Skip to content

Commit

Permalink
stb_image: create_png_image_raw restructuring
Browse files Browse the repository at this point in the history 
Allocate filter_buf for two scan lines that we do all the filter
processing in, then do all other conversions (bit depth,
endianness, inserting alpha=255 values) on the way out.

Separating the two concerns makes everything much clearer.
  • Loading branch information
@rygorous
rygorous authored and Sean Barrett committed Dec 14, 2023
1 parent c6d7c32 commit 4da08a1
Showing 1 changed file with 131 additions and 152 deletions.
283 changes: 131 additions & 152 deletions stb_image.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -4667,13 +4667,38 @@ static int stbi__paeth(int a, int b, int c)

static const stbi_uc stbi__depth_scale_table[9] = { 0, 0xff, 0x55, 0, 0x11, 0,0,0, 0x01 };

// adds an extra all-255 alpha channel
// dest == src is legal
// img_n must be 1 or 3
static void stbi__create_png_alpha_expand8(stbi_uc *dest, stbi_uc *src, stbi__uint32 x, int img_n)
{
int i;
// must process data backwards since we allow dest==src
if (img_n == 1) {
for (i=x-1; i >= 0; --i) {
dest[i*2+1] = 255;
dest[i*2+0] = src[i];
}
} else {
STBI_ASSERT(img_n == 3);
for (i=x-1; i >= 0; --i) {
dest[i*4+3] = 255;
dest[i*4+2] = src[i*3+2];
dest[i*4+1] = src[i*3+1];
dest[i*4+0] = src[i*3+0];
}
}
}

// create the png data from post-deflated data
static int stbi__create_png_image_raw(stbi__png *a, stbi_uc *raw, stbi__uint32 raw_len, int out_n, stbi__uint32 x, stbi__uint32 y, int depth, int color)
{
int bytes = (depth == 16? 2 : 1);
int bytes = (depth == 16 ? 2 : 1);
stbi__context *s = a->s;
stbi__uint32 i,j,stride = x*out_n*bytes;
stbi__uint32 img_len, img_width_bytes;
stbi_uc *filter_buf;
int all_ok = 1;
int k;
int img_n = s->img_n; // copy it into a local for later

Expand All @@ -4685,195 +4710,149 @@ static int stbi__create_png_image_raw(stbi__png *a, stbi_uc *raw, stbi__uint32 r
a->out = (stbi_uc *) stbi__malloc_mad3(x, y, output_bytes, 0); // extra bytes to write off the end into
if (!a->out) return stbi__err("outofmem", "Out of memory");

// note: error exits here don't need to clean up a->out individually,
// stbi__do_png always does on error.
if (!stbi__mad3sizes_valid(img_n, x, depth, 7)) return stbi__err("too large", "Corrupt PNG");
img_width_bytes = (((img_n * x * depth) + 7) >> 3);
if (!stbi__mad2sizes_valid(img_width_bytes, y, img_width_bytes)) return stbi__err("too large", "Corrupt PNG");
img_len = (img_width_bytes + 1) * y;

// we used to check for exact match between raw_len and img_len on non-interlaced PNGs,
// but issue #276 reported a PNG in the wild that had extra data at the end (all zeros),
// so just check for raw_len < img_len always.
if (raw_len < img_len) return stbi__err("not enough pixels","Corrupt PNG");

// Allocate two scan lines worth of filter workspace buffer.
filter_buf = (stbi_uc *) stbi__malloc_mad2(img_width_bytes, 2, 0);
if (!filter_buf) return stbi__err("outofmem", "Out of memory");

// Filtering for low-bit-depth images
if (depth < 8) {
filter_bytes = 1;
width = img_width_bytes;
}

for (j=0; j < y; ++j) {
stbi_uc *cur = a->out + stride*j;
stbi_uc *prior;
// cur/prior filter buffers alternate
stbi_uc *cur = filter_buf + (j & 1)*img_width_bytes;
stbi_uc *prior = filter_buf + (~j & 1)*img_width_bytes;
stbi_uc *dest = a->out + stride*j;
int nk = width * filter_bytes;
int filter = *raw++;

if (filter > 4)
return stbi__err("invalid filter","Corrupt PNG");

if (depth < 8) {
if (img_width_bytes > x) return stbi__err("invalid width","Corrupt PNG");
cur += x*out_n - img_width_bytes; // store output to the rightmost img_len bytes, so we can decode in place
filter_bytes = 1;
width = img_width_bytes;
// check filter type
if (filter > 4) {
all_ok = stbi__err("invalid filter","Corrupt PNG");
break;
}
prior = cur - stride; // bugfix: need to compute this after 'cur +=' computation above

// if first row, use special filter that doesn't sample previous row
if (j == 0) filter = first_row_filter[filter];

// handle first byte explicitly
for (k=0; k < filter_bytes; ++k) {
switch (filter) {
case STBI__F_none : cur[k] = raw[k]; break;
case STBI__F_sub : cur[k] = raw[k]; break;
case STBI__F_up : cur[k] = STBI__BYTECAST(raw[k] + prior[k]); break;
case STBI__F_avg : cur[k] = STBI__BYTECAST(raw[k] + (prior[k]>>1)); break;
case STBI__F_paeth : cur[k] = STBI__BYTECAST(raw[k] + prior[k]); break; // prior[k] == stbi__paeth(0,prior[k],0)
case STBI__F_avg_first : cur[k] = raw[k]; break;
}
}

if (depth == 8) {
if (img_n != out_n)
cur[img_n] = 255; // first pixel
raw += img_n;
cur += out_n;
prior += out_n;
} else if (depth == 16) {
if (img_n != out_n) {
cur[filter_bytes] = 255; // first pixel top byte
cur[filter_bytes+1] = 255; // first pixel bottom byte
}
raw += filter_bytes;
cur += output_bytes;
prior += output_bytes;
} else {
raw += 1;
cur += 1;
prior += 1;
// perform actual filtering
switch (filter) {
case STBI__F_none:
memcpy(cur, raw, nk);
break;
case STBI__F_sub:
memcpy(cur, raw, filter_bytes);
for (k = filter_bytes; k < nk; ++k)
cur[k] = STBI__BYTECAST(raw[k] + cur[k-filter_bytes]);
break;
case STBI__F_up:
for (k = 0; k < nk; ++k)
cur[k] = STBI__BYTECAST(raw[k] + prior[k]);
break;
case STBI__F_avg:
for (k = 0; k < filter_bytes; ++k)
cur[k] = STBI__BYTECAST(raw[k] + (prior[k]>>1));
for (k = filter_bytes; k < nk; ++k)
cur[k] = STBI__BYTECAST(raw[k] + ((prior[k] + cur[k-filter_bytes])>>1));
break;
case STBI__F_paeth:
for (k = 0; k < filter_bytes; ++k)
cur[k] = STBI__BYTECAST(raw[k] + prior[k]); // prior[k] == stbi__paeth(0,prior[k],0)
for (k = filter_bytes; k < nk; ++k)
cur[k] = STBI__BYTECAST(raw[k] + stbi__paeth(cur[k-filter_bytes], prior[k], prior[k-filter_bytes]));
break;
case STBI__F_avg_first:
memcpy(cur, raw, filter_bytes);
for (k = filter_bytes; k < nk; ++k)
cur[k] = STBI__BYTECAST(raw[k] + (cur[k-filter_bytes] >> 1));
break;
}

// this is a little gross, so that we don't switch per-pixel or per-component
if (depth < 8 || img_n == out_n) {
int nk = (width - 1)*filter_bytes;
#define STBI__CASE(f) \
case f: \
for (k=0; k < nk; ++k)
switch (filter) {
// "none" filter turns into a memcpy here; make that explicit.
case STBI__F_none: memcpy(cur, raw, nk); break;
STBI__CASE(STBI__F_sub) { cur[k] = STBI__BYTECAST(raw[k] + cur[k-filter_bytes]); } break;
STBI__CASE(STBI__F_up) { cur[k] = STBI__BYTECAST(raw[k] + prior[k]); } break;
STBI__CASE(STBI__F_avg) { cur[k] = STBI__BYTECAST(raw[k] + ((prior[k] + cur[k-filter_bytes])>>1)); } break;
STBI__CASE(STBI__F_paeth) { cur[k] = STBI__BYTECAST(raw[k] + stbi__paeth(cur[k-filter_bytes],prior[k],prior[k-filter_bytes])); } break;
STBI__CASE(STBI__F_avg_first) { cur[k] = STBI__BYTECAST(raw[k] + (cur[k-filter_bytes] >> 1)); } break;
}
#undef STBI__CASE
raw += nk;
} else {
STBI_ASSERT(img_n+1 == out_n);
#define STBI__CASE(f) \
case f: \
for (i=x-1; i >= 1; --i, cur[filter_bytes]=255,raw+=filter_bytes,cur+=output_bytes,prior+=output_bytes) \
for (k=0; k < filter_bytes; ++k)
switch (filter) {
STBI__CASE(STBI__F_none) { cur[k] = raw[k]; } break;
STBI__CASE(STBI__F_sub) { cur[k] = STBI__BYTECAST(raw[k] + cur[k- output_bytes]); } break;
STBI__CASE(STBI__F_up) { cur[k] = STBI__BYTECAST(raw[k] + prior[k]); } break;
STBI__CASE(STBI__F_avg) { cur[k] = STBI__BYTECAST(raw[k] + ((prior[k] + cur[k- output_bytes])>>1)); } break;
STBI__CASE(STBI__F_paeth) { cur[k] = STBI__BYTECAST(raw[k] + stbi__paeth(cur[k- output_bytes],prior[k],prior[k- output_bytes])); } break;
STBI__CASE(STBI__F_avg_first) { cur[k] = STBI__BYTECAST(raw[k] + (cur[k- output_bytes] >> 1)); } break;
}
#undef STBI__CASE

// the loop above sets the high byte of the pixels' alpha, but for
// 16 bit png files we also need the low byte set. we'll do that here.
if (depth == 16) {
cur = a->out + stride*j; // start at the beginning of the row again
for (i=0; i < x; ++i,cur+=output_bytes) {
cur[filter_bytes+1] = 255;
}
}
}
}
raw += nk;

// we make a separate pass to expand bits to pixels; for performance,
// this could run two scanlines behind the above code, so it won't
// intefere with filtering but will still be in the cache.
if (depth < 8) {
for (j=0; j < y; ++j) {
stbi_uc *cur = a->out + stride*j;
stbi_uc *in = a->out + stride*j + x*out_n - img_width_bytes;
// unpack 1/2/4-bit into a 8-bit buffer. allows us to keep the common 8-bit path optimal at minimal cost for 1/2/4-bit
// png guarante byte alignment, if width is not multiple of 8/4/2 we'll decode dummy trailing data that will be skipped in the later loop
// expand decoded bits in cur to dest, also adding an extra alpha channel if desired
if (depth < 8) {
stbi_uc scale = (color == 0) ? stbi__depth_scale_table[depth] : 1; // scale grayscale values to 0..255 range
stbi_uc *in = cur;
stbi_uc *out = dest;
stbi_uc inb = 0;
stbi__uint32 nsmp = x*img_n;

// note that the final byte might overshoot and write more data than desired.
// we can allocate enough data that this never writes out of memory, but it
// could also overwrite the next scanline. can it overwrite non-empty data
// on the next scanline? yes, consider 1-pixel-wide scanlines with 1-bit-per-pixel.
// so we need to explicitly clamp the final ones

// expand bits to bytes first
if (depth == 4) {
for (k=x*img_n; k >= 2; k-=2, ++in) {
*cur++ = scale * ((*in >> 4) );
*cur++ = scale * ((*in ) & 0x0f);
for (i=0; i < nsmp; ++i) {
if ((i & 1) == 0) inb = *in++;
*out++ = scale * (inb >> 4);
inb <<= 4;
}
if (k > 0) *cur++ = scale * ((*in >> 4) );
} else if (depth == 2) {
for (k=x*img_n; k >= 4; k-=4, ++in) {
*cur++ = scale * ((*in >> 6) );
*cur++ = scale * ((*in >> 4) & 0x03);
*cur++ = scale * ((*in >> 2) & 0x03);
*cur++ = scale * ((*in ) & 0x03);
for (i=0; i < nsmp; ++i) {
if ((i & 3) == 0) inb = *in++;
*out++ = scale * (inb >> 6);
inb <<= 2;
}
if (k > 0) *cur++ = scale * ((*in >> 6) );
if (k > 1) *cur++ = scale * ((*in >> 4) & 0x03);
if (k > 2) *cur++ = scale * ((*in >> 2) & 0x03);
} else if (depth == 1) {
for (k=x*img_n; k >= 8; k-=8, ++in) {
*cur++ = scale * ((*in >> 7) );
*cur++ = scale * ((*in >> 6) & 0x01);
*cur++ = scale * ((*in >> 5) & 0x01);
*cur++ = scale * ((*in >> 4) & 0x01);
*cur++ = scale * ((*in >> 3) & 0x01);
*cur++ = scale * ((*in >> 2) & 0x01);
*cur++ = scale * ((*in >> 1) & 0x01);
*cur++ = scale * ((*in ) & 0x01);
} else {
STBI_ASSERT(depth == 1);
for (i=0; i < nsmp; ++i) {
if ((i & 7) == 0) inb = *in++;
*out++ = scale * (inb >> 7);
inb <<= 1;
}
if (k > 0) *cur++ = scale * ((*in >> 7) );
if (k > 1) *cur++ = scale * ((*in >> 6) & 0x01);
if (k > 2) *cur++ = scale * ((*in >> 5) & 0x01);
if (k > 3) *cur++ = scale * ((*in >> 4) & 0x01);
if (k > 4) *cur++ = scale * ((*in >> 3) & 0x01);
if (k > 5) *cur++ = scale * ((*in >> 2) & 0x01);
if (k > 6) *cur++ = scale * ((*in >> 1) & 0x01);
}
if (img_n != out_n) {
int q;
// insert alpha = 255
cur = a->out + stride*j;

// insert alpha=255 values if desired
if (img_n != out_n)
stbi__create_png_alpha_expand8(dest, dest, x, img_n);
} else if (depth == 8) {
if (img_n == out_n)
memcpy(dest, cur, x*img_n);
else
stbi__create_png_alpha_expand8(dest, cur, x, img_n);
} else if (depth == 16) {
// convert the image data from big-endian to platform-native
stbi__uint16 *dest16 = (stbi__uint16*)dest;
stbi__uint32 nsmp = x*img_n;

if (img_n == out_n) {
for (i = 0; i < nsmp; ++i, ++dest16, cur += 2)
*dest16 = (cur[0] << 8) | cur[1];
} else {
STBI_ASSERT(img_n+1 == out_n);
if (img_n == 1) {
for (q=x-1; q >= 0; --q) {
cur[q*2+1] = 255;
cur[q*2+0] = cur[q];
for (i = 0; i < x; ++i, dest16 += 2, cur += 2) {
dest16[0] = (cur[0] << 8) | cur[1];
dest16[1] = 0xffff;
}
} else {
STBI_ASSERT(img_n == 3);
for (q=x-1; q >= 0; --q) {
cur[q*4+3] = 255;
cur[q*4+2] = cur[q*3+2];
cur[q*4+1] = cur[q*3+1];
cur[q*4+0] = cur[q*3+0];
for (i = 0; i < x; ++i, dest16 += 4, cur += 6) {
dest16[0] = (cur[0] << 8) | cur[1];
dest16[1] = (cur[2] << 8) | cur[3];
dest16[2] = (cur[4] << 8) | cur[5];
dest16[3] = 0xffff;
}
}
}
}
} else if (depth == 16) {
// force the image data from big-endian to platform-native.
// this is done in a separate pass due to the decoding relying
// on the data being untouched, but could probably be done
// per-line during decode if care is taken.
stbi_uc *cur = a->out;
stbi__uint16 *cur16 = (stbi__uint16*)cur;

for(i=0; i < x*y*out_n; ++i,cur16++,cur+=2) {
*cur16 = (cur[0] << 8) | cur[1];
}
}

STBI_FREE(filter_buf);
if (!all_ok) return 0;

return 1;
}

Expand Down

0 comments on commit 4da08a1

Please sign in to comment.