Improve documentation, whitespace, wording
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Dominic Szablewski
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155
qoi.h
155
qoi.h
@ -32,9 +32,8 @@ QOI encodes and decodes images in a lossless format. An encoded QOI image is
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usually around 10--30% larger than a decently optimized PNG image.
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usually around 10--30% larger than a decently optimized PNG image.
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QOI outperforms simpler PNG encoders in compression ratio and performance. QOI
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QOI outperforms simpler PNG encoders in compression ratio and performance. QOI
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images are typically 20% smaller than PNGs written with stbi_image but 10%
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images are typically 20% smaller than PNGs written with stbi_image. Encoding is
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larger than with libpng. Encoding is 25-50x faster and decoding is 3-4x faster
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25-50x faster and decoding is 3-4x faster than stbi_image or libpng.
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than stbi_image or libpng.
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-- Synopsis
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-- Synopsis
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@ -108,92 +107,107 @@ 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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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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The bit length of chunks is divisible by 8 - i.e. all chunks are byte aligned.
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All values encoded in these data bits have the most significant bit (MSB) on the
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left.
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The possible chunks are:
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The possible chunks are:
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| QOI_INDEX |
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- QOI_INDEX -------------
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| Byte + 0 |
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| Byte[0] |
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| 7 6 5 4 3 2 1 0 |
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| 7 6 5 4 3 2 1 0 |
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|-------------------------|
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|-------+-----------------|
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| 0 0 i5 i4 i3 i2 i1 i0 |
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| 0 0 | index |
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i5...i0 forming the 6-bit index into the color index array: 0..63
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2-bit tag b00
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6-bit index into the color index array: 0..63
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| QOI_INDEX |
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- QOI_RUN_8 -------------
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| Byte + 0 |
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| Byte[0] |
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| 7 6 5 4 3 2 1 0 |
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| 7 6 5 4 3 2 1 0 |
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|-------------------------|
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|----------+--------------|
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| 0 1 0 i4 i3 i2 i1 i0 |
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| 0 1 0 | run |
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i4...i0 forming the 5-bit run-length repeating the previous pixel: 1..32
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3-bit tag b010
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5-bit run-length repeating the previous pixel: 1..32
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| QOI_RUN_16 |
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- QOI_RUN_16 --------------------------------------
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| Byte + 0 | Byte + 1 |
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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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| 7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0 |
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|--------------------------------|-------------------------|
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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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| 0 1 1 | run |
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r12...r0 forming the 13-bit run-length repeating the previous pixel: 33..8224
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3-bit tag b011
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13-bit run-length repeating the previous pixel: 33..8224
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| QOI_DIFF_8 |
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- QOI_DIFF_8 ------------
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| Byte + 0 |
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| Byte[0] |
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| 7 6 5 4 3 2 1 0 |
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| 7 6 5 4 3 2 1 0 |
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|-------------------------|
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|-------+-----+-----+-----|
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| 1 0 r1 r0 g1 g0 b1 b0 |
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| 1 0 | dr | db | bg |
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r1...r0 forming the red channel difference between -2..1
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2-bit tag b10
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g1...g0 forming the green channel difference between -2..1
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2-bit red channel difference from the previous pixel between -2..1
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b1...b0 forming the blue channel difference between -2..1
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2-bit green channel difference from the previous pixel between -2..1
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2-bit blue channel difference from the previous pixel 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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The difference to the current channel values are using a wraparound operation,
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"255 + 1" will result in 0.
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so "1 - 2" will result in 255, while "255 + 1" will result in 0.
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| QOI_DIFF_16 |
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- QOI_DIFF_16 -------------------------------------
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| Byte + 0 | Byte + 1 |
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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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| 7 6 5 4 3 2 1 0 | 7 6 5 4 3 2 1 0 |
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|-------------------------|-------------------------|
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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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| 1 1 0 | red diff | green diff | blue diff |
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r4...r0 forming the red channel difference between -16..15
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3-bit tag b110
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g3...g0 forming the green channel difference between -8..7
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5-bit red channel difference from the previous pixel between -16..15
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b3...b0 forming the blue channel difference between -8..7
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4-bit green channel difference from the previous pixel between -8..7
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4-bit blue channel difference from the previous pixel 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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The difference to the current channel values are using a wraparound operation,
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"250 + 7" will result in 1.
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so "10 - 13" will result in 253, while "250 + 7" will result in 1.
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| QOI_DIFF_24 |
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- QOI_DIFF_24 ---------------------------------------------------------------
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| Byte + 0 | Byte + 1 | Byte + 1 |
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| Byte[0] | Byte[1] | Byte[2] |
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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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| 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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|-------------+----------------+--------------+----------------+--------------|
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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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| 1 1 1 0 | red diff | green diff | blue diff | alpha diff |
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r4...r0 forming the red channel difference between -16..15
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4-bit tag b1110
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g4...g0 forming the green channel difference between -16..15
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5-bit red channel difference from the previous pixel between -16..15
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b4...b0 forming the blue channel difference between -16..15
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5-bit green channel difference from the previous pixel between -16..15
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a4...a0 forming the alpha channel difference between -16..15
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5-bit blue channel difference from the previous pixel between -16..15
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5-bit alpha channel difference from the previous pixel 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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The difference to the current channel values are using a wraparound operation,
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"250 + 7" will result in 1.
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so "10 - 13" will result in 253, while "250 + 7" will result in 1.
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| QOI_COLOR |
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- QOI_COLOR -------------
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| Byte + 0 |
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| Byte[0] |
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| 7 6 5 4 3 2 1 0 |
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| 7 6 5 4 3 2 1 0 |
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|-------------------------|
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|-------------+--+--+--+--|
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| 1 1 1 1 r g b a |
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| 1 1 1 1 |hr|hg|hb|ha|
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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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4-bit tag b1111
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it will replace the current color channel value with the value of this byte.
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1-bit red byte follows
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1-bit green byte follows
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1-bit blue byte follows
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1-bit alpha byte follows
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For each set bit hr, hg, hb and ha another byte follows in this order. If such a
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byte follows, it will replace the current color channel value with the value of
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this byte.
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The byte stream is padded with 4 zero bytes. Size the longest chunk we can
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The byte stream is padded at the end with 4 zero bytes. Size the longest chunk
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encounter is 5 bytes (QOI_COLOR with RGBA set), with this padding we just have
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we can encounter is 5 bytes (QOI_COLOR with RGBA set), with this padding we just
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to check for an overrun once per decode loop iteration.
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have to check for an overrun once per decode loop iteration.
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*/
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*/
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@ -385,7 +399,10 @@ void *qoi_encode(const void *data, const qoi_desc *desc, int *out_len) {
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run++;
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run++;
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}
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}
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if (run > 0 && (run == 0x2020 || px.v != px_prev.v || px_pos == px_end)) {
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if (
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run > 0 &&
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(run == 0x2020 || px.v != px_prev.v || px_pos == px_end)
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) {
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if (run < 33) {
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if (run < 33) {
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run -= 1;
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run -= 1;
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bytes[p++] = QOI_RUN_8 | run;
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bytes[p++] = QOI_RUN_8 | run;
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@ -413,26 +430,32 @@ void *qoi_encode(const void *data, const qoi_desc *desc, int *out_len) {
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int va = px.rgba.a - px_prev.rgba.a;
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int va = px.rgba.a - px_prev.rgba.a;
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if (
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if (
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vr > -17 && vr < 16 && vg > -17 && vg < 16 &&
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vr > -17 && vr < 16 &&
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vb > -17 && vb < 16 && va > -17 && va < 16
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vg > -17 && vg < 16 &&
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vb > -17 && vb < 16 &&
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va > -17 && va < 16
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) {
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) {
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if (
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if (
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va == 0 && vr > -3 && vr < 2 &&
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va == 0 &&
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vg > -3 && vg < 2 && vb > -3 && vb < 2
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vr > -3 && vr < 2 &&
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vg > -3 && vg < 2 &&
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vb > -3 && vb < 2
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) {
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) {
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bytes[p++] = QOI_DIFF_8 | ((vr + 2) << 4) | (vg + 2) << 2 | (vb + 2);
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bytes[p++] = QOI_DIFF_8 | ((vr + 2) << 4) | (vg + 2) << 2 | (vb + 2);
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}
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}
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else if (
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else if (
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va == 0 && vr > -17 && vr < 16 &&
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va == 0 &&
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vg > -9 && vg < 8 && vb > -9 && vb < 8
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vr > -17 && vr < 16 &&
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vg > -9 && vg < 8 &&
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vb > -9 && vb < 8
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) {
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) {
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bytes[p++] = QOI_DIFF_16 | (vr + 16);
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bytes[p++] = QOI_DIFF_16 | (vr + 16);
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bytes[p++] = ((vg + 8) << 4) | (vb + 8);
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bytes[p++] = (vg + 8) << 4 | (vb + 8);
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}
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}
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else {
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else {
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bytes[p++] = QOI_DIFF_24 | ((vr + 16) >> 1);
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bytes[p++] = QOI_DIFF_24 | (vr + 16) >> 1;
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bytes[p++] = ((vr + 16) << 7) | ((vg + 16) << 2) | ((vb + 16) >> 3);
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bytes[p++] = (vr + 16) << 7 | (vg + 16) << 2 | (vb + 16) >> 3;
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bytes[p++] = ((vb + 16) << 5) | (va + 16);
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bytes[p++] = (vb + 16) << 5 | (va + 16);
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}
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}
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}
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}
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else {
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else {
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