Draco meshes: the source

since 0.7.0 Formats and I/O
Open the live demo · Read the guide · View on GitHub
Regions the guide quotes: bitstream (line 16), decode (line 35), rebuild (line 38), check (line 99).
1/// Decoding a real Draco bitstream with `decodeDraco`.
2///
3/// Quoted by `draco.md` and shown whole in the Source tab.
4library;

6import 'dart:convert';

8import 'package:flutter3d/flutter3d.dart';
9import 'package:flutter3d_showcase/src/demo/demo.dart';
10import 'package:vector_math/vector_math.dart';

12final class DracoDemo extends ShowcaseDemo {
late final DracoMesh _mesh;
late final MeshData _upload;

// A small edgebreaker-encoded mesh, the same fixture
// `flutter3d_core`'s own decoder test checks against an independently
// computed set of triangles. Edgebreaker connectivity carries no index
// buffer and no vertex order of its own: both come out however the
// encoder's traversal happened to walk the mesh.
static const String _fixtureBase64 =
    'RFJBQ08CAgEBAAAAQDwBPAEBHx8BF2/btm3bfdu2bdv2ruu6ruu6ruu6rusC/wEz/wI1RAL/'
    'AAAAAAABAAkDAAACAQEJAwABAwEBAQAPIwEHAQgBEAEBAQkBFxTABEaQRtBEhyBH5YsP2Cnb'
    'LPkrs79pACDdXi8AUMmBEQBA+gYCgBrctAHA6m9DAYAfsbEGAFY/rh8AqMHMbwDgo5GqAQAA'
    'wPwGAEAarh8AsMEaawDA9WtDAQAfsWkDANd/AwGADW4nBIDtV60BAMzAvQEADuOWBAB3O94D'
    'AMLH6BYAyDZfeQAgfLS1AYC7jckFALodqhUAAACYXAAA3LG1AQC77ZUHAMjH6BYA0DbjPQCA'
    'fNySAMBu/zMAEQKoACR0HgLAYSQDgAgnDQAJjQFAhEMFABpbAugdMwB4+DeAl5UAgFkjBKAM'
    'CQFA0hECYAYaAUDU3gBgehgAMHsJwLdZAYCJMgCQUNIAiJAZAEhIVACcESYA4DADwId9A3CJ'
    'CQBUW0IAyIgQACYtIQBjnBEAQwUOoCUAAAAA/z8AAAAgrr4AUBi/ACCuvgAwcz8OAAMBAQP/'
    'B4E2/5+AAQGrAQP//////58BAf//////o4EBqwEBAQH/m4ATIJkSAFI2CrI77Ywk2kkIW/ru'
    'gP8DAAD/AQAACg==';

@override
Scene build(DemoContext context) {
  final bytes = base64Decode(_fixtureBase64.replaceAll('\n', ''));
  _mesh = decodeDraco(bytes);

  // A decoded `DracoMesh` is not yet a `MeshData`: its attributes are
  // keyed by Draco's own attribute id, not by name, so a caller picks
  // out the ones it wants. This page draws positions only, with a flat
  // normal recomputed per triangle.
  final position = _mesh.attributes.values.firstWhere(
    (attribute) => attribute.type == DracoAttributeType.position,
  );
  final builder = MeshBuilder(
    VertexLayout.standard,
    reserveVertices: _mesh.indices.length,
    reserveIndices: _mesh.indices.length,
  );
  Vector3 pointAt(int index) => Vector3(
    position.values[index * 3],
    position.values[index * 3 + 1],
    position.values[index * 3 + 2],
  );
  for (var t = 0; t + 2 < _mesh.indices.length; t += 3) {
    final a = pointAt(_mesh.indices[t]);
    final b = pointAt(_mesh.indices[t + 1]);
    final c = pointAt(_mesh.indices[t + 2]);
    final cross = (b - a).cross(c - a);
    final normal = cross.length2 > 0.0
        ? cross.normalized()
        : Vector3(0, 0, 1);
    final texcoord = Vector2.zero();
    final i0 = builder.addVertex(
      position: a,
      normal: normal,
      texcoord: texcoord,
    );
    final i1 = builder.addVertex(
      position: b,
      normal: normal,
      texcoord: texcoord,
    );
    final i2 = builder.addVertex(
      position: c,
      normal: normal,
      texcoord: texcoord,
    );
    builder.addTriangle(i0, i1, i2);
  }
  _upload = builder.build();

  return Scene()
    ..add(
      MeshNode(
        DeviceMesh.upload(context.device, _upload),
        Material(baseColor: Vector4(0.75, 0.6, 0.5, 1.0), roughness: 0.6),
        name: 'draco mesh',
      ),
    )
    ..add(
      LightNode(name: 'sun', intensity: 3.0)
        ..setLocalForward(Vector3(-0.4, -1.0, -0.3)),
    );
}

@override
void verify(Scene scene, FrameResult frame) {
  if (_mesh.indices.length != 180) {
    throw StateError(
      'this fixture always decodes to 60 triangles (180 corners); got '
      '${_mesh.indices.length}',
    );
  }
  if (_mesh.pointCount == 0) {
    throw StateError('the decoded mesh has no points');
  }
  if (frame.drawCalls < 1) {
    throw StateError('the decoded mesh was not drawn');
  }
}
112}