ConvRot: Rotation-Based Plug-and-Play 4-bit Quantization for Diffusion Transformers
Paper • 2512.03673 • Published
Krea-2 Turbo — INT4 tensorwise W4A4 (+ConvRot, mixed int8 fallback)
W4A4 quantization of Krea-2 Turbo (bf16 source: Comfy-Org/Krea-2): packed signed INT4 weights with group-256 regular-Hadamard ConvRot rotation (arXiv 2512.03673), activations rotated online and dynamically quantized inside the kernel. Calibration-free.
Mixed recipe: 224 block Linears = 128 INT4 + 96 int8_tensorwise fallback,
selected by measured per-layer sensitivity: each layer was swapped alone to
W4A4 on real sampling inputs, ranked by final-output drift, then chosen by
impact-per-byte at a fixed size budget (all wv/wk, 27/28 wo, the most
sensitive mlp.down/gate/wq layers). +0.95 dB over the class-heuristic
recipe at identical size and speed. The shared modulation projector tproj
and the text-fusion transformer stay bf16.
Samples
Same seed/prompt, BF16 top vs INT4-mixed bottom. Few-step turbo sampling amplifies trajectory divergence (composition shifts) while style, detail and lighting stay at parity — no quantization artifacts.
⚠️ Runtime requirement
Needs the int4_tensorwise runtime, which is not in any released ComfyUI /
comfy-kitchen yet. In-flight:
comfy-kitchen PR #63
(kernels + layout) and the
ComfyUI loader branch
— to try this checkpoint today, run that ComfyUI branch with the PR's
comfy-kitchen branch on PYTHONPATH (or installed). On a stock release it
fails with a clear "format not available" error.
Measurements (NVIDIA L4 / SM 8.9, 1024x1024, 8 steps, cfg 1.0, 3 prompts)
| checkpoint | per image |
|---|---|
| this model (int4 mixed) | 18 s |
| bf16 | 24 s |
Weights: 9.25 GB vs 28 GB bf16 (−67%) — fits a 24 GB card with headroom where bf16 needs weight streaming. PSNR vs bf16 (same seed): 17.7 dB (min 14.5) — few-step turbo models amplify trajectory divergence; visual inspection shows style/quality parity with composition shifts, no quantization artifacts.
Reproduce
comfy-quants export-model-int4-tensorwise \
--config configs/krea2_int4_tensorwise_mixed.yaml \
--source krea2_turbo_bf16.safetensors \
--out krea2_turbo_int4_tensorwise_mixed.safetensors
Produced by comfy-quants
(branch feat/int4-tensorwise). Use with text_encoders/qwen3vl_4b +
vae/qwen_image_vae from Comfy-Org/Krea-2.
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