⚗ Alchemica · Restart

A restart is a small act of self-harm followed by a repair. Partway down the sampling trajectory the image is pushed back up the noise curve - re-noised to a higher sigma than where it currently sits - and then a few extra steps walk it back down. Why would anyone do that? Because the region where texture is decided is worth visiting twice.

How it works

The pack's measurements put the detail window at sigma 0.57 down to 0.13, with the structure of that detail forming roughly half-formed around sigma 0.5–0.65. Re-noising to that range once the composition is already locked gives the model a second chance at the finish: it can add material and micro-relief that a single pass through the region didn't commit to, while the composition - locked at ~0.87, above the restart point - can't move.

Two fields:

restart_steps - how many extra steps after the restart. Default 2, max 20, 0 turns it off.

restart_sigma - how high it re-noises, 0 to 0.95. The default is 0.5 and the useful setting is −1, which means take it from the calibration profile - specifically the sigma where half the detail is born. On a calibrated checkpoint that's a measured number rather than a guess, which is the whole reason this node has a −1 option.

Where it lives in the machinery

Worth knowing because it changes how you wire things: restart is implemented in the schedule, not in the solver loop. The extra steps are appended to the sigma list, with the jump back up inserted at the chosen point. Two consequences:

• It works with the ⚗ Sigmas node, not just the full sampler nodes - the restart is already in the curve you get.
• Its cost is exactly the extra steps you asked for. Restart with 3 extra steps is a longer run, not a cleverer one. The presets are honest about this: alchemica_quality carries 3 restart steps, foto_tarata 2, and foto_tarata_2K 2 - small numbers.

The modo presets in the all-in-one node bake it in the same way: veloce runs 0 restart steps, equilibrato 2, massimo dettaglio 3.

Output and wiring

One ALCHEMICA_OPTS output:

Alchemica · Restart ─opts→ AlchemicaSampler · opts

It overrides the restart group only. The optional opts input lets you chain it with the Schedule, Solver & Detail, Guidance and Two-stages nodes - each takes over its own group, so chain order doesn't matter.

Installing

Part of the pack:

cd ComfyUI/custom_nodes
git clone https://github.com/Nessuno76/ComfyUI-AlchemicaSampler

Restart the ComfyUI server (a page reload won't register the nodes). No dependencies beyond ComfyUI 0.36.0+ itself, and no model files of its own - it just needs whatever your sampler is running.

Traps

Leaving restart_steps at 0 and expecting restart_sigma to do something. 0 is off. That's the whole switch.

Restarting too high. restart_sigma near 0.95 re-noises almost to the start, which means the second pass can rebuild composition and you've bought a whole extra generation. The interesting range is the detail window, which is why −1 exists.

Using it as a quality slider. Restart is not "more steps but better placed". Sometimes the second pass fixes a flat finish; sometimes it just re-rolls texture you already liked. Same seed, restart off, compare - that's a 30-second test and it'll tell you whether your checkpoint likes it.

Stacking it with plunge_sigma. The plunge_restart preset does both - structure stops at 0.75, restart at 0.65 - and it's a deliberately rough recipe with detail 0.60 and CFG 2.25. Copying that combination onto a nice calibrated pipeline is how you find out what "overcooked" means.