Apparatus and method for encoding halftone images

A method for encoding a selected halftone pattern to provide a corresponding first signal with the selected halftone pattern including a first set of pixels arranged in a first block, comprising the steps of:

• providing a second set of pixels arranged in a second block;
• providing the second block with a uniform halftone pattern formed from a selected threshold matrix and a single gray value;
• comparing the selected halftone pattern with the uniform halftone pattern to determine whether the selected halftone pattern substantially matches the uniform halftone pattern;
• encoding the first block with the first signal when the selected halftone pattern substantially matches the uniform halftone pattern;
• and
• encoding the first block with a second signal when the selected halftone pattern is substantially different than the uniform halftone pattern.

A method for encoding a selected halftone pattern to provide a corresponding first signal with the halftone pattern including a set of pixels arranged in a block comprising the steps of:

• setting each pixel to one of a first output state and a second output state;
• providing each pixel with one of a plurality of distinct threshold values, one of the pixels with the first output state having a maximum threshold value and one of the pixels with the second output state having a minimum threshold value;
• comparing the pixel with the first output state having the maximum threshold value to the pixel with the second output state having the minimum threshold value;
• and
• encoding the block with the first signal when the pixel with the first output state having the maximum threshold value is substantially less than the pixel with the second output state having the minimum threshold value.

An apparatus for encoding a selected halftone pattern to provide a corresponding first signal with the halftone pattern including a set of pixels arranged in a block, comprising:

• an image buffer for buffering a threshold matrix and the block;
• means, communicating with said image buffer, for determining whether the selected halftone pattern can be formed with the threshold matrix and a single gray value;
• an encoder for encoding the block with the first signal in response to a determination by said determining means that the selected halftone pattern can be formed with the threshold matrix and the single gray value;
• means for partitioning the block into at least two subblocks in response to a determination by said determining means that the selected halftone pattern cannot be formed with the threshold matrix and the single gray value wherein each of the subblocks is separated by a partition boundary and is characterized by a subblock halftone pattern, and wherein the encoder is adapted to:
• encode the block with a second signal when the selected halftone pattern cannot be formed with the threshold matrix and the single gray value,
• encode each subblock with a third signal when the subblock halftone pattern can be formed with part of the threshold matrix and a single gray value, and
• encode each subblock with a fourth signal when the subblock halftone pattern cannot be formed with part of the threshold matrix and the single gray value.