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Manipulate the assembly (#29)

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feat: Find tangent space of solution variety, use for perturbations

### Tangent space

#### Implementation

The structure `engine::ConfigSubspace` represents a subspace of the configuration vector space $\operatorname{Hom}(\mathbb{R}^n, \mathbb{R}^5)$. It holds a basis for the subspace which is orthonormal with respect to the Euclidean inner product. The method `ConfigSubspace::symmetric_kernel` takes an endomorphism of the configuration vector space, which must be symmetric with respect to the Euclidean inner product, and returns its approximate kernel in the form of a `ConfigSubspace`.

At the end of `engine::realize_gram`, we use the computed Hessian to find the tangent space of the solution variety, and we return it alongside the realization. Since altering the constraints can change the tangent space without changing the solution, we compute the tangent space even when the guess passed to the realization routine is already a solution.

After `Assembly::realize` calls `engine::realize_gram`, it saves the returned tangent space in the assembly's `tangent` signal. The basis vectors are stored in configuration matrix format, ordered according to the elements' column indices. To help maintain consistency between the storage layout of the tangent space and the elements' column indices, we switch the column index data type from `usize` to `Option<usize>` and enforce the following invariants:

1. If an element has a column index, its tangent motions can be found in that column of the tangent space basis matrices.
2. If an element is affected by a constraint, it has a column index.

The comments in `assembly.rs` state the invariants and describe how they're enforced.

#### Automated testing

The test `engine::tests::tangent_test` builds a simple assembly with a known tangent space, runs the realization routine, and checks the returned tangent space against a hand-computed basis.

#### Limitations

The method `ConfigSubspace::symmetric_kernel` approximates the kernel by taking all the eigenspaces whose eigenvalues are smaller than a hard-coded threshold size. We may need a more flexible system eventually.

### Deformation

#### Implementation

The main purpose of this implementation is to confirm that deformation works as we'd hoped. The code is messy, and the deformation routine has at least one numerical quirk.

For simplicity, the keyboard commands that manipulate the assembly are handled by the display, just like the keyboard commands that control the camera. Deformation happens at the beginning of the animation loop.

The function `Assembly::deform` works like this:
1. Take a list of element motions
2. Project them onto the tangent space of the solution variety
3. Sum them to get a deformation $v$ of the whole assembly
4. Step the assembly along the "mass shell" geodesic tangent to $v$
   * This step stays on the solution variety to first order
5. Call `realize` to bring the assembly back onto the solution variety

#### Manual testing

To manipulate the assembly:
1. Select a sphere
2. Make sure the display has focus
3. Hold the following keys:
   * **A**/**D** for $x$ translation
   * **W**/**S** for $y$ translation
   * **shift**+**W**/**S** for $z$ translation

#### Limitations

Because the manipulation commands are handled by the display, you can only manipulate the assembly when the display has focus.

Since our test assemblies only include spheres, we assume in `Assembly::deform` that every element is a sphere.

When the tangent space is zero, `Assembly::deform` does nothing except print "The assembly is rigid" to the console.

During a deformation, the curvature and co-curvature components of a sphere's vector representation can exhibit weird discontinuous "swaps" that don't visibly affect how the sphere is drawn. *[I'll write more about this in an issue.]*

Co-authored-by: Aaron Fenyes <aaron.fenyes@fareycircles.ooo>
Reviewed-on: glen/dyna3#29
Co-authored-by: Vectornaut <vectornaut@nobody@nowhere.net>
Co-committed-by: Vectornaut <vectornaut@nobody@nowhere.net>
This commit is contained in:
Vectornaut 2024-12-30 22:53:07 +00:00 • committed by Glen Whitney
parent b490c8707f
commit 22870342f3
7 changed files with 374 additions and 34 deletions
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98
app-proto/src/display.rs
View file

@ -1,5 +1,5 @@
use core::array;
use nalgebra::{DMatrix, Rotation3, Vector3};
use nalgebra::{DMatrix, DVector, Rotation3, Vector3};
use sycamore::{prelude::*, motion::create_raf};
use web_sys::{
console,
@ -14,7 +14,7 @@ use web_sys::{
wasm_bindgen::{JsCast, JsValue}
};
use crate::{AppState, assembly::ElementKey};
use crate::{AppState, assembly::{ElementKey, ElementMotion}};
fn compile_shader(
context: &WebGl2RenderingContext,
@ -123,6 +123,14 @@ pub fn Display() -> View {
let zoom_out = create_signal(0.0);
let turntable = create_signal(false); /* BENCHMARKING */
// manipulation
let translate_neg_x = create_signal(0.0);
let translate_pos_x = create_signal(0.0);
let translate_neg_y = create_signal(0.0);
let translate_pos_y = create_signal(0.0);
let translate_neg_z = create_signal(0.0);
let translate_pos_z = create_signal(0.0);
// change listener
let scene_changed = create_signal(true);
create_effect(move || {
@ -141,6 +149,7 @@ pub fn Display() -> View {
let mut frames_since_last_sample = 0;
let mean_frame_interval = create_signal(0.0);
let assembly_for_raf = state.assembly.clone();
on_mount(move || {
// timing
let mut last_time = 0.0;
@ -153,6 +162,9 @@ pub fn Display() -> View {
let mut rotation = DMatrix::<f64>::identity(5, 5);
let mut location_z: f64 = 5.0;
// manipulation
const TRANSLATION_SPEED: f64 = 0.15; // in length units per second
// display parameters
const OPACITY: f32 = 0.5; /* SCAFFOLDING */
const HIGHLIGHT: f32 = 0.2; /* SCAFFOLDING */
@ -273,6 +285,14 @@ pub fn Display() -> View {
let zoom_out_val = zoom_out.get();
let turntable_val = turntable.get(); /* BENCHMARKING */
// get the manipulation state
let translate_neg_x_val = translate_neg_x.get();
let translate_pos_x_val = translate_pos_x.get();
let translate_neg_y_val = translate_neg_y.get();
let translate_pos_y_val = translate_pos_y.get();
let translate_neg_z_val = translate_neg_z.get();
let translate_pos_z_val = translate_pos_z.get();
// update the assembly's orientation
let ang_vel = {
let pitch = pitch_up_val - pitch_down_val;
@ -298,6 +318,41 @@ pub fn Display() -> View {
let zoom = zoom_out_val - zoom_in_val;
location_z *= (time_step * ZOOM_SPEED * zoom).exp();
// manipulate the assembly
if state.selection.with(|sel| sel.len() == 1) {
let sel = state.selection.with(
|sel| *sel.into_iter().next().unwrap()
);
let rep = state.assembly.elements.with_untracked(
|elts| elts[sel].representation.get_clone_untracked()
);
let translate_x = translate_pos_x_val - translate_neg_x_val;
let translate_y = translate_pos_y_val - translate_neg_y_val;
let translate_z = translate_pos_z_val - translate_neg_z_val;
if translate_x != 0.0 || translate_y != 0.0 || translate_z != 0.0 {
let vel_field = {
let u = Vector3::new(translate_x, translate_y, translate_z).normalize();
DMatrix::from_column_slice(5, 5, &[
0.0, 0.0, 0.0, 0.0, u[0],
0.0, 0.0, 0.0, 0.0, u[1],
0.0, 0.0, 0.0, 0.0, u[2],
2.0*u[0], 2.0*u[1], 2.0*u[2], 0.0, 0.0,
0.0, 0.0, 0.0, 0.0, 0.0
])
};
let elt_motion: DVector<f64> = time_step * TRANSLATION_SPEED * vel_field * rep;
assembly_for_raf.deform(
vec![
ElementMotion {
key: sel,
velocity: elt_motion.as_view()
}
]
);
scene_changed.set(true);
}
}
if scene_changed.get() {
/* INSTRUMENTS */
// measure mean frame interval
@ -416,7 +471,7 @@ pub fn Display() -> View {
start_animation_loop();
});
let set_nav_signal = move |event: KeyboardEvent, value: f64| {
let set_nav_signal = move |event: &KeyboardEvent, value: f64| {
let mut navigating = true;
let shift = event.shift_key();
match event.key().as_str() {
@ -436,6 +491,23 @@ pub fn Display() -> View {
}
};
let set_manip_signal = move |event: &KeyboardEvent, value: f64| {
let mut manipulating = true;
let shift = event.shift_key();
match event.key().as_str() {
"d" | "D" => translate_pos_x.set(value),
"a" | "A" => translate_neg_x.set(value),
"w" | "W" if shift => translate_neg_z.set(value),
"s" | "S" if shift => translate_pos_z.set(value),
"w" | "W" => translate_pos_y.set(value),
"s" | "S" => translate_neg_y.set(value),
_ => manipulating = false
};
if manipulating {
event.prevent_default();
}
};
view! {
/* TO DO */
// switch back to integer-valued parameters when that becomes possible
@ -447,6 +519,7 @@ pub fn Display() -> View {
tabindex="0",
on:keydown=move |event: KeyboardEvent| {
if event.key() == "Shift" {
// swap navigation inputs
roll_cw.set(yaw_right.get());
roll_ccw.set(yaw_left.get());
zoom_in.set(pitch_up.get());
@ -455,16 +528,24 @@ pub fn Display() -> View {
yaw_left.set(0.0);
pitch_up.set(0.0);
pitch_down.set(0.0);
// swap manipulation inputs
translate_pos_z.set(translate_neg_y.get());
translate_neg_z.set(translate_pos_y.get());
translate_pos_y.set(0.0);
translate_neg_y.set(0.0);
} else {
if event.key() == "Enter" { /* BENCHMARKING */
turntable.set_fn(|turn| !turn);
scene_changed.set(true);
}
set_nav_signal(event, 1.0);
set_nav_signal(&event, 1.0);
set_manip_signal(&event, 1.0);
}
},
on:keyup=move |event: KeyboardEvent| {
if event.key() == "Shift" {
// swap navigation inputs
yaw_right.set(roll_cw.get());
yaw_left.set(roll_ccw.get());
pitch_up.set(zoom_in.get());
@ -473,8 +554,15 @@ pub fn Display() -> View {
roll_ccw.set(0.0);
zoom_in.set(0.0);
zoom_out.set(0.0);
// swap manipulation inputs
translate_pos_y.set(translate_neg_z.get());
translate_neg_y.set(translate_pos_z.get());
translate_pos_z.set(0.0);
translate_neg_z.set(0.0);
} else {
set_nav_signal(event, 0.0);
set_nav_signal(&event, 0.0);
set_manip_signal(&event, 0.0);
}
},
on:blur=move |_| {