Improved adaptive filtering and thresholding arrangement for reducing graininess of images

Claim 2

The method of claim 1 wherein said selecting step comprises at least one of: activating an error filter configured to diffuse said quantization error among a relatively small number of neighbouring pixels when one of said grayscale value is greater than or equal to a predefined grayscale value and the absolute value of said quantization error is less than a predefined error value; activating an error filter configured to diffuse said quantization error among a relatively small number of neighbouring pixels when said grayscale value indicates high-tone image pixels; activating an error filter configured to diffuse said quantization error among a relatively large number of neighbouring pixels when one of said grayscale value is less than a predefined grayscale value and the absolute value of said quantization error is greater than or equal to a predefined error value; and activating an error filter configured to diffuse said quantization error among a relatively large number of neighbouring pixels when said grayscale value indicates mid-tone and shadow-tone image pixels.

Claim 3

The method of claim 1 or 2 wherein said adaptively quantizing step comprises the steps of: selectively modulating a threshold value in response to said determined grayscale value; comparing said grayscale value with said selectively modulated threshold value to produce a binary value; and subtracting said binary value from said grayscale value to generate said quantization error.

Claim 4

The method of claim 3 wherein said step of selectively modulating comprises the step of adding a predetermined quantity of noise errors to said threshold value.

Claim 5

The method of claim 4 wherein said adding step comprises the step of varying said predetermined quantity of noise errors in response to said determined grayscale value.

Claim 6

The method of claim 4 wherein said adding step comprises the step of varying said predetermined quantity of noise errors between 0-5 bits depending upon said determined grayscale value.

Claim 8

The method of claim 7 wherein said processing step comprises the step of quantizing a first encoded pixel to generate a quantization error.

Claim 9

The method of claim 8 wherein said adaptively filtering step comprises the step of spreading said quantization error proportionally among substantially adjacent neighbouring encoded pixels of the array.

Claim 10

The method of claim 9 wherein said substantially adjacent neighbouring pixels excludes those encoded pixels of the array positioned immediately to the right and immediately beneath said first encoded pixel.

Claim 11

The method of claim 9 wherein said substantially adjacent neighbouring pixels includes those encoded pixels of the array positioned immediately to the right and immediately beneath said first encoded pixel.

Claim 12

Apparatus for improving the quality of a halftone image generated from an input stream of continuous-tone image pixels, the apparatus comprising: means (904) for determining grayscale values for pixels of the input stream; and means (960) for adaptively quantizing said grayscale values to generate quantization errors; characterised by: means (960) for selecting error filters (903A-C) from a set of error filters (930) having differing sizes and weighting coefficients in response to said determined grayscale values, said selected error filters being configured to diffuse said quantization errors among neighbouring pixels of said input stream in a manner that improves the quality of said halftone image.

Claim 13

The apparatus of claim 12 wherein said selecting means (960) comprises means for activating error filters configured to diffuse said quantization errors among one of: a relatively small number of neighbouring pixels when one of said grayscale values are greater than or equal to a predefined grayscale value and the absolute values of said quantization errors are less than a predefined error value; and a relatively large number of neighbouring pixels when one of said grayscale values are less than a predefined grayscale value and the absolute values of said quantization errors are greater than or equal to a predefined error value.

Claim 14

The apparatus of claim 13 wherein said activating means comprises one of a pixel detection and control circuit (904) and an error detection and control circuit (960).

Claim 16

The apparatus of claim 15 wherein said selectively modulating means comprises: a threshold circuit (916) for iteratively comparing said grayscale values with a threshold value and generating, from the comparison, binary raster values; and a summing circuit (926) for subtracting said binary raster values from said grayscale values to generate said quantization errors.

Claim 17

The apparatus of claim 16 wherein the threshold circuit (916) is coupled to a noise circuit (920) adapted to randomly modify said threshold value in response to a control signal generated by said pixel detection and control circuit (904).

Claim 19

The system of claim 18 further comprising: an output buffer (950) for storing the binary raster values; and an error buffer (938) for storing said quantization errors of the immediately previous series of pixels, and wherein values of said quantization errors are selected from the error buffer.

Claim 20

A computer system comprising: a memory (104, 106) for storing data and programs; a central processing unit (102) responsive to programs stored in the memory for controlling and coordinating the operation of the computer system; digitizing means responsive to a continuous-tone image for generating a stream of electronically encoded pixel values, each representing a grayscale value of a portion of the continuous-tone image; means responsive to the stream of electronically encoded pixel values for storing the electronically encoded pixel values in a plurality of linear segments, each linear segment comprising pixel values that represent contiguous portions of the continuous-tone image; a pixel detection and control circuit (904) for determining the grayscale values of the electronically encoded pixels; means (960) for adaptively quantizing the grayscale values to process the encoded pixels into a halftone pattern and to generate quantization errors; and a printer (114) for printing the halftone pattern of dots on a print medium to generate a halftone image. characterised by: a filter circuit (930) including a plurality of multiplexed error filters (93A-C), each having a predetermined size and associated weighting coefficients, a selected one of said multiplexed error filters being activated in response to the determined grayscale values to diffuse the quantization errors among neighbouring pixels of the linear segments.