dense-plantain

A per-head attention probe of FLUX.2 Klein 4B testing whether the base model represents canonical body pose as a separable axis from expressive pose deformation, parallel to the canonical-vs-deformed coordinate distinction in dense human-pose models (DensePose, SMPL).

Thesis

Dense human-pose representations canonically encode body shape in a fixed reference pose (T-pose) plus a deformation that maps to the actual pose. Whether image-generation models develop an analogous axis without explicit supervision is the question. dense-plantain pairs descriptions of a body subject in canonical T-pose against the same body subject in an expressive pose, and tests whether per-head attention shifts systematically.

Method

Twenty-five paired prompts holding body subject and setting constant; only pose configuration varies (canonical T-pose vs. expressive pose). Per-head capture protocol identical to the rest of the plantain probe family.

Rigor add-ons: per-head Cohen's d effect size; split-half consistency via 100 random 50/50 stimulus splits.

Results

Metric	Value	Significance
Heads with |t| > 3	8,263 (50.6%)	6.2× empirical null p99
Heads with |t| > 5	4,614 (28.3%)	659× empirical null p99
Heads with |d| > 0.8 (large)	6,164 (37.8%)	—
Split-half r (median)	0.863	[0.85, 0.87] IQR
Max |t|	22.45	—

Top blocks by max |t|:

• single[0]: max|t|=22.45, 283/768 heads at |t|>3, median |d|=0.45
• single[3]: max|t|=22.06, 389/768 heads at |t|>3, median |d|=0.61
• single[9]: max|t|=21.66, 232/768 heads at |t|>3, median |d|=0.39
• joint[3]: max|t|=20.18, 138/192 heads at |t|>3, median |d|=1.02
• joint[4]: max|t|=19.62, 131/192 heads at |t|>3, median |d|=1.01

Interpretation. The body-pose axis is strong (659× null at |t|>5) and highly reproducible (split-half r=0.86). Over half of all 16,320 attention heads register the canonical-vs-expressive pose distinction at |t|>3, and over a third reach Cohen's d > 0.8. Localization is mixed: max signal is in early single blocks (single[0]/[3]/[9]), but joint-block median |d| is highest (≥1.0 in joint[3] and joint[4]), suggesting pose information is initially routed through cross-attention text-image fusion (joint blocks) and then maintained through early single-block transformer processing.

The result places Klein among the models that have an internal canonical-pose representation without explicit pose supervision, parallel to but more diffuse than the structured axis encoded in DensePose-trained networks.

Status

Probe complete. No LoRA training; this is a base-model interpretability finding.

Limitations

The T-pose vs. expressive-pose pair has unmatched articulation complexity — expressive poses involve more anatomical regions deviating from canonical position than T-pose does. A residual contributor to the signal could be "amount of articulation" rather than canonical-vs-deformed status specifically. A follow-up controlling articulation (e.g., A-pose vs. expressive pose with matched limb configurations) would tighten the claim.

Body-type and clothing variations are not factored out across the 25 pairs.

The probe is correlational.

License

Apache 2.0 — matches base FLUX.2 Klein 4B.

References

• Gabeur, V., Long, S., Peng, S., et al. Image Generators are Generalist Vision Learners. arXiv:2604.20329 (2026).
• Black Forest Labs. FLUX.2 Klein. https://bfl.ai/models/flux-2-klein (2025).