Browse Prior Art Database

Direct-to-Palette Dithering

IP.com Disclosure Number: IPCOM000111564D
Original Publication Date: 1994-Mar-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 2 page(s) / 123K

Publishing Venue

IBM

Related People

Crawford, JM: AUTHOR [+3]

Abstract

Color can be represented in many different ways. Current display technology employs the Red-Green-Blue (RGB) color space and a color-limiting scheme called palettization. While reducing the cost of display hardware, palettization complicates the problem of rendering images to a display by reducing the number of discrete colors available. In order to maintain color fidelity, it is necessary to employ a color averaging technique, such as error diffusion or dithering, to simulate the colors of the higher resolution image with the limited colors of the palette.

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Direct-to-Palette Dithering

      Color can be represented in many different ways.  Current
display technology employs the Red-Green-Blue (RGB) color space and a
color-limiting scheme called palettization.  While reducing the cost
of display hardware, palettization complicates the problem of
rendering images to a display by reducing the number of discrete
colors available.  In order to maintain color fidelity, it is
necessary to employ a color averaging technique, such as error
diffusion or dithering, to simulate the colors of the higher
resolution image with the limited colors of the palette.

      Traditionally the palettization process is applied to an image
that is in the same color space as the destination device's palette.
However, other color spaces have characteristics that make them
desirable for capturing, storing, and processing images.  Described
below is one solution to the problem of displaying images,
represented in an arbitrary color space, using a palette that may be
organized in a different color space (that, in turn, may be different
from the color space of the device).

      The problem of rendering images on displays with limited color
capability can be broken down into several steps.  A palette or set
of colors must be selected.  This may be done in the color space of
the display, or in some other more visually uniform color space.
Next a quantizer must be designed that maps each pixel of the image
into one of the palette colors.  This quantizer may transform the
image into a different color space in which to do the quantizing.
Finally, to achieve the most pleasing rendering of an image using a
palette, some form of processing is applied to the image that
modifies the value of pixels based on either the spatial position of
the pixel or the values of surrounding pixels.  This processing
smooths the effects of the quantizing.

      Palettes, quantizers and even the processing to lessen
artifacts may be independent of any particular image, or they may
depend on the values of pixels or statistics of a given image.  The
present article deals with the independent situation, in which the
palette is pre-specified.  The focus is on the quantizer and the
flexibility to quantize to an arbitrary palette (that is in a
different color space than the source image).

      There are two main steps necessary for solving the problem with
a pre-specified palette.  First, a common color space for the palette
and the source image(s) must be selected.  The points in the palette,
referred to as the palettization or simulation colors, are translated
into this common color space.  The second step is the implementation
of the palettization, or simulation technique, including the
selection of a quantizer to reduce the source image points to a more
manageable size, and/or a dither or error diffusion technique to
perform the smoothing.  In the case of the color space of the palette
and the source image needing t...