Patent Publication Number: US-9852357-B2

Title: Clear path detection using an example-based approach

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 12/581,792 filed on Oct. 19, 2009, which is a continuation-in-part of U.S. application Ser. No. 12/474,594 filed on May 29, 2009, which is a continuation-in-part of U.S. application Ser. No. 12/108,581 filed on Apr. 24, 2008. U.S. application Ser. No. 12/581,792 filed on Oct. 19, 2009 claims the benefit of U.S. Provisional Application 61/215,745 filed on May 8, 2009. U.S. application Ser. No. 12/581,792, U.S. application Ser. No. 12/474,594, U.S. application Ser. No. 12/108,581, and U.S. Provisional Application 61/215,745 are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure is related to automated or semi-automated control of a motor vehicle. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Autonomous driving systems and semi-autonomous driving systems utilize inputs regarding the road and other driving conditions to automatically control throttle and steering mechanisms. Accurate estimation and identification of a clear path over which to operate a motor vehicle is critical to successfully replacing the human mind as a control mechanism for vehicle operation. 
     Road conditions can be complex. Under normal operation of a vehicle, the human operator makes hundreds of observations per minute and adjusts operation of the vehicle on the basis of perceived road conditions. One aspect of perceiving road conditions is the perception of the road in the context of objects in and around the roadway and navigating a clear path through any objects. Replacing human perception with technology preferentially includes some means to accurately perceive objects, for example, including stationary objects such as roadside curbs and moving objects such as other vehicles, and road conditions, such as lane markers, potholes, or icy patches upon the roadway, and continue to effectively navigate around such navigational concerns. 
     Technological means for perceiving an object or road conditions include data from visual cameras, radar imaging, LIDAR, vehicle to vehicle communications, vehicle to infrastructure communications, and use of global positioning data with a digital map. Cameras translate visual images in the form of radiation such as light patterns or infrared signatures into a readable data format. One such data format includes pixelated images, in which a perceived scene is broken down into a series of pixels. Radar imaging utilizes radio waves generated by a transmitter to estimate shapes and objects present in front of the transmitter. Patterns in the waves reflecting off these shapes and objects can be analyzed and the locations of objects can be estimated. Once data has been generated regarding the ground in front of the vehicle, the data must be analyzed to estimate the presence of objects or road conditions from the data. By using cameras and radar imaging systems, ground or roadway in front of the vehicle can be analyzed for the presence of objects or road conditions that might need to be avoided. However, the mere identification of potential navigational concerns to be avoided does not complete the analysis. An important component of any autonomous system includes how potential navigational concerns identified in perceived ground data are processed and manipulated to identify a clear path in which to operate the vehicle. 
     One known method to identify a clear path in which to operate the vehicle is to catalog and provisionally identify all perceived navigational concerns and identify a clear path in light of the locations and behaviors of identified concerns. Images may be processed to identify and classify navigational concerns according to their form and relationship to the roadway. While this method can be effective in identifying a clear path, it requires a great deal of processing power, for example, requiring the recognition and separation of different objects in the visual image, for instance, distinguishing between a tree along the side of the road and a pedestrian walking toward the curb. Such methods can be slow or ineffective to process complex situations or may require bulky and expensive equipment to supply the necessary processing capacity. 
     SUMMARY 
     A method for detecting a clear path of travel for a vehicle using a current image generated by a camera includes defining an exemplary vanishing point for each of a plurality of sample images. Features within each of the plurality of sample images are identified, the current image generated by the camera is monitored, and features within the current image are identified. The current image is matched to at least one of the sample images based upon the identified features within the current image and the identified features within the plurality of sample images. A vanishing point for the current image is determined based upon the matching and the exemplary vanishing point for each of the matched sample images. The vanishing point for the current image is utilized to define a clear path of travel, and the clear path of travel is utilized to navigate the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates an exemplary arrangement of a vehicle equipped with a camera and a radar imaging system in accordance with the disclosure; 
         FIG. 2  illustrates a known method to determine a clear path for autonomous driving in accordance with the disclosure; 
         FIG. 3  illustrates an exemplary method to determine a clear path utilizing a likelihood analysis of an image in accordance with the disclosure; 
         FIG. 4  illustrates an exemplary method to analyze an image in accordance with the disclosure; 
         FIG. 5  illustrates an exemplary method to define a classification error by tuning a single threshold in accordance with the disclosure; 
         FIGS. 6A, 6B, and 6C  illustrate an exemplary determination of an image difference by calculating an absolute image intensity difference in accordance with the disclosure; 
         FIG. 7  illustrates an exemplary method to classify a feature as a portion of a clear path and as a detected object at the same time as a method of image analysis in accordance with the disclosure; 
         FIG. 8  further illustrates an exemplary method to classify a feature as a portion of a clear path and as a detected object at the same time as a method of image analysis in accordance with the disclosure; 
         FIG. 9  is a flowchart for an exemplary patch-based method for detecting a clear path in accordance with the disclosure; 
         FIG. 10  is a flowchart for a textureless pixel-based method for detecting a clear path, in accordance with the disclosure; 
         FIG. 11  shows an exemplary image used to illustrate a vehicle detection analysis, in accordance with the disclosure; 
         FIG. 12  shows an exemplary method for feature identification and analysis, in accordance with the disclosure; 
         FIG. 13  graphically illustrates an exemplary feature space including exemplary feature values, in accordance with the disclosure; and 
         FIG. 14  schematically depicts operation of an exemplary example-based method, in accordance with the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same,  FIG. 1  illustrates an exemplary arrangement of camera  110  located on the front of a vehicle  100  and pointed toward the ground in front of the vehicle  100  in accordance with the disclosure. Camera  110  is in communication with processing module  120  containing logic to process inputs from camera  110 . The vehicle  100  may also be equipped with a radar imaging system  130 , which, when present, is also in communication with processing module  120 . It should be appreciated by those having ordinary skill in the art that the vehicle  100  could utilize a number of methods to identify road conditions in addition or in the alternative to the use of camera  110  and the radar imaging system  130 , including GPS information, information from other vehicles in communication with the vehicle  100 , historical data concerning the particular roadway, biometric information such as systems reading the visual focus of the driver, a radar imaging system, or other similar systems. The disclosure herein can be applied to various device arrangements and is therefore not limited thereby. 
     The camera  110  is a device well known in the art capable of translating visual inputs in the form of light, infrared, or other electro-magnetic (EM) radiation into a data format readily capable of analysis, e.g., a digital, pixelated image. In one embodiment, the camera  110  uses a charge coupled device (CCD) sensor to generate images indicating a field-of-view. Preferably, the camera  110  is configured for continuous image generation, e.g., 30 images generated per second. Images generated by the camera  110  may be stored in memory within the camera  110  or transferred to the processing module  120  for storage and/or analysis. Preferably, each image generated by the camera  110  is a two-dimensional image of known pixel dimensions comprising a plurality of identifiable pixels. The plurality of identifiable pixels may be stored and analyzed using an array. Each pixel may be represented in the array as a set of bits or a plurality of sets of bits wherein the bits correspond to a color on a predetermined palette or color map. Each pixel may be expressed as a function of a plurality of color intensity values such as in a red-green-blue (RGB) color model or a cyan-magenta-yellow-key (CMYK) color model. Preferably, each pixel comprises a plurality of sets of bits wherein each set of bits corresponds to a color intensity and a color intensity value e.g., a first set of bits corresponds to a red color intensity value, a second set of bits corresponds to a green color intensity value, and a third set of bits corresponds to blue color intensity value on the RGB color model. 
     The radar imaging device  130  is a device well known in the art incorporating a transmitter capable of emitting radio waves or other EM radiation, a receiver device capable of sensing the emitted waves reflected back to the receiver from objects in front of the transmitter, and means to transfer the sensed waves into a data format capable of analysis, indicating for example range and angle from the objects off which the waves reflected. Alternatively, the radar imaging device  130  may be replaced or supplemented with a light detection and ranging (LIDAR) system configured to transmit and receive optical energy. The received optical energy may be used to determine object geometric dimensions and/or geometrical proximity to the vehicle  100 . It will be noted that radar imaging device  130  is optional and unnecessary to perform many of the methods disclosed herein, wherein processing of visual images is capable of accomplishing clear path detection. The term “clear path” as used herein is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and it is not to be limited to a special or customized meaning), and refers without limitation to a path free of objects exceeding a threshold. 
     The processing module  120  is illustrated in  FIG. 1 , and described herein as a discrete element. Such illustration is for ease of description and it should be recognized that the functions performed by this element may be combined in one or more devices, e.g., implemented in software, hardware, and/or application-specific integrated circuitry. The processing module  120  can be a general-purpose digital computer comprising a microprocessor or central processing unit, storage mediums comprising non-volatile memory including read only memory and electrically programmable read only memory, random access memory, a high speed clock, analog to digital and digital to analog circuitry, and input/output circuitry and devices and appropriate signal conditioning and buffer circuitry. In the alternative, processing module  120  can be a digital signal processing (DSP) unit, such as a customized integrated circuit such as a field programmable gate array. The processing module  120  has a set of processing algorithms, comprising resident program instructions and calibrations stored in the non-volatile memory and executed to provide desired functions. The algorithms are preferably executed during preset loop cycles. Algorithms are executed by the central processing unit and are operable to monitor inputs from the aforementioned sensing devices and execute control and diagnostic routines to control operation of the actuators, using preset calibrations. Loop cycles may be executed at regular intervals, for example each 3.125, 6.25, 12.5, 25 and 100 milliseconds during ongoing engine and vehicle operation. Alternatively, algorithms may be executed in response to occurrence of an event. 
     The processing module  120  executes algorithmic code stored therein to monitor related equipment such as camera  110  and radar imaging system  130  and execute commands or data transfers as indicated by analysis performed within the processing module. Processing module  120  may include algorithms and mechanisms to actuate autonomous driving control by means known in the art and not described herein, or processing module  120  may simply provide information to a separate autonomous driving system. Processing module  120  is adapted to receive input signals from other systems and the operator as necessary depending upon the exact embodiment utilized in conjunction with the control module. 
       FIG. 2  illustrates a known method to determine a clear path for autonomous driving in accordance with the disclosure. Image  10  is generated corresponding to the roadway in front of vehicle  100 . Through one of various methods, objects  40 A,  40 B, and  40 C are identified within image  10 , and each object is categorized and classified according to filtering and trained object behaviors. Separate treatment of each object can be computationally intensive, and requires expensive and bulky equipment to handle the computational load. An algorithm processes all available information regarding the roadway and objects  40  to estimate a clear path available to vehicle  100 . Determination of the clear path depends upon the particular classifications and behaviors of the identified objects  40 . 
       FIG. 3  illustrates an exemplary method to determine a clear path for autonomous or semi-autonomous driving in accordance with the disclosure. Image  10  is depicted including ground  20 , horizon  30 , and objects  40 . Image  10  is collected by camera  110  and represents the road environment in front of vehicle  100 . Ground  20  represents the zone of all available paths open to travel without regard to any potential objects. The method of  FIG. 3  that determines a clear path upon ground  20  starts by presuming all of ground  20  is clear, and then utilizes available data to disqualify portions of ground  20  as not clear. In contrast to the method of  FIG. 2  which classifies every object  40 , the method of  FIG. 3  instead analyzes ground  20  and seeks to define a clear path confidence likelihood from available data that some detectable anomaly which may represent object  40  limits or makes not clear that portion of ground  20 . This focus upon ground  20  instead of objects  40  avoids the complex computational tasks associated with managing the detection of the objects. Individual classification and tracking of individual objects is unnecessary, as individual objects  40  are simply grouped together as a part of the overall uniform limitation upon ground  20 . Ground  20 , described above as all paths open to travel without discrimination, minus limits placed on ground  20  by areas found to be not clear, define clear path  50 , depicted in  FIG. 3  as the area within the dotted lines, or an area with some threshold confidence likelihood of being open for travel of vehicle  100 . 
     Object  40  that creates not clear limitations upon ground  20  can take many forms. For example, an object  40  can represent a discrete object such as a parked car, a pedestrian, or a road obstacle, or object  40  can also represent a less discreet change to surface patterns indicating an edge to a road, such as a road-side curb, a grass line, or water covering the roadway. Object  40  can also include an absence of flat road associated with ground  20 , for instance, as might be detected with a large hole in the road. Object  40  can additionally include an indicator without any definable change in height from the road, but with distinct clear path implications for that segment of road, such as a paint pattern on the roadway indicative of a lane marker. The method disclosed herein, by not seeking to identify object  40  but by taking visual cues from ground  20  and anything in proximity to the ground in image  10 , evaluates a clear path confidence likelihood of clear versus not clear and adjusts the control of vehicle  100  for the presence of any object  40 . 
     Numerous methods for automated analysis of two-dimensional (2D) images are possible. Analysis of image  10  is performed by an algorithm within processing module  120 .  FIG. 4  illustrates one exemplary method which may be applied to analyze image  10  in accordance with the disclosure. This method subdivides image  10  and identifies a sub-image or patch  60  of ground  20  for analysis, extracts features or analyzes the available visual information from patch  60  to identify any interesting or distinguishing features within the patch, and classifies the patch according to a confidence likelihood of being a clear path according to analysis of the features. Patches with greater than a certain threshold of likeliness are classified as clear, and a compilation of patches can be used to assemble a clear path within the image. 
     Patch  60 , as a sub-image of image  10 , can be identified through any known means, such as random search or swarm search of image  10 . Alternatively, information regarding the presence of an object  40  available from some other source of information, such as radar imaging system  130 , can be used to identify a patch to analyze the portion of image  10  which should describe object  40 . Image  10  may require many patches  60  to analyze the whole image. In addition, multiple overlaying patches or patches of different size could be used to fully analyze a region of image  10  containing information of interest. For instance, a small patch  60  might be used to analyze a small dot on the road; however, a large patch  60  might be required to analyze a series of dots which in isolation might seem uninteresting, but in context of the entire series, could indicate an object  40  of interest. In addition, the resolution of patches applied to a particular area may be modulated based upon information available, for instance, with more patches being applied to a region of image  10  wherein an object  40  is thought to exist. Many schemes or strategies can be utilized to define patches  60  for analysis, and the disclosure is not intended to be limited to the specific embodiments described herein. 
     Once a patch  60  has been identified for analysis, processing module  120  processes the patch by applying known feature identification algorithms to the patch. Additionally, processing module  120  may perform analysis of the location of the patch in context to the location of the vehicle. Feature identification algorithms search available visual information for characteristic patterns in the image associated with an object including features defined by line orientation, line location, color, corner characteristics, other visual attributes, and learned attributes. Feature identification algorithms may be applied to sequential images to identify changes corresponding to vehicle motion, wherein changes not associated with ground movement may be identified not clear path. Learned attributes may be learned by machine learning algorithms within the vehicle, but are most frequently programmed offline and may be developed experimentally, empirically, predictively, through modeling or other techniques adequate to accurately train distinguishing attributes. 
     Once features in patch  60  have been extracted, the patch is classified on the basis of the features to determine the confidence likelihood that the patch is a clear path. Likelihood analysis is a process known in the art by which a likelihood value or a confidence is developed that a particular condition exists. Applied to the present disclosure, classification includes likelihood analysis to determine whether the patch represents a clear path or if ground  20  in this patch is limited by an object  40 . Classification is performed in an exemplary embodiment by application of classifiers or algorithms trained with a database of exemplary road conditions and interactions with detected objects. These classifiers allow processing module  120  to develop a fractional clear path likelihood value for patch  60 , quantifying a confidence between zero and one that the features identified within the patch do not indicate a limiting object  40  which would inhibit free travel of vehicle  100 . A threshold confidence can be set, defining the clear path likelihood required to define the patch as a clear path, for instance by the following logic:
 
Confidence=ClearPathLikelihood( i ) If_Confidence&gt;0.5,then_patch=clearpath  [1]
 
In this particular exemplary embodiment, a confidence of 50% or 0.5 is selected as the threshold confidence. This number can be developed experimentally, empirically, predictively, through modeling or other techniques adequate to accurately evaluate patches for clear path characteristics.
 
     The likelihood analysis, as mentioned above, may be performed in one exemplary embodiment by application of trained classifiers to features extracted from a patch. One method analyzes the features a-priori using a training set of images. In this training stage, distinguishing features are selected from a raw feature set, the distinguishing features being defined by methods known in the art, such as Haar wavelet, Gabor wavelet, and Leung-and-Malik filter bank. In addition, 2D image location information based on each feature&#39;s minimal classification errors, calculated as the sum of false acceptance rate (FAR) and false rejection rate (FRR), may be utilized by tuning a single threshold as illustrated in  FIG. 5 . This classification error can be described through the following expression:
 
ClassificationError( i )=FAR i +FRR i   [2]
 
Information from the trained classifiers is used to classify or weight the feature as indicating a clear path or not clear path, the particular classification depending upon the strength of comparisons to the trained data. Classification of the feature, if the feature is the only feature within the patch, may be directly applied to the patch. Classification of a patch with multiple features identified may take many forms, including the patch being defined by the included feature most indicative of the patch being not clear or the patch being defined by a weighted sum of all of the features included therein.
 
     The above method can be utilized to examine an individual image  10  and estimate a clear path  50  based upon visual information contained within image  10 . This method may be repeated at some interval as the vehicle travels down the road to take new information into account and extend the formulated clear path to some range in front of the vehicle&#39;s new position. Selection of the interval must update image  10  with enough frequency to accurately supply vehicle  100  with a clear path in which to drive. However, the interval can also be selected to some minimum value to adequately control the vehicle but also not to unduly burden the computational load placed upon processing module  120 . 
     Clear path detection can be accomplished through a single image  10  as described above. However, processing speed and accuracy can be improved with the addition of a second image taken in close time proximity to the original image, such as sequential images from a streaming video clip. A second image allows direct comparison to the first and provides for updated information regarding progression of the vehicle and movement of detected objects. Also, the change of perspective of camera  110  allows for different analysis of features from the first image: a feature that may not have shown up clearly or was indistinct in the first image may display at a different camera angle, stand out more distinctly, or may have moved since the first image, allowing the classification algorithm an additional opportunity to define the feature. 
     Processing of a second image in relation to the original image  10  can be performed by calculating an image difference. If the image difference of a point of interest, such as a feature identified by radar, is not zero, then the point can be identified as embodying new information. Points where the image difference does equal zero can be eliminated from analysis and computation resources may be conserved. Methods to determine image difference include absolute image intensity difference and vehicle-motion compensated image difference. 
     Determining an image difference by calculating an absolute image intensity difference can be used to gather information between two images. One method of absolute image intensity difference includes determining equivalent image characteristics between the original image and the second image in order to compensate for movement in the vehicle between the images, overlaying the images, and noting any significant change in intensity between the images. A comparison between the images indicating a change in image intensity in a certain area contains new information. Areas or patches displaying no change in intensity can be de-emphasized in analysis, whereas areas displaying clear changes in intensity can be focused upon, utilizing aforementioned methods to analyze patches on either or both captured images. 
       FIGS. 6A, 6B, and 6C  illustrate an exemplary determination of an image difference by calculating an absolute image intensity difference in accordance with the disclosure.  FIG. 6A  depicts an original image.  FIG. 6B  depicts a second image with changes from the original image. In particular the depicted circular shape has shifted to the left. A comparison of the two images as illustrated in  FIG. 6C , an output representing the result of an absolute image intensity difference comparison, identifies one region having gotten darker from the first image to the second image and another region having gotten lighter from the first image to the second image. Such a method can be described as differencing. Analysis of the comparison yields information that some change as a result of movement or change of perspective is likely available in that region of the images. In this way, absolute image intensity difference can be used to analyze a pair of sequential images to identify a potentially not clear path. 
     Likewise, determining an image difference by calculating a vehicle-motion compensated image difference can be used to gather information between two images. Many methods to calculate a vehicle-motion compensated image difference are known. One exemplary method of vehicle-motion compensated image difference includes analyzing a potential object as both a stationary portion of a clear path and a detected object at the same time. Likelihood analysis is performed on features identified corresponding to the potential object from both classifications at the same time, and the classifications may be compared, for example, through the following logic:
 
Confidence( i )=ClearPathLikelihood( i )−DetectedObjectLikelihood( i ) If_Confidence&gt;0,then_patch=clearpath  [3]
 
     In this exemplary comparison, if confidence(i) is greater than zero, then the patch containing the feature is classified as a clear path. If confidence(i) equals or is less than zero, then the patch containing the feature is classified as not a clear path or limited. However, different values may be selected for the confidence level to classify the patch as a clear path. For example, testing may show that false positives are more likely than false negatives, so some factor or offset can be introduced. 
       FIG. 7  illustrates one method to classify a feature as a portion of a clear path and as a detected object at the same time as described above in accordance with the disclosure. Image  10  includes object  40 , trapezoidal projection  70 , and rectangular projection  80 . This method utilizes an assumption projecting object  40  as a flat object on the ground within projection  70  to test the classification of the feature as a portion of a clear path. The method also utilized an assumption projecting object  40  as a vertical object within rectangular projection  80  to test the classification of the feature as a detected object.  FIG. 8  illustrates comparisons made in data collected between the two images to evaluate the nature of object  40  in accordance with the disclosure. Camera  110  at time t 1  observes and captures data from object  40  in the form of a first image. If object  40  is an actual detected object, the profile observed by camera  110  of object  40  at time t 1  will correspond to point  90 A. If object  40  is a flat object in the same plane as ground  20 , then the profile observed by camera  110  of object  40  at time t 1  will correspond to point  90 B. Between times t 1  and t 2 , camera  110  travels some distance. A second image is captured at time t2, and information regarding object  40  can be tested by applying an algorithm looking at visible attributes of the object in the second image in comparison to the first image. If object  40  is an actual detected object, extending upward from ground  20 , then the profile of object  40  at time t 2  will be observed at point  90 C. If object  40  is a flat object in the same plane as ground  20 , then the profile of object  40  at time t2 will be observed at point  90 B. The comparison derived through vehicle-motion compensated image difference can directly assign a confidence by application of classifiers based on the observations of points  90 , or the comparison may simply point to the area displaying change as a point of interest. Testing of the object against both classifications, as a flat object and as an actual detected object, allows either the area including object  40  to be identified for further analysis through analysis of a patch as described above or direct development of a clear path likelihood and a detected object likelihood for comparison, for example as in logic expression (3) above. 
     Information available from analysis of the second image can additionally be improved by integration of information regarding movement of the vehicle, such as speed and yaw-rate. Information regarding vehicle motion is available from a number of sources, including the vehicle speedometer, vehicle dynamic sensors or wheel speed sensors, anti-lock braking mechanisms, and GPS location systems. Algorithms may utilize this vehicle movement information, for example, in conjunction with the projections described in  FIGS. 7 and 8  to project angles which should exist in a feature laying flat on the ground in the second image based upon data from the first image and the measured movement of the vehicle between the images. 
     The number of images utilized for comparison need not be limited to two. Multiple image analysis can be performed at multiple iterations, with an object being tracked and compared over a number of cycles. As mentioned above, computational efficiency can be gained by utilizing image difference analysis to identify points of interest and eliminating areas with zero difference from subsequent analyses. Such efficiencies can be used in multiple iterations, for example, saying that only points of interest identified between a first and second image will be analyzed in the third and fourth images taken. At some point, a fresh set of images will need to be compared to ensure that none of the areas showing zero difference have had any change, for example a moving object impinging upon a previously identified clear path. The utilization of image difference analyses and of focused analyses, eliminating areas identified with zero change, will vary from application to application and may vary between different operating conditions, such as vehicle speed or perceived operating environment. The particular utilization of image difference analyses and of focused analyses can take many different embodiments, and the disclosure is not intended to be limited to the specific embodiments described herein. 
     Multiple methods are herein disclosed for using the clear path detection analysis to identify a clear path including patch-based methods and pixel-based methods. The methods herein are preferably executed in the processing module  120 , but may be combined in one or more devices, e.g., implemented in software, hardware, and/or application-specific integrated circuitry. Patch-based methods are described above and herein with reference to  FIG. 9 . Pixel-based methods are described herein with reference to  FIGS. 11 and 12 . It will be appreciated that pixel-based methods make decisions based upon pixel or a group of pixels. This decision for the component patch being analyzed can be based upon pixels contained within the patch, for example, with the patch being determined to be unclear if any or some minimum number of pixels within the patch are determined to be not clear. Exemplary pixel-based methods include textureless and texture-rich methods. Texture-rich methods analyze pixelated features in an image for distinct interest points based upon a contextual view within the image. The interest points are mapped corresponding to a field-of-view in from on the vehicle  100  and a clear path is determined based upon topography of the interest points within the map. Textureless methods filter from an image non-conforming regions of the image as not belonging to a planar, consistent road surface, remaining pixels correspond to the clear path. As described hereinabove, patch-based methods are computationally, relatively fast, while pixel-based methods are computationally, relatively slow. 
       FIG. 9  illustrates an exemplary patch-based method  200  wherein input from a camera is analyzed to determine a clear path likelihood in accordance with the disclosure. The patch-based method  200  is illustrated in  FIG. 9 , and described herein as comprising discrete elements. Such illustration is for ease of description and it should be recognized that the functions performed by these elements may be combined in one or more devices, e.g., implemented in software, hardware, and/or application-specific integrated circuitry. For example, the patch-based method  200  may be executed as one or more algorithms in the processing module  120 . 
     During operation, the camera  110  generates an image for analysis in the processing module  120  ( 202 ). The processing module  120  identifies patches in the image and selects a patch for analysis ( 204 ). Multiple methods for defining a plurality of patches sufficient to adequately analyze the image are contemplated by this disclosure. As described above, patches may be defined according to random search or swarm search methods. Alternatively, information from some other source of information, such as a radar imaging system, can be used to define a patch to analyze the portion of the image. In addition, multiple overlaying patches can be utilized based upon the perceived area of interest upon the image to be analyzed. Additionally, methods can be utilized to define patches according to anticipated road surface in front of the vehicle and resulting clear path patterns, for example, including a fixed-grid patch method, sectioning off some or all of the image according to a regular patch pattern, and a perspective patch method, similar to the fixed-grid patch method except that the patch sizes and/or shapes are modulated based upon a perspective view of the road and consideration for an amount of road surface contained within each of the patches. Such an anticipated road surface in front of the vehicle can be adjudged by an initial review of the input image, for example, utilizing clearly visible lines that could define lane boundaries as an initial reference to define a patch pattern. In another example, a patch pattern from an immediately previous iteration of the clear path method could be utilized and slowly modulated through the iterations, based upon the iteratively defined clear path and other indications that can be identified within the analyzed images. 
     The fixed-grid patch method identifies and divides the image into a plurality of patches based upon an area of interest and applies a fixed-grid pattern to the area of interest. The fixed-grid pattern sections substantially the entire area of interest in patches. The area of interest preferably encompasses the field-of-view immediately in front of the vehicle, however the area may be defined to encompasses more narrow fields-of-view. In one embodiment, the area of interest encompasses the field-of-view immediately in front of the vehicle and delimited by a horizon line or a vanishing point perceived for the roadway. The fixed-grid patch method can include patch tracking, identification, and analysis via a matrix, wherein each patch may be identified by a number of a number series. 
     The fixed-grid patch may be applied to the image using a number of methods. A first method includes applying the fixed-grid patch to sequential images using equal image coordinates. A second method includes applying the fixed-grid patch to the image using an identified interest point on the image, e.g., the horizon line. A third method includes applying the fixed-grid patch to the image compensating for vehicle movement, e.g., vehicle yaw. 
     The perspective patch method identifies and divides the image into a plurality of patches based upon field-of-view coordinates rather than image coordinates. An area of interest is identified as described hereinabove, wherein a perspective patch pattern is applied to the area of interest based upon estimated field-of-view coordinates, allocating patch size relative to an approximate area of ground covered by each patch. Such perspective patch definitions allow for a more detailed review of patches describing ground closer to the vehicle and less wasteful review of patches describing ground more distant from the vehicle. Patches can be aligned to the perceived lanes of travel upon the roadway, for example, as defined by lane markers and/or curbsides. Such patch definitions will frequently be trapezoidal, with the parallel sides of the trapezoids being parallel to the horizon or horizontal axis of the vehicle, and with the remaining sides of the trapezoids being dependent upon the defined road surface in the image view. Such a patch alignment is efficient to defining the road surface. However, computation of the grid of patches and the analysis of the patches is complicated by the many different shapes. The patches can instead be normalized to rectangle (or square) shapes, still parallel to the horizon or horizontal axis of the vehicle. Such rectangular patches are in ways less efficient in covering the road surface, for example, with portions of the curbside overlapping patches including actually clear roadway, but sufficient resolution of the patches and improved the computational efficiency can make such normalized perspective patches beneficial. 
     A filter or a set of filters may be applied to the selected patch ( 206 ), e.g., a lighting normalization filter. An exemplary normalization filter changes the range of pixel intensity values within the patch, thereby bringing the patch into a range that is more suitable for machine processes. For example, normalizing each pixel value to a zero mean and unit variance enhances the image contrast, specifically in a low lighting environments or when contrast is poor due to glare. A number of exemplary filters and filtering methods useful for image analysis are envisioned, and the disclosure is not intended to be limited to the particular exemplary embodiments described herein. 
     After filtering, feature extraction is executed on the selected patch using feature extraction algorithms ( 208 ). Feature extraction algorithms may analyze the selected patch for predetermined features, e.g., edges, corners, and blobs, and/or shapes, e.g., circles, ellipses, and lines. It will be appreciated that some features have meaning and others do not, and a process of feature selection can be utilized to identify a group of best features for analysis. A classifier training algorithm analyzes each feature and assigns a likelihood value ( 210 ). As mentioned above, classifiers or logic used in developing likelihood values are initially trained offline. Training may optionally be continued in the vehicle based upon fuzzy logic, neural networks, or other learning mechanisms known in the art. These trained classifiers perform a likelihood analysis upon the features extracted, and a likelihood value for the patch determined. This likelihood value expresses a confidence that the selected patch is clear. 
     The likelihood analysis may be augmented using spatially and/or temporally related patches to evaluate identified features during vehicle operation. When the feature extraction algorithm has identified a feature, the processing module  120  may spatially analyze the identified feature for consistency among adjacent or nearby patches. A feature identified in one patch may be compared to surrounding patches to determine if it is an aberration or consistent with the surrounding patches. A feature that is consistent with surrounding patches may be assigned a similar likelihood value to the surrounding patches, whereas a feature not consistent with surrounding patches can be assigned a different likelihood value. Similarly to the spatial analysis, when the feature extraction algorithm has identified a feature, the processing module  120  may temporally analyze the identified feature for consistency among temporally related patches, compensating for vehicle motion. For example, a feature analyzed in several frames and determined to have a high likelihood value through the image frames can be temporally compared to the same feature receiving a low likelihood value in a later frame. If the temporal analysis of the feature reveals new information, such as movement of the feature with respect to the roadway or increased perspective revealing the feature to be indicative of an object situated upon the road surface, then the lower likelihood value for the feature can be confirmed. If no new information is available, then the lower likelihood value for the feature in the present frame can be suspended as not necessarily indicating a not clear path. Subsequent frames can be analyzed similarly to establish the feature as significant or not. Similarly, according to methods described herein, the questionable feature can be analyzed with increased computational emphasis, either in the present image or in subsequent images. 
     The processing module  120  compares the likelihood value to a threshold likelihood value ( 214 ). If the likelihood value is greater than the threshold value, then the patch is identified as a clear path ( 218 ). If the likelihood value is not greater than the threshold value, then the patch is identified as a not clear path. 
     As described above, the patch-based method  200  may be repeated or reiterated in a number of ways, with the same image being analyzed repeatedly with the selection and analysis of different patches, and an identified patch can be tracked and analyzed for change over a number of sequential images. 
       FIG. 10  illustrates an exemplary texture-rich pixel-based method  300  wherein input from a camera is analyzed to determine a clear path likelihood in accordance with the disclosure. The texture-rich pixel-based method  300  is illustrated in  FIG. 10 , and described herein as comprising discrete elements. Such illustration is for ease of description and it should be recognized that the functions performed by these elements may be combined in one or more devices, e.g., implemented in software, hardware, and/or application-specific integrated circuitry. For example, the pixel-based method  300  may be executed as one or more algorithms in the processing module  120 . 
     During operation, the camera  110  generates an image for analysis in the processing module  120  ( 302 ). The processing module  120  analyzes the image for interest points, for example, examining pixel color intensity as described hereinabove and comparing the pixel or a group of pixels with surrounding pixels. An interest point is an identifiable pixel on an image and is associated with a set of visual information, i.e., texture-rich features, and is associated with objects located in the field-of-view. Through methods known in the art, e.g., a scale-invariant feature transform (SIFT), methods employing corner detection or other shape detection, or a Sobel filter, interest points can be identified and utilized to accomplish methods described herein ( 306 ). In one embodiment, redundant interest points, e.g., multiple interest points corresponding to the same feature, are removed for computational efficiency considerations. 
     Once the interest points are identified, the processing module  120  compares sequential images when the vehicle is in motion to match interest points from each image to corresponding points in the sequential images which correspond to the same points in the field-of-view, where possible ( 310 ). Matching includes using correspondence matching programming, e.g., a scale-invariant feature transform (SIFT) feature matching programming and optical flow programming, and may further include locating corresponding points through template matching, taking into account movement of the host vehicle, and making a best estimate whether two points represent the same object or feature visible in the field-of-view. Template matching may be determined using one of multiple methods, including one of several known template matching programming methods to find the corresponding interest points, e.g., Lucas-Kanade or Horn-Schunck. The resulting matched point pairs correspond to a same feature located on both images wherein the same feature is associated with a same object in the field-of-view. While interest points can be matched, not all matched corresponding point pairs represent high quality corresponding point pairs that allow the identification of their three-dimensional positions in the field-of-view for classifications as a clear path for the vehicle to travel through. 
     The processing module  120  filters the matched corresponding point pairs in order to identify high quality corresponding point pairs that can be utilized for three-dimensional position identification with high confidence ( 314 ). Preferential matched point pairs may be selected based upon quality control criteria, e.g., distance between points, distance from image boundary, and color similarities between respective neighboring pixels. Selection of criteria to judge matched pairs can additionally be made based upon conditions such as light level present outside the vehicle, weather, speed of the vehicle, and any other factor affecting an ability to judge matched pairs or an urgency to quickly and accurately define a clear path. 
     The high quality corresponding point pairs are analyzed to determine three-dimensional positions of objects in the field-of-view represented by the corresponding point pairs ( 318 ). It will be appreciated that corresponding points at different heights as compared to ground level will move differently between sequential images. Analyzing movement of the interest points between sequential images can yield estimated three-dimensional coordinates of the interest points. Object position can be determined based upon the high quality corresponding point pairs, sample time between sequential images, and vehicular data such as vehicle speed, and vehicle yaw rate. These methods of triangulation can yield a position of the object in a horizontal plane and a height of the object in relation to a ground level. 
     The determined object positions are utilized to map object positions in front of the host vehicle including an estimated topography of the field-of-view ( 322 ). The topography may be estimated by assigning predetermined spatial dimensions around the object. Preferably, the predetermined spatial dimensions diminish with respect to height at a predetermined rate with respect to distance from the object. Using the map and estimated topography the processing module  120  can determine a clear path in front of the host vehicle ( 326 ). 
     The above method utilizes sequential images to establish a map of object positions and vertical heights in front of the vehicle, such that a clear path can be defined. It will be appreciated that in any two given images, a particular object might not be classified as including two high quality interest points sufficient to be mapped in that particular analysis. However, the above analysis occurs multiple times per second of vehicle travel. As the vehicle travels forward through the clear path, different perspectives upon an object will be gained and a large number of images will be analyzed. Travel over a path and analysis of the multiple iterative images through that path build a confidence through the analyses that no object contradicting the clear path exists in the identified clear path. 
       FIG. 11  illustrates an exemplary textureless pixel-based method  400  wherein input from a camera is analyzed to determine a clear path likelihood in accordance with the disclosure. The textureless pixel-based method  400  is illustrated in  FIG. 11 , and described herein as comprising discrete elements. Such illustration is for ease of description and it should be recognized that the functions performed by these elements may be combined in one or more devices, e.g., implemented in software, hardware, and/or application-specific integrated circuitry. For example, the textureless pixel-based method  400  may be executed as one or more algorithms in the processing module  120 . 
     During operation, the camera  110  generates an image for analysis in the processing module  120  ( 453 ). The processing module  120  analyzes the image using multiple filtering methods to identify and remove non-conforming pixels from the image. Remaining pixels indicate a potential clear path for the vehicle to travel.  FIG. 11  shows the textureless pixel-based method  400  including four exemplary filtering methods to identify and remove non-conforming pixels from the image. A textureless method could be used with some portion of the four exemplary methods and/or can include unnamed but similar methods to process the image. 
     A first exemplary filtering method removes pixels above a horizon or vanishing point, including sky and other vertical features that cannot be part of a road surface ( 456 ). The term “vanishing point” as used herein is a broad term, and is to be given its ordinary and customary meaning to one ordinarily skilled in the art, and refers to an infinite far point on the horizon that is intersected by multiple parallel lines on the ground in the view. Identifying a road surface creating a clear path on which to drive is necessarily below the vanishing point or horizon line. Filtering images to only analyze an area below the horizon line helps to clarify the pixels being analyzed to identify a road surface from irrelevant pixels. As one skilled in the art appreciates, there are many known methods for determining a vanishing point and corresponding horizon line. 
     A second exemplary filtering method applies a filter based upon variance in pixel color intensity, based upon a premise that a road surface will include a large surface with a visual intensity common across the surface ( 459 ). Pixels are removed from the image when an associated pixel color intensity varies greater than a predetermined threshold. For pixels associated with multiple colors, a pixel color intensity of any particular color that varies greater than the predetermined threshold may be removed from the image. The predetermined threshold may be updated based upon historical color intensity of pixels identified as clear. 
     A third exemplary filtering method applies a filter based upon differencing sequential images, allowing analysis of changes between the images ( 462 ). Pixels associated with a pixel color intensity that changes greater than a predetermined threshold from the sequential images may be removed from the image. In one embodiment, the adjustments are made to the images based upon motion of the vehicle so that pixels are differenced and compared as if all pixels correspond to points at a ground level. Known triangulation methods may be used for determining motion adjustments to the images. By using an assumption that all objects in the image are at ground level, non ground level points may be identified by changes not consistent with pixels associated with the ground level. For example, pixels above ground level may move faster between sequential images than would be expected, and this movement may be identified by examining the difference between pixel color intensities between sequential images. 
     A fourth exemplary filtering method applies a filter based upon identifying pixels representing edges or transitions in the visual data ( 465 ). To create the fourth filtered image, the processing module  120  extracts pixels from the image based upon color intensity values that correspond to edges using one of several known edge detection filters, e.g., a Sobel filter. The edge detection filter is preferably executed in the processing module  120 , but may be combined in one or more devices, e.g., implemented in software, hardware, and/or application-specific integrated circuitry. In one embodiment, each pixel is analyzed using a Sobel operator. The Sobel operator computes a gradient vector of color intensity at each pixel resulting in a direction of the largest possible increase from light to dark and the rate of change in that direction. Points corresponding to a rate of change exceeding a threshold and corresponding to gradient vectors at nearby pixels indicate edges and are included in the fourth filtered image. Those pixels are included in the image while the others are removed. 
     Applying the various methods in parallel, the results can be the fused into a single map of the image ( 468 ). Fusion includes pixels identified in each of the various filtering methods. Pixels on the fused clear path map correspond to desirable driving locations in the field-of-view. Locations on the fused clear path map without pixels correspond to undesirable driving locations in the field-of-view. The processing module  120  may analyze the map for visual data indicative of a clear path of travel ( 471 ). 
     The textureless pixel-based method  400 , described in  FIG. 11 , applies various methods to images in parallel to identify features in a view relevant to defining a clear path. However, it will be appreciated that these methods need not be performed in parallel, but rather the methods can be used to process images sequentially in steps or selectively to identify features in a view relevant to defining a clear path. 
     An addition pixel-based clear path detection method includes applying a fused texture-rich and textureless method. 
     The texture-rich and textureless methods can be fused in a number of ways. An image with identified points and determined heights identified with texture-rich methods can be overlaid with a filtered image generated by textureless methods, and agreement of the two methods can be used to define a clear path through the overlaid image. In an alternative method to fuse the two schemes, the data from each of the two schemes can be used to project information upon a programmed overhead map of an area in front of the vehicle, and this overhead map including data gained from analysis of the two schemes can include building confidence indications for regions of the map. In an alternative method to fuse the two schemes, one scheme can be utilized as a primary or dominant scheme, and the second scheme can be utilized or activated to analyze regions in the view identified as ambiguous or unclear. In any method to fuse the two schemes, strengths of one processing scheme can be used to reduce weaknesses of the other processing scheme. If both schemes concur that the path is clear, then the processing module employing the schemes may determine with increased confidence that the path is desirable for the vehicle to traverse. A number of methods to fuse the identified schemes are envisioned, and the disclosure is not intended to be limited to the particular embodiments described herein. Additionally, either scheme or both schemes can be combined with the method employed above utilizing analysis of patches. 
     Hierarchical configurations of multiple clear path detection algorithms may be incorporated into this disclosure, including algorithms arranged based upon computational intensity. Less computationally intensive detection methods can identify clear paths in the image and leave remaining sections of the image not identified as a clear path for more computationally intensive analysis thereby increasing computational efficiency for clear path identification. The particular methods utilized and the hierarchical structure of the particular hierarchical method used can differ or change. In an exemplary hierarchical configuration, a patch-based clear path detection method identifies clear paths in the image before a pixel-based clear path detection method analyzes the remaining sections of the image not identified as a clear path by the patch-based method. A number of different hierarchical configurations are envisioned, and the disclosure is not intended to be limited to the particular embodiments described herein. 
     One skilled in the art will recognize that lighting normalization may be applied to the captured image data. Normalization is a process which changes the range of pixel intensity values. The purpose of the normalization is to bring the image into a range that is more suitable for machine process to improve the reliability. For example, each pixel value is normalized to be a zero mean and unit variance to enhance the image contrast, specifically in a low lighting environment or when contrast is poor due to glare. 
     Methods above analyze an input image an attempt to determine a clear path based upon the analysis of the image. Particular information from the image is used to determine whether a region of the image represents a clear path. An alternative example-based method is disclosed and can be utilized alternatively or additionally to the above methods to define a clear path based upon an input image. An exemplary example-based method can be described by collecting a number of sample images of field-of-views, defining an exemplary clear path for each of the sample images, matching a current image to one or more of the sample images, and determining a clear path based upon the matching. The defined clear paths on the sample images can be defined by any method sufficient to contemplate the context of the images and can, for example, be manually entered in an off-line process. Matching the current image to one or more of the sample images can be accomplished, for example, by identifying features by methods described herein from each of the sample images, identifying features from the current image, comparing the identified features from the current image to a database of identified features from the sample images, and selecting matching sample images to the current image. A clear path can be selected from the best matching sample image or can be determined based upon a combination of the nearest matches to the current image. 
     The sample images are selected to cover a wide variety of geometric layouts, including sample images that are representative of a wide variety of lighting conditions, camera perspectives, traffic conditions, and camera lens radial distortion. In one embodiment, each of the plurality of sample images comprises a different combination of geometric layout, lighting conditions, camera perspectives, traffic conditions, and camera lens radial distortion. The multiple varieties of geometric layouts vary by road curvature and can additionally correspond to different driving environments including urban, rural, highways, gravel roads, tunnels, and/or snow covered roadways. Lighting conditions can be affected by weather conditions and sunlight intensity. Camera perspectives may be affected by vehicle orientation and vehicle lateral offset. Additional sample images may be dynamically selected and stored during vehicle operations based upon unique environmental and/or driving conditions. Sample images may be associated with vehicle position in a mapping database such as global positioning system (GPS). The sample images may then be matched with the current image based upon vehicle position. 
     Image areas on the sample image corresponding to clear path are subjectively defined or may be defined using methods known in the art and checked manually thereafter. The image areas corresponding to clear path are preferably identifiable by image coordinates and a particular associated sample image and may be stored in memory of the processing module  120 . In one embodiment, component patches corresponding to clear path are subjectively defined for the sample images. 
     Features identified on the sample images are preferably determined offline using a feature identification algorithm and stored in memory of the processing module  120 .  FIG. 12  shows an exemplary method  500  for feature identification and analysis. Selected component patches  502  or identifiable image areas are convoluted with a filter bank  504  to obtain a plurality of filter responses  506  as described herein below. The processing module  120  sums absolute responses separately for each filter  508 . Area normalization compensates for different image areas within an image by dividing the image area of each selected patch  510 . As  FIG. 12  shows, the normalized values can be concatenated to form a predetermined dimensional feature vector associated with each patch  512 . 
     Multiple filter banks including, e.g., a Leung-Malik filter bank and a Gabor filter bank, may be used to identify texture features on the sample images and the current image. The Leung-Malik filter bank is a multi-scale, multi-orientated filter bank and includes first and second derivatives of Gaussian filters, Laplacian of Gaussian filters, and Gaussian filters. In one embodiment, 48 filters are included in the Leung-Malik filter bank. The Gabor filter bank includes linear filters associated with filter responses defined by a product of a harmonic function and a Gaussian function. Gabor filters may include multiple orientations and parameters. 
     The identified features may be classified based upon the filter responses. In one embodiment, feature responses may be classified as corresponding to clear path or an obstacle. Classifications may be made based upon a predetermined threshold response. As described herein above, a training set may be used to assign confidence values to filter responses based upon the filter responses and associated error rates. 
     Matching a current image to one or more of the sample images may be accomplished using a refined database. The refined database selects a predetermined number of candidate sample images from the plurality of sample images that most resemble the current image based upon geometric layout, color information, and camera perspective. The candidate sample images are selected based upon feature similarities with features in the current image. Feature similarity is determined based upon spatial proximity to feature values determined for each of the sample images and the current image. Candidate sample images including feature values that are spatially related to feature values of the current image are more likely to have a clear path that corresponds to the current image. 
       FIG. 13  graphically illustrates an exemplary feature space including exemplary feature values. The exemplary feature values are identifiable in an exemplary two-dimensional feature space by x and y coordinates and are determined based upon identified features in an image. However, it will be appreciated that the feature space represented is a simple feature space for ease of demonstration, and more complex feature spaces with multiple dimensions can be utilized to more precisely select sample images as candidates. To select the predetermined number of candidate sample images from the plurality of sample images, feature values of the sample images  550  are compared to the feature value of the current image  551 . A predetermined number of feature values in nearest spatial proximity to the feature value of the current image in the feature space are selected as a candidate sample image. In the exemplary  FIG. 13 , three candidate sample images are selected for candidate sample images. 
     One exemplary equation for evaluating spatial proximity (‘DF’) between a feature value of a sample image and a feature value of a current image may be give as: 
                   DF   =       ∑     i   =   1     N     ⁢           ⁢         f   i   q     ⁡     (       f   i   q     -     f   i   d       )       2               [   4   ]               
Where DF represents the spatial proximity, N represents a number of feature values, f i   d  is the i th  feature value of a candidate sample image, and f i   q  is the i th  feature value of the current image, which also serves as the weight of that feature in distance calculation.
 
     A clear path may be inferred based upon the candidate sample images. Multiple methods may be used to infer a clear path on the current image using the candidate sample images and corresponding clear paths. In one embodiment, clear paths corresponding to the candidate sample images are overlaid, i.e., fused, into a single clear path on the current image. The fused clear path corresponds to image area locations identified as clear path in at least one of the candidate sample images and is associated with image area locations on the current image. The associated image area locations are inferred as the clear path for the current image. Alternate embodiments are envisioned by the disclosure including an embodiment that selects a sample image corresponding to the nearest spatial proximity to the feature value of the current image, selecting a best or closest candidate, and inferring the clear path associated with the sample image as the clear path of the current image. In another embodiment, visible attributes in the entire input image and each of the matched sample images are compared, and a best or closest candidate is selected based upon the visible attributes. Such visible attributes include lighting, horizon position or orientation, perceived lane of travel orientation, traffic density, weather, and other similar factors. 
       FIG. 14  schematically depicts operation of an exemplary example-based method, in accordance with the present disclosure. The example-based method  700  is illustrated in  FIG. 14 , and described herein as comprising discrete elements. Such illustration is for ease of description and it should be recognized that the functions performed by these elements may be combined in one or more devices, e.g., implemented in software, hardware, and/or application-specific integrated circuitry. For example, the pixel-based method  700  may be executed as one or more algorithms in the processing module  120 . 
     Method  700  illustrates comparison of a current image to a number of example or sample images to determine a clear path. As described above, a number of sample images are analyzed through feature identification ( 704 ), and the features determined from filters associated with a filter bank, for example, in an offline training process, are stored in a refined database. Method  700  inputs a current image and performs feature identification upon the image ( 702 ). Features are preferably identified on the current image in real-time during vehicle operation using a feature identification algorithm. The current features are utilized to select one or more matches in the refined database to the current features ( 706 ). In this exemplary embodiment, three matching feature sets are identified. The corresponding images of the matches are then compared to the current image, and a clear path for the current image is determined or inferred based upon the clear path for the matching sample image or images ( 708 ). 
     One having skill in the art will appreciate that an example-based method as described above can be useful to judge situations wherein visual differences between surfaces, for example, similar colors or isotropic textures not having a dominant direction, or less distinctive features in an image might not lend to accurate results from the other exemplary methods described herein. In such instances, the human-defined clear paths or otherwise off-line determined clear paths of the sample images can provide superior input to the location of the clear path. Additionally, it will be appreciated that ambiguities or weaknesses in camera detection, for example, a large radial distortion present in some camera images or clustered background buildings, can distort or create inaccuracies in interpretations of images according to other methods described herein. However, such ambiguities or weaknesses in images exist both in the sample images and the current image used in the disclosed example-based method and, therefore, tend to cancel out. In particular, as described with respect to the hierarchical method disclosed herein, the example-based method provides an alternative method to process an image and identify a clear path in cases wherein a patch-based method and/or a pixel based method cannot clearly identify the clear path. 
     As described above, the exemplary patch-based and pixel-based methods are useful to analyze a patch or pixel details within a patch to judge whether the patch represents a clear path or not a clear path. The example-based method, on the other hand, determines or infers a location of a clear path upon a current image. Further, the example-based method described hereinabove is applied to an entire image; additional embodiments are envisioned including applying the example-based method to image areas to infer a clear path in an identifiable image area. Applied to the hierarchical method described above, an example-based analysis can be operated as one of the layers of the analysis, for example, the determined clear path from the example-based method can be compared to the location of the patch being further analyzed, and the patch can be labeled as clear or not clear based upon the comparison. 
     The above example-based method describes methods to compare a current image to images with clear paths defined thereupon, and selecting a current clear path based upon the comparing. It will be appreciated that a similar comparison can be made with other sample image properties. For example, vanishing points useful to estimate horizons and perceived lane orientations in an image can be determined for a group of sample images. By comparing a current image to the sample images and selecting a group of matched images, an estimate of the vanishing point in the current image can be made. In such an exemplary comparison, a vanishing point for a best match, selected according to factors described above, can be selected as the vanishing point for the current image, an average of the vanishing points from the matched images can be selected as the vanishing point for the current image, or a vanishing point for the current image can similarly be selected based upon the vanishing points from the matched images. Once a vanishing point is selected for the current image has been selected, it will be appreciated that it can be used, for example, to assist in determining a clear path according to the various methods described above. 
     The disclosure has described certain preferred embodiments and modifications thereto. Further modifications and alterations may occur to others upon reading and understanding the specification. Therefore, it is intended that the disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.