rgb565

rgb565 provides deserialization, serialization and conversion routines for the RGB565 pixel format, which stores color information in only 16 bits. The red channel gets 5 bits, the green 6, and blue 5 again. The RGB565 format is often used in embedded devices and microcontrollers for things like e-ink displays that only have a low degree of color reproduction.

rgb565 includes out-of-the-box methods for converting to and from many orderings and endiannesses, with each routine being manually verified, automatically verified and also tested on physical devices that use the RGB565 format.

LUTs

rgb565 comes with the optional ability to make use of look-up tables to gain massive speed increases (up to 20%!) on embedded devices without a graphics coprocessor. sRGB in particular gains a lot from LUTs because you usually need floating-point math to correctly approximate the gamma curve.

You can control the inclusion of look-up tables using Cargo features:

• swap_components_lut to speed up loading and storing to BGR values
• l5_to_l8_lut to speed up converting to 8-bit RGB red/blue channels
• l6_to_l8_lut to speed up converting to 8-bit RGB green channel
• l5_to_s8_lut to speed up converting to 8-bit sRGB red/blue channels
• l6_to_s8_lut to speed up converting to 8-bit sRGB green channel
• l565_to_l888_lut to speed up converting RGB565 values to 8-bit RGB
• l565_to_s888_lut to speed up converting RGB565 values to 8-bit sRGB
• l8_to_l5_lut to speed up converting from 8-bit RGB red/blue channels
• l8_to_l6_lut to speed up converting from 8-bit RGB green channel
• s8_to_l5_lut to speed up converting from 8-bit sRGB red/blue channels
• s8_to_l6_lut to speed up converting from 8-bit sRGB green channels
• l888_to_l565_lut to speed up converting 8-bit RGB values to RGB565
• s888_to_l565_lut to speed up converting 8-bit sRGB values to RGB565

If you won't be using BGR565, then you don't need swap_components_lut. If you'll only be converting all three channels at once, then you don't need the individual l/s#_to_l/s#_lut features.

All LUTs are enabled by default except for l888_to_l565_lut and s888_to_l565_lut, and you'll see why if you read this - the sizes of all the LUTs is as follows:

• swap_components_lut: 128 KiB (131,072 bytes)
• l5_to_l8_lut: 32 bytes
• l6_to_l8_lut: 64 bytes
• l5_to_s8_lut: 32 bytes
• l6_to_s8_lut: 64 bytes
• l565_to_l888_lut: 192 KiB (196,608 bytes)
• l565_to_s888_lut: 192 KiB (196,608 bytes)
• l8_to_l5_lut: 256 bytes
• l8_to_l6_lut: 256 bytes
• s8_to_l5_lut: 256 bytes
• s8_to_l6_lut: 256 bytes
• l888_to_l565_lut: 32 MiB (33,554,432 bytes)
• s888_to_l565_lut: 32 MiB (33,554,432 bytes)

That's because l888_to_l565_lut and s888_to_l565_lut both have to cover the entire 16.777216-million-color space of 24-bit "true color", and I don't think it would be very nice to add bloat like that by default.

Building

$ cargo build

Testing

$ cargo test

License

This repository and all source code it contains is licensed under the MIT License, mainly because I want to include it in libremarkable, which is also licensed under MIT.