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

[pebble <sod...@ya...>]

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