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Switch to Euclidean-invariant projection onto tangent space of solution variety (#34)

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This pull request addresses issues #32 and #33 by projecting nudges onto the tangent space of the solution variety using a Euclidean-invariant inner product, which I'm calling the *uniform* inner product.

### Definition of the uniform inner product

For spheres and planes, the uniform inner product is defined on the tangent space of the hyperboloid $\langle v, v \rangle = 1$. For points, it's defined on the tangent space of the paraboloid $\langle v, v \rangle = 0,\; \langle v, I_\infty \rangle = 1$.

The tangent space of an assembly can be expressed as the direct sum of the tangent spaces of the elements. We extend the uniform inner product to assemblies by declaring the tangent spaces of different elements to be orthogonal.

#### For spheres and planes

If $v = [x, y, z, b, c]^\top$ is on the hyperboloid $\langle v, v \rangle = 1$, the vectors
$$\left[ \begin{array}{c} 2b \\ \cdot \\ \cdot \\ \cdot \\ x \end{array} \right],\;\left[ \begin{array}{c} \cdot \\ 2b \\ \cdot \\ \cdot \\ y \end{array} \right],\;\left[ \begin{array}{c} \cdot \\ \cdot \\ 2b \\ \cdot \\ z \end{array} \right],\;\left[ \begin{array}{l} 2bx \\ 2by \\ 2bz \\ 2b^2 \\ 2bc + 1 \end{array} \right]$$
form a basis for the tangent space of hyperboloid at $v$. We declare this basis to be orthonormal with respect to the uniform inner product.

The first three vectors in the basis are unit-speed translations along the coordinate axes. The last vector moves the surface at unit speed along its normal field. For spheres, this increases the radius at unit rate. For planes, this translates the plane parallel to itself at unit speed. This description makes it clear that the uniform inner product is invariant under Euclidean motions.

#### For points

If $v = [x, y, z, b, c]^\top$ is on the paraboloid $\langle v, v \rangle = 0,\; \langle v, I_\infty \rangle = 1$, the vectors
$$\left[ \begin{array}{c} 2b \\ \cdot \\ \cdot \\ \cdot \\ x \end{array} \right],\;\left[ \begin{array}{c} \cdot \\ 2b \\ \cdot \\ \cdot \\ y \end{array} \right],\;\left[ \begin{array}{c} \cdot \\ \cdot \\ 2b \\ \cdot \\ z \end{array} \right]$$
form a basis for the tangent space of paraboloid at $v$. We declare this basis to be orthonormal with respect to the uniform inner product.

The meanings of the basis vectors, and the argument that the uniform inner product is Euclidean-invariant, are the same as for spheres and planes. In the engine, we pad the basis with $[0, 0, 0, 0, 1]^\top$ to keep the number of uniform coordinates consistent across element types.

### Confirmation of intended behavior

Two new tests confirm that we've corrected the misbehaviors described in issues #32 and #33.

Issue | Test
---|---
#32 | `proj_equivar_test`
#33 | `tangent_test_kaleidocycle`

Co-authored-by: Aaron Fenyes <aaron.fenyes@fareycircles.ooo>
Reviewed-on: glen/dyna3#34
Co-authored-by: Vectornaut <vectornaut@nobody@nowhere.net>
Co-committed-by: Vectornaut <vectornaut@nobody@nowhere.net>
This commit is contained in:
Vectornaut 2025-01-31 19:34:33 +00:00 • committed by Glen Whitney
parent 22870342f3
commit 817a446fad
5 changed files with 432 additions and 54 deletions
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38
app-proto/src/display.rs
View file

@ -130,6 +130,8 @@ pub fn Display() -> View {
let translate_pos_y = create_signal(0.0);
let translate_neg_z = create_signal(0.0);
let translate_pos_z = create_signal(0.0);
let shrink_neg = create_signal(0.0);
let shrink_pos = create_signal(0.0);
// change listener
let scene_changed = create_signal(true);
@ -164,6 +166,7 @@ pub fn Display() -> View {
// manipulation
const TRANSLATION_SPEED: f64 = 0.15; // in length units per second
const SHRINKING_SPEED: f64 = 0.15; // in length units per second
// display parameters
const OPACITY: f32 = 0.5; /* SCAFFOLDING */
@ -292,6 +295,8 @@ pub fn Display() -> View {
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();
let shrink_neg_val = shrink_neg.get();
let shrink_pos_val = shrink_pos.get();
// update the assembly's orientation
let ang_vel = {
@ -323,24 +328,27 @@ pub fn Display() -> View {
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 shrink = shrink_pos_val - shrink_neg_val;
let translating =
translate_x != 0.0
|| translate_y != 0.0
|| translate_z != 0.0;
if translating || shrink != 0.0 {
let elt_motion = {
let u = if translating {
TRANSLATION_SPEED * Vector3::new(
translate_x, translate_y, translate_z
).normalize()
} else {
Vector3::zeros()
};
time_step * DVector::from_column_slice(
&[u[0], u[1], u[2], SHRINKING_SPEED * shrink]
)
};
let elt_motion: DVector<f64> = time_step * TRANSLATION_SPEED * vel_field * rep;
assembly_for_raf.deform(
vec![
ElementMotion {
@ -501,6 +509,8 @@ pub fn Display() -> View {
"s" | "S" if shift => translate_pos_z.set(value),
"w" | "W" => translate_pos_y.set(value),
"s" | "S" => translate_neg_y.set(value),
"]" | "}" => shrink_neg.set(value),
"[" | "{" => shrink_pos.set(value),
_ => manipulating = false
};
if manipulating {