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stb_image: JPEG fast_ac table - decode entire AC at once.

pull/62/head
Fabian Giesen 2014-12-14 01:08:47 -08:00
parent a1bd1f7f1f
commit 1d5652044d
1 changed files with 62 additions and 15 deletions
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77
stb_image.h
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@ -1014,6 +1014,7 @@ typedef struct
stbi__huffman huff_dc[4];
stbi__huffman huff_ac[4];
stbi_uc dequant[4][64];
stbi__int16 fast_ac[4][1 << FAST_BITS];
// sizes for components, interleaved MCUs
int img_h_max, img_v_max;
@ -1085,6 +1086,33 @@ static int stbi__build_huffman(stbi__huffman *h, int *count)
return 1;
}
// build a table that decodes both magnitude and value of small ACs in
// one go.
static void stbi__build_fast_ac(stbi__int16 *fast_ac, stbi__huffman *h)
{
int i;
for (i=0; i < (1 << FAST_BITS); ++i) {
stbi_uc fast = h->fast[i];
fast_ac[i] = 0;
if (fast < 255) {
int rs = h->values[fast];
int run = (rs >> 4) & 15;
int magbits = rs & 15;
int len = h->size[fast];
if (magbits && len + magbits <= FAST_BITS) {
// magnitude code followed by receive_extend code
int k = ((i << len) & ((1 << FAST_BITS) - 1)) >> (FAST_BITS - magbits);
int m = 1 << (magbits - 1);
if (k < m) k += (-1 << magbits) + 1;
// if the result is small enough, we can fit it in fast_ac table
if (k >= -128 && k <= 127)
fast_ac[i] = (stbi__int16) ((k << 8) + (run << 4) + (len + magbits));
}
}
}
}
static void stbi__grow_buffer_unsafe(stbi__jpeg *j)
{
do {
@ -1192,10 +1220,13 @@ static stbi_uc stbi__jpeg_dezigzag[64+15] =
};
// decode one 64-entry block--
static int stbi__jpeg_decode_block(stbi__jpeg *j, short data[64], stbi__huffman *hdc, stbi__huffman *hac, int b)
static int stbi__jpeg_decode_block(stbi__jpeg *j, short data[64], stbi__huffman *hdc, stbi__huffman *hac, stbi__int16 *fac, int b)
{
int diff,dc,k;
int t = stbi__jpeg_huff_decode(j, hdc);
int t;
if (j->code_bits < 16) stbi__grow_buffer_unsafe(j);
t = stbi__jpeg_huff_decode(j, hdc);
if (t < 0) return stbi__err("bad huffman code","Corrupt JPEG");
// 0 all the ac values now so we can do it 32-bits at a time
@ -1209,18 +1240,30 @@ static int stbi__jpeg_decode_block(stbi__jpeg *j, short data[64], stbi__huffman
// decode AC components, see JPEG spec
k = 1;
do {
int r,s;
int rs = stbi__jpeg_huff_decode(j, hac);
if (rs < 0) return stbi__err("bad huffman code","Corrupt JPEG");
s = rs & 15;
r = rs >> 4;
if (s == 0) {
if (rs != 0xf0) break; // end block
k += 16;
} else {
k += r;
int c,r,s;
if (j->code_bits < 16) stbi__grow_buffer_unsafe(j);
c = (j->code_buffer >> (32 - FAST_BITS)) & ((1 << FAST_BITS)-1);
r = fac[c];
if (r) { // fast-AC path
k += (r >> 4) & 15; // run
s = r & 15; // combined length
j->code_buffer <<= s;
j->code_bits -= s;
// decode into unzigzag'd location
data[stbi__jpeg_dezigzag[k++]] = (short) stbi__extend_receive(j,s);
data[stbi__jpeg_dezigzag[k++]] = (short) (r >> 8);
} else {
int rs = stbi__jpeg_huff_decode(j, hac);
if (rs < 0) return stbi__err("bad huffman code","Corrupt JPEG");
s = rs & 15;
r = rs >> 4;
if (s == 0) {
if (rs != 0xf0) break; // end block
k += 16;
} else {
k += r;
// decode into unzigzag'd location
data[stbi__jpeg_dezigzag[k++]] = (short) stbi__extend_receive(j,s);
}
}
} while (k < 64);
return 1;
@ -1406,7 +1449,8 @@ static int stbi__parse_entropy_coded_data(stbi__jpeg *z)
int h = (z->img_comp[n].y+7) >> 3;
for (j=0; j < h; ++j) {
for (i=0; i < w; ++i) {
if (!stbi__jpeg_decode_block(z, data, z->huff_dc+z->img_comp[n].hd, z->huff_ac+z->img_comp[n].ha, n)) return 0;
int ha = z->img_comp[n].ha;
if (!stbi__jpeg_decode_block(z, data, z->huff_dc+z->img_comp[n].hd, z->huff_ac+ha, z->fast_ac[ha], n)) return 0;
#ifdef STBI_SIMD
stbi__idct_installed(z->img_comp[n].data+z->img_comp[n].w2*j*8+i*8, z->img_comp[n].w2, data, z->dequant2[z->img_comp[n].tq]);
#else
@ -1436,7 +1480,8 @@ static int stbi__parse_entropy_coded_data(stbi__jpeg *z)
for (x=0; x < z->img_comp[n].h; ++x) {
int x2 = (i*z->img_comp[n].h + x)*8;
int y2 = (j*z->img_comp[n].v + y)*8;
if (!stbi__jpeg_decode_block(z, data, z->huff_dc+z->img_comp[n].hd, z->huff_ac+z->img_comp[n].ha, n)) return 0;
int ha = z->img_comp[n].ha;
if (!stbi__jpeg_decode_block(z, data, z->huff_dc+z->img_comp[n].hd, z->huff_ac+ha, z->fast_ac[ha], n)) return 0;
#ifdef STBI_SIMD
stbi__idct_installed(z->img_comp[n].data+z->img_comp[n].w2*y2+x2, z->img_comp[n].w2, data, z->dequant2[z->img_comp[n].tq]);
#else
@ -1516,6 +1561,8 @@ static int stbi__process_marker(stbi__jpeg *z, int m)
}
for (i=0; i < n; ++i)
v[i] = stbi__get8(z->s);
if (tc != 0)
stbi__build_fast_ac(z->fast_ac[th], z->huff_ac + th);
L -= n;
}
return L==0;