Merge pull request #43 from MasterQ32/improved_docs
Documentation improval
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Dominic Szablewski
GitHub
commit
bdfda4a5f6
117
qoi.h
117
qoi.h
@ -109,54 +109,87 @@ index matches the current pixel, this index position is written to the stream.
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Each chunk starts with a 2, 3 or 4 bit tag, followed by a number of data bits.
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The bit length of chunks is divisible by 8 - i.e. all chunks are byte aligned.
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QOI_INDEX {
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u8 tag : 2; // b00
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u8 idx : 6; // 6-bit index into the color index array: 0..63
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}
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The possible chunks are:
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QOI_RUN_8 {
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u8 tag : 3; // b010
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u8 run : 5; // 5-bit run-length repeating the previous pixel: 1..32
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}
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| QOI_INDEX |
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| Byte + 0 |
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| 7 6 5 4 3 2 1 0 |
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|-------------------------|
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| 0 0 i5 i4 i3 i2 i1 i0 |
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QOI_RUN_16 {
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u8 tag : 3; // b011
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u16 run : 13; // 13-bit run-length repeating the previous pixel: 33..8224
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}
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i5...i0 forming the 6-bit index into the color index array: 0..63
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QOI_DIFF_8 {
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u8 tag : 2; // b10
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u8 dr : 2; // 2-bit red channel difference: -2..1
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u8 dg : 2; // 2-bit green channel difference: -2..1
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u8 db : 2; // 2-bit blue channel difference: -2..1
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}
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QOI_DIFF_16 {
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u8 tag : 3; // b110
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u8 dr : 5; // 5-bit red channel difference: -16..15
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u8 dg : 4; // 4-bit green channel difference: -8.. 7
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u8 db : 4; // 4-bit blue channel difference: -8.. 7
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}
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| QOI_INDEX |
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| Byte + 0 |
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| 7 6 5 4 3 2 1 0 |
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|-------------------------|
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| 0 1 0 i4 i3 i2 i1 i0 |
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QOI_DIFF_24 {
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u8 tag : 4; // b1110
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u8 dr : 5; // 5-bit red channel difference: -16..15
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u8 dg : 5; // 5-bit green channel difference: -16..15
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u8 db : 5; // 5-bit blue channel difference: -16..15
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u8 da : 5; // 5-bit alpha channel difference: -16..15
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}
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i4...i0 forming the 5-bit run-length repeating the previous pixel: 1..32
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| QOI_RUN_16 |
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| Byte + 0 | Byte + 1 |
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| 7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0 |
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|--------------------------------|-------------------------|
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| 0 1 1 r12 r11 r10 r9 r8 | r7 r6 r5 r4 r3 r2 r1 r0 |
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r12...r0 forming the 13-bit run-length repeating the previous pixel: 33..8224
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| QOI_DIFF_8 |
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| Byte + 0 |
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| 7 6 5 4 3 2 1 0 |
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|-------------------------|
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| 1 0 r1 r0 g1 g0 b1 b0 |
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r1...r0 forming the red channel difference between -2..1
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g1...g0 forming the green channel difference between -2..1
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b1...b0 forming the blue channel difference between -2..1
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The difference to the current channel values are using a wraparound operation, so "1 - 2" will result in 255, while
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"255 + 1" will result in 0.
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| QOI_DIFF_16 |
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| Byte + 0 | Byte + 1 |
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| 7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0 |
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|-------------------------|-------------------------|
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| 1 1 0 r4 r3 r2 r1 r0 | g3 g2 g1 g0 b3 b2 b1 b0 |
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r4...r0 forming the red channel difference between -16..15
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g3...g0 forming the green channel difference between -8..7
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b3...b0 forming the blue channel difference between -8..7
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The difference to the current channel values are using a wraparound operation, so "10 - 13" will result in 253, while
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"250 + 7" will result in 1.
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| QOI_DIFF_24 |
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| Byte + 0 | Byte + 1 | Byte + 1 |
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| 7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0 |
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|-------------------------|-------------------------|-------------------------|
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| 1 1 1 0 r4 r3 r2 r1 | r0 g4 g3 g2 g1 g0 b4 b3 | b2 b1 b0 a4 a3 a2 a1 a0 |
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r4...r0 forming the red channel difference between -16..15
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g4...g0 forming the green channel difference between -16..15
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b4...b0 forming the blue channel difference between -16..15
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a4...a0 forming the alpha channel difference between -16..15
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The difference to the current channel values are using a wraparound operation, so "10 - 13" will result in 253, while
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"250 + 7" will result in 1.
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| QOI_COLOR |
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| Byte + 0 |
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| 7 6 5 4 3 2 1 0 |
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|-------------------------|
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| 1 1 1 1 r g b a |
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for each set bit r, g, b and a another byte follows in the order written here. If such a byte follows,
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it will replace the current color channel value with the value of this byte.
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QOI_COLOR {
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u8 tag : 4; // b1111
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u8 has_r: 1; // red byte follows
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u8 has_g: 1; // green byte follows
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u8 has_b: 1; // blue byte follows
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u8 has_a: 1; // alpha byte follows
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u8 r; // red value if has_r == 1: 0..255
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u8 g; // green value if has_g == 1: 0..255
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u8 b; // blue value if has_b == 1: 0..255
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u8 a; // alpha value if has_a == 1: 0..255
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}
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The byte stream is padded with 4 zero bytes. Size the longest chunk we can
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encounter is 5 bytes (QOI_COLOR with RGBA set), with this padding we just have
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