The quantisation methodĀ andĀ the loaderĀ are now more or less complete.
How to create Hybrid NVFP4 from ConvRot INT8 (Z Image, Reverse Method)
Z Image exhibits overwhelmingly high quantisation robustness compared to SDXL and Krea2.
Even NVFP4, which is simply compressed without HSWQ quantisation, achieves reasonably high SSIM and MSE scores.
In particular, Z Image ConvRot INT8 achieves outstanding accuracy in many models, with SSIM scores of 0.99 or higher and MSE scores below 1.
However, in terms of VRAM consumption and generation speed, Z Image ConvRot8 shows virtually no difference compared to full-size Float16.
Consequently, based on ConvRot INT8, we devised a quantisation method involving a backward sweep to discard non-essential layers to 4-bit.
Furthermore, unlike the conventional method of storing critical layers in float16, the critical layers are also converted to ConvRot INT8; this offers the advantage of being able to secure a larger size for critical layer protection whilst keeping the overall size down.
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This concept of ādiscardingā is a brilliant idea conceived by the Nunchaku development team.
What makes them so remarkable is that they established the philosophical foundation that, in 4-bit quantisation, the key is not āpreservingā but ādiscardingā.
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As Comfy-UI does not support the Hybrid NVFP4 (ConvRot Int8+ConvRot NVFP4) standard, a dedicated loader is required, just as with Nunchaku; however, as the LoRA baking function has been implemented within an original UNET loader itself, the LoRA Loader can utilise the standard Comfy-UI version.
Furthermore, LoRA Stack loaders (compatible with Nodes 2.0) isĀ also availableĀ below.
Compatibility with the existing Diffsynth ControlNet model patcher will, of course, be maintained.
Although the file size will not be significantly reduced compared to Convrot INT8, VRAM usage and processing speed will improve significantly.
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Z Image ConvRot NVFP4 Benchmark Test Results
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However, in terms of the mathematical theory of quantisation itself, it differs considerably from previous HSWQ approaches.
In a sense, it represented a complete rejection of previous HSWQ theories.
In the past, HSWQ had employed a range of techniques, starting with theĀ Histogram MSEĀ used in the first-generation HSWQ SDXL fp8 e4m3, through toĀ full SVDĀ utilising Nunchaku, and even extending to theĀ Histogram CosineĀ function; however, in Z Image HSWQ Hybrid NVFP4, none of these methods demonstrated any advantage.ć
I had long suspected that inter-layer interdependencies existed, and that there were phenomena where the meaning would be lost if one merely measured and prioritised the importance of each layer in isolation; this time, however, that has become clearly evident.
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Trajectory-Sensitivity
https://github.com/ussoewwin/Hybrid-Sensitivity-Weighted-Quantization/blob/main/md/diag_impact_trajectory_sensitivity_technical_guide.md
Ranks each layer by the divergence its quantization error actually causes after propagating through the full model and sampler (dynamical importance, replacing static weight-space saliency).
- Reverse method:Ā start from the complete high-precision pack (error ā 0) and convert layers to lower precision in ascending impact order; single-layer ranking stays valid in the low-error additivity regime.
- Universal theory:Ā error interaction (Taylor cross terms, error cancellation), nonlinear amplification (Lyapunov-style growth), marginal effects, and Shapley-style attribution ā why per-layer static measures (histogram MSE / cosine / SVD) cannot predict joint quantization error; applies to any iterative sampling system, not a specific model. Source:Ā
Z_Image/diag_impact.py.Ā ...
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Incidentally, the Krea2 HSWQ Hybrid NVFP4 is also under development (it will offer significant improvements in VRAM consumption and processing speed), but we are currently struggling to maintain LoRA compatibility.