SCANNING SPEED COMPENSATION

Abstract: The present invention uses a multiple line sensor array (33) for capturing the desired pixels of an object (30). Each sensor is provided with a plurality of photosensitive elements positioned substantially adjacent to each other so as to capture with multiple photosensitive elements the light from one scene pixel area of the scanned media (30). By monitoring the absolute position of the object (30), a feedback signal is generated and used to determine which photosensitive elements are sensing the desired scene pixel area thereby compensating for speed variations in the scanning motion. In another variation of the present invention a weighted average is taken of selected photosensitive elements to obtain a value for the desired scene pixel location. A summation of the signals from the lines of the sensor that are in the correct position multiplied by the correct weighting factor determined by the position sensor feedback will thereby reduce the image degradation caused by varying scanning speeds.

Description: SCANNINS SPEgP COMPENSATION Plel of Invention The present invention is directed to the field of image processing and more particularly to a system for compensating for changes in the scanning speed of a CCD image scanner. BACKGROUND OF THE INVENTION The present invention addresses the problem of reducing the effects of scanning speed variations while relaxing the tolerances required of the scanning mechanism. In currently available state of the art linear scanning systems the media or the scanning head are displaced, or the optical path is modified in order to capture the image one line at a time. Any variations in the rate of the motion between the scanning head and the object results in a distortion of the captured image in the direction of scan. These distortions include, but are not limited to, compression or elongation of discrete portions of the image, intensity variations and edge deformations. In U.S. Patent No. 4,591,727, entitled "Solid State Scanner For A Variable Speed Transport", by G. Gaebelein et al., the scanner timing is synchronized to the speed of the transport moving the document that is being scanned. The output signal gain is then modified at a line rate to compensate for the variable integration times which would otherwise produce variations of intensity in the scene. If the speed of the document is reduced below a fixed threshold the system selects only certain of the scans and discards the data from the others. In U.S. Patent No. 4,878,119 entitled "System For Synchronizing Integration Pulses With Integration Signal in an Asynchronous Raster Input Scanner" by Bel irch et al., there is disclosed a system wherein a raster input scanning array -is operated asynchronously with the integration time remaining constant. At some point in time a trigger is generated which modifies the vertical clock to compensate for the accumulated errors in the scan speed. One of the effects of such an adjustment is that artifacts are produced in the output image. The artifacts will be either an elongation or a compression of the image at random locations. SUMMARY OF THE INVENTION The present invention uses a multiple line sensor array to insure the correct location capture of pixel light from each pixel area of an original scene print. Each sensor column is provided with a plurality M of photosensitive elements positioned substantially adjacent each other so as to " capture the light from an associated scene pixel area of the scanned media. By summing the values of the sensed light from each of the photosensitive elements N that are viewing one scene pixel area the desired value is obtained. In another variation of the present invention the weighted average value of the pixels capturing signal from each desired scene pixel area will yield the desired pixel value. A summation of the signals from the lines of the sensor that are in the correct position multiplied by the correct weighting factor will thereby reduce the image degradation due to motion. The system requires feedback as to the displacement of the mechanism at fixed time intervals corresponding to the line capture rate. By selecting the applicable imager pixels and summing the electrons stored, the tolerances placed on the mechanical drive and object positioning can be greatly simplified and a substantial overall system cost savings will be obtained. From the foregoing it can be seen that it is a primary object of the present invention to provide a scanning speed compensation method for an image scanner. It is a further object of the present invention to provide a system wherein a position sensor, in conjunction with image sensor timing, determines the sensor lines in the correct position at the time of capture. Another object of the present invention is to utilize the provision of position feedback from the position sensor for determining a weighting factor for each scanned line. These and other objects of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein like characters indicate like parts and which drawings form a part of the present specification. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates in block diagram form the preferred embodiment of the invention. Figure 2 illustrates a multi-line linear sensor with an exploded view portion showing the individual photo-elements positioned in a matrix array. Figure 3 illustrates the nominal positioning of the photosensitive elements relative to the original scene pixel. Figure 4 illustrates a right deviation in the object positioning mechanism resulting from mechanical tolerance. Figure 5 illustrates a left deviation in the object positioning mechanism resulting from mechanical tolerance. Figure 6 illustrates the nominal positioning of the photosensitive elements relative to the original scene pixel with a weighted average applied. Figure 7 illustrates a deviation in the object positioning mechanism resulting from a mechanical tolerance relaxation to reduce system cost with the resulting weighted average value calculation. Figure 8 illustrates a more detailed block diagram of the line selection feedback which generates the signal required to determine the correct pixel weighting factors. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 1, a scene 30 is focused by lens 31 onto a CCD image sensor 33. An infrared cutoff filter 42 is situated between scene 30 and lens 31. The analog output of CCD image sensor 33 is processed and digitized in block 36. A position sensor 34 generates a signal, corresponding to the absolute position of scene 30, which is directed to a line selection feedback block 38. The proper lines of the sensor 33 are selected and summed in the line store and ALU block 37 utilizing the output from the line selection feedback block 38. The final captured image reconstruction and storage is accomplished in block 39. From this storage area the image can be outputted to a hardcopy device 40, displayed in a display device 41, or enhanced utilizing image manipulation software. Referring to Figure 2, the CCD image sensor 33 may be of the linear sensor array type comprised of a plurality of M lines 14 each formed from a plurality of photosensitive elements (pixels) . The number of lines 14 in the preferred embodiment is seven. Below the sensor lines 14 is a horizontal shift register 17 which is also formed from a plurality of elements along the length of the sensor array. A single column of photosensitive elements is identified by the number 20. The photosensitive elements in the column 20 are labeled 21 through 27 and an arrow 19 defines the direction of relative motion between the scanned object 30 and the sensor 33. In Figure 3 the photosensitive element 24 of the column 20, and each corresponding pixel element in each of the remaining columns of the sensor defines the position of the center of the sensor, shown by the center line labeled CL. The center line of the linear array is coincident with the center of a scene pixel SP2 on the scanned scene 30. For any given scene there are thousands of pixels positioned in a matrix array that make up the scene. In the scanning process one scene pixel is the pixel of interest at any particular time. That pixel is identified as SP2. The one preceding in interest is labeled SP3 and the one next in interest is labeled SP1. For the condition of Figure 3 no error exists with respect to motion and position sensor 34 indicating that pixel elements 23, 24 and 25 are accurately sensing the illumination from the correct scene pixel. In the preferred embodiment the number of photosensitive elements, for example, 23, 24 and 25 that correspond to one scene pixel SP2 is equal to 3. This number is defined as N. In other words, 3 image sensor pixels are used to cover one scene pixel. With the scanning speed constant at the normal rate the feedback signal will always register these center three pixels when scanning the scene pixel. For the condition where the rate of the mechanism is altered, then the relative positioning is also altered. For example, if the scene were to be slowed down temporarily then the normal center line CL would not reach the point on the scene to be scanned next but a set of lines preceding it would be in the correct position. In this case the position sensor 34 generates a signal indicating that the photosensitive (pixel) elements 24, 25 and 26 are accurately sensing the illumination from the correct scene pixel. This is shown in Figure 4. A similar condition exists when the scanning mechanism is accelerated. In this case the position sensor 34 generates a signal indicating that the pixel elements 22, 23 and 24 are accurately sensing the illumination from the correct scene pixel. This condition is illustrated in Figure 5. The weighted average approach is illustrated in Figures 6 and 7. In Figure 6 the pixel element 24, of the column 20, and therefore the center line of the linear array, is coincident with the center of a scene pixel on the scene 30. In this position no error exists with respect to motion and position sensor 34 indicates that elements 23, 24 and 25 are accurately sensing the illumination from the correct scene pixel. In Figure 7 the scanning mechanism is again accelerated. Improving on the operation shown in Figures 4 and 5 the photosensitive elements are selected and modulated by a weighting factor. In this case the position sensor 34 generates a signal indicating that the pixel elements 22, 23, 24 and 25 are all sensing portions of the illumination from the correct scene pixel. Utilizing the position sensing signal, weighting factors are generated for each of the pixel elements 21, 22, 23, 24, 25, 26 and 27. For the example demonstrated in Figure 7 the following table describes the weighting factors for each photosensitive element. Translating the table above, we can determine the scene pixel value from the following equation: (pix 1*0%) + (pix 2*0%) + (pix 3*100%) + (pix 4*100%) + (pix 5*100%) + (pix 6*0%) + (pix 7*0%) = Scene Pixel Value Using a similar approach the following equation sets forth the weighted average which represents the scene pixel value for the case of Figure 7. (pix 1*0%) + (pix 2*20%) + (pix 3 * 100%) + (pix 4*100%) + (pix 5*80%) + (pix 6*0%) + (pix 7*0%) = Scene Pixel Value To determine the correct pixel selection for the operation shown in Figures 4 and 5, or to determine the necessary weighting factors for the operation shown in Figures 6 and 7, the line selection feedback block 38 of Figure 8 can be used. A stable reference frequency is generated or obtained from the sensor timing which represents the optimum pulse rate which would be seen from the position sensor 34 if the scanning speed where constant. This signal labeled A is generated by a reference frequency generator 81 feeding a counter 79. The signal A is subtracted in a subtractor stage 82 from the position sensor signal B and the difference B-A will represent the relative position with respect to the ideal case. A decoder block 83, which may be a look up table, transforms the B-A signal into a form usable by the Line Store and ALU block 37 by determining the weighting factors required for each photosensitive element. The signal B is generated by inputting the encoder output signal to a counter 80 which counter is identical to counter 79. The following table is a partial example of a lookup table which provides the weighting factors. Input ___. pi 1 p x 2 pix 3 p x 4 pix 5 p i pjx 7 While there has been shown what is considered to be the preferred embodiment of the present invention, it will be manifest that many changes and modifications may be made therein without departing from the essential spirit of the invention. It is intended, therefore, in the annexed claims, to cover all such changes and modifications as may fall within the true scope of the invention.

Claims: ___________ 1. A scanning speed compensator comprising: an image sensor array formed from a plurality of photosensitive elements for providing electrical signals indicative of the intensity of light impinging on each photosensitive element; means for forming said image onto said sensor array; means for detecting the relative position between said image array and the scene being scanned and for providing a signal indicating the position of a pixel area of interest from the scene on the sensor array; means for selecting the signals provided by the photosensitive elements that correspond in position to the pixel area of interest in the original scene as speed compensated signals; and means for storing the selected signals for later use. 2. The scanning speed compensator of Claim 1 and further comprising: means for assigning weighted values to the signal outputs from the photosensitive elements that are in positional alignment with the pixel area of interest on the scanned scene to form a weighted average value for each pixel area of the scene. 3. The scanning speed compensator of Claim 1 wherein said image sensor array is formed of M lines of photosensitive elements such that an scene pixel will be formed across N photosensitive elements. 4. The scanning -speed compensator of Claim 1 wherein said image sensor array is formed of seven lines of photosensitive elements such that an scene pixel will be formed across three photosensitive elements. 5. The scanning speed compensator of Claim 1 wherein said means for detecting the relative position between said image sensor array and the scene being scanned is comprised of: a motion sensing means for providing a signal representing the motion between the image sensor array and the scene being scanned; and means coupled to said motion sensing means for receiving said provided signal and for comparing the magnitude of said provided signal to the magnitude of a reference signal and for providing said signal indicative of the position of a pixel area of interest from the scene on the image sensor array as a function of the comparison. 6. The scanning speed compensator according to Claim 5 wherein said means coupled to said motion sensing means is comprised of: a counter means having an input for receiving said provided signal and for providing a count indicative of the magnitude of the motion; means for generating a stable reference count; and comparing means for receiving said count indicative of the magnitude of the motion and said stable reference count and for providing said signal indicative of the position of a pixel area of interest from the scene on the image sensor array. 7. The scanning speed compensator according to Claim 6 wherein said comparing means is comprised of: a subtractor having inputs for receiving said count indicative of the magnitude and said stable reference count and for providing an output value representing the difference between the signals on its inputs; and a decoder means having as an input the output value from said subtractor for providing a weighting factor to each of said selected signals. 8. A scanning speed compensator for use in a system wherein an image and a sensor are moved with respect to each other comprising: an image; image sensor means for receiving and converting said image into analog signals; means for digitizing said analog signals to provide digitized photosensitive element signals; means for generating positional information regarding the relative motion of the image and sensor; means for selecting photosensitive elements which correspond to particular image pixels; and processing means for combining said digitized photosensitive element signals from said selected photosensitive elements representing said image pixels to generate image pixel values which are speed compensated. 9. The scanning speed compensator of Claim 8 and further comprising: means for assigning weighted values to the signal outputs from the selected photosensitive elements that are in positional alignment with the pixel area of interest on the scanned image to form a weighted average value for each pixel area of the image. 10. The scanning speed compensator of Claim 8 wherein said image sensor means is formed of M lines of photosensitive elements such that an image pixel will be formed across N photosensitive elements wherein N < M. 11. The scanning speed compensator of Claim 8 wherein said image sensor means is formed of seven lines of photosensitive elements such that an image pixel will be formed across three photosensitive elements. 12. The scanning speed compensator of Claim 8 wherein said means for detecting the relative position between said image sensor means and the image being scanned is comprised of: a motion sensing means for providing a signal representing the motion between the image sensor means and the image being scanned;, and means coupled to said motion sensing means for receiving said provided signal and for comparing the magnitude of said provided signal to the magnitude of a reference signal and for providing said signal indicative of the position of a pixel area of interest from the image on the image sensor means array as a function of the comparison. 13. The scanning speed compensator according to Claim 12 wherein said means coupled to said motion sensing means is comprised of: a counter means having an input for receiving said provided signal and for providing a count indicative of the magnitude; means for generating a stable reference count; and comparing means for receiving said count indicative of the magnitude and said stable reference count and for providing said signal indicative of the position of a pixel area of interest from the image on the image sensor means. 14. The scanning speed compensator according to Claim 13 wherein said comparing means is comprised of: a subtractor having inputs for receiving said count indicative of the magnitude and said stable reference count and for providing an output value representing the difference between the signals on its inputs; and a decoder means having as an input the output value from said subtractor for providing a weighting factor to each of said selected signal.

Inventors: GABOURY MICHAEL JOSEPH, GEISBUESCH GORDON, GABOURY, MICHAEL, JOSEPH, GEISBUESCH, GORDON
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1991
Main classification: H04N1
IPC codes: H04N1