Digital halftoning with selectively applied dot-to-dot error diffusion

Claim 1

A method of quantizing pixel values in an image formed by a plurality of pixels, each pixel value representing an optical density of the image at a location therewithin, and having a pixel value selected from one of a set of `c` original optical density values that has a number of members larger than a desired output set of `d` optical density values, including the steps: storing an array of pixel values representing an area of the image; thresholding the pixel values in said array in accordance with a halftone screen pattern, to derive output pixel values, the array representable as an output dot pattern; comparing the output dot pattern with a set of full dot patterns representing a set of incrementally varying gray levels, to determine whether the output dot pattern is a full dot pattern or a partial dot pattern; for a partial dot pattern, directing said array of output pixel values to an output; for a full dot pattern, determining an error value that is a difference between a sum of pixel values in the array before thresholding, and a sum of pixel values in the array thereafter; adding a remainder error stored in a remainder buffer, from previous arrays of pixel values, to the error value to derive a total error; comparing the total error with a reference, to determine whether to vary the output pattern; varying by a preselected increment, the full dot output pattern in response to said comparison; determining the difference between the total error and the preselected incremental value, and storing said difference as a remainder error in the remainder buffer; and directing the array of output pixel values forming the varied full dot output pattern, to an output.

Claim 2

A method of quantizing pixel values in an image formed by a plurality of pixels, each pixel value representing an optical density of the image at a location therewithin, and having a pixel value selected from one of a set of `c` original optical density values that has a number of members larger than a desired output set of `d` optical density values, including the steps: storing an array of pixel values representing an area of the image; thresholding the pixel values in said array in accordance with a halftone screen pattern, to derive output pixel values, the array representable as an output dot pattern; comparing the output dot pattern with a set of full dot patterns representing a set of incrementally varying gray levels, to determine whether the output dot pattern is a full dot pattern or a partial dot pattern; for a partial dot pattern, directing said array of output pixel values to an output; for a full dot pattern, determining an error value that is a difference between a sum of pixel values in the array before thresholding, and a sum of pixel values in the array thereafter; adding a remainder error stored in a remainder buffer, from previous arrays of pixel values, to the error value to derive a total error; comparing the total error with a reference, to determine whether to vary the output pattern; varying by a preselected increment, the full dot output pattern in response to said comparison; determining the difference between the total error and the preselected incremental value, and storing said difference as a remainder error in the remainder buffer; and directing the array of output pixel values forming the varied full dot output pattern, to an output.

Claim 6

A method of quantizing pixel values in an image formed by a plurality of pixels, each pixel value representing an optical density of the image at a location therewithin, and having a pixel value selected from one of a set of `c` original optical density values that has a number of members larger than a desired output set of `d` optical density values, including the steps: storing an array of pixel values representing an area of the image; thresholding the pixel values in said array in accordance with a halftone screen pattern, to derive output pixel values, the array representable as an output dot pattern; determining an error value that is a difference between a sum of pixel values in the array before thresholding, and a sum of pixel values in the array thereafter; comparing the output dot pattern with a set of full dot patterns representing a set of incrementally varying gray levels, to determine whether the output dot pattern is a full dot pattern or a partial dot pattern; for a partial dot pattern, discarding the error value and directing the array of pixels forming the partial dot to an output; for a full dot pattern, adding a remainder error stored in a remainder buffer, from previous arrays of pixel values, to the error value to derive a total error; comparing the total error with a reference, to determine whether to vary the output pattern; varying by a preselected increment, the full dot output pattern in response to said comparison; directing the array of output pixel values forming the varied full dot output pattern, to an output.

Claim 7

A method of quantizing pixel values in an image formed by a plurality of pixels, each pixel value representing an optical density of the image at a location therewithin, and having a pixel value selected from one of a set of `c` original optical density values that has a number of members larger than a desired output set of `d` optical density values, including the steps: storing an array of pixel values representing an area of the image; thresholding the pixel values in said array in accordance with a halftone screen pattern, to derive output pixel values, the array representable as an output dot pattern; determining an error value that is a difference between a sum of pixel values in the array before thresholding, and a sum of pixel values in the array thereafter; comparing the output dot pattern with a set of full dot patterns representing a set of incrementally varying gray levels, to determine whether the output dot pattern is a full dot pattern or a partial dot pattern; for a partial dot pattern, discarding the error value and directing the array of pixels forming the partial dot to an output; for a full dot pattern, adding a remainder error stored in a remainder buffer, from previous arrays of pixel values, to the error value to derive a total error; comparing the total error with a reference, to determine whether to vary the output pattern; varying by a preselected increment, the full dot output pattern in response to said comparison; directing the array of output pixel values forming the varied full dot output pattern, to an output.