PC compatibles with CGA graphic cards When using IBM's
Color Graphics Adapter (CGA) with a
NTSC TV as a
composite monitor, the separation between luminance and chrominance is imperfect, yielding cross-color artifacts. This is especially a problem with 80-column text. It is for this reason that each of the text and graphics modes described above exists twice: Once as the normal "color" version and once as a "monochrome" version. The "monochrome" version of each mode turns off the NTSC color decoding in the viewing monitor completely, resulting in a black-and-white picture, but also no color bleeding, hence, a sharper picture. On RGBI monitors, the two versions of each mode are identical, with the exception of the 320×200 graphics mode, where the "monochrome" version produces the third palette. However, programmers learned that this flaw could be turned into an asset, as distinct patterns of high-resolution dots would turn into consistent areas of solid colors, thus allowing the display of completely new colors. Since these new colors are the result of cross-color artifacting, they are often called "artifact colors". Both the standard 320×200 four-color and the 640×200 color-on-black graphics modes could be used with this technique. Early efforts resulted on a usable resolution of 160×200 with 16 colors. Actual colors depend on the base palette and resolution used, as shown on the gallery below: File:CGA CompVsRGB 320p0.png|320×200 palette 0 (left: RGB, right: composite monitor) File:CGA CompVsRGB 320p1.png|320×200 palette 1 (left: RGB, right: composite monitor) File:CGA CompVsRGB 640.png|640×200 (left: RGB, right: composite monitor) File:CGA CompVsRGB Text.png|CGA 80-column text (left: RGB, right: composite monitor) Later
demonstrations by enthusiasts have increased the maximum number of colors the CGA can display at the same time to 1024. This technique involves a
text mode tweak which quadruples the number of text rows. Certain ASCII characters such as U and ‼ are then used to produce the necessary patterns, which result in non-dithered images with an effective resolution of 80×100 on a composite monitor.
Software support Many of the more high-profile game titles offers graphics optimized for composite color monitors.
Ultima II, the first game in the game series to be ported to IBM PC, uses CGA composite graphics. ''
King's Quest I was also innovative in its use of 16-color graphics. Other titles include Microsoft Decathlon, King's Quest II and King's Quest III''.
TRS-80 Color Computer The
TRS-80 Color Computer (also known as Coco) two color 256×192 graphic mode allows the display of four colors by exploiting NTSC artifacts. It is not possible to reliably display 256 dots across the screen due to the limitations of the NTSC signal and the phase relationship between the graphics chip clock and
color burst frequency. Using the first color set, alternating columns of green and black pixels are not distinct and appear as a muddy green color. However, switching to a white and black color set, instead of a muddy gray as expected, the result is either orange or blue. Reversing the order of the alternating dots will give the opposite color. In effect, the 256x192 two color mode becomes a 128×192 four color mode with black, orange, blue, and white available. Most CoCo games used this mode as it generates more useful colors than the ones provided by the native four color modes. The graphics chip internally can power up on either the rising or falling edge of the clock, so the bit patterns that represent orange and blue are not predictable. Most CoCo games start with a title screen and asks the user to press the reset button until the colors are correct. Readers of
The Rainbow or
HOT CoCo magazine learned that they can use some POKE commands to switch the
Motorola 6847 graphics chip into one of the artifact modes, while
Extended Color BASIC continues to operate as though it were still displaying one of the 128x192 four-color modes. Thus, the entire set of Extended Color BASIC graphics commands can be used with the artifact colors. Some users developed a set of 16 artifact colors using a 4×2 pixel matrix. Use of POKE commands also make these colors available to the graphics commands, although the colors have to be drawn one horizontal line at a time. Some interesting artworks were produced from these effects, especially since the
CoCo Max art package provides them in its palette of colors. The resulting 16 color palette is (approximate colors for illustration purposes only): The
CoCo 3 fixes the clock-edge problem so it is always the same; the user holds the key during reset to alternate the color set. On this computer games can be patched to use a new 128×192 four color mode provided by the
Graphics Interrupt Memory Enhancer (GIME) chip, with hardware colors mapped to the required values.
Apple II Color graphics on the
Apple II uses a quirk of the NTSC television signal standard, which made color display relatively easy and inexpensive to implement. The Apple II display provides two pixels per NTSC subcarrier cycle. When the
color burst reference signal is on and the computer attached to a color display, it can display green by showing one alternating pattern of pixels, magenta with an opposite pattern of alternating pixels, and white by placing two pixels next to each other. Later, blue and orange became available by tweaking the offset of the pixels by half a pixel-width in relation to the color-burst signal. The high-resolution display offers more colors simply by compressing more, narrower pixels into each subcarrier cycle. The coarse, low-resolution graphics display mode works differently, as it can output a pattern of dots per pixel to offer more color options. These patterns are stored in the character generator ROM and replaces the text character bit patterns when the computer is switched to low-res graphics mode. The text mode and low-res graphics mode uses the same memory region and the same circuitry is used for both. but when seen on NTSC colors appear.
Atari 8-bit Graphics 8 mode on early
Atari 8-bit computers with the
Color Television Interface Adaptor (CTIA) chip displayed black or white images at a resolution of 320×192. Programmers quickly discovered that the odd or even patterns of adjacent black and white pixels in this mode would generate one of two additional colors (blue/brown or olive/pink) and software such as
On-Line Systems'
The Wizard and the Princess used this side-effect to display up to four colors at maximum resolution. This technique and its technical underpinnings were documented in
Appendix D of
De Re Atari. When Atari began shipping computers with the improved
Graphic Television Interface Adaptor (GTIA), users found that such programs displayed incorrect colors and required an updated version of the software. In fact, artifact colors were inconsistent across the entire
Atari 8-bit product line complicating playfield design but only affected a handful of titles that used this graphics mode. These color differences can be simulated within Altirra-based emulators. All models with RF or composite output connected to televisions exhibited this effect while those such as the original
Atari 800 or later
XE series with built-in chroma/luma support additionally displayed images without artifacts when connected to a
computer monitor with chroma/luma inputs such as the
Commodore 1701 . For some undocumented reason known only to Atari, they did not enable the chroma pin on the monitor jack of the
800XL although several modifications have been published to incorporate this support.
Other machines Many first generation
MSX computer games use horizontal blurring and dither to generate a palette of 125 simultaneous colors. Companies like
Konami,
Hal Laboratory or
Ponyca used this technique regularly.
Sony's
Graphic Master Lab paint program allowed these 125 colors to be used in user-created drawings. Home computers like the
Atari ST also have graphics prepared with dithering techniques to take advantage of composite TV connections. The
Mega Drive/Genesis takes advantage of composite video horizontal blurring of vertical dither patterns to simulate transparency effects on many games. William Kier (the artist on
Eternal Champions) stated performing manual dither for the graphics on that game, and that it's likely most games dithered in the same fashion. This effect was used so widely used that it can be simulated on modern hardware clones like the
Mega Sg. The Commodore Amiga, when connected over composite video, suffered from noticeable horizontal blurring, specially affecting colored pixels and smoothing out dithered transitions. This technique was used on most titles. Pixel artist Henk Nieborg mentions using dithering on the 1992 Amiga game
Lionheart in order to create additional colors. The technique was also used frequently by British software house
The Bitmap Brothers. The special
Hold-And-Modify is particularly suited for displaying "
high color" TV-like images, taking full advantage of horizontal blurring. ==PAL system==