Research 0121: MCP `compute_vmaf` high-bit-depth input support¶

Question¶

Can the embedded MCP compute_vmaf tool close its documented 10/12-bit YUV gap without widening the whole tool schema to every libvmaf pixel format?

Findings¶

compute_vmaf.c already owns a narrow plain-file reader for YUV420p. It hardcoded byte count, picture allocation, and row reads to 8 bpc.
vmaf_picture_alloc() supports bpc in [8, 16] and doubles the row stride for bpc > 8.
The raw YUV convention used elsewhere in this fork stores high-bit planar samples as little-endian 16-bit words. Reading width * sample_bytes into each plane row matches libvmaf's picture storage on the supported little-endian CI/runtime hosts.
YUV422P/YUV444P need a public pixel_format argument and schema update. Adding them silently would change the MCP tool contract more broadly than the backlog item requires.

Alternatives considered¶

Option	Pros	Cons	Decision
Add optional `bitdepth` only for YUV420p	Closes the HDR/CHUG-adjacent 10/12/16-bit gap; tiny schema change; keeps existing default behaviour	Still no 4:2:2 / 4:4:4	Chosen
Add `pixel_format` plus all planar layouts	Fuller parity with the CLI	Larger schema, more tests/docs, and more room for ambiguous numeric pixfmt names	Deferred
Keep 8-bit only	No code change	Leaves the documented v4 roadmap gap open and blocks high-bit-depth MCP scoring	Rejected

Decision¶

Add an optional integer bitdepth argument to compute_vmaf, accepting 8, 10, 12, or 16 and defaulting to 8. The tool continues to accept YUV420p only. The result JSON echoes bitdepth so clients can audit which path ran.

Validation¶

meson setup core/build-mcp-hbd libvmaf \
  -Denable_mcp=true -Denable_mcp_stdio=true \
  -Denable_mcp_uds=true -Denable_mcp_sse=enabled
meson test -C core/build-mcp-hbd test_mcp_smoke --print-errorlogs

References¶

req: user asked to keep taking the next backlog/gap item after the docs batch.
docs/mcp/embedded.md v4 roadmap row for high-bit-depth compute_vmaf.

Research 0121: MCP compute_vmaf high-bit-depth input support¶