MODEL FIELD NOTES · Direction
Controlling Two-Actor AI Video Prompts: Fixed-Seed A/B/C Tests and Auto-QC False Positives
Directing two synthetic characters to approach, hold hands, or exchange a close reaction is where plain text prompts fail most often. This record documents a fixed-seed A/B/C prompt structure test and explains why automated quality scoring cannot replace 3D motion blocking and human A/B take selection.
阅读本站中文版 · 在黄果制片阅读中文版What changed across the fixed-seed A/B/C prompt test
On September 5, 2026, we ran a controlled comparison on two-character interaction clips using a fixed random seed and identical starting conditions. Variant A used atmospheric prose describing mood and relationship tension; Variant B broke the action into explicit two-second physical beats; Variant C added left-right spatial blocking and camera framing constraints.
We archived the generated clips alongside dialogue transcripts and frame-by-frame human review notes. Beat-segmented physical verbs (Variants B and C) reliably triggered character movement where atmospheric prose left the actors nearly static. However, even the strongest text-only candidate exhibited hand drift and occlusion artifacts during close contact, confirming that prompt wording alone is not a deterministic pose controller.
When two characters occupy overlapping bounding boxes, text encoders struggle to bind specific verb phrases to the left or right actor across all twenty-four frames per second. Explicit spatial tokens reduce ambiguity, yet only a geometric skeleton rig guarantees that Actor A and Actor B maintain distinct torso trajectories throughout the shot.
| Prompt Variant | Structural Strategy | Observed Motion Response | Human Frame Audit |
|---|---|---|---|
| Variant A (Atmospheric prose) | Mood adjectives and emotional intent | Actors remained mostly static with minor sway | Fail: action beat not executed |
| Variant B (Timed physical beats) | Explicit subject-verb-object beats per 2s window | Triggered approach and hand raise, unstable contact | Partial: hand occlusion artifact on approach |
| Variant C (Spatial + framing) | Left/right actor assignment + contact target + lens | Closest match to intended blocking | Best of group, still required A/B take filtering |
Why automated quality checks pass clips that humans immediately reject
During the same experiment, we compared an automated quality-control gate (keyframe sampling plus vision-language alignment scoring) against human inspection. Several clips that received an automatic PASS were rejected within seconds by human reviewers.
Two failure modes consistently fooled automated scoring: a hand crossing in front of a face (which causes transient facial melting or finger fusion between sampled keyframes) and extra limbs appearing during two-actor embraces. Because individual sampled frames still matched the scene description and maintained sharp background textures, the automated gate scored them high despite broken anatomy in motion.
| Failure Mode | Automated QC Verdict | Human Reviewer Verdict | Porncut Studio Mitigation |
|---|---|---|---|
| Hand crossing the face | PASS (high semantic and sharpness score) | REJECT (transient facial distortion during occlusion) | Preview camera orbit and hand trajectory in 3D Director |
| Extra limb on two-actor contact | PASS (missed between sampled frames) | REJECT (duplicate forearm during close hold) | Bind pc_humanoid@1 skeleton + render A/B pair for human pick |
Why Porncut pairs 3D skeleton blocking with A/B takes
These findings shaped Porncut’s core studio loop. Rather than pretending a single prompt will produce a flawless two-actor take on the first try, the workflow separates spatial planning from stochastic sampling so directors can preview blocking before spending GPU credits.
- Block two-actor spacing and hand IK reachability (0.045 m to 0.815 m) on the pc_humanoid@1 rig in the public 3D Motion Catalog (/motions).
- Assign one clear physical beat per shot across a 3-shot storyboard instead of crowding multiple actions into one clip.
- Generate two variants (Take A and Take B) under a fixed credit quote so a human director always chooses the clean cut.