Abstract:
A method to be used within a document image capture device, such as a document scanner ( 10 ) of detecting and measuring the document skew ( 25 ) while the document is still entering the imaging area and then effecting a rotation of the camera to accommodate the document skew so that the resulting image taken is not skewed. This can be accomplished by one of several methods, such as rotating the entire camera, rotating only the sensor element, or rotating another element in the optical path such as a prism. Use of the methods incorporated in this invention does not require the document motion to be stopped to measure and accommodate skew or to take the image.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Reference is made to commonly-assigned co-pending U.S. patent application Ser. No. 09/650,203, filed Aug. 29, 2000, entitled METHOD FOR DETECTION OF SKEW ANGLE AND BORDER OF DOCUMENT IMAGES IN A PRODUCTION SCANNING ENVIRONMENT, by Lee et al., the disclosure of which is incorporated herein. 
   FIELD OF THE INVENTION 
   Image recording devices which capture images of documents, photographs, and other objects, which have orthogonal boundaries or other orthogonal features that can be used to determine when the object is oriented properly to the imaging device. These devices include document scanners, pictorial scanners, and reprographic scanners (image acquisition device/system portion of a digital copier.) 
   BACKGROUND OF THE INVENTION 
   When documents are transported for scanning using a document feeder, they may become skewed while traveling through the paper path. The skew may be caused by a number of factors, including how the paper was placed in the hopper or how the transport rollers grab the leading edge of the paper to move it. Skew may also result from manual placement of documents within the scanner if sufficient care or time is not taken. 
   In the prior art, the image of the misaligned (skewed) document is captured and stored temporarily. Consequently, this image is skewed. The image is then straightened by calculating the angle of skew and manipulating the pixel values that comprise the image. This causes some loss in the sharpness of the image, as well as requiring temporary storage of a skewed image within the image forming device. This invention eliminates this type of de-skewing process. 
   SUMMARY OF THE INVENTION 
   Briefly, according to one aspect of the present invention a method for correcting skew angle in a document scanner comprises transporting a document past a camera. A leading edge of the document is detected. A leading edge skew angle of the document is determined based on the leading edge with a first algorithm. The camera is rotated to align it with the document. It is determined if the document is completely within the field of view of the camera, and when it is the skew angle is recalculated based on the entire document using a second algorithm. The camera is rotated to align it with the document and an image of the document is captured. 
   According to one embodiment of the present invention document skew is detected while the document is still entering the imaging area and then rotate the camera or optical system to accommodate the document skew so that the resulting image taken is not skewed. This can be accomplished by one of several methods, such as rotating the entire camera, the sensor element, or another element in the optical path such as a prism. 
   An advantage of the present invention is that it does not require the document motion to be stopped to take the image or to measure and accommodate skew. 
   When the present invention is used for a non-digital image capture device, for example a film camera or optical copier, an auxiliary digital image sensor would be incorporated for determining the skew in the original document. Since used only for determining document skew, this sensor could be simpler and of lower resolution than what would be needed for capturing the final image, and consequently relatively inexpensive. 
   In a device using this invention the skew can be detected and accommodated as the document enters the imaging area so that the image can be taken without skew error. The image can then immediately be available for transmittal to the host. With the current methods the original (skewed) image must be stored temporarily and then the skew angle must be determined. Then the corrected image must be created from the original image before the image storage space can be used for the next image. This affects the speed at which images can be taken and processed. The impact on process speed is directly related to the size of the image. This method eliminates the loss of sharpness resulting from the prior art methods of image deskew. 
   The present invention may also be used in non-digital photographic applications such as optical copiers and copy stands, and other auto-feeding devices using film cameras. Although the present invention is limited to use in image recording devices which image an area at one time, typically an entire document, the devices utilizing this invention can compete in a broader field with imaging devices which use other types of cameras in accomplishing the same end, which would be to create an image of a document, picture, etc. For the purpose of this disclosure, the term “document” will be used to represent all possible items that are to be imaged. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a document scanner, in this case an area array scanner, which utilizes mechanical skew accommodation according to the present invention. 
       FIG. 2  is a schematic view of the elements of a document scanner according to the present invention. 
       FIG. 3  shows a close-up view of the imaging components and the mechanism for rotating the camera  13 . 
       FIG. 4   a  illustrates a skewed document entering the field-of-view of the camera. 
       FIG. 4   b  illustrates rotation of the camera to accommodate the document skew as the document further enters the field of view of the camera. 
       FIG. 4   c  illustrates the completion of the camera rotation to align the field of view of the camera with the skewed document. 
       FIG. 5  illustrates the flow diagram for one implementation of the method of skew accommodation used in this invention. 
       FIG. 6  is a perspective view of an alternate embodiment of the present invention using an angle image capture device and an auxiliary digital image sensor for determining document skew. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will be directed in particular to elements forming part of, or in cooperation more directly with the apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
   Referring now to  FIG. 1 , one embodiment of this invention is a document scanner  10  which contains one or more digital cameras  13  for capturing images of documents. 
   Referring to  FIG. 3 , for the purposes of this description a digital camera  13  consists of an imaging sensor  14  which converts the optical image of the documents  11  to an electrical file, and a lens  15  which focuses the image of the documents  11  onto the imaging sensor  14 , a circuit board assembly  16  which converts the signal from the imaging sensor to discrete numerical values and provides the image to an image processing system (not shown), and a support means  17  for the imaging sensor  14 , the lens  15 , and the circuit board assembly  16 . 
   Referring back to  FIG. 1  in this scanner, documents  11  are individually picked from an input area, such as an input tray (not shown), and are transported through the field of view  12  of a camera  13 . When the document  11  is entirely within the field of view  12  of the camera  13 , and without stopping the motion of the document  11 , the camera  13  will capture an image of the document  11 . The document  11  continues moving out of the field of view  12  through the remainder of the transport means  18  to an output tray or hopper. Documents  11  follow each other through this transport means, in a continuous manner, and are sequentially imaged. 
   Referring to  FIG. 2  as in most scanners, the imaging sensor  14  is always imaging whenever the system is operating, constantly refreshing the image at regular intervals several times per second. Capturing an image means saving one of these images to an image memory device  19 . In the present invention, while the camera  13  is waiting for a document  11  one out of every so many images is saved to a temporary memory device  20  for the purposes of detecting the presence of the leading edge  22  of a document  11  within that image. The entire image would not be saved in this temporary memory, but the image would be sub-sampled. This means that only one out of every so many picture elements (pixels) within the image would actually be saved. 
   Referring to  FIGS. 4   a – 4   c , as the leading edge  22  of the document  11  enters the field of view  12  the subsampled image will contain this edge as well as the leading portions of the side edges  24 . A computer algorithm determines the presence of these side edges  24  and also determines the angle of skew  25  of the leading edge  22 . Sub-sampled images are used to increase the speed at which the presence of a document and the skew angle  25  can be determined. The scanner can then begin to activate the rotation actuator mechanism  30  to rotate the camera  13  in an appropriate direction to align the imaging sensor  14  with the document  11 . 
   Samples of the image are taken continuously and the skew angle  25  determined. The camera  13  continues to be moved in the appropriate direction until the skew angle  25  is determined to be within an acceptable approach to zero. At that time camera motion is stopped and the scanner continues to process the sub-sampled images until a trailing edge  28  of the document  11  is detected in the image. At that time a more robust skew measuring algorithm, for example, minimum bounding area described in copending U.S. patent application Ser. No. 10/262,049, filed Oct. 1, 2002, can be employed to confirm that the skew angle  25  is within acceptable limits. The camera  13  can be realigned at that time to correct any remaining skew. A final full-resolution image would then be taken and saved to an image memory device  19 . See  FIG. 2 . 
   Generally, scanning takes place while the document  11  is still moving and would be completed before the document began to exit the field of view  12  of the camera  13 . The next document would be imaged in the same manner. 
   For the exceptional document  11  for which the process cannot be accomplished before the document began to exit the field of view  12  of the camera  13 , the transport means  18  would be stopped until the skew angle  25  in the image is within limits and the final image is saved to the image memory device  19 . The transport means  18  would then be restarted and the next document  11  would be imaged in the same manner. 
   Referring again to  FIG. 3 , this invention includes a digital camera  13  which includes an area imaging sensor  14 , which may be a CMOS or a CCD variety. A lens  15  or other optics for focusing the image of the documents onto the imaging sensor. Circuitry for converting the electrical content of the image within the imaging sensor to discrete numerical values (analog to digital conversion) such as the imaging circuit board assembly  16 . A support means for the lens and imaging sensor  17  is provided. A support means for the other elements within the camera are not shown. A rotation actuator mechanism  30  rotates the camera in the axis required to accommodate document skew and constrains motion in all other degrees of freedom. A support means holds the actuator mechanism in position to the document transport means (not shown). 
   Referring back to  FIG. 2  electronic circuitry to control the actuator, such as the actuator control electronics  31 . A processing unit  32 , such as a microprocessor is used to calculate the skew angle and to control the rotation of the actuator. A temporary memory device  20  is used to store subsampled images for the purpose of determining skew angle. An algorithm for the processing unit to follow in determining the rotation required for accommodating the skew is illustrated in  FIG. 5 . An image memory device  19  saves the final image of each document. 
   If the method of choice for capturing the final image is not digital, but uses film or another medium, the invention would also include a camera  50  which utilizes an analog image capture method and medium, such as photographic film, which is shown in  FIG. 6 . 
   In the embodiment within an analog image capture device using film or other media, the process would proceed as described above except an auxiliary digital image camera  40  would be used to detect skew angle  25  of the document leading edge  22 . The field of view  45  of this auxiliary camera would be essentially the same as the field of view  55  of the analog camera. Since auxiliary image sensor  40  is used only to sub-sample images stored on temporary memory for determining skew angle, a low cost digital sensor could be used. When the skew angle requirements are satisfied, the analog image would then be captured to the media employed by analog image capture camera  50 . All other activity would also follow the outline above. 
   The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
   PARTS LIST  
   
       
         10 . Document scanner 
         11 . Document 
         12 . Field of view of camera 
         13 . Camera 
         14 . Imaging sensor 
         15 . Lens 
         16 . Imaging circuit board assembly 
         17 . Lens and imaging sensor support means 
         18 . Transport means 
         19 . Image memory device 
         20 . Temporary memory device 
         22 . Leading edge of document 
         24 . Side edge of document 
         25 . Document skew angle 
         28 . Trailing edge of document 
         30 . Rotation actuator mechanism 
         31 . Actuator control electronics 
         32 . Scanner processing unit 
         40 . Auxiliary digital image sensor 
         45 . Field of view of auxiliary digital image sensor 
         50 . Analog image capture camera 
         55 . Field of view of analog image capture camera