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MPIC

Simple Lossy Compression Image Format for Embedded Platforms

Features

  • Simple.
  • Lossy compression.
  • A typical image compression ratio is somewhere between PNG and JPG.
  • Small memory footprint, only a few hundred bytes of stack memory required for decoding.
  • Designed for 16bpp color images and supports embedded-graphics; add features = ["embedded"] to Cargo.toml.
  • Support for no_std, No alloc is needed for decoding.

Example Apps

/cli: Command Line File Converter

  • Example of a command line application that converts files in MPIC format and other formats such as PNG to each other
$ cargo run -p cli INFILE OUTFILE

/viewer: Image Viewer

  • Example of a GUI application that displays MPIC format files with embedded-graphics
$ cargo run -p viewer FILE_NAME

Other Apps

How it works

  • Divide the image into blocks of 8 x 8 pixels.
  • Convert RGB with 8 bits per channel to YUV with 6 bits per channel.
  • Thin out the U and V channels to 1/4.
  • Because the color difference information is thinned out, even in the worst case, the compression is guaranteed to be more than half of the raw bitmap.
  • Finally, lossless compression is performed using the sliding dictionary method.
  • When decoding, these processes are performed in reverse order.

Comparison with sample images

Mandrill Original Size Converted PNG Size
Original 24bit Bitmap 197KB 155KB
MPIC 72KB 135KB
JPEG 40KB -
Parrots Original Size Converted PNG Size
Original 24bit Bitmap 197KB 105KB
MPIC 49KB 87KB
JPEG 21KB -
Pepper Original Size Converted PNG Size
Original 24bit Bitmap 197KB 117KB
MPIC 58KB 105KB
JPEG 28KB -

File Format

File Header

  • All multi-byte data is encoded in little-endian.
#[repr(C, packed)]
pub struct FileHeader {
    magic: [u8; 4], // b"\x00mpi"
    width: u16,
    height: u16,
    version: u8,    // The current version is 1.
}

Differences between versions

  • In version 0, only multiples of 8 are allowed for width and height.
  • There is no limit to image size in version 1 or later.

Image Data

  • Image data follows the header.
  • Image data is divided into 8 x 8 blocks and stored in chunks.
  • If the image size is not a multiple of 8 x 8, the right and bottom edges are filled with a color interpolated from the surroundings to match a multiple of 8.
  • Number of Chunks = ceil(width / 8) * ceil(height / 8)

Chunk

  • The first byte of each chunk of data indicates the data size, followed by the payload. The chunk size also indicates how the chunks are compressed.
  • For uncompressed chunks, the data size (96), followed by the 64-byte Y channel, 16-byte U channel, and 16-byte V channel. 96 also serves as an identifier for uncompressed data. In practice, normal encoders do not use this mode.
  • The Y channel stores all 8x8 data, while the U and V channels store only 4x4 pixels. The method of thinning the U and V channels is left to the encoder. The decoder should use nearest-neighbor interpolation to expand them by a factor of 2 in height and width.
  • For a 6-bit compacted chunk, the data size is 72. The order of the data is the same as for the uncompressed chunk, but the 6 bits of the uncompressed chunk are compacted into 8 bits, so the data size is 3/4 of the uncompressed chunk.
  • If the data size after compression exceeds 72 with other compression methods, the 6-bit compaction method shall be selected.

Color Conversion Methods

  • RGB888 to YUV666
    y = ((66 * r + 129 * g + 25 * b + 128) >> 10) + 4;

    u = (((-38 * r - 74 * g + 112 * b + 128) / 256) + 128) >> 2;

    v = (((112 * r - 94 * g - 18 * b + 128) / 256) + 128) >> 2;
  • YUV666 to RGB666 or RGB888
fn u6_to_u8(val) {
    (val << 2) | (val >> 4)
}

    y = u6_to_u8(y - 4);
    u = (u6_to_u8(u) - 128);
    v = (u6_to_u8(v) - 128);

    r6 = ((298 * y + 409 * v + 128) >> 10).clamp(0, 63);
    g6 = ((298 * y - 100 * u - 208 * v + 128) >> 10).clamp(0, 63);
    b6 = ((298 * y + 516 * u + 128) >> 10).clamp(0, 63);

    r8 = u6_to_u8(r6)
    g8 = u6_to_u8(g6)
    b8 = u6_to_u8(b6)

LZ Compression Data Encoding

Representation Meaning
00vv_vvvv Raw Value
01nn_nnnn 00mm_mmmm Together with the trailing byte value, it indicates the length (n+3) and offset -(m+1) of the slide dictionary.
01xx_xxxx NNxx_xxxx RESERVED (NN!=00)
1nnm_mmmm Short form of sliding dictionary, it indicates the length (n+2) and offset -(m+1).

License

MIT

(C) 2023 Nerry