Off-axis frustum: the source

since 0.7.0 Views, picking and input
Open the live demo · Read the guide · View on GitHub
Regions the guide quotes: symmetric (line 32), skew (line 37).
1/// A frustum whose axis is not down the middle of the view: what a headset
2/// lens or a portal window needs instead of an ordinary field of view.
3///
4/// Quoted by `off_axis_projection.md` and shown whole in the Source tab.
5library;

7import 'package:flutter3d/flutter3d.dart';
8import 'package:flutter3d_showcase/src/demo/demo.dart';
9import 'package:vector_math/vector_math.dart';

11final class OffAxisProjectionDemo extends ShowcaseDemo {
double shift = 0.5;

late OffAxisProjection _off;

@override
Scene build(DemoContext context) {
  final Material stone = Material(
    name: 'stone',
    baseColor: Vector4(0.68, 0.72, 0.78, 1.0),
    roughness: 0.7,
  );
  final MeshNode ball = MeshNode(
    DeviceMesh.upload(
      context.device,
      SphereShape(segments: 32, rings: 16).build(),
    ),
    stone,
    name: 'ball',
  );

  final OffAxisProjection symmetric = OffAxisProjection.symmetric(
    fovYRadians: 0.9,
    aspect: 16 / 9,
  );

  _off = symmetric.copyWith(
    tanLeft: symmetric.tanLeft - shift,
    tanRight: symmetric.tanRight - shift,
  );
  context.camera.projection = _off;

  return Scene()
    ..add(ball)
    ..add(
      LightNode(name: 'sun', intensity: 3.0)
        ..setLocalForward(Vector3(-0.3, -1.0, -0.4)),
    );
}

@override
void update(DemoContext context, double dt) {
  final OffAxisProjection symmetric = OffAxisProjection.symmetric(
    fovYRadians: 0.9,
    aspect: 16 / 9,
  );
  _off = symmetric.copyWith(
    tanLeft: symmetric.tanLeft - shift,
    tanRight: symmetric.tanRight - shift,
  );
  context.camera.projection = _off;
}

@override
List<DemoControl> controls(DemoContext context) => <DemoControl>[
  SliderControl(
    'Shift',
    min: -0.6,
    max: 0.6,
    value: () => shift,
    onChanged: (double v) => shift = v,
  ),
];

@override
void verify(Scene scene, FrameResult frame) {
  if (frame.drawCalls < 1) {
    throw StateError('the ball was not drawn');
  }
  // The off-centre term in column 2 of the matrix is what shears the
  // frustum. It is zero only when the frustum is symmetric: away from
  // shift == 0 it must be non-zero, or the shift did nothing.
  final Matrix4 matrix = _off.toMatrix(16 / 9);
  final double offCentre = matrix.storage[8];
  if (shift != 0.0 && offCentre == 0.0) {
    throw StateError('a shifted frustum should not be symmetric');
  }
}
89}