Patent ID: 6990242

Claim:
A process for encoding bi-level images, said process comprising: for each pixel location in raster order in the bi-level image, a predicting step for predicting a binary value for the pixel at a pixel location under consideration based on its context, wherein a context of a pixel refers to predicted values of a prescribed pattern of pixels preceding the pixel in raster order, said predicting step comprising, an assigning step for assigning a prescribed initial probability value to each of a set of potential context indexes, wherein a context index is a binary word comprising previously predicted binary values of a prescribed pattern of pixels of the bi-level image preceding in raster order a pixel whose value is currently being predicted, and wherein the initial probability value indicates the probability that the pixel whose value is currently being predicted has a first binary value associated with a first of the two colors of the bi-level image based on the predicted values of the prescribed pattern of pixels preceding the pixel being predicted, and for each pixel location in raster order in the bi-level image, a computing step for computing the context index associated with the prescribed pattern of pixels preceding the pixel location under consideration, wherein pixel locations in the pattern that fall outside the bi-level image are considered to have the first binary value, an identifying step for identifying the probability value assigned to the computed context index, whenever the identified probability value indicates that the pixel location under consideration is more likely than not to have the first binary value, a second assigning step for assigning as the predicted pixel value for that location the first binary value, and whenever the identified probability value indicates that the pixel location under consideration is not more likely than not to have the first binary value, a third assigning step for assigning as the predicted pixel value for that location the second binary value associated with a second of the two colors of the bi-level image, and a determining step for determining whether the predicted pixel value matches the actual pixel value for the pixel location under consideration; and a compressing step for compressing the data concerning at which pixel locations the predicted pixel values match and do not match the actual values using a context-dependent, backward adaptive, Run-Length-Rice encoding technique.