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--- claude:dev_process [2026/05/02 15:50] – add Phase 3 'measurements describe what the system does' + Phase 6 'contract before code' subsections markus_fritsche
+++ claude:dev_process [2026/05/02 15:57] (current) – Phase 6: add worked example for 'state the contract explicitly' (patch 0012, h264 SCALING_MATRIX) markus_fritsche
@@ Line 49: / Line 49: @@
 **Copying from baseline measurements is not implementation. It is transcription of potentially broken behaviour.** A deliverable that matches baseline bytes but violates the API contract is not a deliverable — it is a deferred bug.
+==== What "state the contract explicitly" looks like ====
+Worked example: ''0012-h264-omit-scaling-matrix-frame-based.patch'' in ''~/src/ohm_gl_fix/phase6/step1/''. The commit message opens with the contract before any code:
+<code>
+VAAPI signals "explicit scaling lists are present in the bitstream"
+implicitly: the consumer (ffmpeg-vaapi, mpv, etc.) sends a
+VAIQMatrixBufferH264 alongside RenderPicture iff
+sps_scaling_matrix_present_flag || pps_scaling_matrix_present_flag.
+When the bitstream uses default (flat) scaling, no IQMatrixBuffer
+arrives [...]
+Earlier draft of this patch unconditionally omitted SCALING_MATRIX in
+FRAME_BASED. That's corpus-correct (bbb has no explicit scaling
+lists) but the wrong predicate: the kernel-side gating is by
+"matrix-supplied vs. not," not by decode mode. Streams that signal
+explicit scaling lists must submit SCALING_MATRIX in either mode.
+Contract verification (audit_0008_decode_params_2026-05-01.md +
+hantro_h264.c::assemble_scaling_list): the kernel uses the supplied
+matrix when SCALING_MATRIX is in the control batch and falls back
+to spec-defined defaults when absent. Mode-independent.
+</code>
+What this gets right:
+  * **Contract first** — per-control rules cited from kernel doc (''ext-ctrls-codec-stateless.rst:752''), kernel driver (''hantro_h264.c::assemble_scaling_list''), and sibling implementation (gst-plugins-bad commit 9e3e775) //before// any patch hunks.
+  * **Corpus-correct ≠ spec-correct, called out by name** — the rejected predicate ("omit SCALING_MATRIX in FRAME_BASED") //did// match the BBB baseline. It still got rejected, because the contract said the gate is "matrix-supplied vs. not," not "decode mode." This is exactly the Phase 3-derived-implementation trap.
+  * **Then** the diff implements one branch per contract clause: SPS/PPS/DECODE_PARAMS always, SCALING_MATRIX iff ''matrix_set'', SLICE_PARAMS iff SLICE_BASED, PRED_WEIGHTS iff SLICE_BASED + ''V4L2_H264_CTRL_PRED_WEIGHTS_REQUIRED''.
+Mirror format anywhere reviewable: PR description, commit message body, plan section, or a header comment block. The shape is //contract clauses with citations → code that maps 1:1 to those clauses//.
 ===== Phase 7 — Verification Measurements =====