Monitor color depth

by Joe Gillespie — Nov 1, 1999

Virtually all cathode ray tube-based computer monitors are capable of displaying an infinite number of colours. Each of the Red, Green and Blue components can go from nothing (black) to full brightness just as if you turned a brightness knob on the monitor's control panel. Cathode ray tubes are analog devices and the brightness of the colors on the screen is proportional to the voltages applied to the cathodes.

But computers are digital devices. They don't give a smooth transition from black to full color but work in a number of discrete steps - usually 256 steps for each color. If you multiply 256 x 256 x 256 you get 16,777,216 - that's how many different colors you can get with a 24-bit video card. Every pixel on the screen needs to have three bytes of memory allocated to it to be able to reproduce any of these theoretically possible colours.

For instance, an 800 x 600 monitor has 480,000 pixels. If each pixel has three bytes, then you need at least 1,440,000 bytes of video memory in your computer. A computer running in 8-bits only needs one byte (8 bits= 1 byte) of memory per pixel - 480,000 bytes for an 800 x 600 screen, which is considerably less expensive. It all comes down to cost!

As the screen get larger, the amount of video memory required increases significantly. At 1024 x 768, there are nearly twice as many pixels than there are at 800 x 600 requiring more than 2 Megabytes of video memory - 4 Mb, to be practical .

The compromise situation is the 2 bytes per pixel or 16-bit monitor. Now, if you divide 3 into 16, it doesn't divide evenly - there are 3 x 5 and 1 over. Some 16-bit systems, referred to as 'low color', throw the remaining one away giving 32768 possible colors, better ones use a 5x6x5 system ('high color') giving 65536 possible colors. Green gets the extra bit because the human eye is most sensitive to that color.

In most situations, 16-bit and 24-bit images are indistinguishable from each other, but where there are long, smooth gradations in an image, banding is more obvious on the 16-bit system because of the larger steps between the displayable colors.

To get round this, some 16-bit systems also use dithering to simulate a 24-bit display. 8-bit dithering can be quite crude but 16-bit dithering is very subtle and you need to zoom into the image to be able to see the difference between adjacent pixels, but the overall effect is that you get smoother gradations.

The other problem is that with 16-bit displays, colours that were originally web-safe in 24 or 8-bit modes can be shifted because of the approximation of the 24-bit values. 24 and 8-bit displays will always reproduce web-safe colors exactly but, as noted earlier, more than half of all surfers are using 16-bits and there is not much you can do. Something has to go when you throw away 4 bits of information for each color.

Below is a smooth 24-bit transition between white and black. How well does your system cope with it?

On my Mac, Microsoft Internet Explorer makes a very good job of it in 24, 16 and even in 8 bits. In Netscape - well, the less said the better. Netscape's ability to render this type of image is seriously flawed and has been for several years.

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