Color Graphics Adapter - Further Graphics Modes and Tweaks

Further Graphics Modes and Tweaks

A number of official and unofficial features exist that can be exploited to achieve special effects.

• In 320×200 graphics mode, the background color (which also affects the border color), which defaults to black on mode initialization, can be changed to any of the other 15 colors of the CGA palette. This allows for some variation, as well as flashing effects, as the background color can be changed without having to redraw the screen (i.e. without changing the contents of the video RAM.)
• In 640×200 graphics mode, the foreground color can be changed from its usual white to any of the other 15 colors. The background and border cannot be changed from black.
• In text mode, the border color (displayed outside the regular display area—into the overscan area) can be changed from its usual black to any of the other 15 colors.
• A third 320×200 4-color palette is achieved by disabling the composite color burst bit while in graphics mode. This is what IBM BIOS Mode 5 does, as described above. This switches the current color palette to red, cyan, white and the background color. The intense versions of these colors can also be used and the background color may be changed, but the palette cannot be switched to official palettes 0 or 1 without enabling the composite color signal again. As such, it can only be seen on RGB monitors and will simply appear in grayscale on composite displays. This palette was often used by games because it looked more attractive than the cyan/magenta/white colors. Notably, it is not mentioned in the IBM Reference Guide, and some CGA clones may not support it.
• Through precision timing, it is possible to switch to another palette while the screen being scanned (drawn), allowing the use of any one of the six palettes per scanline. The best example of this in use is the game California Games when run on a stock 4.77 MHz 8088. (Running it on a faster computer does not produce the effect, as the method the programmers used to switch palettes at predetermined locations is extremely sensitive to machine speed.) The same can be done with the background color, to create the river and road in Frogger. Another documented example of the technique is in Atarisoft's port of Jungle Hunt to the PC.
• Additional colors are often approximated using dithering, although the low resolution makes it very apparent. In particular, many Sierra games use palette 0 at low intensity and dark blue as the background color. This gives it the three primary RGB colors to work with (as well as brown).

Some of these above tweaks can even be combined. Examples can be found in several games. Most software titles did not use these possibilities, but there were a few impressive exceptions.