vse-cut-list

VSE Cut List

A runnable example that witnesses the sequencer API rename between Blender 4.5 LTS and 5.x — the single most common way AI-generated VSE code dies on a modern Blender. Every tutorial-era snippet does scene.sequence_editor.sequences.new_effect(..., frame_end=...); on 5.x that path is gone twice over (sequences removed, frame_end kwarg rejected), and on 4.5 the modern spelling (strips, length=) is already the only one the new collection accepts.

What it witnesses: a deterministic cut list — three color programs A/B/C, a GAMMA_CROSS fed by a dedicated source pair T1/T2, a scene strip, and a text strip — asserted against closed-form spans on each version's canonical accessors, then re-asserted after a save/reload round-trip:

• Accessor rename — 5.x: sequence_editor.sequences raises AttributeError; only .strips exists. 4.5: both accessors exist and see the same strips (the transition bridge).
• Creation signature — strips.new_effect(...) ends a strip with frame_end= on 4.5 but length= on 5.x; the wrong kwarg raises TypeError on each side (asserted, not assumed). 5.x also rejects the removed TRANSFORM effect type — per-strip strip.transform (StripTransform) places strips in the frame on both versions.
• Frame-range closed form — every strip's (start, end, duration) matches the authored end-exclusive span: frame_final_* on 4.5; left_handle/right_handle/duration on 5.x, where the frame_final_* names are deprecated aliases (removal announced for 6.0) that must still read equal — the bridge is asserted lossless, with the RNA is_deprecated flag checked on 5.x and its absence checked on 4.5.
• Effect wiring — input_1/input_2 (not input1) on both versions, in authored order, and the cross clamped to exactly the T1/T2 overlap.
• Scene strip — 4-arg new_scene(name, scene, channel, frame_start) on both versions (no length/frame_end kwarg); default span is the source scene's frame range; the strip sources a separate Stage scene.
• Round-trip — spans, channels, colors, GC wiring, text, and mosaic transforms (pixel-unit offsets, tol 1e-3) all survive save_as_mainfile → open_mainfile.
• Compositing (--check-pixels) — a 96×54 render asserts each mosaic cell center carries its strip color (tol 0.1), the cross cell is a true mid blend (strictly between the sources on every channel), and a margin point matches neither cross source — because a consumed input compositing independently would paint it full-frame.

What each check catches on failure:

• Accessor — .sequences returning on 5.x, or the 4.5 bridge seeing different contents (falsified: the legacy path on 5.1.1 raises AttributeError: 'SequenceEditor' object has no attribute 'sequences').
• Creation — Blender re-accepting a removed kwarg silently, or the TRANSFORM enum returning (falsified: calling new_effect(frame_end=...) on 5.1.1 raises TypeError: ... expected (name, type, channel, frame_start, length, input1, input2)).
• Spans — end-inclusive vs end-exclusive off-by-one, retiming drift (falsified: expecting GC span (25, 34) exited 5 with measured GC span (25, 33, 8) != closed form (25, 34, 9) — note the cross was clamped to the 8-frame overlap, itself a witnessed behavior).
• Wiring — swapped or dropped cross inputs (falsified: input1=t2, input2=t1 exited 7 with measured inputs=(T2, T1), expected T1 -> T2).
• Round-trip — serialization dropping strip data (falsified: corrupting the text strip before save exited 10 via the reloaded re-assert).
• Pixels — a broken span or cell transform dropping a cell, a frozen cross, or a consumed input leaking back into the composite (found by authoring: GC below T1/T2 let T2 paint the whole wall teal).

Hazards discovered while authoring (all witnessed by the checks above):

• A GAMMA_CROSS asked to outlast its inputs' overlap is silently clamped to the overlap — request length 9 over a (25, 33) overlap, get (25, 33).
• A scene strip pointing at its own scene is a feedback loop and renders transparent (alpha 0) — the "stage" is silently absent. Source a separate scene.
• Effect strips consume their inputs: input strips never composite on their own channel, and the effect's transform applies on top of the inputs' transforms. Consumption requires the effect on a channel above its inputs; below, they keep painting independently.
• An empty bpy_prop_collection is falsy — se.strips or se.sequences silently falls through to the legacy accessor on an empty timeline. Always branch on hasattr.

Version divergence: the whole example is the divergence — gated on the bpy.app.version tuple (>= (5, 0, 0)), never on version_string ("4.5.11 LTS" is not bare semver). Each side asserts its own canonical contract plus the other side's removal/bridge state. Measured values are identical on 4.5.11 and 5.1.1, including the pixel witness.

Render: the program wall is the sequencer output at frame 29 (mid cross) — crimson A, teal B, amber long-runner C, and the 50/50 cross blend, over the Stage scene strip showing the dark studio. An off-by-one in end-exclusive span math drops its cell to the dark stage; the caption strip carries the closed form. The hero presents that authentic frame on a reference monitor in a dark-studio editing bay — the pixels on the screen are the genuine sequencer output (evidence); only the bay around them is staged (presentation). Rendered locally with EEVEE (GPU host); the checks and --check-pixels need no GPU (Cycles CPU).

Framing measurement

The gallery still renders from the dedicated Bay scene (camera BayCam), not bpy.context.scene — a framing measurement with examples/gallery_framing.py must be handed that (scene, camera) pair explicitly.

Run

# Cheap correctness check (no render) — the CI check:
blender --background --python vse_cut_list.py --

# Compositing witness on a tiny render (Cycles CPU, CI-safe):
blender --background --python vse_cut_list.py -- --check-pixels --engine cycles

# Also render the still (EEVEE on a GPU host; --engine cycles on GPU-less):
blender --background --python vse_cut_list.py -- --output vse.png

It exits non-zero on failure and prints every measured value and tolerance on success, so CI logs carry the numbers. The blender-smoke workflow runs the check and the pixel witness on Blender 4.5 LTS and 5.1.

The --output render path additionally measures framing against the Layer 1 band via examples/gallery_framing.py (exit 10 on violation) before writing the still.