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QOI - The “Quite OK Image” format for fast, lossless image compression
QOI encodes and decodes images in a lossless format. Compared to stb_image and stb_image_write QOI offers 20x-50x faster encoding, 3x-4x faster decoding and 20% better compression.
§Data Format
A QOI file has a 14 byte header, followed by any number of data “chunks” and an 8-byte end marker.
#[repr(C)]
struct QoiHeader {
magic: [u8; 4], // magic bytes "qoif"
width: u32, // image width in pixels (BE)
height: u32, // image height in pixels (BE)
channels: u8, // 3 = RGB, 4 = RGBA
colorspace: u8, // 0 = sRGB with linear alpha, 1 = all channels linear
}Images are encoded from top to bottom, left to right. The decoder and encoder
start with {r: 0, g: 0, b: 0, a: 255} as the previous pixel value. An image is
complete when all pixels specified by width * height have been covered.
Pixels are encoded as
- a run of the previous pixel
- an index into an array of previously seen pixels
- a difference to the previous pixel value in r,g,b
- full r,g,b or r,g,b,a values
The color channels are assumed to not be premultiplied with the alpha channel (“un-premultiplied alpha”).
A running array: [u32; 64] (zero-initialized) of previously seen pixel values is
maintained by the encoder and decoder. Each pixel that is seen by the encoder
and decoder is put into this array at the position formed by a hash function of
the color value. In the encoder, if the pixel value at the index matches the
current pixel, this index position is written to the stream as QOI_OP_INDEX.
The hash function for the index is:
index_position = (r * 3 + g * 5 + b * 7 + a * 11) % 64Each chunk starts with a 2- or 8-bit tag, followed by a number of data bits. The
bit length of chunks is divisible by 8 - i.e. all chunks are byte aligned. All
values encoded in these data bits have the most significant bit on the left.
The 8-bit tags have precedence over the 2-bit tags. A decoder must check for the
presence of an 8-bit tag first.
The byte stream’s end is marked with 7 0x00 bytes followed a single 0x01 byte.
The possible chunks are:
.- QOI_OP_INDEX ----------.
| Byte[0] |
| 7 6 5 4 3 2 1 0 |
|-------+-----------------|
| 0 0 | index |
`-------------------------`2-bit tag 0b00
6-bit index into the color index array: 0..=63
A valid encoder must not issue 7 or more consecutive QOI_OP_INDEX chunks to the
index 0, to avoid confusion with the 8 byte end marker.
.- QOI_OP_DIFF -----------.
| Byte[0] |
| 7 6 5 4 3 2 1 0 |
|-------+-----+-----+-----|
| 0 1 | dr | dg | db |
`-------------------------`2-bit tag 0b01
2-bit red channel difference from the previous pixel between -2..=1
2-bit green channel difference from the previous pixel between -2..=1
2-bit blue channel difference from the previous pixel between -2..=1
The difference to the current channel values are using a wraparound operation,
so 1 - 2 will result in 255, while 255 + 1 will result in 0.
Values are stored as unsigned integers with a bias of 2. E.g. -2 is stored as
0 (0b00). 1 is stored as 3 (0b11).
The alpha value remains unchanged from the previous pixel.
.- QOI_OP_LUMA -------------------------------------.
| Byte[0] | Byte[1] |
| 7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0 |
|-------+-----------------+-------------+-----------|
| 1 0 | green diff | dr - dg | db - dg |
`---------------------------------------------------`2-bit tag 0b10
6-bit green channel difference from the previous pixel -32..=31
4-bit red channel difference minus green channel difference -8..=7
4-bit blue channel difference minus green channel difference -8..=7
The green channel is used to indicate the general direction of change and is
encoded in 6 bits. The red and blue channels (dr and db) base their diffs off
of the green channel difference and are encoded in 4 bits. I.e.:
dr_dg = (last_px.r - cur_px.r) - (last_px.g - cur_px.g)
db_dg = (last_px.b - cur_px.b) - (last_px.g - cur_px.g)The difference to the current channel values are using a wraparound operation,
so 10 - 13 will result in 253, while 250 + 7 will result in 1.
Values are stored as unsigned integers with a bias of 32 for the green channel
and a bias of 8 for the red and blue channel.
The alpha value remains unchanged from the previous pixel.
.- QOI_OP_RUN ------------.
| Byte[0] |
| 7 6 5 4 3 2 1 0 |
|-------+-----------------|
| 1 1 | run |
`-------------------------`2-bit tag 0b11
6-bit run-length repeating the previous pixel: 1..=62
The run-length is stored with a bias of -1. Note that the run-lengths 63 and 64
(0b111110 and 0b111111) are illegal as they are occupied by the QOI_OP_RGB and
QOI_OP_RGBA tags.
.- QOI_OP_RGB ------------------------------------------.
| Byte[0] | Byte[1] | Byte[2] | Byte[3] |
| 7 6 5 4 3 2 1 0 | 7 .. 0 | 7 .. 0 | 7 .. 0 |
|-------------------------+---------+---------+---------|
| 1 1 1 1 1 1 1 0 | red | green | blue |
`-------------------------------------------------------`8-bit tag 0b11111110
8-bit red channel value
8-bit green channel value
8-bit blue channel value
The alpha value remains unchanged from the previous pixel.
.- QOI_OP_RGBA ---------------------------------------------------.
| Byte[0] | Byte[1] | Byte[2] | Byte[3] | Byte[4] |
| 7 6 5 4 3 2 1 0 | 7 .. 0 | 7 .. 0 | 7 .. 0 | 7 .. 0 |
|-------------------------+---------+---------+---------+---------|
| 1 1 1 1 1 1 1 1 | red | green | blue | alpha |
`-----------------------------------------------------------------`8-bit tag 0b11111111
8-bit red channel value
8-bit green channel value
8-bit blue channel value
8-bit alpha channel value
Structs§
- QOI descriptor value.
This value is parsed from image header during decoding.
Or provided by caller to drive encoding. - Color variance value. Wrapping difference between two pixels.
Enums§
- Image color space variants.
- Errros that may occur during image decoding.
- Errors that may occur during image encoding.
Traits§
- Trait for pixel types. Supports byte operations, channels accessing and modifying.