-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy pathqoi.txt
146 lines (112 loc) · 5.49 KB
/
qoi.txt
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
https://qoiformat.org/
A QOI file has a 14 byte header, followed by any number of data "chunks" and an
8-byte end marker.
struct qoi_header_t {
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
};
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
- 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[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 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 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] |
| 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 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 (b00). 1 is stored as 3 (b11).
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 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 and are encoded in 4 bits. 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] |
| 7 6 5 4 3 2 1 0 |
|-------+-----------------|
| 1 1 | run |
`-------------------------`
2-bit tag b11
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
(b111110 and b111111) 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 b11111110
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 b11111111
8-bit red channel value
8-bit green channel value
8-bit blue channel value
8-bit alpha channel value