By now, you are familiar with the Interpress model for producing a page: an ink is pushed through a mask onto the page. Where the ink goes on the page is defined by a mask operator in the Interpress master; what it looks like is determined by the color variable in the Interpress imager. You can use the Interpress facilities for creating masks from characters, geometric shapes and scanned binary images. We will now turn our attention to how you specify and create inks in an Interpress master.

In Interpress, color or ink (the two are used interchangeably) can be black-and-white or gray as well as full color and either constant or sampled. When the color is constant, then the printer fills the area on the page defined by the mask with a uniform ink or flat color. When the color is sampled, the printer uses an ink created from a pixel array or scanned image. The overall ink is a pixel-by-pixel color specification. For example, the makesampledblack operator is used to create a patterned black-and-white ink from a binary pixel array. This chapter will describe how you specify color in an Interpress master, create the ink or color that makes objects visible on a page and produce color images on an Interpress printer.

There are several ways of describing the color content of a page: how it was specified in the design, how it looks to a human observer, how it would be analyzed by an instrument or how it would be physically produced by a printer. Each description is appropriate at a different point in the production cycle of a document, from its design at a workstation, through its specification in an Interpress master to its physical realization on an Interpress printer. Interpress does not specify or limit the descriptions which can be used. You can choose whichever color model or ink description is most convenient for your application and use it in a master, provided your printer supports/understands it.

For a document creator and printer to communicate in color, they have to agree beforehand on the color representations which the master will use. This agreement usually takes the form of a standard description to which both the creator and the printer subscribe. The standard color descriptions which the creator uses in the master are part of the printer's environment. Providing an Interpress master with mechanisms for accessing standard color representations rather than making the representations part of the language itself is a powerful feature of Interpress, as we shall see.

Interpress describes an ideal image which the printer attempts to produce. The image should be specified in a device-independent manner, and not in terms of some particular printer's capabilities. Just as you would normally use device-independent spatial coordinates to describe the position, size and orientation of an object in an Interpress master, you would use device-independent color descriptions of the visual appearance of an object. And just as a particular Interpress printer converts the device-independent spatial description into an equivalent printer-specific description, the printer would convert the color described by the master into a visually equivalent or matching color that could be obtained with the its physical inks. For example, if the master uses the instruction (0.5 MAKEGRAY) to specify a medium gray ink, then a black-and-white printer would convert it to instructions that resulted in a halftone dot pattern on the page whenever the ink was applied (see Figure 1). But a CRT display, capable of producing gray intensity levels would image the object with medium gray intensity.

An Interpress master indicates the color to be used by placing a color specification in the color imager variable (index 13). This imager variable requires an object of type Color. Interpress does not define a standard encoding for this type so that its form depends on a particular printer. This allows the printer to convert from the device-independent description of the color to its particular device-dependent representation at the point where the object is constructed. As a result, there is only a limited number of things you can do with an object of type Color. You cannot open it up and look inside, nor can you alter it. You can move it around on the stack using any of the stack operators, you can store it in a frame with FSET or you can combine it with other elements in a vector using MAKEVEC. You cannot, for example, obtain a third color by adding two objects of type Color together. The most useful thing you can do with an object of type Color is store it in imager variable 13, which is where the printer looks for ink when it executes a mask operator.

The value of color specifies the printer-specific form of the color or ink that will be used to render an object when a mask operator is called. In effect, color is the inkwell into which the operator dips its pen before writing on the page. The color applied through the mask replaces whatever was previously laid down at the same position on the page. (The one exception to this is ink created by MAKESAMPLEDBLACK). At the beginning of each page body, the imager sets color to black.

The following sequence from an Interpress master illustrates some of the legal operations you can perform on inks. It assumes an object of unknown type on top of the stack and checks to see if its type is Color. If it is, then the object is stored in the color imager variable; if it is not, the instructions within the IF body replace the object on the stack with black ink (1 MAKEGRAY) and generate an appearance warning.

Interpress provides several ways of creating ink, such as (1 MAKEGRAY), that are specifically designed to generate objects of type Color.

We used five colors in Plate 1: red, green, blue, light gray and black. If standard names are defined for these colors, then a document creator would use the FINDCOLOR operator in a master to instruct the printer to look up the specified color by name in its environment and return the color on the stack. The formal definition of FINDCOLOR is

<v: Vector of Identifiers> FINDCOLOR ← <col: constantColor>

The vector of identifiers v is the universal name of the color. If we use the name [IPBook, Red] to describe the red ink that we used in Plate 1, then the following statements would create and save the specified ink.

(Identifier "IPBook" Identifier "Red" 2 MAKEVEC) is the name which FINDCOLOR uses to look up the specified ink in the printer's environment. If the printer cannot find it there, then it would substitute another color for it and generate an appearance error. The choice of which color to substitute for the unknown one is at the discretion of the printer. It could, for example, select another color whose name most closely resembles the name of the color it could not find.

Named colors in an Interpress master provide a very efficient interface to simple color printers and pen plotters. You would typically define and use named color in a way that gives the printer the greatest latitude in creating the ink specified by the master. You might use [IPBook, Red] when you want an ink that unambiguously looks red and not magenta, orange or pink. The standard which defines what is meant by [IPBook, Red] can include a tolerance. It can indicate that any shade of red within limits is acceptable. The way that the printer implements [IPBook, Red] would reflect this. A color printer might simply overprint solid areas of magenta and yellow ink to obtain red. Since different printers would use different inks (here we mean physical inks, not Interpress inks), no two reds would look exactly the same, but so long as they were within tolerance they would be valid representations of [IPBook, Red], and would not be confused with magenta or orange or any other color in the document. If you wanted to specify red more precisely, then you would use a more specific color name or standard, such as [nbs, cns, strongRed], which is the color ``Strong Red'' in the National Bureau of Standards Color Naming Scheme. You can use this name in a master if your printer supports this standard. The resulting ink would be specified with tolerances of this standard.

Interpress gives you a great deal of freedom in defining what is meant by a color name. It provides the lookup mechanism, but the definition of actual colors is left to whatever standard the creator and printer can agree upon. Standards for color names which will be used by Xerox Corporation are, at the time of this writing, being developed as part of the Xerox Color Encoding Standard.

Because the mechanism is so flexible, you can even agree to very loose specifications, such as ``any color that is not black, white, or gray.'' Perhaps such specifications should be called pseudo colors. There is an example of such a specification built into the Interpress standard; it is [Xerox, highlight] and is a named ``color'' (or pseudo color) which means some non-neutral hue. It is intended for use with highlight-color printers which have two inks; black, and something else. [Xerox, highlight] says use the ``something else,'' whatever it is.

The practical limit to the size of a set of named colors falls far short of all the colors you can perceive and conceivably use in a document. Even a conservative estimate puts the number of perceptible colors at over a million, with some estimates as high as ten million. Some applications, such as graphic design and computer-aided engineering, are satisfied with anywhere from a dozen to several hundred judiciously selected colors. Other applications, such as computer image generation and color picture processing, need continuously variable color in order to obtain precise color control and smooth shading. Applications that require continuous or, in the case of digital systems, finely quantized colors use color coordinates to specify the colors they use.

We use various color spaces to help us organize how we think about and control color. For example, we could conceive of a color that we would describe as a ``dark grayish red.'' This description illustrates the three variables of subjective color appearance: hue (``red''), saturation or chroma (``grayish'') and lightness (``dark''). Hue is the attribute with a circular scale, labeled with the names blue, green, yellow, red, purple and so on. Lightness is the intensity of the color on a scale from dark through light. Gray or achromatic colors have no hue but can range in lightness from black to white. Chroma is the difference between a color and a gray with the same lightness. Hue, chroma and lightness define a cylindrical coordinate system for color space (see Figure 3). There have been many attempts to devise numeric scales for these variables; the Munsell Color System is one of the earliest and most popular. To define it, subjects selected painted samples which they judged under controlled viewing conditions to be uniformly distributed along the dimensions of hue, chroma and lightness.

Besides subjective measures of color appearance, we can also use objective measures, such as tristimulus values, which ultimately rely on instrument standards. Objective color coordinate systems are based on trichromacy, an experimental law of human color vision that states that the majority of human observers can match any color by adding together suitable amounts of light from three fixed primary colors. The amounts of the primaries used to match a given color are called its tristimulus values. They specify the visual appearance of a color in the sense that two colors that have the same tristimulus values will look the same. Trichromacy explains why color output devices only need three inks or phosphors: to reproduce tristimulus values, they need only three degrees of freedom.

You can use tristimulus values based on any three primaries. In the interests of standardizing the determination of tristimulus values, the International Commission on Illumination1 defined an ideal or Standard Observer, so that instruments rather than human subjects could be used to measure and specify tristimulus values. The tristimulus values of the Standard Observer are labeled X, Y and Z. The formal description of a color C in terms of X,Y,Z values is

C = X X + Y Y + Z Z

The ``='' signifies that the color C is matched by the sum of X units of light from primary X, Y units from Y and Z units from Z. The standard system was designed so that the Y value is the luminance, the correlate of intensity; two different colors with the same luminance will have the same intensity or lightness.

While we could define a color operator that expected X,Y,Z tristimulus values, it makes sense in practical systems to use tristimulus values based on a set of primaries that are close to those used by real color output devices. In particular, if the primaries are red, green and blue, then their tristimulus values R, G and B would represent the red, green and blue content of a color. Since the color matching equation given above is a linear equation, R,G,B values are related to X,Y,Z values by a linear transformation. Most color scanners, many computer graphics programs and all color monitors use R,G,B values, so they are an excellent choice for representing color in an Interpress master.

In the same way that we use (x,y) spatial coordinates to specify the position of a mask on a page, we can use (R,G,B) tristimulus values to specify the location of an ink in color space. Black is at the origin of the coordinate system. The red, green and blue primaries are usually calibrated so that equal amounts give grays along a scale from black to white. The amounts of the primaries that add together to give white are usually normalized so that they have value 1. Therefore, the white point is at (1,1,1) in the coordinate system. The line joining the black and white points in color space is the neutral or gray axis. The point (1,1,0), for example, represents the additive mixture of the red and green primaries, or yellow. (See Figure 4 and Plate 3).

To use R,G,B values in a master, we will invent a color operator [IPBook, RGBOp] and register it in the environment of our printer. [IPBook, RGBOp] converts the color specification (R G B 3 MAKEVEC) into a constant ink. We can then use it in the following master to create the color chart shown in Plate 2.

The sequence (Identifier "IPBook" Identifier "RGBOp" 2 MAKEVEC) is the name that FINDCOLOROPERATOR uses to retrieve the color operator from the printer's environment. The master saves the operator in the frame so that it does not have to look it up every time it converts an RGB vector to an ink. Since Interpress itself does not define standard color operators other than MAKEGRAY, there is no primitive operator to make a color from an RGB specification. However, the sequence (0 FGET DO) in this master gets and executes such an operator that originated in the printer's environment.

The printer that rendered Plate 2 substituted its notion of red, green and blue for the primaries we used to specify the ink. When it is important to use RGB to define a color precisely, then the exact nature of the red, green and blue primaries and the white used to normalize them would have to be specified (usually in terms of CIE Standard tristimulus values, XYZ) and made part of the color operator.

The color operator [IPBook, RGBOp] behaves analogously to the current transformation T in Interpress. In the same way that T converts master coordinates to device coordinates, a color operator converts the master's color representation of an ink to the device-dependent version of the ink (see Figure 5).

The use of named color operators allows each printer to retrieve from its environment an operator that produces a color that is the best interpretation of what the document creator specified in the master. On a color printer, the [IPBook, RGBOp] operator would create an ink using all three color coordinates. On the other hand, the [IPBook, RGBOp] operator on a black-and-white or gray scale printer would create a gray ink that is the printer's best effort at producing the ink specified by the master. It might choose, for instance, to use a gray ink based on the maximum value of the R,G,B values. Or it might choose to do the same thing that black-and-white film or a black-and-white television receiver does when presented with a color scene, which is reproduce the intensity of the colors in the scene but ignore their hue and chroma.

As a document creator, you can ease the burden that full color specifications place on black-and-white printers by using black-and-white compatible color coordinate systems to describe full color inks. A black-and-white compatible color system aligns one of its three coordinate axes with the neutral or gray axis. Therefore, one of the coordinates records the gray intensity or achromatic content of the color while the other two record the chromatic content or hue and chroma of the specified color. As an example of a black-and-white compatible system, we will define a new coordinate system called YES. YES coordinates are related to RGB by the following linear transformation

Y = a1 R + a2 G + (1 - a1 - a2) B

E = 1/2 (R - G)

S = 1/4 (R+G) - 1/2 B

Y is the luminance; a1 and a2 are constants that depend on the red, green and blue primaries and white that define the RGB tristimulus values. We will use values based on the red, green and blue phosphors used in broadcast studio monitors and the standard white illumination used in the graphic arts: a1=0.68 and a2=0.25. For white, Y=1. E and S are opponent-color values: E is red-minus-green and S is yellow-minus-blue. For gray, R,G and B are all equal so that E and S are zero. (See Figure 6 and Plate 4).

To use YES coordinates in a master, we would invent a color operator, call it [IPBook, YESOp] and install it in the environment of our color and black-and-white printers. On a color printer, the operator would be defined so that it created a full color ink, much as if you had used an RGB representation in the master. When a black-and-white printer encounters an ink specified in terms of YES values, it would use its version of the [IPBook, YESOp] to ignore the E and S values and create a gray ink based on the luminance value Y. The effect of executing the color operator [IPBook, YESOp] on a black-and-white printer with the argument (Y E S 3 MAKEVEC) is equivalent to the following statements

The following master encodes color using YES coordinates. It produces Plate 2 when executed on a color printer and Figure 7 when run on a black-and-white printer.

In Interpress, black and white spots can be represented as a vector of binary sample values. One can create pixel arrays from the samples, and used them to print pictures and patterns. Now we have also seen how to specify colors in Interpress. These two ideas can be combined to let us image colored pictures and patterns. We can construct samples vectors where each sample specifies the color of a pixel. Each pixel can have its own color, and the combined raster will form the colored ink. We call this sampled color. Sampled color is used to place full color, gray or black-and-white raster images on a page. The images can come from a document scanner or a computer program; they can be scanned photographs, computer-generated scenes or graphic designs. What they all have in common is that they are represented as a pixel array, where each pixel consists of a color specification. For example, each pixel could have red, green and blue intensity values. This means that there might be more than one sample value per pixel. We would need a sample value for each coordinate so a single pixel in our example would need three samples (one for the red intensity, one for the green, and a third for the blue).

It is also possible to use named colors in the specification of pixels if we first make a table of all the named colors we wish to use. We can then make a samples vector from the indices into the table. In this case, each pixel will have a single sample value, which is a table index. The corresponding entry in the table is the color of the pixel.

In this chapter we have considered color and the ink component of the Interpress imaging model. We have seen how to produce constant gray shades with the primitive MAKEGRAY and SETGRAY operators. We have also seen how constant colors of any hue or tint can be specified either by name, or by means of a color operator from the printer's environment. We learned how color operators can be generated by color model operators. Finally, we saw how full color patterns and images can be represented as arrays of samples where more than one sample per pixel may be needed. The images can be derived from the samples vectors by the MAKESAMPLEDCOLOR and SETSAMPLEDCOLOR (see Figure 9).