Method and apparatus for OCR detection of valuable documents by means of a matrix camera
The invention relates to a method for OCR detection of valuable documents in a cash dispenser in the case of which an image of the valuable document is detected by means of a digital video or matrix camera. A Hough transformation is used to calculate edge lines of the valuable document and a rotation angle is calculated therefrom such that the edges of the valuable document are aligned with the image edges. The detected image is homogenized to compensate an inhomogeneous image background. This is followed by OCR detection of alphanumeric information on the valuable document.
1. A method for optical character recognition (OCR) detection of documents in a self-service machine comprising the steps of:
A) acquiring an image of a document by means of a digital video or matrix camera;
B) performing fine rotation on the acquired image by:
a) reducing the acquired image;
b) applying a median filter to remove details of the acquired image the details including at least one of alphanumeric characters and graphic information;
c) determining a position of a document region corresponding to the document;
d) using an edge filter to determine edge pixels in the acquired image and outputting a binary edge image of the determined edge pixels;
e) detecting straight edge lines of the document region with the aid of the binary edge image of the determined edge pixels by using a Hough transformation;
f) determining a rotation angle by which the document region in the image must be rotated for parallel alignment with edges of a field of view of the digital video or matrix camera; and
g) rotating the document region by the determined rotation angle and cutting off areas outside the detected straight edge lines to establish an intermediate document image; and
C) forming a homogenized background image by:
h) establishing a background image by applying the median filter to each pixel in the intermediate document image, wherein applying the median filter includes replacing a pixel value of each pixel with a median pixel value from a number of pixels neighboring the respective pixel, thereby removing smaller details in the intermediate document image to establish the background image;
i) creating a brightness map from the established background image;
j) subtracting the brightness map from the intermediate document image to form a negative image; and
k) inverting the negative image to form a final document image;
D) binarizing the final document image to segment alphanumeric character information; and
E) OCR detecting the alphanumeric character information;
wherein the steps of forming a homogenized background image include further reducing the intermediate document image before applying the median filter to remove the smaller details and thereby obtaining the brightness map with a coarser resolution.
2. The method of claim 1 , wherein the step of detecting the straight edge lines using the Hough transformation includes determining, for each pixel, which line runs therethrough, and raising an assessment of each of the detected straight edge lines when the pixel is an edge pixel, the straight edge lines corresponding to the lines most highly assessed.
3. An apparatus for optical character recognition (OCR) detection of valuable documents in a self-service machine, comprising:
a digital video or matrix camera to acquire an image of a document;
a median filter to remove details of the acquired image, the details including at least one of alphanumeric characters and graphic information;
an edge filter that determines edge pixels in the acquired image and outputs a binary edge image of the determined edge pixels;
an image processing section that is designed in order:
to reduce the acquired image,
to determine a position of a document region, corresponding to the document, and its edge pixels in the detected image,
to detect straight edge lines of the document region with the aid of the binary edge image of the determined edge pixels by using a Hough transformation,
to determine a rotation angle by which the valuable document region must be rotated in the image for parallel alignment with edges of a field of view of the digital video or matrix camera,
to rotate the document region by the determined rotation angle and to cut off areas outside the detected straight edge lines to establish an intermediate document image,
to establish a background image by applying the median filter to each pixel in the intermediate document image, wherein applying the median filter includes replacing a pixel value of each pixel with a median pixel value from a number of pixels neighboring the respective pixel, thereby removing smaller details in the intermediate document image to establish the background image,
to create a brightness map from the established background image,
to subtract the brightness map from the intermediate document image to form a negative image,
to invert the negative image to form a final document image, and
to binarize the final document image to detect alphanumeric character information in the final document image by OCR detection,
wherein the intermediate document image is further reduced before applying the median filter to remove the smaller details and thereby obtaining the brightness map with a coarser resolution.
4. The apparatus of claim 3 , wherein the Hough transformation determines, for each pixel, which line runs therethrough, and raises an assessment of each of the detected straight edge lines when the pixel is an edge pixel, the straight edge lines corresponding to the lines most highly assessed.
5. The apparatus of claim 3 , wherein the image processing section is further designed to further reduce the intermediate document image before applying the median filter to remove smaller details to obtain the brightness map with a coarser resolution.