MiniMax H3 R40 · Isolated Stage-2 Sampling

By the R40 generation of IAMCCS's MiniMax H3 pipeline, the architecture got more opinionated: stage-2 refinement became something you tune separately, driven by a Shot Lab plan instead of the main settings. This node is the stage-2 sampler for that world - the "isolated" in its display name means its seed, sigmas and acceleration are fully controlled by a separate shot_lab object so you can iterate on refine passes shot-by-shot without re-running or re-encoding stage 1.

The R40 Shot Lab concept (the IAMCCS_H3_R40_SHOT_LAB input) is essentially a per-shot control deck: it carries stage2_seed, stage2_seed_stride, a sigma preset, sparse-attention settings, and a stage-2 LoRA. Think of it as the difference between tweaking a global setting that affects every shot vs. dialing in "this specific take" the way a director would.

How it works

Same shape as the R38 refine sampler - builds noise, sampler, sigmas and a patched model for a custom sampler - but with R40's tuning knobs layered on:

• Seed = shot_lab's stage2_seed + segment_index × stage2_seed_stride. Independent of stage 1, so refining shot 4 doesn't disturb shots 1–3.
• LoRA stack: turbo LoRA, then secondary LoRA, then the shot_lab's own stage-2 LoRA. Three application layers, all from the plan.
• Sigma shift via MiniMaxH3SigmaShift (video shift 12 / audio 3 from sampling config).
• Sparse attention (H3SparseAttention) if the shot lab asks: a video-budget fraction (default 0.30) with denser early/late steps. This is the "how much attention can we skip" lever that buys speed on long clips.
• Sigma preset: seedhunter_3 (default), seedhunter_4, seedhunter_5 - hand-tuned sigma schedules that end at 0.0. These replace the "just trust the scheduler" approach with explicit step boundaries tuned for the refine regime.
• Default sampler er_sde.

The _apply_sparse path depends on an H3-Optimizations node being registered; if you request sparse and it isn't available, it raises telling you exactly that.

Inputs

• model, cine_linx, segment_index - as with R38.
• shot_lab - the IAMCCS_H3_R40_SHOT_LAB object from the Shot Lab control node upstream. This is what makes it "isolated".

Outputs

model, noise, sampler, sigmas, report - feed the first four to SamplerCustomAdvanced.

Install

Pack install (git clone https://github.com/IAMCCS/IAMCCS-nodes.git), MiniMax H3 support, and - only if you use sparse mode - the external H3-Optimizations node. The seedhunter sigma presets are built in; nothing to download for those.

Gotchas

• This node needs the R40 Shot Lab input. There's no fallback if shot_lab is missing - the node exists specifically because the author wanted stage-2 sampling decoupled from the main plan, so don't wire it like an R38 node.
• Sparse attention budget is a budget (0.30 = aim to skip ~70% of video attention). Crank it too aggressive and refinement quality dips; the denser-early/late-steps default is there to protect the parts of the schedule that matter.
• sigma preset names come from IAMCCS's own seedhunter tuning runs. If you override the preset to manual, you're on the list of hard-coded schedules - there isn't a free-form custom sigma editor on this node.