mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-18 14:26:43 +00:00
41 lines
1.9 KiB
Text
41 lines
1.9 KiB
Text
|
Basic parsing method
|
||
|
|
====================
|
||
|
|
|
||
|
|
In an MPEG-2 Program Stream, every chunk of data starts with at least 23
|
||
|
|
zeros and a one. This is followed by a byte of ID. At any given point I
|
||
|
|
can search for the next 3-byte string equal to 1 to find the next chunk.
|
||
|
|
I assume these start codes are aligned on byte boundaries. I might be
|
||
|
|
wrong, in which case this thing has to be rewritten at some point in the
|
||
|
|
future.
|
||
|
|
|
||
|
|
This means there are two basic modes of operation. The first is the
|
||
|
|
simple search for the next start code. The second is a continuation mode,
|
||
|
|
where data from the previous buffer is available to attempt to complete a
|
||
|
|
chunk. Hopefully the majority of time will be spent in the first mode, as
|
||
|
|
that is where the most efficiency is, since there's no copying of partial
|
||
|
|
chunks.
|
||
|
|
|
||
|
|
The parsing is done as a state machine, as long as there's data left in
|
||
|
|
the buffer, something is attempted. What is attempted is based on the
|
||
|
|
state of the parser (gee, so that's why they call it a state machine <g>).
|
||
|
|
The stages are:
|
||
|
|
|
||
|
|
1) looking for sync (have_sync == FALSE)
|
||
|
|
a) have some zeros (zeros > 0)
|
||
|
|
2) getting ID (have_sync == TRUE, id == 0)
|
||
|
|
3) decoding the chunk contents (have_sync == TRUE, id != 0)
|
||
|
|
|
||
|
|
Mechanism for handling cross-buffer chunks of data
|
||
|
|
==================================================
|
||
|
|
|
||
|
|
The problem: if data were to come to the parser in 64-byte chunks, the
|
||
|
|
pack head would be split across at least two buffers, possibly three. Up
|
||
|
|
front I will make the assumption that no one will be sending buffers of
|
||
|
|
less size than the largest chunk (header, PES packet), such that no chunk
|
||
|
|
will be split across more than two buffers.
|
||
|
|
|
||
|
|
If we take the pack header as an example, when the stream starts out, we
|
||
|
|
can assume that the it starts at the beginning of the buffer and doesn't
|
||
|
|
exceed the bounds of it. However, if we're mid-stream and starting
|
||
|
|
another pack, it can be split across two buffers.
|