DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Raster images
There are four distinct types of raster image data:
• Bilevel, by definition 1 bit per pixel
• Grayscale, typically 8 bits per pixel
• Truecolor, typically 24 bits per pixel
• Pseudocolor, typically 8 bits per pixel
Hicolor, with 16 bits per pixel, is a variant of truecolor.
Grayscale and truecolor systems are capable of representing
continuous tone. Video systems use only truecolor
(and perhaps grayscale as a special case).
In the following sections, I will explain bilevel, grayscale,
hicolor, truecolor, and pseudocolor in turn. Each
description is accompanied by a block diagram that
represents the hardware at the back end of the framebuffer
or graphics card (including the digital-to-analog
converter, DAC). Alternatively, you can consider each
block diagram to represent an algorithm that converts
image data to monitor R’, G’, and B’ components.
Bilevel 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-andwhite,
or grayscale, 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 a CRT 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 grayscale, the
maximum pixel value) conventionally represents black.
34 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES