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[Previous](32-05.html) [Table of Contents](index.html) [Next](33-02.html)
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Chapter 33\
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Yogi Bear and Eurythmics Confront VGA Colors {#Heading1}
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---------------------------------------------
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### The Basics of VGA Color Generation {#Heading2}
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Kevin Mangis wants to know about the VGA’s 4-bit to 8-bit to 18-bit
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color translation. Mansur Loloyan would like to find out how to generate
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a look-up table containing 256 colors and how to change the default
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color palette. And surely they are only the tip of the iceberg; hordes
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of screaming programmers from every corner of the planet are no doubt
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tearing the place up looking for a discussion of VGA color, and venting
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their frustration at my mailbox. *Let’s have it*, they’ve said, clearly
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and in considerable numbers. As Eurythmics might say, who is this humble
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writer to disagree?
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On the other hand, I hope you all know what you’re getting into. To
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paraphrase Yogi, the VGA is smarter (and more confusing) than the
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average board. There’s the basic 8-bit to 18-bit translation, there’s
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the EGA-compatible 4-bit to 6-bit translation, there’s the 2- or 4-bit
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color paging register that’s used to pad 6- or 4-bit pixel values out to
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8 bits, and then there’s 256-color mode. Fear not, it will all make
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sense in the end, but it may take us a couple of additional chapters to
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get there—so let’s get started.
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Before we begin, though, I must refer you to Michael Covington’s
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excellent article, “Color Vision and the VGA,” in the June/July 1990
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issue of *PC TECHNIQUES*. Michael, one of the most brilliant people it
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has ever been my pleasure to meet, is an expert in many areas I know
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nothing about, including linguistics and artificial intelligence. Add to
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that list the topic of color perception, for his article superbly
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describes the mechanisms by which we perceive color and ties that
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information to the VGA’s capabilities. After reading Michael’s article,
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you’ll understand what colors the VGA is capable of generating, and why.
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Our topic in this chapter complements Michael’s article nicely. Where he
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focused on color perception, we’ll focus on color generation; that is,
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the ways in which the VGA can be programmed to generate those colors
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that lie within its capabilities. To find out why a VGA can’t generate
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as pure a red as an LED, read Michael’s article. If you want to find out
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how to flip between 16 different sets of 16 colors, though, don’t touch
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that dial!
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I would be remiss if I didn’t point you in the direction of two more
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articles, these in the July 1990 issue of *Dr. Dobb’s Journal*. “Super
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VGA Programming,” by Chris Howard, provides a good deal of useful
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information about SuperVGA chipsets, modes, and programming. “Circles
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and the Digital Differential Analyzer,” by Tim Paterson, is a good
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article about fast circle drawing, a topic we’ll tackle soon. All in
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all, the dog days of 1990 were good times for graphics.
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### VGA Color Basics {#Heading3}
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Briefly put, the VGA color translation circuitry takes in one 4- or
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8-bit pixel value at a time and translates it into three 6-bit values,
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one each of red, green, and blue, that are converted to corresponding
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analog levels and sent to the monitor. Seems simple enough, doesn’t it?
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Unfortunately, nothing is ever that simple on the VGA, and color
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translation is no exception.
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#### The Palette RAM {#Heading4}
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The color path in the VGA involves two stages, as shown in Figure 33.1.
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The first stage fetches a 4-bit pixel from display memory and feeds it
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into the EGA-compatible palette RAM (so called because it is
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functionally equivalent to the palette RAM color translation circuitry
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of the EGA), which translates it into a 6-bit value and sends it on to
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the DAC. The translation involves nothing more complex than the 4-bit
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value of a pixel being used as the address of one of the 16 palette RAM
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registers; a pixel value of 0 selects the contents of palette RAM
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register 0, a pixel value of 1 selects register 1, and so on. Each
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palette RAM register stores 6 bits, so each time a palette RAM register
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is selected by an incoming 4-bit pixel value, 6 bits of information are
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sent out by the palette RAM. (The operation of the palette RAM was
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described back in Chapter 29.)
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The process is much the same in text mode, except that in text mode each
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4-bit pixel value is generated based on the character’s font pattern and
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attribute. In 256-color mode, which we’ll get to eventually, the palette
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RAM is not a factor from the programmer’s perspective and should be left
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alone.
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#### The DAC {#Heading5}
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Once the EGA-compatible palette RAM has fulfilled its karma and
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performed 4-bit to 6-bit translation on a pixel, the resulting value is
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sent to the DAC (Digital/Analog Converter). The DAC performs an 8-bit to
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18-bit conversion in much the same manner as the palette RAM, converts
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the 18-bit result to analog red, green, and blue signals (6 bits for
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each signal), and sends the three analog signals to the monitor. The DAC
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is a separate chip, external to the VGA chip, but it’s an integral part
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of the VGA standard and is present on every VGA.
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\
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**Figure 33.1** *The VGA color generation path.*
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(I’d like to take a moment to point out that you can’t speak of “color”
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at any point in the color translation process until the output stage of
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the DAC. The 4-bit pixel values in memory, 6-bit values in the palette
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RAM, and 8-bit values sent to the DAC are all attributes, not colors,
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because they’re subject to translation by a later stage. For example, a
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pixel with a 4-bit value of 0 isn’t black, it’s attribute 0. It will be
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translated to 3FH if palette RAM register 0 is set to 3FH, but that’s
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not the color white, just another attribute. The value 3FH coming into
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the DAC isn’t white either, and if the value stored in DAC register 63
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is red=7, green=0, and blue=0, the actual *color* displayed for that
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pixel that was 0 in display memory will be dim red. It isn’t color until
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the DAC says it’s color.)
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The DAC contains 256 18-bit storage registers, used to translate one of
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256 possible 8-bit values into one of 256K (262,144, to be precise)
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18-bit values. The 18-bit values are actually composed of three 6-bit
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values, one each for red, green, and blue; for each color component, the
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higher the number, the brighter the color, with 0 turning that color off
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in the pixel and 63 (3FH) making that color maximum brightness. Got all
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that?
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------------------------ --------------------------------- --------------------
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[Previous](32-05.html) [Table of Contents](index.html) [Next](33-02.html)
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------------------------ --------------------------------- --------------------
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* * * * *
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Graphics Programming Black Book © 2001 Michael Abrash
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