Data compression method for graphics images
First Claim
1. A method for compressing data representing a graphics image encoded in binary form having at least first and second bit planes respectively containing a most significant bit and at least significant bit for each pixel in the image indicating whether the pixel has an extreme intensity value or one of two intermediate intensity values, comprising the steps of:
- identifying edge pixels from the change in value of the binary state of successive bits in said first bit plane of said image;
generating at least a single binary bit for each adjacent edge pixel so identified;
comparing the bits in the first and second bit planes for each of said edge pixels to determine the binary state of each of said binary bits generated, for indicating by said state whether each edge pixel has an extreme intensity value or an intermediate intensity value; and
replacing said second bit plane with said indicative binary bits to compress the image data.
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Abstract
Graphic images are generally considered to be those images comprised of text and/or drawings. Data compression of graphics images is desired whenever a fast image transmission speed is desired in a limited band width channel. It is also used for storage of a large number of images in a limited capacity storage system. A high compression ratio is achieved by thresholding the graphics image to a bilevel black-white image at one bit per pixel and then employing a second data compression on the black-white image. At low resolution, bilevel images have poor quality at edges and a quality improvement is needed. The data compression apparatus and method disclosed separates the graphics image into at least first and second bit planes identifies edge pixels from the first bit plane indicating a black/white change, locates the edge pixels and generates at least a single bit for each edge pixel indicating whether the edge pixel has a maximum intensity value such as black or white or an intermediate gray intensity value. Intermediate values are not allowed except at edge pixels which enhances both quality and compressibility of the resulting graphics image.
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Citations
8 Claims
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1. A method for compressing data representing a graphics image encoded in binary form having at least first and second bit planes respectively containing a most significant bit and at least significant bit for each pixel in the image indicating whether the pixel has an extreme intensity value or one of two intermediate intensity values, comprising the steps of:
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identifying edge pixels from the change in value of the binary state of successive bits in said first bit plane of said image; generating at least a single binary bit for each adjacent edge pixel so identified; comparing the bits in the first and second bit planes for each of said edge pixels to determine the binary state of each of said binary bits generated, for indicating by said state whether each edge pixel has an extreme intensity value or an intermediate intensity value; and replacing said second bit plane with said indicative binary bits to compress the image data. - View Dependent Claims (2, 3, 4)
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5. Apparatus for compressing binary data representing a graphics image having at least first and second bit planes respectively containing a most significant bit and a least significant bit for each pixel in the image, comprising:
- means for identifying edge pixels from the change in value of the binary state of successive bits in said first bit plane of said image; and
means for comparing corresponding bits in said first and second bit planes for each said edge pixel and generating at least a single bit for each said edge pixel, indicating whether said edge pixel has a maximum intensity value or an intermediate inensity value, which single bits replace said second bit plane. - View Dependent Claims (6, 7, 8)
- means for identifying edge pixels from the change in value of the binary state of successive bits in said first bit plane of said image; and
Specification