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stb_image: Small PNG filter simplification

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Paeth on the first scanline is not a separate filter, it
happens to be equivalent to the "sub" filter which is already
an option.
This commit is contained in:
Fabian Giesen 2023-05-01 17:10:45 -07:00 committed by Sean Barrett
parent d373674115
commit e5f0e18d0f
1 changed files with 5 additions and 9 deletions
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14
stb_image.h
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@ -4639,9 +4639,8 @@ enum {
STBI__F_up=2,
STBI__F_avg=3,
STBI__F_paeth=4,
// synthetic filters used for first scanline to avoid needing a dummy row of 0s
STBI__F_avg_first,
STBI__F_paeth_first
// synthetic filter used for first scanline to avoid needing a dummy row of 0s
STBI__F_avg_first
};
static stbi_uc first_row_filter[5] =
@ -4650,7 +4649,7 @@ static stbi_uc first_row_filter[5] =
STBI__F_sub,
STBI__F_none,
STBI__F_avg_first,
STBI__F_paeth_first
STBI__F_sub // Paeth with b=c=0 turns out to be equivalent to sub
};
static int stbi__paeth(int a, int b, int c)
@ -4719,9 +4718,8 @@ static int stbi__create_png_image_raw(stbi__png *a, stbi_uc *raw, stbi__uint32 r
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] + stbi__paeth(0,prior[k],0)); 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;
case STBI__F_paeth_first: cur[k] = raw[k]; break;
}
}
@ -4753,13 +4751,12 @@ static int stbi__create_png_image_raw(stbi__png *a, stbi_uc *raw, stbi__uint32 r
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;
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;
STBI__CASE(STBI__F_paeth_first) { cur[k] = STBI__BYTECAST(raw[k] + stbi__paeth(cur[k-filter_bytes],0,0)); } break;
}
#undef STBI__CASE
raw += nk;
@ -4776,7 +4773,6 @@ static int stbi__create_png_image_raw(stbi__png *a, stbi_uc *raw, stbi__uint32 r
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;
STBI__CASE(STBI__F_paeth_first) { cur[k] = STBI__BYTECAST(raw[k] + stbi__paeth(cur[k- output_bytes],0,0)); } break;
}
#undef STBI__CASE