Re: [mpeg2-dev] How to reduce Intra block DC bits prior to IDCT

[Keith W. <ke...@mi...>]

Hi pebble,

I'm not sure exactly where you're getting the [0, 32768] (the DC
component can be negative), but I think you may probably barking up
the wrong tree. IEEE 1180 only defines the IDCT statistically, so
different MPEG-2 decoders can produce slightly different output.
libmpeg2 may even produce slightly different output depending on
whether you're using the C IDCT, MMX, SSE2, etc. The precision is
probably not the issue; both libmpeg2 and the reference decoder use
the precision specified in the coded bitstream for each component.

You might find that switching the IDCT in use (look at
mpeg2_idct_init() in idct.c, or modify cpu_accel.c) gives you
something closer to the results of the reference decoder. In the end,
though, these are all conforming decoders. Trying to get one to match
the other _exactly_ may require swapping out one IDCT for another and
tracking exactly where the divergence occurs.

Good luck!

Best,
Keith

On Wed, Aug 3, 2011 at 11:03 PM, pebble <sod...@ya...> wrote:
>
> Hi there,
>
> I need to hook into libmpeg2 so that I may do some processing on mpeg2 bitstream level. I have a working algorithm developed with the reference decoder (mssg's mpeg2v12.zip), but I would like to try on libmpeg2 because of its very fast decoding speed.
>
> To decode the same mpeg2 video file, I have tried the reference decoder and libmpeg2's mpegdec. I found that both decoders have produced nonidentical frames (a few pixels at various position gets different values). Furthermore, by checking the IDCT process, I found that, for Intra block, DC value in DCT coefficient matrix, the reference decoder (getpic.c) is using signed 11 bits [-1024, +1024] but libmpeg2 (slice.c) is using unsigned 15 bits [0, 32768].
>
> I thought this could be one of the reasons why the decoded frames are nonidentical.
>
> Why libmpeg2 needs to use a larger bit size ? Is there any way to configure libmpeg2 to use signed 11 bits for Intra block DC value, so that its behavior could become a bit closer to that of reference decoder?
>
> Thanks.
>
> Best Regards,
>
> pebble
>
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