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stb_image: header scan mode always needs to check for tRNS 

For paletted images, header-scanning mode (used by stbi_info) kept
going after the image header to see if a tRNS (transparency) chunk
shows up to correctly determine the channel count, but we would
immediately return after IHDR for non-paletted images.

This is incorrect. We also change our reported channel count on
RGB images with a tRNS chunk, therefore non-paletted images also
have to resume scanning further chunks.

Update the logic to keep scanning regardless and report the
correct channel count from stbi_info for RGB images with tRNS
(constant alpha channel).

Fixes issue #1419.
This commit is contained in:
Fabian Giesen 2023-01-23 00:46:33 -08:00
parent 24fa1ff2e9
commit 40eb865b3b
1 changed files with 9 additions and 3 deletions
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12
stb_image.h
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@ -5115,14 +5115,13 @@ static int stbi__parse_png_file(stbi__png *z, int scan, int req_comp)
if (!pal_img_n) {
s->img_n = (color & 2 ? 3 : 1) + (color & 4 ? 1 : 0);
if ((1 << 30) / s->img_x / s->img_n < s->img_y) return stbi__err("too large", "Image too large to decode");
if (scan == STBI__SCAN_header) return 1;
} else {
// if paletted, then pal_n is our final components, and
// img_n is # components to decompress/filter.
s->img_n = 1;
if ((1 << 30) / s->img_x / 4 < s->img_y) return stbi__err("too large","Corrupt PNG");
// if SCAN_header, have to scan to see if we have a tRNS
}
// even with SCAN_header, have to scan to see if we have a tRNS
break;
}
@ -5154,6 +5153,8 @@ static int stbi__parse_png_file(stbi__png *z, int scan, int req_comp)
if (!(s->img_n & 1)) return stbi__err("tRNS with alpha","Corrupt PNG");
if (c.length != (stbi__uint32) s->img_n*2) return stbi__err("bad tRNS len","Corrupt PNG");
has_trans = 1;
// non-paletted with tRNS = constant alpha. if header-scanning, we can stop now.
if (scan == STBI__SCAN_header) { ++s->img_n; return 1; }
if (z->depth == 16) {
for (k = 0; k < s->img_n; ++k) tc16[k] = (stbi__uint16)stbi__get16be(s); // copy the values as-is
} else {
@ -5166,7 +5167,12 @@ static int stbi__parse_png_file(stbi__png *z, int scan, int req_comp)
case STBI__PNG_TYPE('I','D','A','T'): {
if (first) return stbi__err("first not IHDR", "Corrupt PNG");
if (pal_img_n && !pal_len) return stbi__err("no PLTE","Corrupt PNG");
if (scan == STBI__SCAN_header) { s->img_n = pal_img_n; return 1; }
if (scan == STBI__SCAN_header) {
// header scan definitely stops at first IDAT
if (pal_img_n)
s->img_n = pal_img_n;
return 1;
}
if (c.length > (1u << 30)) return stbi__err("IDAT size limit", "IDAT section larger than 2^30 bytes");
if ((int)(ioff + c.length) < (int)ioff) return 0;
if (ioff + c.length > idata_limit) {