[FFmpeg-devel] [PATCH 1/5] mlpenc: fix lossless check error in number_sbits
we need two bits instead of one bit to represent -1 in bitstream
Signed-off-by: Jai Luthra
---
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++;
return av_log2(FFABS(number)) + 1 + !!number;
--
2.25.0
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Re: [FFmpeg-devel] [PATCH 1/5] mlpenc: fix lossless check error in number_sbits
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
---
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
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
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Re: [FFmpeg-devel] [PATCH 1/5] mlpenc: fix lossless check error in number_sbits
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
> ---
> 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?
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[FFmpeg-devel] [PATCH 1/5] mlpenc: fix lossless check error in number_sbits
we need two bits instead of one bit to represent -1 in bitstream
Signed-off-by: Jai Luthra
---
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++;
return av_log2(FFABS(number)) + 1 + !!number;
--
2.22.0
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