Optical Adaptive Thresholder for Converting Analog Signals to Binary Signals

1. Field of Invention

This invention relates to document scanners and converting their analog video signals to binary video signals.

2. Background of Prior Art

In the field of document scanning and image processing, the output of linear array scanners are analog video signals. These analog video signals represent continuously varying shades of gray. One objective is to convert these analog video signals to binary video signals by comparing a thresholding signal with the analog video signal on a pixel by pixel basis. The generation of the thresholding signal is critical if an accurate binary video representation of printed document is desired. Generally, an aperture or window is electrically synthesized from the primary video signal. Several scan lines are synchronously stored until the aperture is filled. The aperture makes available a matrix of pixels in the local vicinity of and including the pixel of interest. A function such as median[AR], weighted average[AR], or local peak/valley[AK] operates on the matrix of pixels and generates a threshold level for a given pixel. The next pixel of interest will have an aperture with a different matrix of pixels and so on. Most threshold functions are adaptive in that each threshold level is calculated from its aperture on a pixel by pixel basis. The aperture can be of one dimension(1D) or two dimension(2D) and any number of pixels in either dimension. When linear CCD's are used as scanners, one dimensional aperture synthesis generally requires some sort of delay lines or temporary storage of data and additional circuitry. When a two-dimensional aperture synthesis system is employed, several scans of data must be stored as well and then synchronously made available as required by the aperture. Complexity and quantity of electronic circuitry is proportional to any increase of the aperture in either dimension.

It is an object of this invention to generate a binary video image of a document by an optically synthesized local adaptive threshold on a pixel by pixel basis.

A benefit of this invention is a decrease in electronic circuitry and its associated advantages in manufacturing and maintenance.

Another benefit of this invention is a higher frequency of operation for a given power consumption.

Another benefit of this invention is ease of changing aperture size.

FIG. 1 describes an overview of the two CCD scanner system.

FIG. 2 shows a comparison of the primary video waveform and the 2D adaptive threshold waveform. The output of the voltage comparator is the binary representation on a pixel by pixel basis.

1 Test document

2 Illumination

3 Video Lens (focused)

4 Thresh. Lens (defocused)

5 CCD scanner (primary)

6 CCD scanner (adp. thrsh)

7 Control Logic & Timing

8 Odd pixels (adp. thrsh)

9 Even Pixels (adp. thrsh)

10 Odd pixels (primary)

11 Even pixels (primary)

12 Gain amplifier

13 Offset amplifier

14 Gain amplifier

15 Offset amplifier

16 Odd pixels (adp. thrsh)

17 Even pixels (adp. thrsh)

18 Odd pixels (primary)

19 Even Pixels (primary)

20 Sample/Hold Multiplexor

21 Sample/Hold Multiplexor

22 Capacitor

23 Capacitor

24 FET buffer

25 FET buffer

26 Adaptive thresh. signal

27 Primary video signal

28 Voltage Comparator

29 Binary video signal

FIG. 1 shows a paper document 1, illumination 2, lenses 3 and 4, and the two identical CCD scanners 5 and 6, both synchronously scanning precisely the same line of the document 1. Note that 3 and 5 are on the optical axis while 4 and 6 are slightly off axis. CCD scanner 5 generates the primary video waveform through a finely focused lens 3. CCD scanner 6 optically synthesizes the adaptive threshold waveform through a defocused lens 4. The degree of defocusing determines the lack of contrast and consequently the size of the effective synthesized aperture. A more finely focused lens results in a smaller synthesized aperture. Conversely, a coarsely focused lens results in a larger synthesized aperture. The main point here is that, since the defocused light through 4 onto 6 contains all the information for a synthesized aperture, the equivalent electronic circuitry in prior art, such as median[AR] or weighted average [AR] can now be omitted. Each scanner 5 and 6 is a linear CCD with two analog video outputs 8 and 9 or 10 and 11. One output of a scanner can be considered even numbered pixels and the other output odd numbered pixels. The odd and even pixels must be multiplexed or interlaced in the proper sequence by the video multiplexors 20 and 21 to represent the original features on the document. The two outputs are bufferred with gain and offset amplifiers 12, 13, 14, 15 to facilitate matching the two signals 16 and 17 or 18 and 19. Signals 16 and 17 are multiplexed by 20. Signals 18 and 19 are multiplexed by The multiplexors, 20 and 21 are equivalent to sample and hold circuits, which store their signals on capacitors 22 and 23. FET input buffers 24 and 25 are used to transfer the signals to the inputs of comparator 28. Comparator 28 compares the adaptive threshhold signal 26 with the primary video signal 27. The output 29 of the comparator is a binary representation of this comparison on a pixel by pixel basis.