Adaptive filtering and thresholding arrangement for reducing graininess of images
First Claim
1. A method for improving the quality of a halftone image generated from an input stream of continuous-tone image pixels, the method comprising the steps of:
- determining a grayscale value for a selected one of the pixels of the input stream;
adaptively quantizing said gray scale value of the selected pixel to generate a quantization error;
providing;
a first error filter corresponding to a first range of quantized grayscale values and having a first size and a first weighting coefficient, anda second error filter corresponding to a second range of quantized grayscale values and having a second size and a second weighting coefficient; and
selecting one of;
the first error filter when the quantized grayscale value of the selected pixel is within the first range, andthe second error filter when the quantized grayscale value of the selected pixel is within the second range,wherein said selected error filter is configured to diffuse said quantization error among neighboring pixels of said input stream in a manner that improves the quality of said halftone image.
0 Assignments
0 Petitions
Accused Products
Abstract
An adaptive filtering and thresholding arrangement provides a set of error filters having different sizes and associated weighted coefficients for diffusing quantization errors among neighboring pixels in predetermined tonal areas of an image to achieve a smooth halftone image quality. Each error filter circuit is optimally applied to a particular pixel area depending upon the grayscale tone of that area and the desired output print resolution. The arrangement further provides for the addition of "noise" errors to threshold values at selected input image pixel ranges and at the intersection of two differently filtered areas to eliminate visible pattern distortion.
106 Citations
25 Claims
-
1. A method for improving the quality of a halftone image generated from an input stream of continuous-tone image pixels, the method comprising the steps of:
-
determining a grayscale value for a selected one of the pixels of the input stream; adaptively quantizing said gray scale value of the selected pixel to generate a quantization error; providing; a first error filter corresponding to a first range of quantized grayscale values and having a first size and a first weighting coefficient, and a second error filter corresponding to a second range of quantized grayscale values and having a second size and a second weighting coefficient; and selecting one of; the first error filter when the quantized grayscale value of the selected pixel is within the first range, and the second error filter when the quantized grayscale value of the selected pixel is within the second range, wherein said selected error filter is configured to diffuse said quantization error among neighboring pixels of said input stream in a manner that improves the quality of said halftone image. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
-
-
12. A method for improving the quality of a printed halftone image having a desired resolution, the method comprising the steps of:
-
storing an array of electronically encoded pixels representative of tonal values associated with a source image; adaptively filtering a selected one of said encoded pixels in accordance with a multiplexed filter circuit that is enabled, in response to said representative tonal values and said desired resolution, for activation by one of; a first error filter corresponding to a first range of quantized grayscale value and having a first size and a first weighting coefficient and a second error filter corresponding to a second range of quantized grayscale values and having a second size and a second weighting coefficient, wherein the first error filter is selected when the quantized grayscale value of the selected pixel is within the first range and the second error filter is selected when the quantized grayscale value of the selected pixel is within the second range; processing the selected one of said encoded pixels into a halftone pattern which, when printed, emphasizes fine details of the source image at highlight and upper midtone regions, and refines halftones at darker and lower midtone regions; and printing said halftone pattern onto a recording medium with said desired resolution. - View Dependent Claims (13, 14, 15, 16)
-
-
17. Apparatus for improving the quality of a halftone image generated from an input stream of continuous-tone image pixels, the apparatus comprising:
-
means for determining grayscale values for a selected one of the pixels of the input stream; means for adaptively quantizing said grayscale value of the selected pixel to generate quantization errors; a first error filter corresponding to a first range of quantized grayscale values and having a first size and a first weighting coefficient; a second error filter corresponding to a second range of quantized grayscale values and having a second size and a second weighting coefficient; and means for selecting one of; said first error filter when said quantized grayscale value of the selected pixel is within the first range, and said second error filter when the quantized grayscale value of the selected pixel is within the second range, wherein said selected error filter is configured to diffuse said quantization errors among neighboring pixels of said input stream in a manner that improves the quality of said halftone image. - View Dependent Claims (18, 19, 20, 21, 22, 23, 24)
-
-
25. A computer system comprising:
-
a memory for storing data and programs; a central processing unit 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 for determining the grayscale value of a selected one of the electronically encoded pixels; means for adaptively quantizing the grayscale value of the selected pixel to process the encoded pixels into a halftone pattern and to generate quantization errors; a filter circuit including a first error filter corresponding to a first range of quantized grayscale values and having a first size and a first weighting coefficient, and a second error filter corresponding to a second range of quantized grayscale values and having second size and a second weighting coefficient, wherein the first error filter is selected when the quantized grayscale value of the selected pixel is within the first range and the second error filter is selected when the quantized grayscale value of the selected pixel is within the second range; and a printer responsive to the adaptively quantizing means and the filter circuit for printing the halftone pattern of monochrome dots on a print medium to generate a halftone image.
-
Specification