in Telerobotic System.” 
       FIG.  1    illustrates a schematic of the stereo geometry for two image capturing elements, e.g., left and right optical lens  101 ,  102 , which are separated by a baseline distance “b”. Left and right image planes  121 ,  122  are shown at a focal length “f” (i.e., a depth at which the left and right images are focused). The image planes  121 ,  122  represent stereo images that are captured by the lens  101 ,  102  and are bounded by their fields of view. The focal length may be adjusted within a focusing range, but the baseline distance is fixed for the stereoscopic camera. 
     A point “P” at a depth “Z” from the lens  101 ,  102  is seen at different points on the image planes  121 ,  122 . In particular, the point “P” is projected at a position “d1” on the left image plane  121  and projected at a position “d2” on the right image plane  122 . The difference or disparity “D” between the two positions “d2” and “d1” can be determined from the following well-known relationship: 
     
       
         
           
             
               D 
               b 
             
             = 
             
               f 
               z 
             
           
         
       
     
     Thus, as the depth “Z” gets smaller and smaller, the disparity “D” gets larger and larger. 
     Stereo images captured by the stereoscopic camera are displayed on a stereo viewer. As an example, the stereo viewer may have left and right display screens upon which left and right stereo images are respectively displayed. The stereo viewer in this case, may also have left and right eyepieces through which a user places his/her left and right eyes to respectively view the left and right display screens. 
     When objects are viewed at very close range by the stereoscopic camera, even after or while adjusting the camera focus, the user may have difficulty fusing his/her eyes on an image of the object being displayed on the stereo viewer due to the large disparity “D” between corresponding points on the left and right display screens. In addition, after the camera focus control has reached the end of its range, a practical limit may be placed on how close an object may be viewed relative to the stereoscopic camera. 
     To address this problem, another stereoscopic camera with a smaller baseline “b” (i.e., closer spacing between the image capturing elements) may be used at very close range to reduce the disparity “D” between displayed stereo images and consequently, allow a user to comfortably see the stereo images being displayed on the stereo viewer. However, the use of multiple stereoscopic cameras with different baselines adversely adds to the cost of the stereo imaging system and increases the difficulty of its use by an operator. 
     SUMMARY OF THE INVENTION 
     The embodiments of the invention are summarized by the claims that follow below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates a schematic of the stereo geometry for two image capturing elements of a stereo imaging system. 
         FIG.  2    illustrates a block diagram of a stereo imaging system utilizing aspects of the present invention. 
         FIG.  3    illustrates a flow diagram of a method for providing automatic disparity adjustment for both normal and close range viewing in a stereo viewer of a stereo imaging system utilizing aspects of the present invention. 
         FIG.  4    illustrates a schematic of a region of interest in a stereo view of a stereo imaging system utilizing aspects of the present invention. 
         FIG.  5    illustrates a schematic of a stereo view constructed from a pair of stereo images in a stereo imaging system utilizing aspects of the present invention. 
         FIGS.  6   a - 6   d    schematically illustrate steps performed in a first embodiment of a method for modifying stereo images as part of the method of  FIG.  3    utilizing aspects of the present invention. 
         FIGS.  7   a - 7   c    schematically illustrate steps performed in a second embodiment of a method for modifying stereo images as part of the method of  FIG.  3    utilizing aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  2    illustrates, as an example, a block diagram of a stereo imaging system  200  in which a method  300  utilizing the invention is implemented. A stereoscopic camera  201  is preferably a high-definition digital stereo camera which generates a video stream of stereo images captured at a frame rate of the camera, such as thirty frames per second. Each frame of stereo images includes a left stereo image  211  and a right stereo image  212  which has been captured at a high resolution, such as 1920×1080 pixels. An image processor  202  modifies the stereo images received from the stereoscopic camera  201  according to the method  300 . A stereo viewer  204  has left and right display screens for respectively displaying the modified left stereo image  221  and modified right stereo image  222  received from the image processor  202 . The resolution of the left and right display screens is typically a lower resolution, such as 1280×1024 pixels, than that of the camera  201 . 
     A depth sensing system  203  determines depth values for a region of interest in the stereo images and provides the depth value to the image processor  202  so that it may be used by the method  300 . The region of interest may be predefined as a default region in the stereo images, for example, the center region, or it may be user defined. As an example, the user may define the region of interest using a telestrator  231 , so that the user may draw a region of interest over one of the left and right stereo images being displayed at the time on the telestrator  231 . Details on such a telestration system may be found, for example, in U.S. 2007/0156017 entitled “Stereo Telestration for Robotic Surgery”, which is incorporated herein by reference. Alternatively, the user may define the center of a region of interest using a gaze tracking unit  232  which tracks the user’s gaze point on one or both of the display screens of the stereo viewer  204 . Details for such a gaze tracking system may be found, for example, in U.S. Application No. 61/554,741 entitled “Method and System for Stereo Gaze Tracking”. The location and/or dimensions of the region of interest may be predefined or definable by the user using any conventional means such as a Graphical User Interface (GUI). Regardless of how the region of interest is defined, it may be displayed for the convenience of the user on the stereo viewer  204  at its proper location as an overlay to any three-dimensional objects or surface topology being displayed therein at the time. The overlay may be a three-dimensional overlay at the same depths and following the contour of the underlying objects or surface topology or it may be a two-dimensional overlay floating over the underlying objects or surface topology at a specified depth value. 
     The depth sensing system  203  may determine the depth values for the region of interest in the stereo images using one or a combination of known methods. As an example, a structured light technique may be used in which a known light pattern is projected onto a three-dimensional scene and the relative light intensities on the scene tracked to derive a depth map for the scene. See, e.g., Daniel Scharstein and Richard Szeliski, “High-Accuracy Stereo Depth Maps Using Structured Light,” IEEE Computer Society Conference on Computer Vision and Pattern Recognition (CVPR 2003), vol. 1, pages 195-202, Madison, Wis., June 2003. As another example, the depth value may be determined by determining corresponding points in stereo images using a robust sparse image matching algorithm, determining disparities between the corresponding points, and converting the disparities to depths using a predetermined disparity to depth mapping. See, e.g., U.S. Pat. No. 8,184,880 entitled “Robust Sparse Image Matching for Robotic Surgery”, which is incorporated herein by reference. As yet another example, a laser range finder may be used for determining depth values of a three-dimensional scene. The depth value may be an average depth value for the surface topology appearing in the region of interest. When tools, which are being used to interact with objects of the surface topology, appear above the surface topology, depth values for the tools which occlude part of the surface topology may be included or excluded from the calculation. 
       FIG.  3    illustrates, as an example, a flow diagram of a method  300  for modifying stereo images captured at close range to aid a user to fuse his/her eyes on objects in the modified stereo images when they are displayed on a stereo viewer. Typically, the method may be used while the user is adjusting the distance of the camera lens from one or more objects comprising a surface topology. Manual or auto focus may be employed during the distance adjustment. The method  300  is preferably implemented as program code executed by the image processor  202 . In this example, the method is only active when the mode is turned ON, giving a user the option to turn OFF the mode and not use the method. Alternatively, the mode may always be ON, giving the user no option to turn it OFF. To help describe the method,  FIGS.  4 ,  5 ,  6   a - 6   d , and  7   a - 7   c    are provided. 
     In block  301 , the method determines a depth value for a region of interest appearing in a current frame in a video stream of stereo images being received from a stereoscopic camera. The depth value is determined, for example, by the depth sensing system  203  as previously described. 
     As an example of a region of interest,  FIG.  4    illustrates a region of interest  430  which is cylindrically shaped and extends within the stereo view  410  to a surface topology (not shown). The stereo view  410  is the intersection of the fields of view  401 ,  402  of the image capturing elements  101 ,  102 . As shown in  FIG.  5   , the region of interest  430  is centered within the stereo view  410 , but offset in the left and right stereo images  211 ,  212  by a disparity corresponding to the average depth value of the surface topology. Note that the area  420  outside the stereo view  410  is only viewable in monovision since it is viewable by only the image capturing element  101 . 
     In block  302 , a determination is made whether the depth value is less than a first threshold value Z1 (as shown in  FIG.  4   ). The first threshold value Z1 is a depth value at which it becomes difficult for a user viewing the stereo images on the stereo viewer to visually fuse the stereo images with his/her eyes. Typically, this depth value is a function of the baseline distance “b” between the image capturing elements of the stereoscopic camera. It may also be a function of the focal lengths and fields of view of the image capturing elements. As an example, a depth value of 5 centimeters has been empirically determined as such a point for a medical robotic system employing a stereoscopic endoscope. Thus, the first threshold value Z1 may be empirically determined and pre-programmed into the stereo imaging system as a default value. Additionally, or alternatively, it may be specified and/or altered by the operator in a conventional manner to accommodate specific user preferences. 
     If the determination in block  302  is NO, then the method loops back to block  301  to process a next frame in the video stream of stereo images without any disparity adjustment so that the stereo imaging system operates in a normal viewing mode. 
     On the other hand, if the determination in block  302  is YES, then in block  303 , the method determines a target disparity adjustment. It may do this, for example, by using either an empirically determined equation which is a function of depth values or using an empirically determined look-up table which is indexed by depth values. When using the look-up table, linear or best curve fitting interpolation between look-up table values may also be performed as necessary. 
     To empirically determine the target disparity adjustment for a depth value, the left and right stereo images may be shifted in horizontal directions to reduce the disparity between them until a user viewing the stereo images on a stereo viewer is able to comfortably fuse his/her eyes on the stereo images being displayed at the time. As the depth value increasingly becomes smaller, more horizontal shifting may be required to achieve comfortable fusing of stereo images. Thus, target disparity adjustments as a function of depth value may be empirically determined in this manner starting with the target depth value Z1 and ending with a minimum depth value Z2 (as shown in  FIG.  4   ). The minimum depth value Z2 is reached when the intersection of the fields of view of the two image capturing elements becomes too small for practical stereo viewing and/or too small to accommodate the horizontal pixel shifting for the target disparity adjustments. Note that in determining the target disparity adjustments in this manner, conversion between the low resolution display screens of the stereo viewer and the high resolution stereo images is implicitly accommodated. As an example of an empirically determined function for target disparity adjustments, a medical robotic system employing a stereoscopic endoscope has been found to have satisfactory results with a target disparity adjustment which is linearly scaled from 24 pixels at 5 cm (the target depth value) to 250 pixels at 2 cm (the minimum depth value). As with the first threshold depth value, the minimum depth value Z2 may be pre-programmed into the stereo imaging system or it may be user specified and/or user alterable using conventional human/computer interaction means. Once the minimum depth value is reached, the target disparity adjustment may remain fixed as the depth value further becomes smaller. Alternatively, the stereo imaging system may restrict the depth value from becoming less than the minimum depth value. 
     In block  304 , the method optionally filters the target disparity adjustment to avoid an abrupt change and/or jitter in the stereo images to be displayed on the stereo viewer. As an example, a maximum pixel shift per frame may be specified, such as 1-3 pixels per frame. The use of low pass filters, moving averages, and other well known smoothing techniques may also be used. 
     In block  305 , the method modifies the stereo images. In modifying the stereo images, the method takes into account the target disparity adjustment determined in block  303 , the optional filtering performed in block  304 , and the respective resolutions of the stereo images and the left and right display screens of the stereo viewer. The modification may be performed using the sub-blocks  311 ,  312 ,  313 . 
     In a first embodiment, as shown in  FIGS.  6   a - 6   d   , all three sub-blocks  311 - 313  are used. 
     In sub-block  311 , a shift step may be performed such as shown in  FIGS.  6   a - 6   b   . In this case, all pixels are shifted horizontally in both the left and right stereo images  211 ,  212  so as to reduce the disparity between corresponding points by the filtered target disparity adjustment. As a result, shifted-out pixel columns  603 ,  604  are not used (e.g., discarded) and shifted-in pixel columns  601 ,  602  are filled with filler pixel values such as a specific color. A plus sign “+” is shown in the center of each stereo image to provide a reference point. 
     In sub-block  312 , an image cropping step is performed such as shown in  FIG.  6   c   . In this first embodiment, the shifted-in pixel columns  601 ,  602  are cropped out along with areas  611 ,  612  surrounding areas  621 ,  622 , which will be described further with respect to the following zooming step. Note that the plus sign “+” is in the same place before and after the cropping, because the same amount is cropped off each pair of opposing ends. 
     In sub-block  313 , an image zooming step is performed such as shown in  FIG.  6   d   . In this example, the areas  621 ,  622  remaining after the cropping step of sub-block  312  are zoomed-out so that the resulting left and right stereo images  221 ,  222  have the same resolutions as their respective left and right display screens of the stereo viewer. With this in mind, the areas  621 ,  622  are determined so that when zoomed-out, proper filtered disparity adjustments result in the resulting left and right stereo images  221 ,  222  which are to be displayed in the stereo viewer. 
     In a second embodiment, as shown in  FIGS.  7   a - 7   c   , only sub-blocks  312  and  313  are used since sub-block  311  is eliminated with a modification to sub-block  312 . 
     In modified sub-block  312 , the stereo images  211 ,  212  are cropped so as to directly result in the areas  621 ,  622 , which are zoomed out as previously described with respect to sub-block  313 . 
     Finally, in block  306 , the method displays the modified stereo images on the left and right display screens of the stereo viewer. The method then loops back to block  301  to process the next frame of stereo images. 
     Although the various aspects of the present invention have been described with respect to a preferred embodiment, it will be understood that the invention is entitled to full protection within the full scope of the appended claims.