Murray, James D., and William vanRyper (1996), Encyclopedia of Graphics File Formats, Second Edition (Sebastopol, Calif.: O’Reilly & Associates).

and some systems use other techniques. For a system to operate in a perceptually uniform manner, similar to or compatible with video, its LUTs need to be loaded with suitable transfer functions. If the LUTs are loaded with transfer functions that cause code values to be proportional to intensity, then the advantages of perceptual uniformity will be diminished or lost.

Many different file formats are in use for each of these representations. Discussion of file formats is outside the scope of this book. To convey photo- graphic-quality colour images, a file format must accommodate at least 24 bits per pixel. To make maximum perceptual use of a limited number of bits per component, nonlinear coding should be used, as I outlined in Perceptual uniformity, on page 30.

Symbolic image description

Many methods are used to describe the content of a picture at a level of abstraction higher than directly enumerating the value of each pixel. Symbolic image data is converted to a raster image by the process of

rasterizing. Images are rasterized (or imaged or rendered)

Historical hicolour systems assigned 5 bits to each of red, green, and blue, which – with one bit of overhead – formed a 16-bit pixel. (Some systems assigned 6 bits to green and had no overhead bit.) The hicolour scheme is obsolete.

Bilevel

Greyscale

Truecolour

Greyscale and truecolour systems are capable of representing continuous tone. Video systems use only truecolour (and perhaps greyscale as a special case).

In the following sections, I will explain bilevel, greyscale, truecolour, and pseudocolour in turn. The truecolour and pseudocolour descriptions are accompanied by block diagrams that represent the hardware at the back end of the framebuffer or graphics card. (Historically, this hardware would have included digital-to-analog converters, DACs; today, digital display interfaces such as DVI, HDMI, and DisplayPort are used.) Alternatively, you can consider each block diagram to represent an algorithm that converts image data to display R’, G’, and B’ components.

Each pixel of a bilevel (or two-level) image comprises one bit, which represents either black or white – but nothing in between. In computing this is often called monochrome. (That term ought to denote shades of

a single hue; however, in common usage – and particularly in video – monochrome denotes the black-and- white, or greyscale, component of an image.)

Since the invention of data communications, binary zero (0) has been known as space, and binary one (1) has been known as mark. A “mark” on an electronic display device emits light, so in video and in computer graphics a binary one (or the maximum code value) conventionally represents white. In printing, a “mark” deposits ink on the page, so in printing a binary one (or in greyscale, the maximum pixel value) conventionally represents black.

A greyscale image represents an effectively continuous range of tones, from black, through intermediate shades of grey, to white. A greyscale system with a sufficient number of bits per component, 8 bits or more, can represent a black-and-white photograph.

A greyscale system may or may not have a lookup table (LUT); it may or may not be perceptually uniform.

In printing, a greyscale image is said to have continuous tone, or contone (distinguished from line art or type). When a contone image is printed, halftoning is ordinarily used.

A truecolour system has separate red, green, and blue components for each pixel. In most truecolour systems, each component is represented by a byte of 8

CLUTs are necessary for pseudocolour. Most truecolour systems have LUTs as by-products of their capability to handle pseudocolour.

Figure 6.2 Truecolour (24-bit) graphics usually involves three programmable lookup tables (LUTs). The numerical values shown here are from the default Macintosh LUT (prior to Mac OS X 10.6). In video, R’G’B’ values are transmitted to the display with no intervening lookup table. To make a truecolour computer system display video properly, the LUTs must be loaded with ramps that map input to output unchanged. Such

a “unity” mapping (or “ramp”) is the default in PCs.

Poynton, Charles (1998), “The rehabilitation of gamma,” in Proc. SPIE 3299 (Human Vision and Electronic Imaging III, Bellingham, Wash.: SPIE).

Concerning alpha, see page 387.

Pseudocolour

I reserve the term CLUT for pseudocolour. In greyscale and truecolour systems, the LUTs store transfer functions, not colours. In Macintosh, pseudocolour CLUT values are roughly, but not optimally, perceptually coded.

will be evident in many images, as I mentioned on page 30. Having 24-bit truecolour is not a guarantee of good image quality. If a scanner claims to have 30 bits (or 36 bits) per pixel, obviously each component has 10 bits (or 12 bits). However, it makes a great deal of difference whether these values are coded physically (as linearlight luminance, loosely “intensity”), or coded perceptually (as a quantity comparable to lightness).

In video, either the LUTs are absent, or each is set to the identity function. Studio video systems are effectively permanently wired in truecolour mode with perceptually uniform coding: Code values are presented directly to the DACs, without intervening lookup tables.

It is easiest to design a framebuffer memory system where each pixel has a number of bytes that is a power of two; so, a truecolour framebuffer often has four bytes per pixel – “32-bit colour.” Three bytes comprise the red, green, and blue colour components; the fourth byte is used for purposes other than representing colour. The fourth byte may contain overlay information, or it may store an alpha (α) or key component representing opacity from black (0, fully transparent) to white (1, fully opaque). In computer graphics, the alpha component modulates components (usually RGB) that are coded in the linear-light domain. In video, the linear key signal modulates nonlinear (gamma-corrected) R’G’B’ signals or luma and colour difference components such as Y’CBCR.

In a pseudocolour (or indexed colour, or colourmapped) system, several bits – usually 8 – comprise each pixel in an image or framebuffer. This provides

a moderate number of unique codes – usually 256 – for each pixel. Pseudocolour involves “painting by numbers,” where the number of colours is rather small. In an 8-bit pseudocolour system, any particular image, or the content of the framebuffer at any instant in time, is limited to a selection of just 28 (or 256) colours from the universe of available colours.

Each code value is used as an index into a colour lookup table (CLUT, colourmap, or palette) that retrieves

R’G’B’ components; the DAC translates these linearly into voltage levels that are applied to the display. (Macintosh is an exception: Image data read from the

Figure 6.3 Pseudocolour (8-bit) graphics systems use a limited number of integers, usually 0 through 255, to represent colours. Each pixel value is processed through a colour lookup table (CLUT) to obtain red, green, and blue output values to be delivered to the display.

R’

G’

B’

The browser-safe palette forms a radix-6 number system with RGB digits valued 0 through 5.

216 = 63

CLUT is in effect passed through a second LUT.) Pseudocolour CLUT values are effectively perceptually coded.

The CLUT and DACs of an 8-bit pseudocolour system are sketched in Figure 6.3 above. A typical lookup table retrieves 8-bit values for each of red, green, and blue, so each of the 256 different colours can be chosen from a universe of 224, or 16777216, colours. (A CLUT may return 4, 6, or more than 8 bits for each component.) Pseudocolour image data is always accompanied by the associated colourmap (or palette). The colourmap may be fixed, independent of the image, or it may be specific to the particular image (adaptive or optimized). A popular choice for a fixed CLUT is the browser safe palette comprising the 216 colours formed by combinations of 8-bit R’, G’, and B’ values chosen from the set {0, 51, 102, 153, 204, 255}. This set of 216 colours fits nicely within an 8-bit pseudocolour CLUT; the colours are perceptually distributed throughout the R’G’B’ cube. Pseudocolour is appropriate for images such as maps, schematic diagrams, or cartoons, where each colour or combination is either completely present or completely

absent at any point in the image. In a typical CLUT, adjacent pseudocolour codes are generally completely unrelated; for example, the colour assigned to code 42 has no necessary relationship to the colour assigned to code 43.