packingTexture function

Future<Image?> packingTexture(List<Mesh> meshes)

Reference：https://observablehq.com/@mourner/simple-rectangle-packing

Implementation

Future<Image?> packingTexture(List<Mesh> meshes) async {
  // generate a key for a mesh.
  String getMeshKey(Mesh mesh) {
    if (mesh.texture != null) return mesh.texturePath ?? '${mesh.textureRect}';
    return toColor(mesh.material.diffuse.bgr).toString();
  }

  // only pack the different textures.
  final allMeshes = meshes;
  final textures = <String, Mesh>{};
  for (Mesh mesh in allMeshes) {
    if (mesh.vertices.isEmpty) continue;
    final String key = getMeshKey(mesh);
    textures.putIfAbsent(key, () => mesh);
  }
  // if there is only one texture then return the texture directly.
  meshes = textures.values.toList();
  if (meshes.length == 1) return meshes[0].texture;
  if (meshes.isEmpty) return null;

  // packing
  double area = 0;
  double maxWidth = 0;
  for (Mesh mesh in meshes) {
    area += mesh.textureRect.width * mesh.textureRect.height;
    maxWidth = math.max(maxWidth, mesh.textureRect.width);
  }
  meshes.sort((Mesh a, Mesh b) => b.textureRect.height.compareTo(a.textureRect.height));

  final double startWidth = math.max(math.sqrt(area / 0.95), maxWidth);
  final List<Rect> spaces = <Rect>[];
  spaces.add(Rect.fromLTWH(0, 0, startWidth, double.infinity));

  for (Mesh mesh in meshes) {
    for (int i = spaces.length - 1; i >= 0; i--) {
      final Rect block = mesh.textureRect;
      final Rect space = spaces[i];
      if (block.width > space.width || block.height > space.height) continue;
      mesh.textureRect = Rect.fromLTWH(space.left, space.top, block.width, block.height);
      if (block.width == space.width && block.height == space.height) {
        final Rect last = spaces.removeLast();
        if (i < spaces.length) spaces[i] = last;
      } else if (block.height == space.height) {
        spaces[i] = Rect.fromLTWH(space.left + block.width, space.top, space.width - block.width, space.height);
      } else if (block.width == space.width) {
        spaces[i] = Rect.fromLTWH(space.left, space.top + block.height, space.width, space.height - block.height);
      } else {
        spaces.add(Rect.fromLTWH(space.left + block.width, space.top, space.width - block.width, block.height));
        spaces[i] = Rect.fromLTWH(space.left, space.top + block.height, space.width, space.height - block.height);
      }
      break;
    }
  }

  // get the packed texture size
  int textureWidth = 0;
  int textureHeight = 0;
  for (Mesh mesh in meshes) {
    final Rect box = mesh.textureRect;
    if (textureWidth < box.left + box.width) textureWidth = (box.left + box.width).ceil();
    if (textureHeight < box.top + box.height) textureHeight = (box.top + box.height).ceil();
  }

  // get the pixels from mesh.texture
  final texture = Uint32List(textureWidth * textureHeight);
  for (Mesh mesh in meshes) {
    final int imageWidth = mesh.textureRect.width.toInt();
    final int imageHeight = mesh.textureRect.height.toInt();
    Uint32List pixels;
    if (mesh.texture != null) {
      final Uint32List data = await getImagePixels(mesh.texture!);
      pixels = data.buffer.asUint32List();
    } else {
      final int length = imageWidth * imageHeight;
      pixels = Uint32List(length);
      // color mode then set texture to transparent.
      const int color = 0; //mesh.material == null ? 0 : toColor(mesh.material.kd.bgr, mesh.material.d).value;
      for (int i = 0; i < length; i++) {
        pixels[i] = color;
      }
    }

    // break if the mesh.texture has changed
    if (mesh.textureRect.right > textureWidth || mesh.textureRect.bottom > textureHeight) break;

    // copy pixels from mesh.texture to texture
    int fromIndex = 0;
    int toIndex = mesh.textureRect.top.toInt() * textureWidth + mesh.textureRect.left.toInt();
    for (int y = 0; y < imageHeight; y++) {
      for (int x = 0; x < imageWidth; x++) {
        texture[toIndex + x] = pixels[fromIndex + x];
      }
      fromIndex += imageWidth;
      toIndex += textureWidth;
    }
  }

  // apply the packed textureRect to all meshes.
  for (Mesh mesh in allMeshes) {
    final String? key = getMeshKey(mesh);
    if (key != null) {
      final Rect? rect = textures[key]?.textureRect;
      if (rect != null) mesh.textureRect = rect;
    }
  }

  final c = Completer<Image>();
  decodeImageFromPixels(texture.buffer.asUint8List(), textureWidth, textureHeight, PixelFormat.rgba8888, (image) {
    c.complete(image);
  });
  return c.future;
}