A method and system for detecting grids in a digital image

Abstract: A method for detecting grids in a digital image having a plurality of pixels for capturing an image, comprises the steps of performing a high pass filter operation on a sample of pixels for filtering out the image from the sampled pixels; and performing a statistical F-test operation on the results of the high pass filter operation for determining when the digital image contains grids.

Description: Background Of The Invention Field of Invention The invention relates generally to a method and system for detecting grids in a digital image. Background of the Related Art There are methods and systems, which are well known in the art, for viewing internal portions of an object by directing radiation signals, generally referred to in the art as x-rays, onto the object and by receiving the radiation signals which pass through the object onto a recording medium, typically film. Ideally, the radiation signals will pass from the radiation source to the film in a straight line. However, as the radiation signals pass through the object, some of the signals are scattered so that they no longer travel in a straight line, which scattering causes reduced image quality. To reduce this effect, radiographic grids are placed between the object and film. Radiation grids are lead foil strips placed in a spaced apart relationship with each other, which spaced apart relationship is typically maintained by placing aluminum between the strips. The strips and aluminum are then packaged in a suitable covering. These grids, though effective for reducing radiation scattering, occasionally introduce artifacts such as grid lines into the image. The film containing the captured image is processed in a variety of well known and utilized techniques for permitting on observer to view the image captured on the film. One such technique is to convert the image into a digitized form for displaying the image on a computer display and similar devices. To convert the image into digitized form, typically, a digitizer converts the film into digital form for permitting the image to be displayed on a computer display by well known techniques. Before displaying the image on the computer screen, grids lines, if there are any, should be removed. There are two well known methods in the art for reducing the effects of grid lines. One method utilizes moving the grids when the object of interest is radiated. The other method is to apply a processing algorithm to the computer system which is displaying the image. Although the above methods for reducing grid lines are satisfactory, they are not without drawbacks. Moving grids involve a costly procedure and, in addition, require the object to be exposed to the radiation for a longer period of time than is normally required. This obviously is a drawback if the object is a person. For these reasons, they are not a preferred method of reducing the effects of grid lines. The processing algorithm is also costly and, as a secondary effect, can reduce the quality of the image. Consequently, a need exists for a method of detecting grids so that costly processing algorithms for removing grids are applied only when there are grids in the image. Summary Of The Invention It is an object of the present invention to provide a method and system for detecting grids. It is an advantage of the present invention to provide such a grid detection method and system which are cost effective and involve a minimum of time to implement. It is a feature of the present invention to implement a high pass filter for removing the capture image from the digitized representation and to apply a statistical operation on the results for detecting the presence of grids. The above and other objects, features and advantages 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 invention. With this object in view, the present invention resides in a method for detecting grids in a digital image having a plurality of pixels, comprising the steps of (a) performing a horizontal and vertical high pass filter operation on a sample of pixels within the image; and (b) performing a F-test on the results of the high pass filter operation for determining if the digital image contains grids, wherein the F-test operation comprises computing a ratio of variances of the horizontal and vertical filtered samples, and wherein it is determined that vertical grids are present if the F-test operation indicates that the horizontal variance is less than the vertical variance, and wherein it is determined that horizontal grids are present if the F-test operation indicates that the horizontal variance is greater than the vertical variance. Brief Description Of The Drawings In the course of the following detailed description, reference will be made to the attached drawings in which: Fig. 1 is a drawing illustrating the environment of the present invention; Fig. 2 is a flowchart of the method in accordance with the present invention. Fig. 3 is an example of a plot pixel value versus pixel location; Fig. 4 is a plot in which Fig. 3 is smoothed; and Fig. 5 is the subtraction of Fig. 4 from Fig. 3, a high pass filter. Detailed Description Of The Invention Referring to Fig. 1, there is illustrated a system for implementing the present invention. The system includes a high voltage generator 10 which powers an X-ray tube 20 for generating a beam of radiation signals 30, generally referred to as X-rays, for irradiating an object 40. The radiation signals pass through the object and are detected by a recording medium, typically film 50. As is illustrated by the dashed lines, some of the radiation signals are scattered as they pass through the object 40. These scattered signals reduce the image quality, and as a result, a well known and utilized radiographic grid 60 is placed between the object 40 and film 50 for reducing the effects of scattering. The film 50, which now contains the captured image, is placed in a digitizer 70 for converting it into a digital format. During this conversion, the digitizer 70 scans one portion of the film 50, generally referred to in the art as a pixel 80, and assigns a pixel value to the pixel 80, which value represents the contents on the film 50 at that particular pixel 80. The digitizer 70 scans the entire image in the above described manner for converting each portion of the film 50, or pixel 80, into a pixel value. In essence, the digitized data represents each pixel 80 location and its corresponding pixel value. The film 50 is typically scanned in such a manner that in pixel representation the image includes a plurality of contiguous pixels 80 having a checkered array configuration. The digitized data is passed to an image processor 90 where the data is stored in pixel format in a first 100 or second memory 110. In other words, a pixel location and its associated pixel value are stored in either of the two memories 100 and 110. The image processor 90 also contains an arithmetic unit 120 for performing arithmetic functions and other typical software functions. The image processor 90 upon command from a user can display the image stored in the memories 100 and 110 on a computer display 130. However, before the image is displayed on the screen 130, a method of the present invention analyzes the image for the presence of grids which may have been created by the radiographic grids 60. Referring to Fig. 2, the method is illustrated in flowchart form. The image processor 90 loads a software program of the present invention which randomly samples 140 two percent of the pixel values stored in memory 100 and 110 for determining the mean value 150 of the sampled pixels 80, which mean value is an estimate of the mean of the entire image. It is instructive to note that two percent sampling is merely the percentage of the preferred embodiment and that any percentage of the pixels 80 may, in fact, be sampled for this estimation. Pixels 80 with a value of the mean or higher are an estimation of where it is easier to detect grids. Next, an array of pixels 80, preferably a 16 x 16 array, having a pixel value of the mean or higher is selected 160 for determining the mean value 170 of the 16 x 16 array. Again, although a 16 x 16 array is preferred, any size array may be used for this sample. If the array mean is below the estimated mean of the image 180, the array is discarded and another 16 x 16 array is sampled 160. If the array mean is at or above the estimated mean of the entire image 180, the array will be further analyzed for the presence of grids. The 16 x 16 array is further sampled by sampling two or more lines of pixels which are parallel to an imaginary x-axis or ordinate 190 which is through the center of the array. Each pixel 80 from each of the sampled lines which have a same ordinate is then averaged 200 together so that the result is an average pixel value along the ordinate. This result is plotted as a graph of pixel value versus pixel location along the ordinate. An example of such a plot is shown in Fig. 3. Referring back to the flowchart, this plot is smoothed 210 which is well known in the art and an illustration of which is shown in Fig. 4. (See Digital Picture Processing Picture , Rosenfeld and Kak, Academic Press 1982 for further disclosure of smoothing) The smoothed plot is subtracted 220 from the averaged plot for forming a high pass filter of the resultant pixels. An example of a plot of the high pass filter is shown in Fig. 5. Referring to Fig. 2, the resultant high pass plot is autocorrelated 230 by techniques which are well known in the art for enhancing the high pass plot (See Detection of Signals in Noise, Whalen 1971 for further disclosure of autocorrelation). For further enhancement, the autocorrelated 240 plot is again autocorrelated for further still better quality. The variance, which is a well known statistical technique, is then calculated 230 from the result of this autocorrelation (See Probability and Statistics, DeGroet Addison Wesley 1975 for details of variance calculation). Steps 190 through 250 are then repeated for sampling along the y-axis or abscissa, and are recited below for thoroughness of understanding. For example, the 16 x 16 array further samples two or more lines of pixels which are parallel to an imaginary the abscissa 260 which is also through the center of the array. Each pixel from each of the sampled lines which have a same abscissa is then averaged 270 together so that the result is an average pixel value along the abscissa. This result is plotted as a graph of pixel value versus pixel location along the abscissa and the plot is smoothed 280. The smoothed plot is subtracted 290 from the averaged plot for forming a high pass filter of the resultant pixels. The resultant high pass plot is autocorrelated 300 for enhancing the high pass plot. For further enhancement, the autocorrelated plot is again autocorrelated 310 for further still better quality. The variance is then calculated 320 from the result of this autocorrelation. After the variances in both directions are calculated, a F-test 330 is used for determining whether grids are present in the 16 x 16 sampled array of pixels. The F-test is calculated as follows: F = σ 2 Horizontal σ 2 Vertical Equation (1) If the F-test score is greater than 1, this results in a determination that vertical grids are present in the sampled array, and a vote of positive one is given. If the F-test score is less than 1, this results in a determination that horizontal grids are present in the sampled array, and a vote of negative one is given. A F-test score of zero means that neither vertical nor horizontal grids are present, and a vote of zero is given. If all of a predetermined percentage of pixels 80 with a mean value at or greater than the mean pixel value of the entire image have been included in a 16 x 16 array for analysis 340, the average of all the votes 350 are taken and the result indicates whether any grids are present and, if so, what direction they rest. The result of the average has the same meaning as the F-test score: positive one means vertical grids, negative one means horizontal grids are present, and a zero indicates no grids are present. If all the pixels have not been included in a 16 x 16 array for analysis 340, the above steps are repeated until they have been included in a 16 x 16 array analysis. It is also obvious for the man skilled in the art that said invention could also be used for circular grids. In such a case the average of the pixel value is made according to a predetermined pattern dependent on the location of the 16x16 sampled array with respect to the center of the grid. Referring back to Fig. 1, if a grid is present in the image, the image processor 90 applies a filter to the image for removing them. Such filters are well known in the art.

Claims: A method for detecting the presence of grids (60) in a digital image having a plurality of pixels (80), comprising the steps of: (a) performing a horizontal and vertical high pass filter operation on a sample of pixels within the image; and (b) performing a F-test (330) on the results of the high pass filter operation for determining if the digital image contains grids (60), wherein the F-test operation comprises computing a ration of variances of the horizontal and vertical filtered samples, and wherein it is determined that vertical grids are present if the F-test operation indicates that the horizontal variance is less than the vertical variance, and wherein it is determined that horizontal grids are present if the F-test operation indicates that the horizontal variance is greater than the vertical variance. The method as in claim 1 wherein the high pass filter operation includes the steps of : (a1) forming a plot of pixel location versus the pixel value averaged, at the corresponding pixel location, according to a predetermined pattern dependent on the grid used; (a2) smoothing (210, 280) the results of the plotting operation; and (a3) subtracting (220, 290) each plotted pixel value and each smoothed pixel value which are from the same pixel location for obtaining the high pass filter. The method as in claim 2 wherein step (al) includes: (ai) further sampling from the sampled pixels at least two spaced apart lines of pixels; (aii) averaging (200, 270) pixel values from each of the spaced apart lines which have a matching abscissa for removing noise from the digital image; and (aiii) forming the plot in step (a1) from the averaged pixel values. The method as in claim 3 wherein step (aii) includes averaging pixel values from each of the spaced apart lines which have a matching ordinate for removing noise from the digital image. The method as in claim 4 wherein the plot is autocorrelated for enhancing the plot. The method as in claim 1 wherein no grids are present in the digital image when either of the conditions in step (b) are not met. The method as in claim 6 wherein each of the sampled pixels designate a first signal if vertical grids are detected and designate a second signal if horizontal grids are detected for determining which of the first or second signals have a majority which, in turn, determines whether the digital image has vertical or horizontal signals. A system for detecting horizontal and vertical grids (60) in a digital image having a plurality of pixels (80) for capturing an image, comprising: (a) means (90, 120) for performing a horizontal and vertical high pass filter operation on a sample of pixels for filtering out the image from the sampled pixels; and (b) means (90, 120) for performing a F-test operation on the results of the high pass filter operation for determining if the digital image contains grids; wherein the F-test comprise computing a ratio of variances of the horizontal and vertical filtered samples, and wherein it is determined vertical grids are present if the F-test operation indicates that the horizontal variance is less than the vertical variance and wherein it is determined horizontal grids are present if the F-test indicates that the horizontal variance is greater than the vertical variance. The system as in claim 8 further comprising: (a1) means for forming a plot of pixel location versus the pixel value averaged, at the corresponding pixel location, according to a predetermined pattern dependent on the grid used; (a2) means for smoothing the results of the plotting operation; and (a3) means for subtracting each plotted pixel value and each smoothed pixel value which are from the same pixel location for obtaining the high pass filter.

Inventors: GABORSKI ROGER S, PAWLICKI THADDEUS F, GABORSKI, ROGER S., PAWLICKI, THADDEUS F., Gaborski, Roger S., c/o Eastman Kodak Company, Pawlicki, Thaddeus F., c/o Eastman Kodak Company
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1996
Main classification: G06T5
IPC codes: G06T7, G01N23, G06K9, G06T1