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目的是将maya中的相机无缝导入到pytorch3d中

坑1: 直接导出maya中相机的欧拉角以及Trans, 然后直接导入pytorch3d中

问题所在: maya中直接导出的相机旋转角以及Trans是 c2w, 而根据文档https://pytorch3d.org/docs/cameras 中的一句话, 经过R可以实现world to view的变换, 所以可以确定这个地方需要输入的参数其实是w2c
$M_{c2w}=\left[\begin{array}{cc}R& T\\ 0& 1\end{array}\right]$, $M_{w2c}=M_{c2w}^{-1}=\left[\begin{array}{cc}{R}^T& -R^{T}T\\ 0& 1\end{array}\right]$
转换后输入即可

坑2: 坐标系不同, 需要先进行坐标系变换

各个系统的坐标系如上图所示, 其中maya的坐标系与OpenGL相同, pytorch3d的坐标系需要将xz轴反向, 具体实现代码如下:

        R_py3d = R.clone().TT_py3d = T.clone()R_py3d[:, [0, 2]] *= -1  # 变换坐标系T_py3d[[0, 2]] *= -1

这里的实现参考了 pytorch3d/renderer/camera_conversions.py 中的_cameras_from_opencv_projection 方法, 这个方法是将opencv的相机变成pytorch3d, 这2个变换的相似点是都有一个轴固定, 然后旋转180°

def _cameras_from_opencv_projection(R: torch.Tensor,tvec: torch.Tensor,camera_matrix: torch.Tensor,image_size: torch.Tensor,
) -> PerspectiveCameras:focal_length = torch.stack([camera_matrix[:, 0, 0], camera_matrix[:, 1, 1]], dim=-1)principal_point = camera_matrix[:, :2, 2]# Retype the image_size correctly and flip to width, height.image_size_wh = image_size.to(R).flip(dims=(1,))# Screen to NDC conversion:# For non square images, we scale the points such that smallest side# has range [-1, 1] and the largest side has range [-u, u], with u > 1.# This convention is consistent with the PyTorch3D renderer, as well as# the transformation function `get_ndc_to_screen_transform`.scale = image_size_wh.to(R).min(dim=1, keepdim=True)[0] / 2.0scale = scale.expand(-1, 2)c0 = image_size_wh / 2.0# Get the PyTorch3D focal length and principal point.focal_pytorch3d = focal_length / scalep0_pytorch3d = -(principal_point - c0) / scale# For R, T we flip x, y axes (opencv screen space has an opposite# orientation of screen axes).# We also transpose R (opencv multiplies points from the opposite=left side).R_pytorch3d = R.clone().permute(0, 2, 1)T_pytorch3d = tvec.clone()R_pytorch3d[:, :, :2] *= -1T_pytorch3d[:, :2] *= -1return PerspectiveCameras(R=R_pytorch3d,T=T_pytorch3d,focal_length=focal_pytorch3d,principal_point=p0_pytorch3d,image_size=image_size,device=R.device,)

其他变换相关的参考资料
blender to pytorch3d camera: https://github.com/facebookresearch/pytorch3d/issues/1105

坑3: 内参的导入问题

读取代码

        focal_length = torch.tensor([K[0, 0], K[1, 1]], dtype=torch.float32).unsqueeze(0)principal_point = torch.tensor([K[0, 2], K[1, 2]], dtype=torch.float32).unsqueeze(0)cameras = PerspectiveCameras(device=self.device, R=R_py3d.unsqueeze(0), T=T_py3d.unsqueeze(0),focal_length=focal_length, principal_point=principal_point, in_ndc=False, image_size=[[1024, 1024]])

这里需要注意, 当输入focal_length与principal_point时, 一定要注意 in_ndc 这个选项!
还需要注意分辨率要与maya设置的渲染分辨率对上, 根据目前的观测, 就是K[0][2] * 2

最终完整代码:

maya相机参数通过如下代码获得:

    def query_cam_pos(self, camera_name):cam, cam_shape = self.scene["cameras"][camera_name]focal_length = cmds.camera(cam_shape, q=True, fl=True)inches_to_mm = 25.4app_horiz = cmds.camera(cam_shape, q=True, hfa=True) * inches_to_mmapp_vert = cmds.camera(cam_shape, q=True, vfa=True) * inches_to_mmpixel_width = self.config['resolution'][0]pixel_height = self.config['resolution'][1]focal_length_x_pixel = pixel_width * focal_length / app_horizfocal_length_y_pixel = pixel_height * focal_length / app_verttranslate = cmds.getAttr(cam + ".translate")[0]eular_rot = cmds.getAttr(cam + ".rotate")[0]# convert_to_opencv_matrixK = np.eye(3)K[0, 0] = focal_length_x_pixelK[1, 1] = focal_length_y_pixelK[0, 2] = pixel_width / 2.0K[1, 2] = pixel_height / 2.0R = utils.eulerAngleToRoatationMatrix((math.radians(eular_rot[0]), math.radians(eular_rot[1]), math.radians(eular_rot[2])))return K, R, translate

pytorch3d中相机参数的读取代码

		# 将c2w变成w2cR = R.TT = - R @ T# 然后变换坐标轴, 绕y轴转180°R_py3d = R.clone().TT_py3d = T.clone()R_py3d[:, [0, 2]] *= -1  # 变换坐标系T_py3d[[0, 2]] *= -1focal_length = torch.tensor([K[0, 0], K[1, 1]], dtype=torch.float32).unsqueeze(0)principal_point = torch.tensor([K[0, 2], K[1, 2]], dtype=torch.float32).unsqueeze(0)cameras = PerspectiveCameras(device=self.device, R=R_py3d.unsqueeze(0), T=T_py3d.unsqueeze(0), focal_length=focal_length, principal_point=principal_point, in_ndc=False,image_size=[[1024, 1024]])