Plot3D Rust version

Rust utilities for reading, writing, and analysing NASA PLOT3D structured grids. The crate draws heavily on the excellent plot3d Python project maintained by NASA. If you are looking for a battle-tested Python implementation with a rich set of examples, start there. This repository is a Rust reimagining that keeps the same data model while taking advantage of Rust’s type safety, performance, and interoperability.

Features

• Parse ASCII and binary PLOT3D files into strongly typed Block structures
• Compute face connectivity, including periodic interfaces and exterior surfaces
• Reduce meshes via common divisors to accelerate matching operations
• Rotate blocks with arbitrary axes and angles and detect rotational periodicity
• Export meshes back to PLOT3D formats
• Utilities for translational periodicity, block merging, and lightweight graph analyses

Many algorithms mirror the behaviour of the Python utilities one-for-one, making it straightforward to port workflows between languages or compare outputs across implementations.

Installation

Add the crate to your Cargo.toml:

[dependencies]
plot3d = "0.1"

Can also do add by running cargo add plot3d

The crate uses the 2021 edition of Rust and depends on common ecosystem crates such as serde, ndarray, and reqwest for optional test helpers.

Quick Start

use plot3d::{read_plot3d_ascii, connectivity_fast};

fn main() -> anyhow::Result<()> {
    // Read an ASCII PLOT3D file into blocks
    let blocks = read_plot3d_ascii("VSPT_ASCII.xyz")?;

    // Compute face-to-face connectivity and remaining outer faces
    let (matches, outer_faces) = connectivity_fast(&blocks);

    println!("Found {} matched interfaces", matches.len());
    println!("Remaining outer faces: {}", outer_faces.len());

    Ok(())
}

For rotational periodicity detection:

use plot3d::{read_plot3d_ascii, connectivity_fast, rotated_periodicity};

fn main() -> anyhow::Result<()> {
    let blocks = read_plot3d_ascii("VSPT_ASCII.xyz")?;
    let (matches, outer) = connectivity_fast(&blocks);

    // Rotate about the x-axis by 360/55 degrees, reducing the mesh by the shared GCD
    let (periodic, remaining) = rotated_periodicity(&blocks, &matches, &outer, 360.0 / 55.0, 'x', true);

    println!("Periodic interfaces: {}", periodic.len());
    println!("Remaining outer faces: {}", remaining.len());

    Ok(())
}

Orientation & Permutation Matrices

When two block faces share an interface, their parametric (u, v) coordinate systems may differ by any combination of axis reversal and transposition. The crate encodes all 8 possible orientations as a 3-bit index into the PERMUTATION_MATRICES constant array of 2x2 matrices.

Bit encoding

The permutation_index stored in [Orientation] is built from three boolean flags:

permutation_index = u_reversed | (v_reversed << 1) | (swapped << 2)

Index	Binary	u_reversed	v_reversed	swapped	Matrix	Effect
0	000	no	no	no	[[ 1, 0],[ 0, 1]]	identity
1	001	yes	no	no	[[-1, 0],[ 0, 1]]	flip u
2	010	no	yes	no	[[ 1, 0],[ 0,-1]]	flip v
3	011	yes	yes	no	[[-1, 0],[ 0,-1]]	flip both
4	100	no	no	yes	[[ 0, 1],[ 1, 0]]	transpose
5	101	yes	no	yes	[[ 0,-1],[ 1, 0]]	transpose + flip u
6	110	no	yes	yes	[[ 0, 1],[-1, 0]]	transpose + flip v
7	111	yes	yes	yes	[[ 0,-1],[-1, 0]]	transpose + both

Why 8 permutations?

In-plane matches (both faces share the same constant axis, e.g. both K-constant) can usually be described by simply reversing one or both diagonal corners. Cross-plane matches (e.g. a K-constant face abutting a J-constant face) additionally require a swap of the u and v axes, giving the full set of 8 orientations.

Accessing orientation from a FaceMatch

After running connectivity_fast, each FaceMatch carries an optional orientation field:

use plot3d::{read_plot3d_ascii, connectivity_fast, PERMUTATION_MATRICES};

let blocks = read_plot3d_ascii("grid.xyz").unwrap();
let (matches, _outer) = connectivity_fast(&blocks);

for m in &matches {
    if let Some(ref orient) = m.orientation {
        let idx = orient.permutation_index;
        let matrix = &PERMUTATION_MATRICES[idx as usize];
        println!(
            "block {} <-> block {}: permutation {}, matrix {:?}, plane {:?}",
            m.block1.block_index, m.block2.block_index,
            idx, matrix, orient.plane,
        );
    }
}

The Orientation struct also provides convenience accessors: u_reversed(), v_reversed(), swapped(), and matrix().

Verification Pipeline

After computing connectivity or periodicity, use the verification functions in verification.rs to correct diagonal ordering and determine orientation:

1. verify_connectivity — extracts a canonical 2D grid from each face pair, tries all 8 permutation matrices, and picks the one that aligns nodes point-by-point within tolerance. Sets Orientation { permutation_index, plane } on each verified match.

2. verify_periodicity — same approach but rotates block1's face by the periodicity angle before comparing grids.

3. align_face_orientations — for same-dimension in-plane matches, walks all 8 diagonal orientations to find the one where directed I/J/K traversal matches node-by-node. Cross-axis matches pass through trusting corner verification.

The connectivity_finder binary in the companion grid-packed repository demonstrates the full pipeline: connectivity_fast -> face_matches_to_dict -> verify_connectivity -> align_face_orientations -> rotated_periodicity -> verify_periodicity.

JSON Output Formats

The serialization module provides two JSON output formats controlled by a --diagonal flag in the connectivity_finder binary:

Default format (lo/hi)

Face bounds are ascending. Each match includes permutation_index (0-7) indicating which PERMUTATION_MATRICES entry transforms face B to match face A.

{
  "block1": { "block_index": 0, "lo": [0,0,0], "hi": [0,101,33] },
  "block2": { "block_index": 30, "lo": [0,0,0], "hi": [0,101,33] },
  "permutation_index": 3
}

Diagonal format (lb/ub, --diagonal)

Designed for GlennHT compatibility:

• In-plane matches (perm 0-3): block2's lb/ub encodes traversal direction via reversed indices. permutation_index: -1 (direction is fully encoded in the bounds).
• Cross-plane matches (perm 4-7): ascending lb/ub with the actual permutation_index, since bounds alone cannot encode an axis swap.

{
  "block1": { "block_index": 0, "lb": [0,0,0], "ub": [0,101,33] },
  "block2": { "block_index": 30, "lb": [0,101,33], "ub": [0,0,0] },
  "permutation_index": -1
}

The permutation_matrices_json() helper embeds the full 8-matrix array in the JSON output header so consumers can reconstruct orientations without hard-coding the table.

Relationship to the Python Project

The original Python implementation includes comprehensive notebooks, example data, and a GUI. plot3d-rs strives to remain API-compatible where possible:

• File I/O routines mirror the signatures of plot3d.read_plot3D and friends
• Connectivity pipelines (connectivity, connectivity_fast, periodicity detection) follow the same logic and produce comparable results
• Many structs (e.g., FaceRecord, FaceMatch, PeriodicPair) are direct translations of the Python dictionaries used in the NASA project

When uncertain about the expected behaviour, use the Python utilities as ground truth. The Rust crate is intentionally lightweight and pragmatic, making it well-suited for embedding PLOT3D workflows in larger Rust applications or integrating with other numerical codes.

Documentation

• Unverified Connectivity Faces: Root Cause Analysis — why cross-plane face connections need orientation flags beyond lb/ub, and how plot3d-rs handles them
• Presentation (PowerPoint) — visual walkthrough of the 2D→3D combinatorics and the orientation fix

Contributing

Bug reports, feature suggestions, and pull requests are welcome. If you find a discrepancy between this crate and the Python reference, please open an issue referencing the relevant Python behaviour so we can keep the implementations aligned.

License

This project is licensed under the MIT license.