Crate fidget

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Fidget is a library of infrastructure and algorithms for function evaluation, with an emphasis on complex closed-form implicit surfaces.

An implicit surface is a function f(x, y, z), where x, y, and z represent a position in 3D space. By convention, if f(x, y, z) < 0, then that position is inside the shape; if it’s > 0, then that position is outside the shape; otherwise, it’s on the boundary of the shape.

A closed-form implicit surface means that the function is given as a fixed expression built from closed-form operations (addition, subtraction, etc), with no mutable state. This is in contrast to ShaderToy-style implicit surface functions, which often include mutable state and make control-flow decisions at runtime.

Finally, complex means that that the library scales to expressions with thousands of clauses.

Details on overall project status are in the project’s README; the rest of this page is a quick tour through the library APIs.

§Shape construction

A “shape” is a closed-form function of (x, y, z) with a single output. For example, a circle of radius 1 could be expressed as sqrt(x*x + y*y) - 1.

Shapes are constructed within a fidget::context::Context. A context serves as an arena-style allocator, doing local deduplication and other simple optimizations (e.g. constant folding).

Shapes can be constructed manually, using functions on a context:

use fidget::context::Context;

let mut ctx = Context::new();
let x = ctx.x();
let y = ctx.y();
let sum = ctx.add(x, y)?;

This is efficient, but is awkward to write. It’s also possible to construct shapes without a Context using the Tree type, then import the tree into a context:

use fidget::context::{Context, Tree};

let t = Tree::x() + Tree::y();
let mut ctx = Context::new();
let sum = ctx.import(&t);

As a third alternative, Fidget includes bindings to Rhai, a simple Rust-native scripting language, in the fidget::rhai namespace. These bindings allow shapes to be constructed from a script, adding flexibility:

let t = fidget::rhai::eval("x + y")?;
let mut ctx = Context::new();
let sum = ctx.import(&t);

§Evaluation

The main operation performed on an implicit surface is evaluation, i.e. passing it some position (x, y, z) and getting back a result. This will be done a lot, so it has to be fast.

Evaluation is deliberately agnostic to the specific details of how we go from position to results. This abstraction is represented by the Function trait, which defines how to make both evaluators and tapes.

An evaluator is an object which performs evaluation of some kind (point, array, gradient, interval). It carries no persistent data, and would typically be constructed on a per-thread basis.

A tape contains instructions for an evaluator.

At the moment, Fidget implements two kinds of functions:

fidget::vm::VmFunction evaluates a list of opcodes using an interpreter. This is slower, but can run in more situations (e.g. in WebAssembly).
fidget::jit::JitFunction performs fast evaluation by compiling expressions down to native code.

The Function trait requires four different kinds of evaluation:

Single-point evaluation
Interval evaluation
Evaluation on an array of points, returning f32 values
Evaluation on an array of points, returning partial derivatives with respect to input variables

These evaluation flavors are used in rendering:

Interval evaluation can conservatively prove large regions of space to be empty or full, at which point they don’t need to be considered further.
Array-of-points evaluation speeds up calculating occupancy (inside / outside) when given a set of voxels, because dispatch overhead is amortized over many points.
At the surface of the model, partial derivatives represent normals and can be used for shading.

§Functions and shapes

The Function trait supports arbitrary numbers of variables; when using it for implicit surfaces, it’s common to wrap it in a Shape, which binds (x, y, z) axes to specific variables.

Here’s a simple example of multi-point evaluation, using a VmShape to evaluate the function X + Y at four sample locations:

use fidget::{
    context::Tree,
    shape::{Shape, EzShape},
    vm::VmShape
};

let tree = Tree::x() + Tree::y();
let shape = VmShape::from(tree);
let mut eval = VmShape::new_float_slice_eval();
let tape = shape.ez_float_slice_tape();
let out = eval.eval(
    &tape,
    &[0.0, 1.0, 2.0, 3.0], // X
    &[2.0, 3.0, 4.0, 5.0], // Y
    &[0.0, 0.0, 0.0, 0.0], // Z
)?;
assert_eq!(out, &[2.0, 4.0, 6.0, 8.0]);

§Shape simplification

Interval evaluation serves two purposes. As we already mentioned, it can be used to prove large regions empty or filled, which lets us do less work when rendering. In addition, it can discover sections of the tape that are always inactive in a particular spatial region.

Consider evaluating f(x, y, z) = max(x, y) with x = [0, 1] and y = [2, 3]:

use fidget::{
    context::Tree,
    shape::EzShape,
    vm::VmShape
};

let tree = Tree::x().min(Tree::y());
let shape = VmShape::from(tree);
let mut interval_eval = VmShape::new_interval_eval();
let tape = shape.ez_interval_tape();
let (out, trace) = interval_eval.eval(
    &tape,
    [0.0, 1.0], // X
    [2.0, 3.0], // Y
    [0.0, 0.0], // Z
)?;
assert_eq!(out, [0.0, 1.0].into());

In the evaluation region x = [0, 1]; y = [2, 3], x is strictly less than y in the min(x, y) clause. This means that we can simplify the tape from min(x, y) → x.

Interval evaluation is a kind of tracing evaluation, which returns a tuple of (value, trace). The trace can be used to simplify the original shape:

assert_eq!(shape.size(), 4); // min, X, Y, output
// (same code as above)
let new_shape = shape.ez_simplify(trace.unwrap())?;
assert_eq!(new_shape.size(), 2); // just the X term, then the output

Remember that this simplified tape is only valid for points (or intervals) within the interval region x = [0, 1]; y = [2, 3]. It’s up to you to make sure this is upheld!

§Rasterization

Fidget implements both 2D and 3D rasterization of implicit surfaces, implemented in the fidget::render module.

Here’s a quick example:

use fidget::{
    context::{Tree, Context},
    render::{BitRenderMode, ImageSize, ImageRenderConfig},
    vm::VmShape,
};

let x = Tree::x();
let y = Tree::y();
let tree = (x.square() + y.square()).sqrt() - 1.0;
let cfg = ImageRenderConfig {
    image_size: ImageSize::from(32),
    ..Default::default()
};
let shape = VmShape::from(tree);
let out = cfg.run::<_, BitRenderMode>(shape).unwrap();
let mut iter = out.iter();
for y in 0..cfg.image_size.height() {
    for x in 0..cfg.image_size.width() {
        if *iter.next().unwrap() {
            print!("XX");
        } else {
            print!("  ");
        }
    }
    println!();
}

// This will print
//           XXXXXXXXXX
//       XXXXXXXXXXXXXXXXXX
//     XXXXXXXXXXXXXXXXXXXXXX
//   XXXXXXXXXXXXXXXXXXXXXXXXXX
//   XXXXXXXXXXXXXXXXXXXXXXXXXX
// XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
// XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
// XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
// XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
// XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
//   XXXXXXXXXXXXXXXXXXXXXXXXXX
//   XXXXXXXXXXXXXXXXXXXXXXXXXX
//     XXXXXXXXXXXXXXXXXXXXXX
//       XXXXXXXXXXXXXXXXXX
//           XXXXXXXXXX

§Meshing

Fidget implements Manifold Dual Contouring, which converts from implicit surfaces to triangle meshes.

This is documented in the fidget::mesh module.

§WebAssembly notes

The getrandom backend must be selected with RUSTFLAGS, e.g. RUSTFLAGS='--cfg getrandom_backend="wasm_js"'. This can be specified on the command line or in a .cargo/config.toml configuration file (e.g. this file in Fidget itself).

See the getrandom docs for more details on why this is necessary.

§Feature flags

jit (enabled by default) — Enables fast evaluation via a JIT compiler. This is exposed in the fidget::jit module, and is supported on macOS, Linux, and Windows (i.e. all supported platforms except WebAssembly). There’s no way to disable the feature on specific platforms (Cargo issue); users will have to disable it manually via default-features = false.
rhai (enabled by default) — Enable Rhai bindings, in the fidget::rhai module
eval-tests — Enable eval-tests if you’re writing your own evaluators and want to unit-test them. When enabled, the crate exports a set of macros to test each evaluator type, e.g. float_slice_tests!(...).

Modules§

compiler: Compiler infrastructure
context: Infrastructure for representing math expressions as trees and graphs
eval: Traits and data structures for function evaluation
gui: Platform-independent GUI abstractions
jitjit and non-WebAssembly: Compilation down to native machine code
mesh: Octree construction and meshing
render: 2D and 3D rendering
rhairhai: Rhai bindings to Fidget
shape: Data structures for shape evaluation
shapes: Standard library of shapes and transforms
solver: Solver for systems of equations expressed as sets of Function objects
types: Custom types used during evaluation
var: Input variables to math expressions
vm: Simple virtual machine for shape evaluation

Structs§

Context: A Context holds a set of deduplicated constants, variables, and operations.

Enums§

Error: Universal error type for Fidget

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