On Wed, Jul 10, 2019 at 12:14:56AM +0200, Lynne wrote:
Jul 9, 2019, 9:18 PM by m...@jailuthra.in:we need two bits instead of one bit to represent -1 in bitstream Signed-off-by: Jai Luthra <m...@jailuthra.in> --- libavcodec/mlpenc.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/libavcodec/mlpenc.c b/libavcodec/mlpenc.c index deb171645c..f4948451f1 100644 --- a/libavcodec/mlpenc.c +++ b/libavcodec/mlpenc.c @@ -466,7 +466,7 @@ static void default_decoding_params(MLPEncodeContext *ctx, */ static int inline number_sbits(int number) { - if (number < 0) + if (number < -1) number++;This is different from the first patch's version. Sure its correct now?
Yep. Previous patch produced valid bitstream too, but this provides better compression [1] by representing numbers of the form -2^x with one less bit for x >= 1.
This makes more sense, as we can represent -2 in two-bit twos-complement notation as `10` so output should be 2 bits (instead of 3 by previous patch). (similarly for -4, -8, -16, ...)
The lossless errors were being caused when a block of samples were all either -1 or 0. This function implied all samples could be represented as single bit each, but down the pipeline after huff vlc calculations, the encoder pushed <nothing> on the bitstream for all samples, which was always interpreted as 0 by the decoder and never -1.
NB: One can argue -1 and 0 in fact can be represented in a single bit as two's complement, 0 being `0` and -1 being `1`. But imho, single bit two's complement is a weird boundary case, and not considering it solves the issue here. If someone has a better idea pls suggest.
[1]: tested using both patches on https://samples.ffmpeg.org/flac/When%20I%20Grow%20Up.flac. Previous patch compressed it to 28788216 byte MLP stream, this one compresses it to 28787834 byte MLP stream. Both streams are valid and decode to lossless bit-exact output.
thx -- jlut _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
