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WebP/Lossy: Add function for inverse 4x4 DCT from spec

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Nico Weber 2023-05-29 12:59:16 -04:00 • committed by Andreas Kling
parent 8bb6a820e2
commit fe6960286c
1 changed files with 55 additions and 0 deletions
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55
Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossyTables.h
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@ -674,4 +674,59 @@ inline void vp8_short_inv_walsh4x4_c(i16* input, i16* output)
}
}
// https://datatracker.ietf.org/doc/html/rfc6386#section-14.4 "Implementation of the DCT Inversion"
inline void short_idct4x4llm_c(i16* input, i16* output, int pitch)
{
static constexpr int cospi8sqrt2minus1 = 20091;
static constexpr int sinpi8sqrt2 = 35468;
i16* ip = input;
i16* op = output;
int shortpitch = pitch >> 1;
for (int i = 0; i < 4; i++) {
int a1 = ip[0] + ip[8];
int b1 = ip[0] - ip[8];
int temp1 = (ip[4] * sinpi8sqrt2) >> 16;
int temp2 = ip[12] + ((ip[12] * cospi8sqrt2minus1) >> 16);
int c1 = temp1 - temp2;
temp1 = ip[4] + ((ip[4] * cospi8sqrt2minus1) >> 16);
temp2 = (ip[12] * sinpi8sqrt2) >> 16;
int d1 = temp1 + temp2;
op[shortpitch * 0] = a1 + d1;
op[shortpitch * 3] = a1 - d1;
op[shortpitch * 1] = b1 + c1;
op[shortpitch * 2] = b1 - c1;
ip++;
op++;
}
ip = output;
op = output;
for (int i = 0; i < 4; i++) {
int a1 = ip[0] + ip[2];
int b1 = ip[0] - ip[2];
int temp1 = (ip[1] * sinpi8sqrt2) >> 16;
int temp2 = ip[3] + ((ip[3] * cospi8sqrt2minus1) >> 16);
int c1 = temp1 - temp2;
temp1 = ip[1] + ((ip[1] * cospi8sqrt2minus1) >> 16);
temp2 = (ip[3] * sinpi8sqrt2) >> 16;
int d1 = temp1 + temp2;
op[0] = (a1 + d1 + 4) >> 3;
op[3] = (a1 - d1 + 4) >> 3;
op[1] = (b1 + c1 + 4) >> 3;
op[2] = (b1 - c1 + 4) >> 3;
ip += shortpitch;
op += shortpitch;
}
}
}