Colors

OpenDoors uses the traditional IBM-PC four-bit color numbers. A text attribute contains a foreground nibble in bits 0–3 and a background value in bits 4–6; bit 7 is the traditional blink bit. The same numerical constants therefore have slightly different names and effects depending on whether they are passed as a foreground or background.

od_set_color() accepts separate foreground and background values and constructs the attribute. od_set_attrib() accepts an already combined attribute byte. The color fields in od_control also store combined attributes unless their individual reference says otherwise.

Dark foreground colors

The D_* names are values 0 through 7. They select normal-intensity foregrounds. When used as a background, the same values select the eight ordinary background colors.

D_BLACK

Value 0. As a foreground it selects black; against a black background this is invisible. As a background it selects black. A default monochrome-style attribute often combines D_GREY with D_BLACK.

D_BLUE

Value 1. Selects dark blue as a foreground or blue as a background.

D_GREEN

Value 2. Selects dark green as a foreground or green as a background.

D_CYAN

Value 3. Selects dark cyan as a foreground or cyan as a background.

D_RED

Value 4. Selects dark red as a foreground or red as a background.

D_MAGENTA

Value 5. Selects dark magenta as a foreground or magenta as a background.

D_BROWN

Value 6. Selects brown as a foreground. The same bit pattern selects the brown/yellow member of the ordinary background palette.

D_GREY

Value 7. Selects light grey—historically called dark white—as a foreground, or grey/white as a background. This is the conventional default text foreground.

Light foreground colors

The L_* names are values 8 through 15. Bit 3 raises the foreground intensity. All eight are valid foreground choices. They do not select an additional set of bright backgrounds in the established OpenDoors attribute model.

L_BLACK

Value 8. Selects high-intensity black, conventionally displayed as dark grey.

L_BLUE

Value 9. Selects bright blue.

L_GREEN

Value 10. Selects bright green.

L_CYAN

Value 11. Selects bright cyan.

L_RED

Value 12. Selects bright red.

L_MAGENTA

Value 13. Selects bright magenta.

L_YELLOW

Value 14. Selects yellow. It is the high-intensity form of D_BROWN.

L_WHITE

Value 15. Selects bright white. It is the high-intensity form of D_GREY.

Blinking background names

When values 8 through 15 are supplied as the background argument to od_set_color(), their low three bits select the background and their high bit requests blink. The B_* aliases make that use clear. Their numerical values are identical to the corresponding L_* foreground values.

B_BLACK

Alias of L_BLACK, value 8. Selects a black background with the blink bit set.

B_BLUE

Alias of L_BLUE, value 9. Selects a blue background with blink.

B_GREEN

Alias of L_GREEN, value 10. Selects a green background with blink.

B_CYAN

Alias of L_CYAN, value 11. Selects a cyan background with blink.

B_RED

Alias of L_RED, value 12. Selects a red background with blink.

B_MAGENTA

Alias of L_MAGENTA, value 13. Selects a magenta background with blink.

B_BROWN

Alias of L_YELLOW, value 14. Selects a brown background with blink. The name describes the three-bit background color; it is not a bright yellow background.

B_GREY

Alias of L_WHITE, value 15. Selects a grey background with blink.

B_YELLOW and B_WHITE are compatibility spellings for values 14 and 15. They do not introduce bright-background semantics.

Combining an attribute

To combine a foreground and background explicitly, keep the foreground's low four bits and move the background into the upper nibble:

BYTE attribute = (BYTE)(L_YELLOW | (D_BLUE << 4));
od_set_attrib(attribute);

The equivalent high-level call is:

od_set_color(L_YELLOW, D_BLUE);

Use the high-level call when choosing colors by name. Use a combined byte when storing a complete attribute or passing it to an API which explicitly accepts one.

Terminal representation

These values describe the logical IBM-PC attribute even on Windows, macOS, and Unix-like systems. OpenDoors translates that attribute into the remote terminal's ANSI/AVATAR output and into the local virtual-screen representation. The actual RGB color, intensity, or blink effect is chosen by the terminal.

Many modern terminals treat blink as a bright-background request or disable blinking entirely. OpenDoors preserves the established bit and wire-level intent; it cannot require a terminal emulator to animate it. Doors which need the broadest readability should prefer normal backgrounds and use foreground intensity for emphasis.

Color selection does not change character encoding. CP437-to-UTF-8 conversion, terminal emulation, and display attributes are independent stages of output.