Image analysis – Color image processing – Image segmentation using color
Reexamination Certificate
1996-09-06
2001-07-24
Boudreau, Leo (Department: 2621)
Image analysis
Color image processing
Image segmentation using color
C382S173000, C382S254000
Reexamination Certificate
active
06266439
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to image processing apparatus and methods for processing digital colour data representing a colour or black-and-white image to enhance the rendition of text in presentations of said image, and in particular, but not exclusively, to such apparatus and methods for use in a colour scanner printer, copier, facsimile or the like.
In many existing designs of colour copier, the original image is scanned and digitised to provide red, green and blue (RGB) digital signals representing the relative intensities of these three colours at each pixel of the scanned image. The RGB data is converted to cyan, magenta, yellow and black (CMYK) signals by a colour processing pipeline, and the CMYK signals are used to control the amount of cyan, magenta, yellow and black ink or toner deposited at the appropriate pixel in the printed image.
Whilst the colour processing pipeline is able to deal appropriately with continuous tone colour image data, it does not perform well with areas of text, i.e. the processing of simple text on a white or plain coloured background. Due to a mixture of scanner inaccuracies and the non-linearities in the colour mapping between RGB and CMYK, which are desirable for continuous tone and to push near white background regions to white, the text tends to be rendered both too light and with too much colour in and around it. The adverse effects on coloured text or black text on coloured background are perceived as being less critical. Similar problems arise in equipment other than copiers, for example laser or ink-jet colour printers, colour display units, colour scanners and the like, where the image is represented as a collection of chromatic signals.
U.S. Pat. No. 5,189,523 discloses a system in which an image is scanned in a first scan to provide a discrimination signal, for example, using an edge extraction filter and a “black judgement” step. The discrimination data is stored in a bit map memory which is then used during a second scan to control whether data from the second scan is enhanced or used raw.
European Published Application No. 463,844 discloses a system in which an input image is processed to enhance the boundary region between a text letter or item and the background. Both these earlier proposals concentrate on the edge or interface between black and white; they do not take into account further features within the image components or connectivity, and so in some instances may highlight the edge of a text glyph whilst degrading its overall form.
SUMMARY OF THE INVENTION
We have developed an improved apparatus and method for text enhancement in which the physical regions bounded by a potential text item or glyph are determined and the colour data therein monitored, to discriminate black text items.
Accordingly, in one aspect, this invention provides image processing apparatus for processing colour signals representing a colour or black-and-white image, to enhance the rendition of black text in a subsequent presentation of said image, said apparatus comprising:
(i) means for analysing at least one of said colour signals over said image or a part thereof to identify within said image or part thereof one or more groups of neighbouring pixels, each of which defines a potential text item;
(ii) colour discriminating means for monitoring at least some of the colour signals of pixels within and/or around each potential text item to discriminate one or more black text items, and
(iii) means for enhancing said one or more discriminated black text items in a subsequent presentation of said image.
For convenience, the phrases “black text item” and the like are used to define black text items, near black text items, or indeed any other foreground text items, including white foreground on a black background.
The analysing means preferably compares the intensity of said colour signal in each pixel with a first threshold and classifies each pixel having an intensity below said first threshold as belonging to a potential text item, and the apparatus preferably includes means for storing data identifying each potential text item and the pixels belonging thereto.
Where the colour signals comprise the usual red, green and blue colour signals, the means for analysing preferably analyses the green colour signals.
The colour discriminating means preferably monitors the red, green and blue intensities of each of the pixels making up the potential text item. In a preferred embodiment, said colour discriminating means includes, for each colour channel, means for accumulating a value representing an estimate for each colour channel.
The means for enhancing preferably sets each of the pixels within each discriminated black text item to black. We have found it beneficial to configure the apparatus so that the means for enhancing sets to black only those pixels where the intensity of said colour signal is below a second threshold, lower than the first threshold mentioned above.
Preferably, said means for enhancing includes means for setting the colour signals of selected pixels adjacent said discriminated black text item to make up white or another suitable background colour.
The image processing apparatus preferably includes means for classifying the image into potential text areas and non-text areas, so that the analysing means may analyse the colour signals only in text regions.
Said means for classifying preferably includes means for defining selected cells or regions within said image, means for monitoring within each cell or region the values of the colour signals for selected pixels to determine data representing the statistical distribution of the intensities of the colour signals for said selected pixels, and means for identifying said selected region as a text area or a non-text area dependent on said statistical distribution. Said means for monitoring may determine peaks in the statistical distribution data for each channel.
Said means for identifying preferably includes means for comparing said statistical distribution data with a black intensity maximum threshold and a white intensity minimum threshold, said white intensity minimum threshold being set as required to capture both pure white and off-white backgrounds. A selected region is identified as a text region if it does not include a peak between said black intensity maximum threshold and said white intensity minimum threshold, and if it meets one or both of the following conditions:
(i) it includes a peak above the white minimum threshold, and
(ii) it includes a peak below the black maximum threshold.
The above technique indicates black on white or white on black. For more general colour discrimination, in one embodiment, to avoid visiting each possible triplet combination (256
3
in all for 8-bit RGB), the apparatus includes means for determining intensity statistical distribution data for each colour signal separately, means for identifying peaks in the statistical distribution data for each colour signal, means for selecting restricted search spaces corresponding to said peaks, each restricted search space preferably being defined using the intensity values for said peaks, and means for assessing the population in each of said search spaces, whereby the presence of a highly-populated search space corresponding to combinations of colour signals making up black or near black, and another highly-populated search space corresponding to colour signals making up white, near white or a lightly-coloured background (as defined by the white intensity minimum threshold), indicates that the selected region is a text area.
To reduce the size of memory required for processing, and to avoid the need to store a complete image or page before being able to start outputting processed colour signals, the apparatus preferably includes means for storing a sliding window or swath of the image, with said means for analysing and said colour discriminating means operating on said sliding window or swath.
In another aspect, this invention provides an image processing method for processing colo
Pedersen Claus Henrik
Pollard Stephen Bernard
Boudreau Leo
Hewlett--Packard Company
Sherali Ishrat
LandOfFree
Image processing apparatus and methods does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Image processing apparatus and methods, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Image processing apparatus and methods will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2566242