Full Dense Depth Map Image for Known Positioned Camera from Lidar Point Cloud

Lidar sensors can supply us great information about circumferences and that information are very crucial for much automatic robotic application such as a self-driving car. Although Lidar sensor gives us 360 degrees of viewpoint cloud and it is quite dense, if we want to match any camera images within those point cloud, the depth map for certain camera become pretty sparse and it is far behind to use that matched depth information for any purpose.

In this project, we are focusing on reading point cloud, camera image and calibration parameters from sample Kitti dataset [1] and create dense depth image for certain camera whose translation and rotations are known. Here is a view of the point cloud.

In the dataset, there are camera2 and camera3's RGB images and their projection's in the calibration file. We read that calibration and create 3x4 dimensioned projection matrix P which transform [X;Y;Z;1] vector into pixel location into camera2 frame. Here is the camera image in the following image.

To do that projection, we use standard projection process which P,X,Y,Z are known but λ, x, y parameters are to be estimated.

λ.[x;y;1]=P.[X;Y;Z;1]

we can solve above equation and find each point's projection on to the image plane shown by (x,y). So we can set λ into (x,y)'s depth information. Basically, we can create a map whose element is Map(x,y)=λ.

But this Map become too sparse. According to our test, we can see only 6% of the Map's indexes have value. To get rid of that problem, we calculated not just depth info of (x,y) location but also ngird number of left and right bottom and up locations depth information too. For instance, if ngrid=4, it means we used 9x9 neighborhood and we calculated weighted depth information according to the distance of the neighborhood. Basically, we used the distance of the neighborhood as a weight of sum process. Here is the result of the initial depth map without any process, and depth map with 1,2,3,4 and 5 grid size.

As you can see, the quality of depth map increases by grid size. But note that the larger grid size, the more computational time. In our tests, it needs almost 1 second for ngrid=4 with single CPU implementation. We tried to write Matlab code efficient to avoid using loops.

To evaluate depth and image matching by sight, you can see both the inverse of the depth map and camera image together in the following image.

You can find c++ implementation of another method in reference[2].

You can basically launch the main script by following Matlab command.

> main

Reference

[1] Geiger, Andreas, et al. "Vision meets robotics: The KITTI dataset." The International Journal of Robotics Research 32.11 (2013): 1231-1237.

[2] Premebida, Cristiano, et al. "High-resolution LIDAR-based depth mapping using bilateral filter." arXiv preprint arXiv:1606.05614 (2016).