IMAGE PROCESSING METHOD USING VARIANCE ANALYSIS

Abstract: An apparatus for processing images comprise means (2) to scan in an image. The image data is stored and a variance calculator (6) calculates a variance for the scanned image. An image categoriser (16) determines what further processing of the image is appropriate in dependence on the variance.

Description: IMAGE PROCESSING METHOD USING VARIANCE ANALYSIS This invention relates to a method and apparatus for automated processing of images. It is particularly useful with images of documents for identifying the types of document shown in the image, such as may be used in an automated mail processing system. The complexity of document images is highly variable. This is particularly apparent in mail processing systems where envelopes can have many forms. The simplest type would be a plain white envelope with only an address indicated on it. More complex envelopes may have other material printed on the envelope such as a return address. There are also, e.g., different coloured envelopes, envelopes with windows which enable the address to be seen, plastic envelopes with material printed on them and which also have partial transparency thereby allowing some of the documents within to be seen. It has been appreciated that it is often most effective to apply different image processing methods to images of different degrees of complexity when performing subsequent analysis. For example, when locating and attempting to read an address on an envelope, different processing methods will be required for the different types of envelope. A plain white envelope would be most simple as the address can be located very easily and then optical character recognition applied to it. A more complex envelope such as a plastic and partially transparent envelope will require additional processing to identify the address prior to optical character recognition. Previous methods for automated mail processing have used the same type of image processing for all types of envelopes. We have appreciated that this is computationally inefficient as some documents are subjected to processing which is not appropriate to them because of the simplicity of their images. A preferred embodiment of the present invention therefore seeks to provide a method and apparatus for identifying images of different types of document so that subsequent processing may be applied to each image in dependence on its document type. By doing this, it is possible to ensure that the most appropriate subsequent analysis process is applied to each document. It is important that this analysis of document type is computationally efficient since the intention is to reduce the overall computational overhead of processing. The invention is defined with more precision in the appended claims to which reference should now be made. A preferred embodiment of the invention will now be described in detail by way of example with reference to the accompanying drawings in which: Figure 1 shows a block diagram of apparatus embodying the invention; Figure 2 shows an item of mail scanned when positioned on a uniform background; Figure 3 shows an image of a white envelope; Figure 4 shows an image of a transparent envelope with printing on it; and Figure 5 shows an image of a buff envelope . The block diagram of Figure 1 shows schematically the apparatus and processes which are required to categorise an image, as would be used in a mail processing system. Initially, an image scanner 2 scans in an image of the document being processed and produces image values for each pixel of the image. These are usually grey scale but may be colour. In its simplest form grey scale could be a single bit with two values. However, we have found that this does not give particularly good results and a higher number of bits are required to achieve good categorisation of images . Figure 2 illustrates the scanning in of the image. As can be seen, the particular image in this example is a complex envelope with additional information printed on it. It occupies the top left-hand corner of the scanning area. The grey scale scanned image is then divided in a block divider 4. Typically, blocks of 20 x 20 pixels have been found to be a suitable block size at 200 dots per inch (dpi) or 1/10 of the scanned resolution but other sizes are possible. The image data for each block is then passed to a block variance calculator 6. This produces a variance figure for the grey scale data in each block. The variance data is cropped to the size of the document. After this, the variance values for the relevant blocks are passed to a summation unit 10 which adds all the variances for the blocks together and are also passed to a unit 12 which calculates the variance of variances between the blocks . A normaliser 14 divides the sum of the variances by the number of blocks to produce a normalised variance for the whole image. This and the output of the calculator for the variance of variances form an input to an image categoriser 16. This has an image type output which produces a signal dependent on the category of image determined from the normalised variance and the variance of variances. This signal can then be used in further processing to ensure that the correct type of further processing is used for the image. Figure 3 shows an image of a white envelope. When this is analysed by the circuitry of Figure 1 the normalised variance of this comes to a low level, e.g., 162. The variance of variances, i.e., the range of variances included within the image will be relatively high so that there will be one or more blocks surrounding the address area and the printed material of the base of the image which have a high variance while the remainder of the image which is white will have a low variance. Thus, the inputs to the image categoriser will be a low normalised variance per block and a relatively high variance of variances. The image categoriser will interpret this as representing a white envelope with little printing on it and will allocate the appropriate image type signal to this image. This is used for setting up further processing to be performed on the image. Figure 4 shows an image of a transparent envelope through which much of the internal material can be seen and which has the address label in the bottom left-hand corner. The normalised variance of this is detected as being 1552, thereby indicating that there is a lot of activity in the image and that the image is therefore probably quite complex. However, the variance of variance figure will be relatively low because there will be little variation in variance from block to block. Therefore, a high figure for normalised variants will be input to the image categoriser and a low figure for variance of variances. The image categoriser will in response to this set an image type to this image indicating that it is a complex image with much activity distributed relatively evenly over its surface and will therefore send an appropriate signal to the further processing circuitry to ensure that the correct type of processing is performed on it. Figure 5 shows a buff envelope. Buff envelopes tend not to be relatively but are textured in greyscale this translates to variations in grey level in a small scale. The overall normalised variance of the envelope should be very low because the variations in grey level occur on a scale much smaller than the block size. However, the texture in the buff material adds to the normalised variance and so this will normally be higher than, for example, the white envelope of Figure 4. However, because much of the activity comes from the envelope texture and colour, and this is fairly consistent across the image, the variance of the variance is relatively low for the image. Thus, the input to the image categoriser will be a low variance of variances figure and a normalised variance which is higher than would usually be expected for white envelopes. The image categoriser should then decide the image is of a buff type envelope and will produce an image type signal accordingly to ensure that it then receives the appropriate type of further processing. The invention finds particular useful application in automated mail processing systems where it can be used to increase the computational efficiency of those systems by ensuring that the further processing applied to items of mail is tailored according to the type of image, thereby avoiding computation time which would be appropriate to complex images when processing e.g., plain envelopes. The invention can be implemented in dedicated hardware or in software as will the apparent to those skilled in the art.

Claims: CLAIMS : 1. A method for processing images comprising the steps of scanning an image to be processed, storing the scanned image data, determining a variance for the image data of the scanned image, and determining what further processing of the image is appropriate in dependence on the thus determined variance. 2. A method according to claim 1 in which the step of determining a variance for the image data comprises subdividing the image into a plurality of blocks of pixels, determining a variance for each block, summing the block variances, and normalising the result of the summation to produce a variance for the image. 3. A method according to claim 2 including the further step of determining a variance for the block variances and in which the step of determining what further processing is appropriate uses this variance in addition to the block variance in making the determination . 4. A method according to any preceding claim in which the image data is grey scale data. 5. A method according to claim 2 or 3 in which the block style used is 20 pixels by 20 pixels. 6. A method according to any preceding claim in which the step of determining the variance of the image includes the step of determining which portion of the scanned image includes relevant data and determining the variance only for this portion of the image. 7. A method according to any preceding claim in which the images are images of items of mail and the further processing is dependent on the type of mail item determined from the variance. 8. Apparatus for processing images comprising means for scanning an image, means for storing the scanned image data, means for determining a variance for the image data of the scanned image, and means for determining what further processing of the image is appropriate in dependence on the thus determined variance. 9. A method according to claim 10 in which the means for determining a variance for the image data comprises means for subdividing the image into a plurality of blocks of pixels, means for determining a variance for each block and means for summing the block variances, and means for normalising the result of the summation to produce a variance for the image. 10. Apparatus according to claim 9 further including means for determining a variance for the block variances and wherein the means for determining what further processing is appropriate uses this variance in addition to the block variance in making the determination . 11. Apparatus according to any of claims 7-10 in which the image data is grey scale data. 12. Apparatus according to claim 9 in which the block size used is 20 pixels by 20 pixels. 13. Apparatus according to any of claims 8-12 in which the means for determining the variance of the image includes means for determining which portion of the scanned image includes relevant data and determining the variance only for this portion of the image. 14. Apparatus according to any of claims 8-13 in which the images are images of items of mail and the further processing is dependent on the type of mail item determined from the variance.

Inventors: SWANN ROBERT EDWARD MEREDITH, TAN ROBERT WEI LIANG, YOUNG ROBERT WILLIAM, SWANN, ROBERT, EDWARD, MEREDITH, TAN, ROBERT, WEI, LIANG, YOUNG, ROBERT, WILLIAM
Applicants: CONSIGNIA PLC, SWANN ROBERT EDWARD MEREDITH, TAN ROBERT WEI LIANG, YOUNG ROBERT WILLIAM, SWANN, ROBERT, EDWARD, MEREDITH, TAN, ROBERT, WEI, LIANG, YOUNG, ROBERT, WILLIAM
Application year: 2001
Main classification: H04N1
IPC codes: H04N1