Hi,
While testing my application that uses Live555 for MPEG2 Video input ("MPV"), I
noticed MPEG1or2VideoRTPSource presents some slice data with the expected size,
but others are presented as individual 1384 bytes chunks, just like at the
network level.
Looking into the code, I see the following fragment which looks suspicious
(MPEG1or2VideoRTPSource::processSpecialHeader() in MPEG1or2VideoRTPSource.cpp,
line 50):
u_int32_t sBit = header&0x00002000; // sequence-header-present
u_int32_t bBit = header&0x00001000; // beginning-of-slice
u_int32_t eBit = header&0x00000800; // end-of-slice
fCurrentPacketBeginsFrame = (sBit|bBit) != 0;
fCurrentPacketCompletesFrame = ((sBit&~bBit)|eBit) != 0;
sBit and bBit are looking at different bits of the header, and as each can only
have the one specific bit set and all other bits are always 0, (sBit&~bBit) is
in fact identical to sBit (*).
I believe the operation should be performed at the logical level rather than
bitwise:
fCurrentPacketCompletesFrame = ((sBit != 0) && (bBit == 0)) || (eBit != 0);
Changing the code as above did allow me to receive slice data correctly (if my
understanding is correct) aggregated according to the S, B and E bits of the
header.
Is the above correct and if so, could this change be applied for a next release?
Thanks,
-David
(*) in sBit, all bits are 0 except - possibly - the 'S' bit; hence the only bit
that could be set in (sBit&~bBit) is the 'S' bit; and in ~bBit, all other bits
than the 'B' bit are always 1, hence the 'S' bit in ~bBit is always 1.
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