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Starts to improve the documentation.

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This commit is contained in:
Felix "xq" Queißner 2021-11-28 01:08:52 +01:00
parent be8d23c574
commit 7053672d3a
1 changed files with 66 additions and 42 deletions
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108
qoi.h
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@ -109,54 +109,78 @@ index matches the current pixel, this index position is written to the stream.
Each chunk starts with a 2, 3 or 4 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.
QOI_INDEX {
u8 tag : 2; // b00
u8 idx : 6; // 6-bit index into the color index array: 0..63
}
The possible chunks are:
QOI_RUN_8 {
u8 tag : 3; // b010
u8 run : 5; // 5-bit run-length repeating the previous pixel: 1..32
}
| QOI_INDEX |
| Byte + 0 |
| 7 6 5 4 3 2 1 0 |
|-------------------------|
| 0 0 i5 i4 i3 i2 i1 i0 |
QOI_RUN_16 {
u8 tag : 3; // b011
u16 run : 13; // 13-bit run-length repeating the previous pixel: 33..8224
}
i5...i0 forming the 6-bit index into the color index array: 0..63
QOI_DIFF_8 {
u8 tag : 2; // b10
u8 dr : 2; // 2-bit red channel difference: -2..1
u8 dg : 2; // 2-bit green channel difference: -2..1
u8 db : 2; // 2-bit blue channel difference: -2..1
}
QOI_DIFF_16 {
u8 tag : 3; // b110
u8 dr : 5; // 5-bit red channel difference: -16..15
u8 dg : 4; // 4-bit green channel difference: -8.. 7
u8 db : 4; // 4-bit blue channel difference: -8.. 7
}
| QOI_INDEX |
| Byte + 0 |
| 7 6 5 4 3 2 1 0 |
|-------------------------|
| 0 1 0 i4 i3 i2 i1 i0 |
QOI_DIFF_24 {
u8 tag : 4; // b1110
u8 dr : 5; // 5-bit red channel difference: -16..15
u8 dg : 5; // 5-bit green channel difference: -16..15
u8 db : 5; // 5-bit blue channel difference: -16..15
u8 da : 5; // 5-bit alpha channel difference: -16..15
}
i4...i0 forming the 5-bit run-length repeating the previous pixel: 1..32
| QOI_RUN_16 |
| Byte + 0 | Byte + 1 |
| 7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0 |
|--------------------------------|-------------------------|
| 0 1 1 r12 r11 r10 r9 r8 | r7 r6 r5 r4 r3 r2 r1 r0 |
r12...r0 forming the 13-bit run-length repeating the previous pixel: 33..8224
| QOI_DIFF_8 |
| Byte + 0 |
| 7 6 5 4 3 2 1 0 |
|-------------------------|
| 1 0 r1 r0 g1 g0 b1 b0 |
r1...r0 forming the red channel difference between -2..1
g1...g0 forming the green channel difference between -2..1
b1...b0 forming the blue channel difference between -2..1
| QOI_DIFF_16 |
| Byte + 0 | Byte + 1 |
| 7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0 |
|-------------------------|-------------------------|
| 1 1 0 r4 r3 r2 r1 r0 | g3 g2 g1 g0 b3 b2 b1 b0 |
r4...r0 forming the red channel difference between -16..15
g3...g0 forming the green channel difference between -8..7
b3...b0 forming the blue channel difference between -8..7
| QOI_DIFF_24 |
| Byte + 0 | Byte + 1 | Byte + 1 |
| 7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0 |
|-------------------------|-------------------------|-------------------------|
| 1 1 1 0 r4 r3 r2 r1 | r0 g4 g3 g2 g1 g0 b4 b3 | b2 b1 b0 a4 a3 a2 a1 a0 |
r4...r0 forming the red channel difference between -16..15
g4...g0 forming the green channel difference between -16..15
b4...b0 forming the blue channel difference between -16..15
a4...a0 forming the alpha channel difference between -16..15
| QOI_COLOR |
| Byte + 0 |
| 7 6 5 4 3 2 1 0 |
|-------------------------|
| 1 1 1 1 r g b a |
for each set bit r, g, b and a another byte follows in the order written here. If such a byte follows,
it will replace the current color channel value with the value of this byte.
QOI_COLOR {
u8 tag : 4; // b1111
u8 has_r: 1; // red byte follows
u8 has_g: 1; // green byte follows
u8 has_b: 1; // blue byte follows
u8 has_a: 1; // alpha byte follows
u8 r; // red value if has_r == 1: 0..255
u8 g; // green value if has_g == 1: 0..255
u8 b; // blue value if has_b == 1: 0..255
u8 a; // alpha value if has_a == 1: 0..255
}
The byte stream is padded with 4 zero bytes. Size the longest chunk we can
encounter is 5 bytes (QOI_COLOR with RGBA set), with this padding we just have