QOI (image format)
The alpha value remains unchanged from the previous pixel.
|
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
b11111111
(255) - 8-bit red channel value
- 8-bit green channel value
- 8-bit blue channel value
- 8-bit alpha channel value
QOI_OP_INDEX
Byte[0] (Range: 0 .. 63) | |||||||
---|---|---|---|---|---|---|---|
7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
0 | 0 | index |
- 2-bit tag
b00
- 6-bit index into the color index array:
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 |
- 2-bit tag
b01
- 2-bit red channel difference from the previous pixel
-2..1
- 2-bit green channel difference from the previous pixel
-2..1
- 2-bit blue channel difference from the previous pixel
-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 |
- 2-bit tag
b10
- 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. 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 |
- 2-bit tag
b11
- 6-bit run-length repeating the previous pixel
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]
References
- ^ a b "Lossless Image Compression in O(n) Time". Phoboslab.org. 2021-11-24. Archived from the original on 2022-05-08. Retrieved May 1, 2022.
- ^ "QOI The Quite OK Image Format". qoiformat.org. 2023-12-14. Archived from the original on 2023-12-16. Retrieved December 14, 2023.
- ^ "ImageMagick - Image Formats". Archived from the original on January 2, 2022. Retrieved May 4, 2022.
- ^ "History of IrfanView Changes/Versions". www.irfanview.com. Archived from the original on 2021-01-14. Retrieved 2022-05-10.
- ^ "FFmpeg Changelog - Gitweb". ffmpeg.org. Archived from the original on 2022-07-13. Retrieved 2022-07-13.
- ^ "GraphicConverter Release Notes version 11.8 (build 5762)". Lemke Software. Archived from the original on 11 February 2023. Retrieved 21 February 2023.
- ^ "Release v0.76.0". Microsoft. Retrieved 26 March 2024.
- ^ "PowerToys File Explorer add-ons utility for Windows". learn.microsoft.com. 2023-12-04. Retrieved 2024-02-05.
- ^ James Hein. "Moving images to the next level". Bangkok Post. Retrieved April 1, 2022.
- ^ "Version 2022.1.0.609". GameMaker Release Notes. YoYo Games. 26 January 2022. Retrieved 26 March 2024.
- ^ "Texture Groups". GameMaker Manual. YoYo Games. Retrieved 26 March 2024.
- ^ Simon Sharwood. "Developer creates 'Quite OK Image Format' – but it performs better than just OK". The Register. Archived from the original on 2023-06-02. Retrieved 2023-12-30.
- ^ Szablewski, Dominic (2022-01-05). "The Quite OK Image Format Specification" (PDF). Archived (PDF) from the original on 2022-04-30. Retrieved 2022-06-05. This article incorporates text from this source, which is in the public domain.
External links
- Format website: C Source code and benchmark results
- 1 page PDF specification
- GitHub repository (including C implementation)
- How PNG Works: Compromising Speed for Quality - YouTube A video comparing compression techniques in PNG and QOI with animations and examples.
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