Tuning print prep¶

Print prep is the last step of every mini: it turns the 3D engine's model into a print-ready STL, sized, on a base, in one solid piece, with its previews. This page says what it does step by step, which options it takes, and how to run it by hand to try settings the app doesn't offer.

It's Mimic's own Swift code (app/Sources/MimicCore/Prep.swift, whose header lists every option), run as a separate program so Stop can end it like any other step. It replaced a Blender script: on the dwarf (32 mm, 25 mm base, 0.2 mm nozzle) Blender took 58 s and 6.0 GB of memory, print prep takes 6 s and 1.5 GB. At 100 mm on a 0.4 mm nozzle, 157 s and 13.2 GB against 14 s and 2.9 GB.

What it does, step by step¶

Reads the model (model.glb, written by the 3D engine or by File → Import Model…).
Turns it to face the front (--turn), the way a slicer's front view looks at it. Pixal3D builds figures facing the other way from TRELLIS.2, so Mimic turns them 180°.
Stands an object up (objects only, --ground bottom). A model that leans a few degrees is levelled by up to 30°, then set on a side it can stand on: see Objects resting on a flat side.
Refuses a flat sheet. If its thinnest side is under 2% of its longest, print prep stops with "The 3D model came out flat, like a sheet of paper." A flat drawing needs the grey sculpt. A model with no size at all, or with points that aren't numbers, stops with "The 3D model has no shape to print".
Scales it. A character is scaled so --height is the distance from the ground to its top. The ground is where most of the bottom is (0.5% of the surface lies below it), not the lowest point, so a trailing wisp sinks into the base rather than holding the figure up on a pin. An object (--fit longest) is scaled so its longest side, whichever way it runs, is --height. Floating specks don't count towards it. A character standing on a base of the 3D engine's own, copied from one in its picture, is measured from that base's top: see A base in the picture.
Centres it. A character is centred on the solid cross-sections through its lower body (at 3, 6, 9 and 12% of its height), so a raised sword or a cape doesn't pull it off the base. An object is centred on its whole shadow. This is also where it measures the footprint: how far the figure reaches out near its feet.
Sinks the feet 0.6 mm into the base, so the two are one solid.
Makes one solid. The model becomes a signed distance field on a grid --voxel apart, built in slabs so memory follows the surface rather than the whole box, and one surface is drawn from it with marching cubes, so the result is closed and manifold by construction. In the same field it:
- inflates the surface by --inflate, a true offset that thickens blades, staffs and capes by twice that;
- adds the base: round, square or hex, --base across (across the flats for a hex), --base-height tall, with a rounded top edge;
- presses a floor into the base's top for --base-style (seams 0.45 mm deep);
- cuts the magnet hole underneath;
- cuts off the 3D engine's own base, when it made one, so only Mimic's is printed;
- cuts everything below --flatten off, so the bottom is flat however bumpy the model's own base was.
Keeps the largest piece and drops the rest: see Parts left out.
Lowers it by --flatten, so the bottom of the base is at zero.
Trims it to about --faces triangles (800,000), by collapsing edges in a way that never breaks the solid. A base with a floor gets a sixteenth more, for its seams.
Draws the previews: front, left, right and back, grey, 900 × 900, on a transparent background, beside the print file. They're drawn in software, so they don't compete with the 3D engine for the graphics chip. They come before the STL on purpose: a resize stopped while they're drawn keeps the old print file.
Writes the STL, to a .part.stl first, then puts it in place in one go.
Notes where it put the model in placement.json beside the STL: one matrix for the turn, the levelling, the scale and every shift, from the 3D model to the print file. Export for Virtual Tabletop reads it to find each part's colour on the 3D model. See Where your files are.
Says how it went: the size, how many loose pieces it dropped, and any warnings. See What it says.

The options¶

mimic _prep in.glb out.stl [--height 32] [--base 25] [--base-height 3] [--nozzle 0.4]
    [--inflate MM] [--voxel MM] [--faces 800000] [--no-base] [--flatten 0.4]
    [--fit height|longest] [--ground feet|bottom] [--turn DEG] [--base-shape round|square|hex]
    [--base-style plain|stone|wood|cobble] [--base-seed N] [--magnet 5x2|6x2|8x3|none]

Sizes are millimetres.

Option	Default	What it does
`--height MM`	32	Figure height, ground to top. With `--fit longest`, the longest side.
`--fit height` / `longest`	`height`	Size by the height, or by the longest side (objects).
`--ground feet` / `bottom`	`feet`	Centre on the feet's cross-sections, or on the whole shadow (objects). `bottom` also levels the model and stands it on a steady side.
`--base MM`	25	Base width: a round base's diameter, a square's side, a hex's width across the flats.
`--base-shape`	`round`	`round`, `square` or `hex`. A square or hex faces the figure with a flat side.
`--base-style`	`plain`	`plain`, or a floor pressed into the top: `stone` (flagstones), `wood` (planks) or `cobble` (cobblestones).
`--base-seed N`	0	Lays out the floor's stones or planks.
`--base-height MM`	3	The base before the bottom is cut flat: 3 mm prints as 2.6 mm, of which the feet take 0.6. Raised to fit a magnet hole.
`--magnet`	`none`	A hole underneath for a round magnet: `5x2`, `6x2` or `8x3` (across × tall, mm).
`--no-base`	off	No base: keep the model's own. No magnet hole either.
`--nozzle MM`	0.4	Your printer's nozzle. Sets `--inflate`, `--voxel`, the magnet hole's room and the floor's seams.
`--inflate MM`	0.4 × nozzle	How far the surface is pushed out. More keeps thin parts whole; too much softens faces.
`--voxel MM`	nozzle ÷ 4	Grid spacing. Smaller keeps more detail and takes much more time and memory.
`--flatten MM`	0.4	How much is sliced off the bottom.
`--faces N`	800000	About how many triangles the STL keeps.
`--turn DEG`	0	Turns the model about its vertical axis first.

Anything else is refused ("unknown option"). So is a size print prep couldn't make on a Mac: --height takes 5 to 500, --base 5 to 200, --base-height 0 to 50, --nozzle 0.1 to 1, --inflate 0 to 2, --voxel 0.01 to 5 and --flatten 0 to 50. That's wider than New Mini and Resize let you choose, to experiment with. --faces takes a whole number from 1000 to 5000000, and --turn a number of 0 or more.

How the nozzle feeds in¶

The nozzle you pick in New Mini or Resize (--nozzle in Terminal) sets the rest unless you set them yourself:

Nozzle	Extra thickness (`--inflate`)	Grid (`--voxel`)	Magnet hole room	Floor seams
0.2 mm	0.08 mm	0.05 mm	+0.2 mm	0.4 mm wide
0.4 mm	0.16 mm	0.1 mm	+0.2 mm	0.8 mm wide
0.6 mm	0.24 mm	0.15 mm	+0.3 mm	1.2 mm wide, stones 1.5 × larger

The inflate of 0.4 × nozzle was tuned on a 0.2 mm nozzle, where 0.08 mm keeps cloth whole and 0.15 mm already looked melted. A wider nozzle drops thinner walls, so it gets more. A grid of a quarter of the nozzle is finer than any line the printer can make. A fixed 0.05 mm made a 100 mm figure on a 0.4 mm nozzle take many minutes and 8 GB.

In the app, the inflate is Extra thickness for thin parts, under Size & printer → Advanced in New Mini and Resize (0 to 0.4 mm). Mimic sends it only once you've moved it; otherwise print prep works it out from the nozzle. In Terminal it's --inflate.

Magnets¶

The hole is the magnet's width plus room for the nozzle (a printed hole comes out a little smaller than drawn), and its height plus 0.2 mm for glue, up from the bottom. At least 0.8 mm of solid is kept over it, so the base grows when it has to:

Magnet	Hole (0.2 or 0.4 mm nozzle)	Base height, plain top	With a floor
5 × 2 mm	5.2 mm wide, 2.2 mm deep	3.4 mm	3.85 mm
6 × 2 mm	6.2 mm wide, 2.2 mm deep	3.4 mm	3.85 mm
8 × 3 mm	8.2 mm wide, 3.2 mm deep	4.4 mm	4.85 mm

The base heights are before the bottom is cut flat, as --base-height is.

What Mimic passes¶

Mimic runs print prep with the sizes saved in the mini's settings.json (requested) and adds what the mini needs:

Flags	When
`--height`, `--base`, `--nozzle`	Whatever was chosen. A mini made in Terminal without sizes gets print prep's defaults.
`--inflate`	Only when it was set by hand.
`--no-base`, or `--base-shape`, `--base-style`, `--magnet`	Only when they aren't the default.
`--fit longest --ground bottom`	For anything that isn't a character.
`--turn 180`	For a mini made with Pixal3D. Never for an imported model, which faces whichever way its file has it.
`--base-seed N`	With a floor on the base: the mini's own variation number (42 if it has none), so Try Again lays the stones the same way and another version differently.

So a TRELLIS.2 tiefling at 38 mm on a 0.2 mm nozzle, on a hex base with a stone floor, runs:

mimic _prep model.glb tiefling.stl --height 38.0 --base 25.0 --nozzle 0.2 \
    --base-shape hex --base-style stone --base-seed 42

Run it by hand¶

mimic _prep is a hidden command: it isn't in mimic --help, it's for experimenting, and its options may change between versions. With the command-line tool installed (see Mimic from a terminal) it's mimic _prep; without it, run the app's own binary, /Applications/Mimic.app/Contents/MacOS/mimic _prep.

Warning

Work on a copy, never in a mini's own folder. Print prep overwrites the STL, its previews and placement.json, and leaves a prep-result.json beside them. To give a mini new sizes, use Resize This Mini… or mimic resize instead.

mkdir -p ~/Desktop/prep-test
cp ~/Documents/Mimic/dwarf-cleric/model.glb ~/Desktop/prep-test/
cd ~/Desktop/prep-test
mimic _prep model.glb dwarf.stl --height 32 --nozzle 0.2 --turn 180 --faces 400000

Add --turn 180 when the mini was made with Pixal3D (its settings.json says "model": "pixal3d-sv", or has no model at all); leave it out for TRELLIS.2. The input has to be a GLB: for an STL, import it with File → Import Model… or mimic import first, and use the model.glb that makes.

It writes dwarf.stl, dwarf_front.png, dwarf_left.png, dwarf_right.png, dwarf_back.png, placement.json and prep-result.json, and prints what it did. It exits with 0 when it worked, or prints mini_prep: FAILED: … and exits with 1, an option it doesn't take or a size out of range included (not 64, as the other mimic commands do for something typed wrong).

Things worth trying that the app doesn't offer:

--voxel smaller than nozzle ÷ 4, for a large display piece on a fine nozzle. Time and memory grow fast.
--faces lower, for a smaller file to share, or higher, to keep more of a big figure's detail.
--flatten higher, when a model's own base is very uneven and you used --no-base.
--base-height taller, for a chunkier base.

What it says¶

Print prep's report goes to the mini's prep.log, with how long each part took. For example (times and sizes vary):

prep: read 980630 triangles 0.1 s
prep: placed 0.1 s
prep: solid 289x262x516 grid, 2237348 triangles 0.6 s
prep: largest piece 0.2 s
prep: trimmed to 800000 triangles 1.1 s
prep: drew the previews 1.0 s
prep: written 0.0 s
mini_prep: …/dwarf-cleric.stl  size 28.3 x 25.5 x 51.2 mm  faces 800000  loose pieces dropped 150  footprint 20.2 mm

Other lines it may add:

Line	Meaning	Shown in the app as
`prep: left out the 3D model's own base (5% of its height)`	The engine made a base, copied from the picture, and it was cut off.	Nothing.
`prep: levelled by 2.8°`	An object was levelled.	Nothing.
`prep: set on its most stable side (turned 90°)`	An object was set on another side.	Nothing.
`mini_prep: magnet hole 5.2 mm wide, 2.2 mm deep, for a 5 × 2 mm magnet; base 3.40 mm tall`	The hole, and the base height it needed.	Nothing.
`mini_prep: no base, so no magnet hole`	A magnet was asked for without a base.	Nothing.
`mini_prep: WARNING footprint 41.3 mm is wider than the 40 mm base; raise the base to at least 43 mm`	The figure reaches out past its base.	"The bottom of the figure reaches past the edge of its base. Resize This Mini with a bigger base size to fit it on."
`mini_prep: WARNING part: …`	A part was left out.	The same words, on the mini's page.
`mini_prep: WARNING stand: …`	An object without a base can't stand.	The same words, on the mini's page.
`mini_prep: FAILED: …`	Print prep stopped.	Why the mini didn't finish.

An object on a base never gets the footprint warning: its base is a plinth, sized to its shadow.

The app doesn't read prep.log: it reads the same warnings from prep-result.json, so rewording a log line changes nothing it shows. A mini's page keeps showing the last run's warnings, after a relaunch too, until a resize replaces them.

A base in the picture¶

When a character's picture shows it on a base, the 3D engine copies that base: a drum under its feet, usually wider than Mimic's base and hollow underneath. Printed on top of Mimic's base, it hung over the edge on supports and came out with a ragged rim. So print prep looks for one: a wide flat top in the lowest 12% of the model, with hardly anything flat just above it. Feet, robes and rocks don't have one. When it finds one, the figure is sized from that top, stands on Mimic's base from there, and the engine's base is left out (sunk a little deeper into Mimic's base, by the inflate and two voxels, so none of its skin shows).

With --no-base, the engine's base is kept, as the character's own. Objects are never looked at.

Objects resting on a flat side¶

Anything that isn't a character goes through two more steps before it's sized:

Levelling. The downward-facing surface in its lowest tenth is turned to face straight down, in up to three passes. More than 30° is more likely an object lying on its side on purpose than a lean, so that's left alone. A teapot the 3D engine leaned 5° prints on its foot, not on the edge of it.
Resting. Its possible sides are the faces of its convex hull (built from the main pieces only, so floating specks don't hold it up), where hull faces lying within 3° of each other count as one side. It stands on a side if it survives an 8° tilt there. If it already stands on a side within 10° of straight down, it's left as it is, however much steadier lying down would be: a vase, a pillar or a statue with a flat back is never laid down. Otherwise it goes onto the steadiest side within 30° of down, and never further.

If no side near its bottom can hold it up (a figure on small feet, a bird on a perch), the levelling is undone too and it stays exactly as the 3D engine made it. Without a base it then gets the "can't stand on its own" warning; with Add a base it stands on the base. Characters never go through any of this: they stand on their feet.

Parts left out¶

Print prep keeps only the largest connected piece. The rest are of two kinds, told apart by the sign of their volume rather than their size:

The inner walls of hollows. A hollow's inner surface is a piece of its own, inside out, so its volume is negative. Dropping it fills the hollow. These can be big (a whole body's length).
Solid pieces. Mostly specks the 3D engine left floating, but sometimes something the figure holds that the engine didn't join to its hands. A solid piece at least a tenth of the figure's height long is counted as a part. It's still left out, because it would print floating where the hands held it, needing supports and glue. But Mimic says so: "A part came out separate from the figure (about 30 mm long) and was left out. Try Make Another Version. If you use Pixal3D, TRELLIS.2 (Settings → 3D Model) joins held things more reliably."

The tenth was measured: a lost bow was 93% of its figure's height, and the largest solid speck on seven real minis was under 1%. In the comparison that made TRELLIS.2 the default, it got the held part right 9 times out of 9, against 4 out of 10 for Pixal3D.