Abstract:
A digital scanner or copier is provided that can dynamically adjust the black point of an image to be scanned based on detected background level of the document. A pre-scan acts on a small scanning window near a leading edge of the document to obtain a histogram of grey-level values. From this, the background level of the document can be obtained to determine a white point of the image. Additionally, a black point of the image is originally set to a default offset value. However, an adjustment factor based on the detected white point is used to adjust the default offset value to a value that increases the dynamic range of the output image, while still maintaining solid black for black areas of the image.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to methods and apparatus for determining background content of an image of a scanned document and using this content to dynamically adjust the black point of the image. 
     2. Description of Related Art 
     In conventional scanners and digital copiers, the black point of an image is adjusted statically using a fixed offset value. Such a method works fine for images with white or light background. However, when the input image has a darker background, such as colored paper, the blackest point is also lower and using a fixed offset value causes the background to not be completely suppressed. That is, the use of a single fixed offset for different types of input images (images on various colored backgrounds) does not work very well in suppressing both the background while reproducing quality black text. 
     One approach to this is to provide several preset modes, each having different offset values. However, picking a different mode with a lower offset value causes the dark areas to not be reproduced as solid black. Another problem with this approach is that a user has to manually set or pick a correct mode. This is inconvenient. 
     SUMMARY OF THE INVENTION 
     In copier and scanner systems, copying a document or an original while suppressing the substrate of the original is often required. For example, when the original is printed on colored paper. Such suppression can be achieved by detecting the background color automatically. Background detection can be performed on just the leading edge of the document or the whole document. However, whole page background detection requires pre-scanning of the entire original image. The detected background can be removed by adjusting the gain of the scanned image and clipping the values that exceed the system processing range. 
     Automatic background suppression senses the background and automatically suppresses it before final printing. Conventional automatic background suppression systems generate a histogram of the document using standard methods and then calculate the mean and standard deviation. Other systems estimate the standard deviation and mean value. Exemplary background detection systems and methods can be found in co-pending U.S. patent application Ser. No. 08/886,205 filed Jul. 1, 1997, copending U.S. application Ser. No. 09/159,038 filed Sep. 23, 1998, and U.S. Pat. No. 5,881,166, the disclosures of which are incorporated herein by reference in their entirety. 
     The above systems and methods have been used to suppress background and to adjust a tone reproduction curve (TRC). However, such systems have not heretofore used such background information to scale the offset value. Applicants have discovered that the background value determined during a scan of the leading edge of the document can be used to dynamically adjust the fixed offset value for the image. This helps in increasing the dynamic range of the output image without sacrificing the black areas of the image. Moreover, as the histogram only acts on the leading edge, the scan and approximations can be achieved in real-time, thus avoiding the delays necessary to scan an entire image to determine precise white point and black point values. 
     Thus, a method of scanning a document according to the invention may include: scanning at least a portion of a document; automatically detecting a background of the document to form a white point WP of the image; providing a default fixed offset value FOV used as an initial black point BP of the image; providing a scaled adjustment factor AF dependent on the detected white point WP; providing a scaled black point BP by multiplying the fixed offset value FOV by the scaled offset adjustment factor AF; computing a gain G according to the equation G=255/(WP−BP); and scanning the image and applying the gain G and black point BP to all pixels of the image. 
     Also, a digital scanner or digital copier according to the invention may include: a scanning unit that can scan the document; a histogram generator that generates a histogram from scanned data corresponding to at least a portion of the scanned document; a computing unit; a fixed offset value FOV stored in memory; and an offset adjuster that calculates an offset adjustment factor AF based on a detected white point WP of the document, wherein the computing unit calculates the white point WP based on the histogram, calculates a black point BP based on the fixed offset value FOV and the detected white point WP, and calculates a gain based on the calculated white point WP and black point BP. 
     These and other features and advantages of this invention are described in or are apparent from the following detailed description of various exemplary embodiments of the systems and methods according to this invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various exemplary embodiments of systems and methods according to this invention will be described in detail, with reference to the following figures, wherein: 
     FIG. 1 is a diagram illustrating components of a digital scanner; 
     FIG. 2 is a block diagram illustrating the electronic architecture of a digital scanner coupled to a workstation, network, storage medium and image output terminal in accordance with embodiments of the invention; 
     FIG. 3 is a graphical representation of a sample window used to generate histogram data from an input document according to the present invention; 
     FIG. 4 is a histogram of the input document; 
     FIG. 5 is a graphical representation of the histogram of FIG. 4; 
     FIG. 6 is a smoothed histogram of the input document; 
     FIG. 7 is a flowchart of an offset adjustment process according to the present invention; and 
     FIG. 8 is a continuation of the flowchart of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention relates generally to methods and systems for dynamically adjusting a black point during scanning and outputting of an image from a digital scanner. The digital scanner is capable of being connected to a wide array of copiers, printers, computers, networks, facsimile machines, etc. and capable of scanning and producing complex and interesting images to be stored, printed and displayed. The images may include text, graphics, and scanned or computer-generated images. With this scanner, high quality image output can be achieved by automatically determining an image background and using this information to dynamically adjust a fixed offset used to provide a black point of the image. 
     FIG. 1 illustrates components of a scanning unit  20  of a digital scanner. In the scanning unit  20 , a light source  21  is used to illuminate a document  22  to be scanned. In a platen-type scanning situation, the document  22  usually rests upon a glass platen  24 , which supports the document  22  for scanning purposes. The document may be placed in the glass platen  24  by an operator. Alternatively, the scanning unit may include a feeder or document handler  29 , which places the document on the glass  24 . Another example of a feeder is shown in U.S. Pat. No. 5,430,536. 
     On top of the glass platen  24  and the document  22 , a backdrop portion (platen cover)  26  is placed so as to prevent stray light from leaving the scanning area to provide a background from which an input document can be distinguished. The backdrop portion  26  is part of document handler  29 . The backdrop portion  26  is the surface or surfaces that can be scanned by an image-sensing unit  28  when a document is or is not present in the scanning station. The light reflected from the document passes through a lens subsystem (not shown) so that the reflected light impinges upon the image sensing unit  28 , such as a charge coupled device (CCD) array or a full width array. 
     Examples of a full width arrays are found in U.S. Pat. Nos. 5,473,513; 5,748,344; 5,552,828; 5,691,760; 5,031,032; 5,545,913; and 5,604,362. A full width array typically comprises one or more linear arrays of photosites, wherein each linear array may be sensitive to one or more colors. In a full color digital scanner, the linear arrays of photosites are used to produce electrical signals which are converted to color image data representing the document that is being scanned. However, in a black-and-white scanner, preferably only one linear array of photosites is used to produce electrical signals, which are converted to black and white image data representing the image of the document that was scanned. 
     FIG. 2 is a block diagram illustrating the electronic architecture of digital scanner  30  including scanning unit  20 . The digital scanner  30  is coupled to a workstation  50  by way of a scanner interface  40 . An example of a suitable scanner interface is a SCSI interface. Examples of the workstation  50  include a personal computer and a computer terminal. The workstation  50  includes and/or has access to a storage medium  52 . The workstation  50  is preferably adapted to communicate with a computer network  54 , and to communicate with the Internet either directly or through computer network  54 . The digital scanner  30  is preferably coupled to at least one image output terminal (IOT)  60 , such as a printing system. 
     The scanning unit  20  scans an image and converts the analog signals received by the image sensing unit  28  into digital signals (digital data). An image processing unit  70  registers each image, and preferably executes signal correction to enhance the digital signals. As the image processing unit  70  continuously processes the digital data, a first-in first-out (FIFO) buffer  75  temporarily stores the digital data outputted by the image processing unit  70 , and transmits the digital data to the International Telecommunications Union (ITU) G3/G4  80  and Joint Photographic Experts Group (JPEG)  85  in bursts, so that the processed data is compressed. Other data compression units may be substituted for ITU G3/G4  80  and JPEG  85 . The compressed digital data is stored in memory  100 , preferably by way of a Peripheral Component Interconnect Direct Memory Access (PCI/DMA) Controller  90  and a video bus  95 . Alternatively, an operator may not wish to compress the digital data. The operator may bypass the compression step so that the digital data processed by the image processing unit  70  is sent through FIFO  75  and directly stored in memory  100  by way of PCI DMA controller  90 . 
     A computing unit  110 , such as a microprocessor, is coupled to a scanner interface  40 , memory  100  and PCI DMA controller  90  by way of the video bus  95  and a video bus bridge  120 . The computing unit  110  is also coupled to a flash  130 , static RAM  140  and a display  150 . The computing unit  110  is also connected to the scanning unit  20  and the image processing unit  70  by way of a control/data bus. For example, the computing unit  110  may be communicating with the image processing unit  70  through the video bus  95  and/or PCI DMA controller  90 . Alternatively, the computing unit  110  may communicate directly with different components, such as the image processing unit  70  by way of control/data buses (not shown). 
     As mentioned previously, automatic background suppression is used in digital copiers/scanners to detect the background value of the input document and to automatically suppress the background without any user intervention or adjustment. This background detection is performed by analyzing the lead edge statistics of a document, such as document  22  in FIG. 3, wherein a group of scanlines are collected to generate a histogram for the input document. Rather than examining the entire document, a small sampling window  200  is applied to the leading edge of the input document  22 . A suitable window may be approximately 4,000 pixels by four scanlines and applied to the input document to generate the histogram. The histogram, preferably an 8-bit 256-value histogram, is shown in FIGS. 4 and 5 and represents the eight-bit grey levels of the input document, wherein a grey-level of 0 represents the black pixels and a grey-level of 255 represents the white pixels. 
     The pixel value having the highest frequency in the sampling window represents the mean grey-level of the background. For example, the mean grey-level of exemplary input document  22  is “201” as shown in FIGS. 4 and 5. The histogram represents the lead edge statistics for the input document. This generated histogram is then preferably smoothed and compressed. A suitable smoothing and compression process is detailed in co-pending Ser. No. 08/886,205, the disclosure of which is incorporated herein by reference in its entirety, to obtain a background value BKG. As shown in FIG. 6, after smoothing and compression, the example of FIGS. 4-5 is shifted to a value of “225” from a mean grey-scale value of “201”. This value becomes the background value BKG=225, which also is known as the white point WP for the image. That is, all pixel values of the image greater than the BKG (lighter) are clipped/suppressed and considered to be part of the background. 
     If the document is scanned through a DADF (feeder  29 ), the histogram is collected at approximately 1.25 mm into the registered document. On the other hand, if the document is placed on the platen  24  without any document sensing feature enabled, a short prescan is performed. A histogram is then collected at approximately 10 mm from the platen registration corner, wherein the scanlines have a greater offset from the lead edge than in the auto feed example. 
     During platen scanning, the scanner  20  takes its initial histogram at the 10 mm point because of lack of knowledge of the exact position of the document  22 . For example, documents can get skewed while closing the platen cover  26 , resulting in collection of data from the platen cover  26  if the histogram is collected at 1.25 mm from the registration corner. Another reason is due to incorrect information present within the first few millimeters of the registration corner due to an integrating cavity effect from the underside of the platen cover  26 , which is a common problem with most document scanners. 
     A process for scanning an image and obtaining a printed output therefrom according to the invention will be described with reference to FIGS. 7-8. The process starts at step S 700  and advances to step S 705  where a scan setup is received from graphical user interface (GUI)  160 . A suitable GUI can be found in co-pending Ser. No. 09/511,992 filed concurrently herewith, the subject matter of which is incorporated herein by reference in its entirety. From step S 705 , flow advances to step S 710  where a scan image is set up and a scan image command is submitted to the scanner  30  from workstation  50 . Flow then advances to step S 715  where the scanner  30  checks for receipt of a scan image command. If not received, flow returns to step S 715 . If, however, a scan image command is received by the scanner, flow advances to step S 720  where the scanner  30  is initialized and then at step S 725  the image scan is started. Initially, only a small scan window  200  is scanned, which preferably is on the leading edge of the image. The scan window  200  is defined by four corners. The top left corner is the start corner for both the fast scan direction and the slow scan direction (FS START and SS START). The top right corner is the fast scan end corner (FS END) and continues to be the start in the slow scan direction (SS START). The bottom left corner is the end of the slow scan direction (SS END) and continues to be the start of the fast scan direction (FS START). The bottom right corner is the end of both scan directions (FS END and SS END). 
     To achieve this leading edge scanning to determine a histogram, the scanner increments first in the fast scan direction and then in the slow scan direction. At step S 730 , the scanner checks to see whether the slow scan start has been reached. If not, the scanner increments and flow returns to step S 730 . If it has, flow advances to step S 735  where it is checked whether the slow scan end has been reached. If not, flow advances to step S 740  where the current pixel is checked to see whether it is beyond the fast scan direction start (FS START). If not, the scanner increments in the fast scan direction and flow returns to step S 740 . If it has, flow advances to step S 745  where it is determined whether the current pixel is less than the fast scan direction end (FS END). If it is, the scanner stores the grey-level value, increments to the next pixel, and the flow returns to step S 745 . If the current pixel is greater than the FS END, the fast scan row is completed and flow advances to step S 750 , where the pixel values for that fast scan direction row are collected. After step S 750 , the scanner is incremented in the slow scan direction and flow returns to step S 735 , where steps S 735 -S 750  are repeated until the entire scan window has been scanned. At that point where the scanner crosses the SS END position, flow advances from step S 735  to step S 755 , at which time the entire video histogram has been collected. 
     From step S 755 , flow advances to step S 800 , where a background peak BKG (also known as a white point WP) is determined, such as by the methods disclosed in co-pending U.S. applications Ser. Nos. 08/886,205 and 09/159,038 mentioned previously. From step S 800 , flow advances to step S 810 , where a default fixed offset value FOV is obtained for a selected mode. Conventionally, a single fixed offset value FOV for a black point is set. However, it is also possible for different black point offset values to be selected depending on the particular mode of operation, i.,e., photo, text or graphics scanning. In the case of a single fixed offset value FOV, a suitable fixed offset grey-scale value is about 20. However, this fixed offset may be set between 0 and 30, depending on the specific mode selected. Selection of the fixed offset value FOV may be performed through GUI  160  or may be performed automatically (i.e., preset). 
     From step S 810 , flow advances to step S 820 , where it is determined whether the suppression is strong. That is, whether the detected background BKG from step S 800  is light or dark. There are likely to be problems with both suppressing the background and reproducing black text with sufficient clarity and darkness when the detected background is dark and strong suppression is needed. The detection in step S 820  can be performed by comparing the detected background BKG against a fixed threshold, which in an 8-bit grey-level system may be a grey-level of 190, although the threshold is not limited to such a specific threshold value. It has been found that for fairly light backgrounds having a grey-level of greater than 190, use of the fixed offset value FOV for the black point can be retained. Thus, if at step S 820 , the background is found to be greater than the threshold (signifying a light background and weak suppression), flow advances to step S 830 , where a gain is computed based on the detected background peak and the default fixed offset value FOV. A suitable formula for gain is: 
     
       
           G= 255/( WP−BP ).  (1) 
       
     
     where WP is the white point (detected background BKG) and BP corresponds to the offset value FOV. In the example of FIG. 6 with a background BKG of 225 and a fixed default offset value FOV of 20, the gain becomes 255/(201−20)=1.41. From step S 830 , flow advances to step S 840 , where hardware of the scanner is programmed with the computed gain G and the default offset value BP. From step S 840 , flow advances to step S 880 , where the image processing unit  70  applies the gain G and offset BP to the remainder of the image. From step S 880 , flow advances to step S 890 , where an image is printed from IOT  60  based on the input document. After step S 890 , the process stops at step S 895 . 
     If, however, in step S 820  the suppression is determined to be strong, i.e., a dark background BKG below the threshold of 190, a scaling process is performed at step S 850  to adjust the offset to expand the dynamic range of the input and produce better black image quality when a dark background is detected, such as printing on a colored background. In particular, the scaling is based on the detected background BKG (WP). The adjustment may be through use of a linear or non-linear equation. A suitable formula for obtaining the offset adjustment factor scaling is: 
     
       
           AF= ( WP+ 64)/255,  (2) 
       
     
     when the detected mean grey-level WP is less than or equal to the threshold of 190. Then, a new black point BP can be determined by setting: 
     
       
           BP= OffsetScale*Default Offset  (3) 
       
     
     From step S 850 , flow advances to step S 860  where the gain is computed using equation 1. However, in step S 860 , the adjusted BP is used rather than the default offset. From step S 860 , flow advances to step S 870  where the hardware of the scanner is programmed with the computed gain G and the scaled black point BP. From step S 870 , flow advances to step S 880 , where the image processing unit  70  applies the gain G and offset BP to the remainder of the image. From step S 880 , flow advances to step S 890 , where an image is printed from IOT  60  based on the input document. After step S 890 , the process stops at step S 895 . 
     Alternatively, rather than branching to steps S 730  and S 740 , step S 750  can scale the offset value based on whether or not the background peak is above or below the threshold. For example, if above the threshold of 190, the OffsetScale can be set to a fixed value of 1 (i.e., no adjustment). However, if equal to or below the threshold of 190, the OffsetScale can be set according to equation (2). 
     The formula for the OffsetScale is not limited to the above exemplary linear equation, and may be a non-linear equation. Moreover, the fixed threshold of 190 is meant to be illustrative and not limiting. 
     The operations and determinations discussed above can be implemented using a programmed general purpose computer. However, the various operations and determinations described above can also be implemented on a special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit elements, an ASIC or other integrated circuit, a digital signal processor, a hardwired electronic or logic circuit such as a discrete element circuit, a programmable logic device such as a PLD, PLA, FPGA or PAL, or the like. In general, any device capable of implementing a finite state machine that is in turn capable of implementing the operations and determinations discussed above can be used to implement these operations and determinations. 
     The above system and methods are suitable for use with digital scanners and copiers. Such methods and systems can be used with other processes and systems for adjusting and manipulating output from a scanner. For example, the invention may be used in conjunction with setting or adjustment of a tone reproduction curve (TRC) of the scanner. Examples of methods and systems of TRC adjustment can be found in co-pending applications Ser. Nos. 09/512,769; 09/512,889; and 09/512,888, filed concurrent herewith and incorporated by reference herein in their entirety. 
     While the systems and methods of this invention have been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. For example, while described with reference to a black and white system, the invention is also applicable to a color system, wherein the black point adjustment essentially would make luminance adjustments to the black point. Accordingly, the exemplary embodiments of the systems and methods of this invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.