1. Field of the Invention
The present invention concerns a system for providing output signals representative of color corrected color densities of coloring agents used in reproducing a colored original image. More particularly, the present invention automatically provides the output signals representative of the color corrected color densities in response to input signals representative of the primary color readings of the original image.
2. Description of the Prior Art
The art of color reproduction strives to faithfully reproduce the colors of the original image. More particularly, color reproduction strives to create a reproduced image which reflects the same spectral colors, typically measured in terms of red, green and blue, as the original. Instruments such as a densitometer allow quantified readings of the primary spectral colors and thereby enable the quality of the reproduction to be quanified in numerical terms. A densitometer produces the three primary color readings of red, green, and blue by separately sensing the light transmitted through separate red, green, and blue filters. That is to say, a red reading corresponds to the amount of standardized light passing through a standarized red filter after being reflected from a particular portion of the original image. Similarly, green and blue readings are produced by sensing the light transmitted through respective green and blue filters. The red, green, and blue filters are designed to respectively absorb one-third of the spectrum reflected from the image centered about the three primary colors red, green, and blue. As a result, primary color readings are a subtractive process which readings represent the remaining light transmitted through red, green, and blue filters.
In principle, the objective of color reproduction is to impress transparent coloring agents on white paper such that the coloring agents act as ideal primary color filters. Color agents are typically identified by their process colors of cyan, magenta and yellow corresponding respectively to red, green, and blue. Ideally these process colors act as perfect red, green, and blue filters which absorb only their designated portion of the spectrum and reflect or transmit the rest.
Unfortunately, practically available commercial coloring agents do not act as perfect red, green, and blue filters. That is to say, typical coloring agents are not strongly absorbing in one-third of the spectrum and strongly reflecting or transmitting in the other two-thirds of the spectrum For example, a typical cyan coloring agent which ideally would absorb light only in the red third of the spectrum, also absorbs light in the green portion and blue portion. Similarly, typical magenta coloring agents, while predominately absorbing light in the green part of the spectrum, also absorb in the red and blue portions, and yellow coloring agents absorb not only the blue portion of the spectrum but also in the green portion and the red portion. Hence, coloring agents are said to contain so-called "dirt" in reference to the fact that coloring agents do not perform as ideal red, green, and blue filters.
In order to quantify the non-ideal characteristics of coloring agents, each process color for a particular type of coloring agent can be quantified as having a certain proportion or percentage of the other two colors For example, cyan ink at a given density can be said to include a density level or percentage of magenta and yellow. That is to say, the green filtering action in the cyan ink can be characterized as a certain percentage of magenta in the cyan and the blue filtering action can be characterized as a certain percentage of yellow. Analogously, a magenta ink at a given density level can be said to include a certain percentages of cyan and of yellow, and the yellow ink can be said to have a certain percentages of magenta and cyan therein at a given density. Thus, when used in combination, each coloring agent contributes color to the other agent colors and the amount of contribution is different at different densities. This contribution effect must be taken into account in the color correction process.
The color correction process is additionally complicated by the fact that typical coloring agents experience so-called "linearity" failures such as "proportionality" failure and "additivity" failure. Proportionality failure refers to the fact that the color density of an agent color as measured by a densitometer does not increase linearly as the printing density of the coloring agent increases. That is, when the printing or coverage density of a cyan ink is increased, the measured cyan color density does not increase linearly therewith. Additionally, as the cyan ink density increases the magenta and yellow contributions also do not increase linearly.
Additivity failure refers to the fact that when two or more coloring agents are combined, the contribution one makes to the other does not increase or add linearly. For example, a cyan ink at a given density level has a certain cyan color density. A magenta ink at a given density level also contains a certain cyan density. If the cyan from the cyan ink and the cyan from the magenta ink combined linearly, the total cyan density reading of the combination would be a simple sum of the cyan density from the cyan ink and the cyan density contributed by the magenta ink. However, the resulting combination typically produces a net cyan density reading less than the sum of the two cyan sources. Thus, the cyan densities fail to add linearly.
As can be appreciated from the discussion above, the problems of color correction are substantial. In lithography, color correction is typically achieved by masking techniques in which the lithographer produces color separation masks used for producing printing plates which are designed to produce the desired color correction. The masking techniques, while generally effective, are also time consuming and expensive and may be prohibitively expensive for short runs of printing material.
Another color correction technique concerns the use of a microcomputer and associated electronic memory in which primary color reading data is stored therein corresponding to color charts of the coloring agents to be used in the reproduction process. The color agent charts for a given type of coloring agents are composed of patches of various combinations of the three colors of coloring agents at various color densities. If fifteen density steps of the three agent colors are used to make the charts, 3,375 combinations are possible with each producing corresponding red, green, and blue readings This data is typically arranged in the form of a table and the patch is selected having red, green, and blue readings which correspond most closely with the desired red, green, and blue readings.
As those skilled in the art will appreciate, a lookup table so constructed is limited in its color correction accuracy. For example, fifteen density steps for each process color produces increments of 6.67% for each step. With this large a gap between steps, the color correction may be inaccurate by at least this amount. If a minimum 3% accuracy is required, data must be produced for about 33 steps of density for each process color. This increases the number of color patches to almost 36,000. If accuracy of 1% is desired, data corresponding to nearly 1,000,000 patches must be stored.
Thus, to increase the color correction accuracy using such tables, the amount of memory space increases by the cube of the number of density steps. Additionally, six bytes of data must be stored for each patch corresponding to a byte of data for each process color in the combination and a byte of data for each primary color reading produced by that color patch. Thus, for 1% accuracy, at least 8,000,000 bytes of data must be stored. As those skilled in the art will appreciate, this volume of data becomes unwieldy to produce and enter, expensive to store, and time consuming to retrieve Furthermore, this data is usable only for a particular set of inks and must be re-entered for different inks. That is to say, a new look-up table is required if the inks are changed. As a result of these problems, electronic color correction is not in widespread use.