Process and device for analyzing and restoring image data in a digital signal from a scanned document

An image scanning device comprising:

• a) a scanning reading device for scanning a document and for providing a primary digital image data signal representative of pixel intensity values for each discrete pixel of said scanned document;
• and
• b) a binarizing device for binarizing said primary image data signal, including means for adjusting a threshold value to each pixel and means for comparing each of said pixel intensity values with the threshold value associated to the pixel and for delivering a binary image signal based on said comparison,
• said threshold adjusting means comprising means for assigning to each pixel a background parameter value and an information parameter value, wherein said assigning means comprises
• window memory means for memorizing said background and information parameter values assigned to at least one of the previously scanned pixels situated immediately adjacent to the current pixel,
• first decaying means for diminishing the stored background parameter value assigned to said one of said previously scanned pixels and providing the diminished result as a decayed background level and an associated first setting means for setting the background parameter of the current pixel at a value which is the highest one of the intensity value of the current pixel and said decayed background level,
• second decaying means for increasing the stored information parameter value assigned to said one of said previously scanned pixels and providing the increased result as decayed information level, and associated second setting means for setting the information parameter of the current pixel at a value which is the lowest one of the intensity value of the current pixel and said decayed information level,
• means for calculating the threshold value as a function of said set current background and information parameter values;
• and
• c) a device for the binarized evaluation of a document comprising:
• means for registering the intensity values of pixels distributed in at least two concentric rings around each elementary area of successively scanned elementary areas, at least in one sector centered on said elementary area, determining contrast values each equal to the difference of the registered intensity values between a pixel of an inner ring and a pixel of an outer ring, and assigning a positive and negative difference sign to respectively the positive and negative contrast values thus determined having an absolute value higher than a predetermined contrast threshold, wherein a contrast threshold value which is a function of the difference between the stored background and information parameter values of at least one of said previously scanned pixels is substituted for the predetermined contrast threshold;
• and
• means for counting distinctively said positive and negative difference signs in the vicinity of said elementary area and means for assigning a light or dark quality to said elementary area respectively when said counted difference signs contain a predominant number of positive or negative difference signs conforming to a predetermined minimum condition.

An image scanning device comprising:

• a) a scanning reading device for scanning a document and for providing a primary digital image data signal representative of pixel intensity values for each discrete pixel of said scanned document;
• b) a binarizing device for binarizing said primary image data signal, including means for adjusting a threshold value to each pixel and means for comparing each of said pixel intensity values with the threshold value associated to the pixel and for delivering a binary image signal based on said comparison,
• said threshold adjusting means comprising means for assigning to each pixel a background parameter value and an information parameter value, wherein said assigning means comprises
• window memory means for memorizing said background and information parameter values assigned to at least one of the previously scanned pixels situated immediately adjacent to the current pixel,
• first decaying means for diminishing the stored background parameter value assigned to said one of said previously scanned pixels and providing the diminished result as a decayed background level and an associated first setting means for setting the background parameter of the current pixel at a value which is the highest one of the intensity value of the current pixel and said decayed background level,
• second decaying means for increasing the stored information parameter value assigned to said one of said previously scanned pixels and providing the increased result as decayed information level, and associated second setting means for setting the information parameter of the current pixel at a value which is the lowest one of the intensity value of the current pixel and said decayed information level,
• means for calculating the threshold value as a function of said set current background and information parameter values,
• means for providing a guard band value, and means responding to the difference between the stored background and information parameter values of at least one of said previously scanned pixels for disabling said first and second decaying means whenever said difference is lower than said guard band value;
• and
• c) a device for the binarized evaluation of a document comprising:
• means for registering the intensity values of pixels distributed in at least two concentric rings around each elementary area of successively scanned elementary areas, at least in one sector centered on said elementary area, determining contrast values each equal to the difference of the registered intensity values between a pixel of an inner ring and a pixel of an outer ring, and assigning a positive and negative difference sign to respectively the positive and negative contrast values thus determined having an absolute value higher than a predetermined contrast threshold, wherein a contrast threshold value which is a function of the difference between the stored background and information parameter values of at least one of said previously scanned pixels is substituted for the predetermined contrast threshold;
• and
• means for counting distinctively said positive and negative difference signs in the vicinity of said elementary area and means for assigning a light or dark quality to said elementary area respectively when said counted difference signs contain a predominant number of positive or negative difference signs conforming to a predetermined minimum condition.

A process for providing a characterization binary output from a signal indicative of intensity values of distributed discrete pixels of a scanned image of a document emitted by a scanning device, wherein each of successively scanned pixels is characterized as being black or white depending on a comparison between the intensity value of the scanned pixel and a threshold assigned to said pixel, said process comprising successively assigning to each pixel a background level and an information level and calculating said threshold as a function of said background level and said information level at that pixel, wherein each of said assigning and calculating steps comprises the substeps of:

• determining the whitest one of the background levels assigned to those of the previously scanned pixels which are situated immediately adjacent to the current pixel in predetermined directions around it and the darkest one of the information levels assigned to said previously scanned pixels,
• decaying said whitest background level by subtracting a first decay rate from this level and decaying said darkest information level by adding a second decay rate to this level,
• setting the background level of the current pixel at a value which is the highest one of the intensity value of the current pixel and said decayed background level, and setting the information level of this pixel at a value which is the lowest one of the intensity value of the current pixel and said decayed information level, and
• memorizing said set current background level and said set current information level for use in the calculation of following thresholds for further pixels.