RGB55

RGB55 is a color representation in which the red, green, and blue components are each encoded with 5 bits. This results in 15 bits of color information per pixel, allowing 32 × 32 × 32 = 32,768 possible colors. In practice, RGB55 is usually stored in a 16-bit word, with the remaining bit typically unused or repurposed as alpha in related schemes such as ARGB1555, which uses a single bit for transparency.

Bit layout for RGB55 can vary by hardware, but a common arrangement places red in the most significant 5 bits, green in the middle 5 bits, and blue in the least significant 5 bits. Some systems may swap channel positions or present the data in different endianness. The exact mapping is an implementation detail that affects how software reads or writes pixel data.

Each channel’s 5-bit value ranges from 0 to 31. To convert to standard 8-bit-per-channel color, a typical approach is to scale by 255/31 or to use the expansion formula 8-bit = (5-bit << 3) | (5-bit >> 2), which approximates full 8-bit precision.

RGB55 is commonly used in memory- or bandwidth-constrained environments where 16-bit pixels are advantageous, such as older graphics hardware, embedded displays, or retro-style graphics modes. It offers a modest color space compared to 24-bit RGB but reduces memory usage and simplifies some graphics pipelines. Limitations include reduced color fidelity and smoother gradient support, which often necessitates dithering to approximate intermediate tones.

See also RGB555, RGB565, and 15-bit color.