26 What are true color and high color?
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This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
26 What are true color and high color?
[From: Ralph Valentino (ralf@wpi.wpi.edu), Mike Scott (scott@bme.ri.ccf.org) and corrections by Ethan Royael Nicholas (ernichol@unity.ncsu.edu)]
The color of a pixel is formed by mixing three colors: Red, Green and Blue. The number of discrete intensities that the video card is capable of generating for each color determines the maximum number of colors that can be displayed. For most graphics cards, the intensity of each of these colors ranges from 0 to 255, an 8 bit value. So, the total number of unique colors available is 16.7 million (2 ^ 24).
Depending on the implementation, a subset of these colours may be available for display at a given pixel addressability. The original VGA controllers had three 6-bit DAC's (Digital to Analog Converter) allowing up to 256 colours to be simultaneously displayed (in certain video modes - others could display fewer colours) from a colour space of 2^18 = 262144 unique colours. Because of the 6-bit DAC, up to 2^6 or 64 shades of gray could be displayed. With some newer VGA cards and many SVGA video cards that have 8-bit DAC's, you can pick any 256 colours from a palette of 2^24 = 16.7 million, though these modes are not available through the VGA controller - usually they are achieved via a graphics accelerator in an environment like Windows or OS/2. If a picture that you want to display has more than 256 unique colors, various methods can be used to come up with the 256 closest colors, and when combined with dithering reasonable results can be achieved for some images. Images displayed with 8 bit color often look grainy or patchy.
An improvement on this was high color. This provides either 15 or 16 bits of colour depth by using 5 bits for each of red, green and blue or 5 bits of red and blue and 6 bits of green, respectively. As a result, up to 32768 (15 bit) or 65536 (16 bit) colours can be simultaneously displayed. This provides an enormous visual improvement over 8 bit color, and can be noticed immediately when viewing most images. For many folks (me included) the differences between high color and true color (explanation below) are almost indiscernable unless you look closely.
Most newer cards are capable of displaying the full 16.7 million colors simultaneously at certain (usually lower) pixel addressabilities. The ability to display all 16.7 million colors at a time instead of a limited palette of those available is called 24-bit or true color. Since it has a larger gamut of colors to choose from, it can display colors much closer to the true picture colors. The true color label refers to the belief that most people can not perceive more than 16.7 million different colors, and so a 24 bit representation of an image will look as good as the original with respect to color reproduction. Others disagree, feeling that 32, 48 or even 64 bit colour is necessary.
Since it doesn't have to use close colors, it displays the true picture colors, thus the name 'true color'. Note that many picture formats, including GIF, also have the 256 color limitation, so a true color card won't improve the picture viewing at all. More recent picture formats, such as JPEG, support 24 bit color.
When used in truecolor mode, some video cards actually operate in a 32 bit mode. This is due to the fact that the video processor is often optimized to move 32 bit words around, and that the memory bus is often 32 bits wide to each RAM bank. In almost all cases, the result is that while the card is effectively operating in 32 bit mode, only 24 bit color is displayed. On cards that run this way, the calculation of the amount of video RAM required is different than you might think since instead of 24 bits, the card actually uses 32 bits for truecolor. To provide 24 bit color while minimizing video RAM requirements, many video processors implement a packed-pixel mode. This results in the card operating in 24 bit mode, but may have an associated performance penalty.
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