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Improve documentation, whitespace, wording

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Dominic Szablewski 2021-11-28 17:36:05 +01:00
parent f0a38c19e5
commit 80356a5aaa
1 changed files with 101 additions and 78 deletions
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155
qoi.h
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@ -32,9 +32,8 @@ QOI encodes and decodes images in a lossless format. An encoded QOI image is
usually around 10--30% larger than a decently optimized PNG image.
QOI outperforms simpler PNG encoders in compression ratio and performance. QOI
images are typically 20% smaller than PNGs written with stbi_image but 10%
larger than with libpng. Encoding is 25-50x faster and decoding is 3-4x faster
than stbi_image or libpng.
images are typically 20% smaller than PNGs written with stbi_image. Encoding is
25-50x faster and decoding is 3-4x faster than stbi_image or libpng.
-- Synopsis
@ -108,92 +107,107 @@ 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.
All values encoded in these data bits have the most significant bit (MSB) on the
left.
The possible chunks are:
| QOI_INDEX |
| Byte + 0 |
- QOI_INDEX -------------
| Byte[0] |
| 7 6 5 4 3 2 1 0 |
|-------------------------|
| 0 0 i5 i4 i3 i2 i1 i0 |
|-------+-----------------|
| 0 0 | index |
i5...i0 forming the 6-bit index into the color index array: 0..63
2-bit tag b00
6-bit index into the color index array: 0..63
| QOI_INDEX |
| Byte + 0 |
- QOI_RUN_8 -------------
| Byte[0] |
| 7 6 5 4 3 2 1 0 |
|-------------------------|
| 0 1 0 i4 i3 i2 i1 i0 |
|----------+--------------|
| 0 1 0 | run |
i4...i0 forming the 5-bit run-length repeating the previous pixel: 1..32
3-bit tag b010
5-bit run-length repeating the previous pixel: 1..32
| QOI_RUN_16 |
| Byte + 0 | Byte + 1 |
- 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 |
|----------+----------------------------------------|
| 0 1 1 | run |
r12...r0 forming the 13-bit run-length repeating the previous pixel: 33..8224
3-bit tag b011
13-bit run-length repeating the previous pixel: 33..8224
| QOI_DIFF_8 |
| Byte + 0 |
- QOI_DIFF_8 ------------
| Byte[0] |
| 7 6 5 4 3 2 1 0 |
|-------------------------|
| 1 0 r1 r0 g1 g0 b1 b0 |
|-------+-----+-----+-----|
| 1 0 | dr | db | bg |
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
2-bit tag b10
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.
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.
| QOI_DIFF_16 |
| Byte + 0 | Byte + 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 |
|----------+--------------|------------ +-----------|
| 1 1 0 | red diff | green diff | blue diff |
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
3-bit tag b110
5-bit red channel difference from the previous pixel between -16..15
4-bit green channel difference from the previous pixel between -8..7
4-bit blue channel difference from the previous pixel between -8..7
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.
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.
| QOI_DIFF_24 |
| Byte + 0 | Byte + 1 | Byte + 1 |
- QOI_DIFF_24 ---------------------------------------------------------------
| Byte[0] | Byte[1] | Byte[2] |
| 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 |
|-------------+----------------+--------------+----------------+--------------|
| 1 1 1 0 | red diff | green diff | blue diff | alpha diff |
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
4-bit tag b1110
5-bit red channel difference from the previous pixel between -16..15
5-bit green channel difference from the previous pixel between -16..15
5-bit blue channel difference from the previous pixel between -16..15
5-bit alpha channel difference from the previous pixel between -16..15
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.
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.
| QOI_COLOR |
| Byte + 0 |
- QOI_COLOR -------------
| Byte[0] |
| 7 6 5 4 3 2 1 0 |
|-------------------------|
| 1 1 1 1 r g b a |
|-------------+--+--+--+--|
| 1 1 1 1 |hr|hg|hb|ha|
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.
4-bit tag b1111
1-bit red byte follows
1-bit green byte follows
1-bit blue byte follows
1-bit alpha byte follows
For each set bit hr, hg, hb and ha another byte follows in this order. If such a
byte follows, it will replace the current color channel value with the value of
this byte.
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
to check for an overrun once per decode loop iteration.
The byte stream is padded at the end 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 to check for an overrun once per decode loop iteration.
*/
@ -385,7 +399,10 @@ void *qoi_encode(const void *data, const qoi_desc *desc, int *out_len) {
run++;
}
if (run > 0 && (run == 0x2020 || px.v != px_prev.v || px_pos == px_end)) {
if (
run > 0 &&
(run == 0x2020 || px.v != px_prev.v || px_pos == px_end)
) {
if (run < 33) {
run -= 1;
bytes[p++] = QOI_RUN_8 | run;
@ -413,26 +430,32 @@ void *qoi_encode(const void *data, const qoi_desc *desc, int *out_len) {
int va = px.rgba.a - px_prev.rgba.a;
if (
vr > -17 && vr < 16 && vg > -17 && vg < 16 &&
vb > -17 && vb < 16 && va > -17 && va < 16
vr > -17 && vr < 16 &&
vg > -17 && vg < 16 &&
vb > -17 && vb < 16 &&
va > -17 && va < 16
) {
if (
va == 0 && vr > -3 && vr < 2 &&
vg > -3 && vg < 2 && vb > -3 && vb < 2
va == 0 &&
vr > -3 && vr < 2 &&
vg > -3 && vg < 2 &&
vb > -3 && vb < 2
) {
bytes[p++] = QOI_DIFF_8 | ((vr + 2) << 4) | (vg + 2) << 2 | (vb + 2);
}
else if (
va == 0 && vr > -17 && vr < 16 &&
vg > -9 && vg < 8 && vb > -9 && vb < 8
va == 0 &&
vr > -17 && vr < 16 &&
vg > -9 && vg < 8 &&
vb > -9 && vb < 8
) {
bytes[p++] = QOI_DIFF_16 | (vr + 16);
bytes[p++] = ((vg + 8) << 4) | (vb + 8);
bytes[p++] = (vg + 8) << 4 | (vb + 8);
}
else {
bytes[p++] = QOI_DIFF_24 | ((vr + 16) >> 1);
bytes[p++] = ((vr + 16) << 7) | ((vg + 16) << 2) | ((vb + 16) >> 3);
bytes[p++] = ((vb + 16) << 5) | (va + 16);
bytes[p++] = QOI_DIFF_24 | (vr + 16) >> 1;
bytes[p++] = (vr + 16) << 7 | (vg + 16) << 2 | (vb + 16) >> 3;
bytes[p++] = (vb + 16) << 5 | (va + 16);
}
}
else {