gs_utils.py

import numpy as np
import os
from plyfile import PlyElement, PlyData
import open3d as o3d

def get_f_dc(colors):
    return RGB2SH(colors)[:, :, np.newaxis]

def get_f_rest(points, max_sh_degree=3):
    f_rest_shape = (points.shape[0], (max_sh_degree + 1) ** 2 - 1, 3)
    return np.zeros(f_rest_shape)

def get_opacity(points):
    return inverse_sigmoid(0.5 * np.ones((points.shape[0], 1)))

def get_scales(points):
    scales = np.ones((points.shape[0], 3)) * 0.0015
    scales[:, 2] = 1e-6
    
    return np.log(scales)

def get_rotation(normals):
    if normals is not None and np.any(normals):
        return normal2rotation(normals)
    else:
        return np.zeros((normals.shape[0], 4))

def RGB2SH(rgb):
    return (rgb - 0.5) / 0.28209479177387814

def inverse_sigmoid(x):
    return np.log(x / (1 - x))

def normal2rotation(n):
    n = n / np.linalg.norm(n, axis=1, keepdims=True)
    w0 = np.tile([[1, 0, 0]], (n.shape[0], 1))
    R0 = w0 - np.sum(w0 * n, axis=1, keepdims=True) * n
    R0 *= np.sign(R0[:, :1])
    R0 /= np.linalg.norm(R0, axis=1, keepdims=True)
    R1 = np.cross(n, R0)
    R1 *= np.sign(R1[:, 1:2]) * np.sign(n[:, 2:])
    R = np.stack([R0, R1, n], axis=-1)
    q = rotmat2quaternion(R)
    return q

def rotmat2quaternion(R, normalize=False):
    tr = R[:, 0, 0] + R[:, 1, 1] + R[:, 2, 2] + 1e-6
    r = np.sqrt(1 + tr) / 2
    q = np.stack([
        r,
        (R[:, 2, 1] - R[:, 1, 2]) / (4 * r),
        (R[:, 0, 2] - R[:, 2, 0]) / (4 * r),
        (R[:, 1, 0] - R[:, 0, 1]) / (4 * r)
    ], axis=-1)
    if normalize:
        q /= np.linalg.norm(q, axis=-1, keepdims=True)
    return q

def point2gs(path, pcd, scale=None, max_sh_degree=1):
    # Ensure the directory exists
    os.makedirs(os.path.dirname(path), exist_ok=True)

    # Get point cloud data
    xyz = np.asarray(pcd.points)
    normals = np.asarray(pcd.normals) if pcd.has_normals() else np.zeros_like(xyz)
    colors = np.asarray(pcd.colors) if pcd.has_colors() else np.ones_like(xyz)

    # Generate additional attributes
    f_dc = get_f_dc(colors).reshape(xyz.shape[0], -1)
    f_rest = get_f_rest(xyz, max_sh_degree).reshape(xyz.shape[0], -1)
    opacities = get_opacity(xyz)
    if scale is not None:
        scale = np.log(scale)
    else:
        scale = get_scales(xyz)
    rotation = get_rotation(normals)
    
    # Construct list of attributes
    attribute_names = ['x', 'y', 'z', 'nx', 'ny', 'nz']
    attribute_names.extend([f'f_dc_{i}' for i in range(f_dc.shape[-1])])
    attribute_names.extend([f'f_rest_{i}' for i in range(f_rest.shape[-1])])
    attribute_names.append('opacity')
    attribute_names.extend([f'scale_{i}' for i in range(scale.shape[1])])
    attribute_names.extend([f'rot_{i}' for i in range(rotation.shape[1])])

    # Create dtype for numpy structured array
    dtype_full = [(attribute, 'f4') for attribute in attribute_names]

    # Combine all attributes
    attributes = np.concatenate((
        xyz, normals, 
        f_dc,
        f_rest,
        opacities, scale, rotation
    ), axis=1)
    
    # Ensure attributes match the dtype
    assert attributes.shape[1] == len(dtype_full), f"Mismatch in attribute count. Expected {len(dtype_full)}, got {attributes.shape[1]}"

    # Create structured array
    elements = np.empty(xyz.shape[0], dtype=dtype_full)
    elements[:] = list(map(tuple, attributes))

    # Create PlyElement and save
    el = PlyElement.describe(elements, 'vertex')
    PlyData([el]).write(path)