Abstract: Improved processes for quantizing a set of mean-preserving Tone Transfer Functions (TTFs) for a multi-level halftoning system which minimize or at least significantly reduce the overall quantization error in the system as a function of a prespecified system halftone cell size, P, and the number of micro output levels, L.sub.y, supported by the multi-level halftoning system. The processes call for dividing the range of possible input contone representative intensity values into T subintervals, where T=PL.sub.y ; determining the values of the centers of the T-subintervals; and quantizing each of the normalized continuous mean preserving TTFs as a function of the values of the centers of the T-subintervals. According to a preferred embodiment, the quantizing is performed such that the quantized TTFs approximate the normalized continuous mean preserving TTFs and ideally satisfy the criteria that the maximum quantization error between any quantized and unquantized TTF is minimized.

Abstract: A convex interpolation apparatus and method to map source color signals in a n-dimensional color space to target color signals in a m-dimensional color space includes finding a set of sample signals in the source color space whose convex hull encloses the given signal; determining the coefficients needed to express the given signal as a convex combination of the set of sample signals; and interpolating the source signal to obtain a target signal in the target color space by using the coefficients and the sample signals is the target color space that correspond to the sample signals selected in the source color space. The sample signals in both color spaces can be lattice points or non-lattice points. A method and apparatus are also provided which can not only determine if a point is enclosed by a convex hull of a set of points in a n-dimensional space, but also simultaneously derive the coefficients needed to express the given point as a convex combination of the set of points.

Abstract: A method of calibrating a four color printer capable of printing three colors and black, includes the steps of: forming a minimum black data structure representing a minimum black strategy; forming a maximum black data structure representing a maximum black strategy; for each printable black level, forming a fixed black data structure; and for a specific output color value, finding the minimum and maximum black levels using the minimum and maximum black data structures, determining a desired black level between the minimum and maximum black levels according to a defined black strategy, and determining the three printing color levels from the fixed black data structure corresponding to the desired black level.

Abstract: A process for selecting output levels for multi-level halftones optimizes the stability of an output device, even though the system contains unstable components. First, a function which quantifies undesirable mean level image artifacts for the unstable display device is defined, to stabilize the tone transfer curve of the unstable device. Then density levels are chosen to minimize the function and optimize stability of the output device.

Abstract: Apparatus and a method for generating a multi-level halftone image (104) from a digitally sampled continuous tone (contone) image (101). The apparatus includes a control circuit (140), a preference matrix (150) having as its matrix elements addresses of a plurality of look-up tables (160) and a plurality of look-up tables (160) in the form of a look-up table stack (155). The control circuit instructs the preference matrix to select a specific look-up table from the look-up table stack in a pre-defined manner. The selected table is used to convert an intensity value (102) into each multi-level pixel value (106) in the halftone image. To accomplish the conversion, each look-up table contains a quantized one-dimensional transfer function (165') having as an input the magnitude of the intensity value. The transfer functions are automatically generated such that the sum of the derivatives of the transfer functions equals the number of pixel values in a halftone cell.

Abstract: Apparatus and a method for generating a multi-level halftone image (104) from a digitally sampled continuous tone image (101). The apparatus includes a control circuit (140), a preference matrix (150) having as its matrix elements addresses of a plurality of look-up tables (160) and a plurality of look-up tables (160) in the form of a look-up table stack (155). Generally, the control circuit (140) instructs the preference matrix (150) to select a look-up table (160) from the look-up table stack (155) in a pre-defined manner. The selected table (160) is used to convert an intensity value (102) into a multi-level pixel value (106) in the halftone image (104). To accomplish the conversion, each look-up table (160) contains a quantized one-dimensional transfer function (165') having as an input the magnitude of the intensity value (102). The magnitude is mapped into an output level by the transfer function as one of a plurality of available levels.

Abstract: An image processor performs error diffusion by thresholding an incoming gray pixel to black or white, comparing the difference between the black or white print value and the incoming pixel gray level and superimposing the difference on the next pixel gray level. The threshold value is read out from a threshold matrix which has been modulated with a pattern which is complementary to the known artifact pattern of the error diffusion process. Typically, the threshold matrix has sparce threshold values along one of its diagonals, the direction of the diagonal being perpendicular to the prevailing direction of artifacts generated by the error diffusion algorithm.

Abstract: A halftoning method for creating a color binary image from a continuous tone color image or computer generated graphics signal is improved by diffusing color vector error and by including the visual color blur functions in the recursive error propagation algorithm so that perceived color vector rather than binary color error is propagated.

Abstract: An image processor converts a multiple gray-level image to a bi-tonal image using both error diffusion and ordered dither to enhance image quality. The error diffusion process is attenuated smoothly as the local area intensity approaches a light or dark maximum, while the ordered dither process is attenuated smoothly as the local area high spatial frequency content increases.

Abstract: A digital halftone image is produced by providing a binary bit image pattern having a minimum visual noise for each density level in an image. The patterns are produced by employing a stochastic combinatorial minimization technique and a human visual system modulation transfer function (MTF) weighting function to generate a halftone pattern for each density level of the multi-level digital image signal. A halftone image is produced by modularly addressing these patterns with each pixel value in the image.