Abstract: A custom palette comprises colors on a plurality of luminance levels consisting of colors of uniform luminance. The spacing in luminance between levels varies in relation to the luminance, the spacing between levels being closer at low luminance and increasing at greater luminance. The palette preferably includes a gray color from each luminance level, highly saturated colors at the vertices of every alternate level, and a selection of lower saturation colors encircling the gray color on each level. The selection of lower saturation colors preferably includes a higher concentration of flesh and sky colors.

Abstract: A high color resolution image is accurately approximated with a low resolution image through a unique combination of ordered dithering and error diffusion. The true color of each pixel is modified with error from previously rendered pixels and then dithered to an intermediate color of 15 bits. The intermediate color is then mapped to the nearest displayable color in a displayable color palette using a precomputed look-up table. Any error between a displayed color of a pixel and its true color is calculated and spread among neighboring pixels not yet rendered.

Abstract: A system and method for facsimile data encoding uses vectors indicating the locations of changing elements along a horizontal scan line. The change vectors are stored in a vector table. The system may use a reference vector table and a coding vector table if CCITT recommendation T.6 two dimensional encoding is used. The system encodes horizontal scan lines using the vectors to determine the location of the changing elements. The value of the reference changing element is easily determined using the vector tables. The system determines the locations of all changing elements on a horizontal scan line before performing any encoding of that particular scan line thus simplifying the encoding process. The system can receive and decode facsimile data to generate change vectors. The change vectors provide a simple mechanism for regenerating the raw data.

Abstract: A high color resolution image is accurately approximated with a low resolution image through ordered dithering to colors of a displayable color palette. The colors of the displayable color palette are located at points of a grid aligned with three orthogonal axes in a three dimensional color model, including a luminance axis between black and white. The grid points are defined by the intersections of three sets of parallel planes, each set being perpendicular to one of the three axes. The set of planes perpendicular to the luminance axis are spaced nonuniformly, preferably according to a cubic model. The ordered dithering maps the true color of a pixel in the image to the color in the displayable palette at one of eight adjacent grid points. When ordered dithering of a pixel's true color would map the true color to a grid point outside the color gamut, the pixel is instead mapped to the nearest color on the color gamut's border to the grid point.

Abstract: A custom palette comprises colors on a plurality of luminance levels consisting of colors of uniform luminance. The spacing in luminance between levels varies in relation to the luminance, the spacing between levels being closer at low luminance and increasing at greater luminance. The palette preferably includes a gray color from each luminance level, highly saturated colors at the vertices of every alternate level, and a selection of lower saturation colors encircling the gray color on each level. The selection of lower saturation colors preferably includes a higher concentration of flesh and sky colors.

Abstract: A high color resolution image is rendered using a palette having a lower color resolution. The color of each pixel in the image is mapped to the nearest of the palette colors located on or between levels of uniform luminance adjacent to the pixel color. The mapping of the pixel color is further constrained to exclude palette colors for which double the error to the pixel color is outside the color gamut. The mapping is performed efficiently by a look-up table, the size of which is reduced by first dithering the pixel color to a color in an intermediate palette and then indexing the table using the intermediate palette color to find the reduced palette color.