mirror of
https://github.com/freemkv/libfreemkv.git
synced 2026-10-07 18:13:10 +00:00
Matthew Jackson
6e14fc0c0c
Security - Remove the unsafe Vec::set_len over recycled sector buffers — UB reachable from safe code (GHSA-j8ww-f5fg-9pmh, low). Uses Vec::resize; pooling unchanged. CI / tooling - clippy on stable with the MSRV single-sourced from Cargo.toml; cargo-deny dependency-audit gate with first-party git sources allowed (#15); audience-based comment-guard (#17). Docs - README (keydb ownership, BD-J parser count), drive-access transport/reset notes, mux timeline module-doc links.
76 lines
3.4 KiB
Markdown
76 lines
3.4 KiB
Markdown
# VC-1 elementary stream parser — internal notes
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Relocated prose from `src/mux/codec/vc1.rs` internal (non-pub) items, kept
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here in full since it exceeds the 3-line internal-comment cap.
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## `parse_vc1_interlace`
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Read the advanced-profile sequence header's `INTERLACE` flag (SMPTE 421M
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§6.1.1): bit 41 after the start code — after PROFILE(2) LEVEL(3)
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COLORDIFF_FORMAT(2) FRMRTQ(3) BITRTQ(5) POSTPROCFLAG(1) MAX_CODED_WIDTH(12)
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MAX_CODED_HEIGHT(12) PULLDOWN(1). `None` for simple/main profile or a header
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too short / over-escaped to reach the bit. De-escapes emulation-prevention
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bytes first (as `parse_vc1_resolution` does) so the bit offset is exact.
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## `vc1_progressive_ptype`
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Decode the advanced-profile **progressive** picture PTYPE VLC (SMPTE 421M
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§7.1.1.4, Table): `0`=P, `10`=B, `110`=I, `1110`=BI (intra → I), `1111`=
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Skipped (predicted, no residual → P). Only valid when the sequence is
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progressive — for interlaced an FCM code (and, for field pictures, a combined
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FPTYPE) precedes/replaces PTYPE, so the caller declines those.
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## `vc1_frame_coding_type`
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Measure the coding type of an advanced-profile frame from its picture header.
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`frame_rbsp` starts immediately after the frame start code (`00 00 01 0D`).
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Decodes PTYPE only for a PROGRESSIVE sequence (where PTYPE is the first
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picture-layer field); declines (`None`) for interlaced/simple-main/unknown
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rather than guess at the wrong bit offset.
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## `handle_header`
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Handle a seq_header or entry_point start-code unit (Annex B raw bytes).
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Decision is against the currently-ACTIVE body `cur`, not the codecPrivate
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copy `first`:
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- First of its type → seeds codecPrivate; stripped (decoder gets it from
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the BITMAPINFOHEADER extra data at init).
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- Equal to the active set `cur` → redundant; stripped.
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- Different from `cur` (a change in EITHER direction, including reverting
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to the codecPrivate/first value) → prepended into `prefix` in Annex B
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form and `cur` updated.
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Returns `true` when the unit was emitted into `prefix`.
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## `parse_vc1_resolution`
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Parse width and height from a VC-1 advanced profile sequence header.
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The sequence header starts with 00 00 01 0F. After the start code:
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byte 0 bits 7-6: profile (3 = advanced). Advanced profile seq-header
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layout (SMPTE 421M, from sh[4]): PROFILE(2)+LEVEL(3)+COLORDIFF_FORMAT(2)+
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FRMRTQ_POSTPROC(3)+BITRTQ_POSTPROC(5)+POSTPROCFLAG(1)=16 bits, then
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MAX_CODED_WIDTH(12)+HEIGHT(12) = 40 bits total = 5 bytes.
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## `make_ap_seq_header` (test helper)
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Build an advanced-profile VC-1 sequence header encoding the given
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width/height. Layout from sh[4]: PROFILE(2)=3, LEVEL(3), COLORDIFF(2),
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FRMRTQ(3), BITRTQ(5), POSTPROCFLAG(1) = 16 bits, then
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MAX_CODED_WIDTH(12) = width/2 - 1, MAX_CODED_HEIGHT(12) = height/2 - 1.
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## `vc1_emits_entry_point_revert_to_first_value` (test)
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Regression: entry_point is redefined from A (== codecPrivate) to B, then
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switched BACK to A. A streaming decoder applied codecPrivate at init and
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is now on B; the revert to A must be emitted IN-BAND even though A ==
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codecPrivate, or the A-segment decodes against the wrong entry point.
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## `vc1_keyframe_prefix_order_seq_unchanged_entry_redefined` (test)
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Regression: keyframe where seq_header is UNCHANGED (stripped by scan) but
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entry_point is REDEFINED (changed). Before the fix, the old code appended
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entry_point during the scan, then reassert() appended seq_header AFTER it,
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producing [entry_point, seq_header] — entry_point before seq_header,
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violating SMPTE 421M. After the fix, assembly is always seq-then-entry.
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