> Then I do not see how you can make a "generic" parser without
> knowing in advance whether the start code is 3 or 4 bytes.
No, it’s actually quite easy. If you know that the start code is either 0x00
0x00 0x01 or 0x00 0x00 0x00 0x01, then the following code (taken from our
H.264/5 stream parser, used in “H264or5VideoStreamFramer”) does the trick:
while (next4Bytes != 0x00000001 && (next4Bytes&0xFFFFFF00) != 0x00000100)
{
// We save at least some of "next4Bytes".
if ((unsigned)(next4Bytes&0xFF) > 1) {
// Common case: 0x00000001 or 0x000001 definitely doesn't begin
anywhere in "next4Bytes", so we save all of it:
save4Bytes(next4Bytes);
skipBytes(4);
} else {
// Save the first byte, and continue testing the rest:
saveByte(next4Bytes>>24);
skipBytes(1);
}
setParseState(); // ensures forward progress
next4Bytes = test4Bytes();
}
// Assert: next4Bytes starts with 0x00000001 or 0x000001, and we've saved
all previous bytes (forming a complete NAL unit).
// Skip over these remaining bytes, up until the start of the next NAL
unit:
if (next4Bytes == 0x00000001) {
skipBytes(4);
} else {
skipBytes(3);
}
}
Ross Finlayson
Live Networks, Inc.
http://www.live555.com/
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