Dynamic threshold system for multiple raster content (MRC) representation of documents

1. A method for dynamically thresholding an image signal, the method comprising the operations of:

(a) receiving, at a computing block, the image signal and a minimum and a maximum within each of a set of windows centered on the current pixel in the image signal;

(b) computing, for each of the windows, based on the current pixel and the respective minimum and maximum, a respective indicator representing the distance and direction of the current pixel relative to a respective threshold plane; and

(c) outputting a control signal based on the indicators.

2. The method of claim 1 wherein operation (b) comprises the operations of:

(i) computing, for each of the windows, a respective contrast vector; and

(ii) computing, for each of the windows, a bias vector and a dot product of the respective contrast vector and a respective thresholded pixel vector representing the current pixel thresholded by the bias vector, the dot product representing the respective indicator.

3. The method of claim 2 wherein, for one of the windows, operation (ii) comprises the operation of computing the average between the respective maximum and the respective minimum to form the bias vector.

4. The method of claim 2 wherein, for one of the windows, operation (ii) comprises the operation of computing the average between a vector representing a lowpass filtered neighborhood of the current pixel and the average between the respective maximum and the respective minimum to form the bias vector.

5. The method of claim 1 wherein operation (b) comprises:

(1) computing a first indicator for a first window, via a first logic block, the first indicator representing the distance and direction of the current pixel relative to a first threshold plane;

(2) computing a second indicator for a second window, via a second logic block, the second indicator representing the distance and direction of the current pixel relative to a second threshold plane;

(3) thresholding the current pixel in the image signal via a third logic block and outputting a third logic block signal; and

wherein operation (c) comprises generating the control signals based on the first and second indicators and the third logic block signal.

6. The method of claim 5 wherein operation (b) further comprises computing a first activity measure indicating activity in the first window, using the first logic block, and computing a second activity measure indicating activity in the second window using the second logic block.

7. The method of claim 5 wherein operation (c) comprises:

comparing the first activity measure with the second activity measure and outputting a select signal, using a comparator block; and

receiving the first indicator, the second indicator and the third logic block signal, selecting and outputting one of the first indicator, the second indicator and third logic block signal in accordance with the select signal, using a multiplexer.

8. A system for dynamically thresholding an image signal, the system comprising:

a computing block receiving the image signal and a minimum and a maximum within each of a set of windows centered on the current pixel in the image signal, and computing, for each of the windows, based on the current pixel and the respective minimum and maximum, a respective indicator representing the distance and direction of the current pixel relative to a respective threshold plane, and outputting a control signal based on the indicators.

9. The system of claim 8 wherein the computing block comprises:

a first module computing, for each of the windows, a respective contrast vector; and

a second module computing, for each of the windows, a bias vector and a dot product of the respective contrast vector and a respective thresholded pixel vector representing the current pixel thresholded by the bias vector, the dot product representing the respective indicator.

10. The system of claim 9 wherein, for one of the windows, the second module computes the bias vector by computing the average between the respective maximum and the respective minimum.

11. The system of claim 9 wherein, for one of the windows, the second module computes the bias vector by computing the average between a vector representing a lowpass filtered neighborhood of the current pixel and the average between the respective maximum and the respective minimum.

12. The system of claim 8 wherein the computing block comprises:

(1) a first logic block computing a first indicator for a first window, the first indicator representing the distance and direction of the current pixel relative to a first threshold plane;

(2) a second logic block computing a second indicator for a second window, the second indicator representing the distance and direction of the current pixel relative to a second threshold plane;

(3) a third logic block thresholding the current pixel in the image signal and outputting a third logic block signal; and

(4) a decision module generating the control signals in communication with the first, second, and third logic blocks, the control signals being based on the first and second indicators and the third logic block signal.

13. The system of claim 12 wherein the first logic block computes a first activity measure indicating activity in the first window and wherein the second logic block computes a second activity measure indicating activity in the second window.

14. The system of claim 12 wherein the decision module comprises:

a comparator block comparing the first activity measure with the second activity measure and outputting a select signal; and

a multiplexer receiving the first indicator, the second indicator and the third logic block signal, selecting and outputting one of the first indicator, the second indicator and third logic block signal in accordance with the select signal.

15. An article of manufacture comprising:

a machine usable medium having program code embedded therein, the program code being used for dynamically thresholding an image signal, the program code comprising:

(a) machine readable code to receive the image signal and a minimum and a maximum within each of a set of windows centered on the current pixel in the image signal;

(b) machine readable code to compute, for each of the windows, based on the current pixel and the respective minimum and maximum, a respective indicator representing the distance and direction of the current pixel relative to a respective threshold plane; and

(c) machine readable code to output a control signal based on the indicators.