EP0313789A1

EP0313789A1 - Method and apparatus for representing three-dimensional color data in a one-dimensional reference system

Info

Publication number: EP0313789A1
Application number: EP19880115124
Authority: EP
Inventors: John Charles Dalrymple; Scott William Bigger
Original assignee: Tektronix Inc
Current assignee: Tektronix Inc
Priority date: 1987-10-26
Filing date: 1988-09-15
Publication date: 1989-05-03
Anticipated expiration: 2008-09-15
Also published as: EP0313789B1; DE3876212T2; JPH01147497A; DE3876212D1

Abstract

A method and apparatus are disclosed for representing and displaying the true (actual) color of an image in an index color system. The user specifies the number of color levels necessary to represent the range of displayable colors, and specifies the color (the red, green, blue primary color values) desired for an image. From the specified number, the apparatus calculates RGB (red, green, blue) color values, and stores them in memory in a manner suitable for referencing by an index type address. From the specified color, the apparatus calculates an index suitable for referencing the desired RGB color values from the stored color values. The apparatus comprises a keyboard and a port to a host computer for inputting index color data and true color data, a processing means coupled to the keyboard and/or host computer for producing index-type data from true color data, a memory means for storing the input data and data produced by the processing means, and a display means for displaying stored data.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to the display of color images on a computer terminal and, more particularly, to a method of processing, storing, and referencing data representing image color.
In many computer terminal systems, image color data are stored as R,G,B (red, green, blue) color component values in predetermined memory locations. Often, red values are stored in one group of memory locations, green values in another, and blue values in a third group. To select a color, made up of one color value from each of the groups, the user usually specifies a color number or index. The index, which represents a single (one-dimensional) address, permits access to the particular set of RGB values representing the selected color. One such prior art "single index" system is described, for example, in U.S. Patent 4,509,043 issued April 2, 1985 to inventor P.X. Mossaides.
Another type of prior art system is the "true color" system, in which the true or actual colors of an object are sought to be displayed. One such true color system is described, for example, in the Model One/25 Programming Guide published December 12, 1983 by Raster Technologies Corporation. In such a true color system, the user generally specifies the RGB color values themselves (rather than color numbers or indexes) to represent a selected color. The "color values" approach is representative of the true color system--a three-dimensional color referencing system. The "color number" approach is representative of the index color system--a one-dimensional color referencing system.
True color systems are generally used to produce shaded images, to display scanned-in images, and to simulate physical phenomena such as colored lights illuminating a scene composed of colored objects. Indexed color systems are generally used to depict color of icon symbols, to represent scalar values, or to distinguish image segments (e.g.,. primitives such as points, lines, arcs, circles, rectangles, polygons, and text).
Often it is desirable to display symbolic icons, specified in index color, superimposed on shaded images specified in true color. To accomplish this, what is needed is a system which would permit both true-color representation and index color representation.

SUMMARY OF INVENTION

In accordance with the illustrated preferred embodiment of the invention, a method and apparatus is disclosed which provides for the display of both true-color images and index color images. The user may specify either a color number to denote index color, or RGB color values to denote true color. Index color operations and true color operations are performed in an index color environment. To perform true color operation in an index color environment, the apparatus computes one-dimensional index-type data from user-supplied three-dimensional true color data. The user specifies the number of color levels to be used in representing the range of displayable red, green and blue colors, and specifies the RGB values of the color to be displayed. The apparatus derives true color data (RGB color level values) from the number of color levels specified by the user, stores the derived data in memory in a predetermined arrangement suitable for referencing by a single address or index, and derives from the user-specified RGB values a single address for referencing the stored data.
The apparatus comprises a keyboard and a port to a host computer for inputting index color data and true color data, a processing means coupled to the keyboard and/or host computer for producing index-type data from true color data, a memory means for storing the input data and data produced by the processing means, and a display means for displaying stored data.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is block diagram of a system of the prior art capable of performing index color operations.
Fig. 2 is a block diagram showing an arrangement of data in a memory of the system of Fig. 1;
Fig. 3 is a block diagram of a system of the prior art capable of performing true color operations;
Fig. 4 is a block diagram of the apparatus of the present invention for performing index color operations and true color operations in an index color environment;
Fig. 5 is a block diagram showing a color map memory of the apparatus of Fig. 4;
Fig. 6 is a block diagram showing the range of displayable colors as a three-dimensional color coordinate space;
Fig. 7 is a block diagram of a concatenation mask used in the apparatus of Fig. 4; and
Fig. 8 is a block diagram showing multiple bit planes, a true color surface and a single. index surface.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 1 shows a system of the prior art capable of performing index color operations. Fig. 2 shows a color map memory of the system, RGB data typically stored in the memory, and a one-dimensional (single color index) address scheme for referencing stored data. Fig. 3 shows a three-dimensional address scheme of the prior art for referencing stored data. From user-supplied RGB values stored in image memory, references are made to color values stored in three look-up tables (LUTs). Fig. 2 shows a one address (index) scheme, while Fig. 3 shows a three address (true color) scheme.
Fig. 4 shows a true-color one-address scheme, the scheme utilized by the apparatus of the present invention. Like the prior art system described in U.S. patent 4,509,043, the apparatus of the present invention is capable of performing index color operations. Unlike prior art systems, the apparatus is capable of performing true color operations in an index color environment.
The apparatus comprises a keyboard 10 and port to a host computer for inputting index color data and true color data, a processing means 11 coupled to the keyboard and/or host computer for producing index-type data from true color data, a memory means 13 for storing the input data and data produced by the processing means, and a display means 15 for displaying stored data.
The processing means 11 includes a microprocessor 12_"having a ROM (read only memory, not shown) with a stored program, and a vector generator 18. The memory means 13 includes a memory 14 for storing index information, a video display memory 20, and a color map memory 22. The display means 15 includes a video timing and control circuit 16, and a CRT (cathode ray tube) display with associated deflection circuit and D to A (digital-to-analog) converter.
The apparatus of the present invention is much like the system described in U.S. patent 4,509,043 for performing index operations, and the description of that system applies as well to the present apparatus. However, in the performing of true color operations in and color map memory of the present apparatus are designed to operate differently from said system, as described below.
Fig. 5 shows color map memory 22 of the present apparatus in greater detail. Like the prior art, the RGB values are arranged in tabular form in memory, with each memory location containing one set of R,G,and B values addressable by a single address. Unlike the prior art, the RGB values in the tables are calculated (derived) by the apparatus from data supplied to the apparatus. The calculation is performed by processor 12 under control of the stored program. (Examples of the stored program are presented in Appendixes A and B.) Also unlike the prior art, the single address used to reference the RGB values in the table is derived by processor 12 of the apparatus from RGB values specified by the user. Thus, in the present invention, both the RGB table values and the single address for referencing the RGB table values are derived by the apparatus.
The initial user of the apparatus (e.g., the supplier of the apparatus) specifies the RGB color values to be used in populating the table, by uniformly quantizing the closed interval from zero to one for each color. For example, the user may specify the number of the displayable levels to be Q (i.e., Q discrete quantization levels of red, green, and blue displayable primary colors). Thereafter, the apparatus calculates the actual table entries (i.e., the true-color RGB values which, when referenced subsequently by a single index-type address, produces the original user-specified values).
As shown in Table I, one set of displayable primary color levels may be:
The five quantization levels occupy the range from 0% to 100% of displayable red; green, and blue primary colors.
As shown in Fig. 6, the range of displayable colors may be viewed as a three-dimensional color space, where (0,0,0) represents black, i.e., 0% red, 0% green, 0% blue, (1,0,0) represents 100% red, (0,1,0) represents 100% green, (0,0,1) represents 100% blue, and (1,1,1) represents white, i.e., 100% red, 100% green and 100% blue.
As shown in Fig. 6, the user of the apparatus may select:

Q, levels of Red
Q₂ levels of Green
Q₃ levels of Blue

when representing a true-color object within the limits of the displayable colors of the apparatus. From the displayable levels specified by the user, the apparatus derives (calculates) color values suitable for single index referencing (single addressing), and stores the derived values in tables in memory for later use.
The apparatus calculates the color values (table entries) and populates the RGB tables as follows:

Red (address) = i/(Qi - 1)
Green (address) = j/(Q₂- 1)
Blue (address) = k/(Q₃- 1)
where:
Q₁ represents the number of quantization levels specified for red
Q₂ represents the number of quantization levels specified for green
Q₃ represents the number of quantization levels specified for blue
quantization level i = 0 to Q₁ - 1
quantization level j = 0 to Q₂- 1
quantization level k = 0 to Q₃- 1
address = i + Q, J + Q, QW2Q k
n represents the number of permissible table entries.
and
(Q₁ × Q₂ × Q₃) < n.
or
Q₁ x Q₂ x Q₃ ≦ 2^m, where m represents the number of bit planes, as shown for example in Figs. 2 and 8.

The following RGB tables show color data produced (derived) by processor 12 in response to user-specified quantization levels. Processor 12 stores the derived data in memory 22 in a predetermined sequence (arrangement), as shown in Table II below.
The sequence in which the data is stored in memory 22 ensures that, from the RGB values specified by the user, the processor 12 will derive the appropriate index (single address) for referencing the correct RGB values in memory 22. Processor 12 derives each single address (index) by means of the following formula:

Index = [R (Q₁ - 1)] rounded + Q₁ [G (Q₂ -1)] rounded + Q₁ Q₂ [B (Q₂ -1)] rounded.

Thus, as shown in Table I above, from the three-dimensional true color value (.25, .5, .75) specified by the user, the address 86 would be derived by processor 12, permitting access to the correct RGB values in the table.
An alternative formula for deriving single addresses (indexes) from specified (input) RGB values is:

[RQ₁ ]truncated + Q₁ [G Q₂]truncated + Q₁ Q₂ [B Q3]truncated
where: 0≦R< 1;0≦G< 1;0≦B< 1
and Q₁, Q₂, Q₃, are powers of two.
Given B, bits of red, 8₂ bits of green and B₃ bits of blue, and given an m-bit plane surface, as shown for example in Fig. 8, where m = b₁ + b₂ + b₃ and b₁ ≦ B₁, b₂ ≦ B2, b₃ ≦ B3, then the above-mentioned truncation operation may be performed as shown in Fig. 7, where the b₁ most significant bits (MSB) of the R field, and the b₂ MSB of the G field, and the b₃ MSB of the B field, are concatenated, and shifted to align it with the m-bit

In the foregoing, the method for converting from true color data to single index data is described. The method include the steps of deriving true color data (i.e., RGB color level values) from the number of color levels specified by the user, storing the derived data in memory in predetermined arrangement suitable for referencing by a single address or index, and deriving from user-supplier RGB color values a single address for referencing the stored data.
Appendix A shows RGB values, and associated single index addresses, derived for:

Q₁ = 5 red levels
Q₂ = 5 green levels
Q₃ = 5 blue levels,
the RGB values being in the range 0 to 255, representing 0% to 100%.

Appendix B shows RGB values, and associated single index addresses, derived for:

Q, = 7 red levels
Q₂ = 6 green levels
Q₃ = 5 blue levels,
the RGB values being in the range 0 to 255, representing 0% to 100%.

In appendix C below, index and true color data (stored in binary form in color map memory 22) are shown for both a single index surface (surface 1) and a true color surface (surface 2). For example, of the address "00 01 10 11 shown in Appendix C and in Fig.. 8, the "00" represents a color index for surface 1, and the "01 10 11 represents RGB color values for surface 2. The address was computed (derived) from user-supplied index and true color values. The address is used to reference the RGB values "01010101 10101010 11111111".
Depending on which surface has priority over the other, the RGB values may differ for a given address. For example, given the computed address "01 11 11 11", if the RGB values "10000000 10000000 10000000" had been predetermined to represent the color index "01", and the RGB values "11111111 11111111 11111111 " had been predetermined to represent the RGB color level "11 11 11", then the RGB values "10000000 10000000 10000000" would be used if the indexed color surface had priority, and the RGB values "11111111 11111111 11111111 " would be used if the true color surface had priority.

Claims

1. An apparatus for representing three-component color data in a single index reference system comprising:

processing means responsive to applied data for deriving a single index and a plurality of primary color component values, the applied data representing selected numbers of color levels of color components and representing color component values of a desired color of an image; and

memory means for storing the derived primary color component values in a sequence such that the single index references the color component values of the desired color.