//  [python]
def _extract_eig(
    inp: Tuple[int, str], 
    K: int, 
    images_root: str,
    output_dir: str,
    which_matrix: str = 'laplacian',
    which_features: str = 'k',
    normalize: bool = True,
    lapnorm: bool = True,
    which_color_matrix: str = 'knn',
    threshold_at_zero: bool = True,
    image_downsample_factor: Optional[int] = None,
    image_color_lambda: float = 10,
):
    index, features_file = inp

    # Load 
    data_dict = torch.load(features_file, map_location='cpu')
    image_id = data_dict['file'][:-4]
    
    # Load
    output_file = str(Path(output_dir) / f'{image_id}.pth')
    if Path(output_file).is_file():
        print(f'Skipping existing file {str(output_file)}')
        return  # skip because already generated

    # Load affinity matrix
    feats = data_dict[which_features].squeeze().cuda()
    if normalize:
        feats = F.normalize(feats, p=2, dim=-1)

    # Eigenvectors of affinity matrix
    if which_matrix == 'affinity_torch':
        W = feats @ feats.T
        if threshold_at_zero:
            W = (W * (W > 0))
        eigenvalues, eigenvectors = torch.eig(W, eigenvectors=True)
        eigenvalues = eigenvalues.cpu()
        eigenvectors = eigenvectors.cpu()
    
    # Eigenvectors of affinity matrix with scipy
    elif which_matrix == 'affinity_svd':        
        USV = torch.linalg.svd(feats, full_matrices=False)
        eigenvectors = USV[0][:, :K].T.to('cpu', non_blocking=True)
        eigenvalues = USV[1][:K].to('cpu', non_blocking=True)

    # Eigenvectors of affinity matrix with scipy
    elif which_matrix == 'affinity':
        W = (feats @ feats.T)
        if threshold_at_zero:
            W = (W * (W > 0))
        W = W.cpu().numpy()
        eigenvalues, eigenvectors = eigsh(W, which='LM', k=K)
        eigenvectors = torch.flip(torch.from_numpy(eigenvectors), dims=(-1,)).T

    # Eigenvectors of matting laplacian matrix
    elif which_matrix in ['matting_laplacian', 'laplacian']:

        # Get sizes
        B, C, H, W, P, H_patch, W_patch, H_pad, W_pad = utils.get_image_sizes(data_dict)
        if image_downsample_factor is None:
            image_downsample_factor = P
        H_pad_lr, W_pad_lr = H_pad // image_downsample_factor, W_pad // image_downsample_factor

        # Upscale features to match the resolution
        if (H_patch, W_patch) != (H_pad_lr, W_pad_lr):
            feats = F.interpolate(
                feats.T.reshape(1, -1, H_patch, W_patch), 
                size=(H_pad_lr, W_pad_lr), mode='bilinear', align_corners=False
            ).reshape(-1, H_pad_lr * W_pad_lr).T

        ### Feature affinities 
        W_feat = (feats @ feats.T)
        if threshold_at_zero:
            W_feat = (W_feat * (W_feat > 0))
        W_feat = W_feat / W_feat.max()  # NOTE: If features are normalized, this naturally does nothing
        W_feat = W_feat.cpu().numpy()
        
        ### Color affinities 
        # If we are fusing with color affinites, then load the image and compute
        if image_color_lambda > 0:

            # Load image
            image_file = str(Path(images_root) / f'{image_id}.jpg')
            image_lr = Image.open(image_file).resize((W_pad_lr, H_pad_lr), Image.BILINEAR)
            image_lr = np.array(image_lr) / 255.

            # Color affinities (of type scipy.sparse.csr_matrix)
            if which_color_matrix == 'knn':
                W_lr = utils.knn_affinity(image_lr)
            elif which_color_matrix == 'rw':
                W_lr = utils.rw_affinity(image_lr)
            
            # Convert to dense numpy array
            W_color = np.array(W_lr.todense().astype(np.float32))
        
        else:

            # No color affinity
            W_color = 0

        # Combine
        W_comb = W_feat + W_color * image_color_lambda  # combination
        D_comb = np.array(utils.get_diagonal(W_comb).todense())  # is dense or sparse faster? not sure, should check

        # Extract eigenvectors
        if lapnorm:
            try:
                eigenvalues, eigenvectors = eigsh(D_comb - W_comb, k=K, sigma=0, which='LM', M=D_comb)
            except:
                eigenvalues, eigenvectors = eigsh(D_comb - W_comb, k=K, which='SM', M=D_comb)
        else:
            try:
                eigenvalues, eigenvectors = eigsh(D_comb - W_comb, k=K, sigma=0, which='LM')
            except:
                eigenvalues, eigenvectors = eigsh(D_comb - W_comb, k=K, which='SM')
        eigenvalues, eigenvectors = torch.from_numpy(eigenvalues), torch.from_numpy(eigenvectors.T).float()

    # Sign ambiguity
    for k in range(eigenvectors.shape[0]):
        if 0.5 < torch.mean((eigenvectors[k] > 0).float()).item() < 1.0:  # reverse segment
            eigenvectors[k] = 0 - eigenvectors[k]

    # Save dict
    output_dict = {'eigenvalues': eigenvalues, 'eigenvectors': eigenvectors}
    torch.save(output_dict, output_file)