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speedhq: fix decoding artifacts

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The quantization table is stored in the natural order, but when we
access it, we use an index that's in zigzag order, causing us to read
the wrong value. This causes artifacts, especially in areas with
horizontal or vertical edges. The artifacts look a lot like the
DCT ringing artifacts you'd expect to see from a low-bitrate file,
but when comparing to NewTek's own decoder, it's obvious they're not
supposed to be there.

Fix by simply storing the scaled quantization table in zigzag order.
Performance is unchanged.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
This commit is contained in:
Steinar H. Gunderson 2017-02-18 19:41:02 +01:00 committed by Michael Niedermayer
parent ed69cb83f8
commit e3c14eaa54
1 changed files with 1 additions and 1 deletions
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2
libavcodec/speedhq.c
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@ -409,7 +409,7 @@ static int decode_speedhq_field(const SHQContext *s, const uint8_t *buf, int buf
static void compute_quant_matrix(int *output, int qscale)
{
int i;
for (i = 0; i < 64; i++) output[i] = unscaled_quant_matrix[i] * qscale;
for (i = 0; i < 64; i++) output[i] = unscaled_quant_matrix[ff_zigzag_direct[i]] * qscale;
}
static int speedhq_decode_frame(AVCodecContext *avctx,