Image preprocessing by modified median filter

{
  "publication_number": "US4736439A",
  "country": "US",
  "kind": "A",
  "title": "Image preprocessing by modified median filter",
  "abstract": "Preprocessing of a noisy image, such as is obtained by a raster scan, by tracting the median of the pixel values of one or more lines from the medians of predetermined pixel neighborhoods.",
  "claims": [
    "1. In a system providing an image represented digitally as a plurality of parallel vectors and in which components of the vectors have values corresponding to the brightness of points of a scene and to noise present therein or occurring in the system, the method of median filtration comprising: calculating for each vector of said plurality a vector median value which is the median of the values of predetermined components of the vector; and when there is a significant difference between the vector median value of one vector of said plurality and the vector median value of the vector of said plurality adjacent to said one vector at one side thereof, subtracting from one component of said one vector the median of the vector median values of said one vector and one or more other vectors included in said plurality and contiguously related to said one vector at said one side thereof.",
    "2. The method of claim 1 further comprising setting the value of said one component to a predetermined value when the value of said one component, after subtracting said median, is less than a value corresponding to a predetermined such brightness."
  ],
  "description_excerpt": "1. Field of the Invention\n\nThis invention pertains to the field of image analysis. More particularly, it pertains to image transformation or pre-recognition processing using adaptive quantization based on a local average.\n\n2. Description of the Prior Art\n\nThe digital representation of an image by pixel values dispersed in a matrix is, well known, as are various digital techniques for the removal of noise from the image, \"noise\" being used in the sense of anything which degrades an ideal image and including high frequency noise, which affects one or a few pixels, and low frequency noise, such as a systemic error which causes a brightness change between opposite sides of an image.\n\nAn effective way of correcting an unrepresentative value of one pixel is \"median filtration\", which is the substitution, for the value of one pixel, of the median of pixel values in a predetermined neighborhood of pixels containing the one pixel. For example, the median value of pixels arranged in a cross or a square centered on the one pixel may be substituted for the value thereof. Such neighborhood median filtration is, however, relatively ineffective for low frequency noise and does not take advantage of lengthy discontinuities, such as the horizon or extended object portions, actually present in an imaged scene.\n\nThe subject invention involves image preprocessing by median filtration in which the median value of one or more rows of pixels of a digitized image represented by a matrix, as might result from a raster scan, is subtracted from the values of individual pixels to filter low and high frequency noise.",
  "cpc": [
    "G06T 5/20"
  ],
  "ipc": [
    "G06T 5/20"
  ],
  "assignees": [
    "US Department of Navy"
  ],
  "inventors": [
    "Bruce O. May"
  ],
  "filing_date": "1985-05-24",
  "publication_date": "1988-04-05",
  "grant_date": "1988-04-05",
  "priority_date": "1985-05-24",
  "application_number": "US-73760685-A",
  "family_id": "24964549",
  "cited_by_count": 69,
  "citations": [
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}