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WebP/Lossy: Clamp right after summing IDCT output, instead of later
https://datatracker.ietf.org/doc/html/rfc6386#section-14.5 says: """ The summing procedure is fairly straightforward, having only a couple of details. The prediction and residue buffers are both arrays of 16-bit signed integers. Each individual (Y, U, and V pixel) result is calculated first as a 32-bit sum of the prediction and residue, and is then saturated to 8-bit unsigned range (using, say, the clamp255 function defined above) before being stored as an 8-bit unsigned pixel value. """ It's IMHO not 100% clear if the clamping is supposed to happen immediately (so that it affects prediction inputs for the next macroblock) or later. But vp8_dixie_idct_add() on page 173 in https://datatracker.ietf.org/doc/html/rfc6386#section-20.8 does: recon[0] = CLAMP_255(predict[0] + ((a1 + d1 + 4) >> 3)); So it does look like it should happen immediately. (I'm a bit confused why the spec then says "The prediction and residue buffers are both arrays of 16-bit signed integers", since the prediction buffer can just be an u8 buffer now, without changing behavior.
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1 changed files with 4 additions and 4 deletions
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@ -1051,7 +1051,7 @@ void add_idct_to_prediction(Span<i16> prediction, Coefficients coefficients, int
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for (int py = 0; py < 4; ++py) {
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for (int px = 0; px < 4; ++px) {
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i16& p = prediction[(4 * y + py) * N + (4 * x + px)];
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p += idct_output[py * 4 + px];
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p = clamp(p + idct_output[py * 4 + px], 0, 255);
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}
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}
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}
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@ -1123,11 +1123,11 @@ void convert_yuv_to_rgb(Bitmap& bitmap, int mb_x, int mb_y, ReadonlySpan<i16> y_
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// "is then saturated to 8-bit unsigned range (using, say, the
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// clamp255 function defined above) before being stored as an 8-bit
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// unsigned pixel value."
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u8 Y = clamp(y_data[y * 16 + x], 0, 255);
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u8 Y = y_data[y * 16 + x];
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// FIXME: Could do nicer upsampling than just nearest neighbor
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u8 U = clamp(u_data[(y / 2) * 8 + x / 2], 0, 255);
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u8 V = clamp(v_data[(y / 2) * 8 + x / 2], 0, 255);
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u8 U = u_data[(y / 2) * 8 + x / 2];
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u8 V = v_data[(y / 2) * 8 + x / 2];
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// XXX: These numbers are from the fixed-point values in libwebp's yuv.h. There's probably a better reference somewhere.
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int r = 1.1655 * Y + 1.596 * V - 222.4;
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