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Image compression file format From Wikipedia, the free encyclopedia
The Quite OK Image Format (QOI) is a specification for lossless image compression of 24-bit (8 bits per color RGB) or 32-bit (8 bits per color with 8-bit alpha channel RGBA) color raster (bitmapped) images, invented by Dominic Szablewski and first announced on 24 November 2021.[1]
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Filename extension |
.qoi |
---|---|
Magic number | qoif (4 bytes, ASCII) |
Developed by | Dominic Szablewski |
Initial release | 24 November 2021 |
Latest release | 1.0 5 January 2022 |
Type of format | Lossless bitmap image format |
Standard | Specification |
Open format? | Yes |
Free format? | Yes |
Website | qoiformat.org |
The intended purpose was to create an open source lossless compression method that was faster and easier to implement than PNG. Figures specified in the blog post announcing the format claim twenty to fifty times faster encoding, and three to four times faster decoding speed compared to PNG, with similar file sizes.[1] The author has donated the specification to the public domain (CC0).[2]
QOI is supported natively by ImageMagick,[3] Imagine (v1.3.9+), IrfanView (v4.60+),[4] FFmpeg (v5.1+),[5] and GraphicConverter (v11.8+).[6] Microsoft PowerToys (v0.76+) for Windows 10 and 11 adds support for previewing QOI images to the Windows File Explorer.[7][8] Community made plugins are available in GIMP, Paint.NET and XnView MP.[9]
The game engine GameMaker designates bzip2 + QOI as the default format of texture groups since version 2022.1.0.609, to achieve the better compression but still quicker to decompress, while the standalone QOI and PNG formats are optional for the even faster performance and better compabilities respectively.[10][11]
There are also implementations for various languages such as Rust, Python, Java, C++, C# and more.[12] A full list can be found on the project's Git(Hub) repository README.
A QOI file consists of a 14-byte header, followed by any number of data “chunks” and an 8-byte end marker.
qoi_header {
char magic[4]; // magic bytes "qoif"
uint32_t width; // image width in pixels (BE)
uint32_t height; // image height in pixels (BE)
uint8_t channels; // 3 = RGB, 4 = RGBA
uint8_t colorspace; // 0 = sRGB with linear alpha
// 1 = all channels linear
};
The colorspace and channel fields are purely informative. They do not change the way data chunks are encoded.
Images are encoded row by row, left to right, top to bottom. 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:
QOI_OP_RUN
)QOI_OP_INDEX
)QOI_OP_DIFF
or QOI_OP_LUMA
)QOI_OP_RGB
or QOI_OP_RGBA
)The color channels are assumed to not be premultiplied with the alpha channel (“un-premultiplied alpha”). A running array[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) % 64
Each 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 by a single 0x01
byte.
The possible chunks are:
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 |
b11111110
(254)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 |
b11111111
(255)QOI_OP_INDEX
Byte[0] (Range: 0 .. 63) | |||||||
---|---|---|---|---|---|---|---|
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
0 | 0 | index |
b00
0..63
A valid encoder must not issue 2 or more consecutive QOI_OP_INDEX
chunks to the same index. QOI_OP_RUN
should be used instead.
QOI_OP_DIFF
Byte[0] (Range: 64 .. 127) | |||||||
---|---|---|---|---|---|---|---|
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
0 | 1 | dr | dg | db |
b01
-2..1
-2..1
-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 (b00
). 1 is stored as 3 (b11
). The alpha value remains unchanged from the previous pixel.
QOI_OP_LUMA
Byte[0] (Range: 128 .. 191) | Byte[1] | ||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
1 | 0 | dg | dr - dg | db - dg |
b10
-32..31
-8..7
-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. I.e.:
dr_dg = (cur_px.r - prev_px.r) - (cur_px.g - prev_px.g)
db_dg = (cur_px.b - prev_px.b) - (cur_px.g - prev_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] (Range: 192 .. 253) | |||||||
---|---|---|---|---|---|---|---|
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
1 | 1 | run |
b11
The run-length is stored with a bias of −1. Note that the runlengths 63 and 64 (b111110
and b111111
) are illegal as they are occupied by the QOI_OP_RGB
and QOI_OP_RGBA
tags.[13]
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