Advanced mode

Form finding

Instead of modelling a shape, describe the conditions it must satisfy and let it settle into whatever satisfies them. This is how real shells, membranes and cable nets are designed, and it produces forms that are correct rather than merely plausible.

What form finding is

Antoni Gaudí famously hung chains from a ceiling, photographed them, and turned the photograph upside down. A hanging chain is in pure tension; inverted, the same curve is in pure compression. The Sagrada Família is that experiment made permanent.

Rhea does the same thing numerically. You set constraints — edges want to be this long, this boundary is fixed, gravity pulls down, faces want to be flat — and a position-based solver iterates until everything is as satisfied as it can simultaneously be. The result is a shape you did not draw and could not easily have drawn.

The five forms

Minimal surface: The shape a soap film takes across a wire frame: the least area that spans the boundary. Every point is a saddle. Structurally efficient and, not coincidentally, beautiful.
Hanging chain: Gravity plus rest lengths, boundary pinned. Flip the result vertically and you have a compression-only shell — the Gaudí method, and still the correct way to design a masonry vault.
Tensile membrane: A prestressed fabric surface pulled between fixed points. The shape of every stadium canopy and shade sail you have ever stood under.
Inflated cushion: Internal pressure pushing outward against edge constraints. ETFE pillows, air-supported roofs, and anything that looks like it is holding its breath.
Planarised quads: Every quad pulled onto its own best-fit plane while the overall shape is held. This is the constraint that makes a curved surface actually buildable from flat glass. See also Panelize.

Custom leaves the weights as set, so the presets are starting configurations rather than fixed modes.

The constraints

Every one has a weight. They are solved simultaneously, so the result is a compromise between all of them, and the weights determine who wins where they disagree.

Rest length: A spring holding each edge at its rest length. The base constraint of almost every setup — without it nothing holds together.
Keep current lengths: Uses the mesh's present edge lengths as the rest lengths, so the solve starts from equilibrium and only the other constraints push it away. Turn this on when you are refining an existing form rather than finding a new one.
Rest scale: Multiplies the rest lengths. Below 1 the mesh is prestressed in tension and pulls itself taut — this is what makes a membrane a membrane rather than a slack sheet. Above 1 it goes slack and buckles.
Bending: Resistance to curvature change across edges. Higher values give a stiffer surface that resists creasing; zero gives something with no bending stiffness at all, like fabric.
Planarity: Pulls each face onto its own best-fit plane. Fights against shape preservation, and the balance between them is the whole art of panelisation.
Area: Drives faces toward uniform area. Evens out a mesh that has bunched up during the solve.
Gravity / Load: Downward force, and how far each vertex is pulled per iteration. Gravity is the weight; load is the step size. Raising load rather than gravity is usually how you make a solve converge faster.
Pressure / Inflation: Outward force along the normals, and its per-iteration distance. Negative inflation deflates, which is a legitimate and under-used option.
Collision / Radius: Pushes apart vertices that come within the radius, so a folding surface does not pass through itself. Expensive, and necessary as soon as the form starts to curl.
Surface: Holds vertices against a target mesh. This is how you form-find within a constraint volume, or keep a relaxing pattern on the surface it started on.

Pinning

A solve with nothing held in place collapses to a point, and this is the most common way a first attempt fails.

Pin boundary: Holds open boundary vertices in place. For a minimal surface this is exactly right: the boundary is the wire frame and everything else is the soap film.
Pin group: A vertex group whose members are held, in addition to or instead of the boundary. This is how you specify support points — the columns a canopy hangs from, the anchors a cable net ties to.

Solver controls

Iterations1 – 10,000: How long to run. Watch the residual rather than guessing: when it stops falling, more iterations are not buying anything.
Relaxation0.01 – 1.5: How much of each iteration's correction is actually applied. Below 1 is under-relaxation — slower and more stable. Above 1 is over-relaxation, which converges faster and can oscillate. If a solve refuses to settle, lower this first.

Reading the residual

The panel reports the convergence residual from the final iteration: how far the constraints are from being satisfied. It should fall steadily and level off.

Falling and levelling — converged. Stop.
Still falling — needs more iterations.
Oscillating — relaxation is too high, or two constraints are fighting. Lower relaxation, then rebalance the weights.
Rising — diverging. Lower relaxation substantially, and check that something is pinned.
Stuck high — the constraints cannot all be satisfied. That is information: it means the design as specified is not achievable, which is exactly what form finding is for telling you.

Constraints that cannot both be true

Full planarity and full shape preservation on a doubly-curved surface are mutually exclusive — that is a geometric fact, not a solver limitation. A saddle cannot be built from flat quads without changing shape. The solver will find the best available compromise and the residual will tell you what it cost you.

A worked sequence

The architectural pipeline, start to finish, using only what is in Rhea:

Add a Saddle — a gridshell roof form to begin with.
Apply the Diagrid effect for the structural lattice, then apply it so there is real geometry.
Form-find with Hanging chain, boundary pinned. Flip the result vertically: the lattice is now in compression.
Planarise so every panel is flat, and check the analysis overlay for panels that resist.
Generate a waffle or contour set, nest it, and export SVG for the cutter.

It is not only for buildings

Cloth that stays put. A tensile solve gives you a draped, taut surface with no simulation to bake and no frame dependency.
Organic smoothing. Rest length plus bending with no load relaxes a lumpy sculpt into something with surface tension.
Detangling. Collision alone will push a self-intersecting mesh apart, which is often faster than fixing it by hand.
Growth finishing. A differentially grown result relaxed with bending and collision keeps its folds and loses its noise.