In this article, we will describe how to process a group of images captured with the Leica BLK3D Imager. With the Leica BLK3D function "Image Group" it is possible to capture up to 500 stereo-images which can be used to generate dense point clouds, polygonal models, orthomsoaic and etc. The point cloud will be automatically scaled and oriented thanks to the BLK3D calibrated stereo camera and tilt sensor. More information can be found on the Leica Geosystems website.


The project data contains images of a car accident. The tutorial will show the procedure of loading images to the project, data processing, and performing measurements on the reconstructed model performed in Agisoft Metashape Professional.


This article describes the following processing steps:

Import data

In this case, it is important to use the multi-camera system approach at the stage of adding photos into the project. To add photos select Add Folder... command from the Workflow menu (point to "ImageGroup YYYY-MM-DD TTTTTT" folder contents). In the Add Photos dialog select Multi-camera system data layout in the Add Photos dialog - thus Metashape will automatically load the offset information for stereo-camera system according to the information from BLK3D metadata. 



Loaded images will be displayed on the Photos pane (View Menu > Photos):



Note: when any of imported images is poor quality or wrong pose there is no need to delete this image from the project. Just select and disable it.


Camera calibration 

Photogrammetric calibration of cameras is performed to determine the values of the interior orientation of the cameras, including the parameters of distortion of the camera lens. Metashape automatically divides them into calibration groups according to the image resolution and/or EXIF meta data like camera type and focal length.  


We recommend to keep the Precalibrated values in the Camera Calibration dialog window for each camera in multi-camera system (loaded automatically for BLK3D): 



The offsets for the slave camera will be automatically uploaded to Metashape. You can view them on the Slave offset tab in the Camera calibration dialog window: 



Align photos

Metashape estimates the camera position and orientation for each multi-camera system and generates a sparse point cloud consisting of the tie points. 


1. Select Align Photos... command from the Workflow menu. 


2. In the Align Photos dialog box select the desired alignment options. The settings presented on the image below are set based on the particular dataset parameters and can be referred to as example values, the settings may differ depending on the project.



3. Click OK button.


4. The progress dialog box will appear displaying the current processing status. To cancel the processing click Cancel button.


The result of the align photos operation will be shown in the Model view as the estimated camera locations and tie point cloud, representing the matching points between the images.

(Each blue rectangle on the screenshot represents the estimated exterior orientation for Master camera in stereo-camera system).



After the alignment, next to the chunk label in the Workspace pane you will see the [S] or [R] mark. 

[S] - means that the chunk is scaled and relative measurements can be already carried out.

[R] - an indicator of a chunk being georeferenced.  



Build Dense Cloud

1. If necessary, adjust the reconstruction volume (bounding box) in the Model view mode using Rotate Region, Resize Region, and Move Region instruments. 


2. Select the Build Dense Cloud... command from the Workflow menu.


3. In the Build Dense Cloud dialog box select the desired reconstruction parameters. The settings presented on the image below are set based on the particular dataset parameters and can be referred to as example values, the settings may differ depending on the project. 



4. Click OK button when done.

 

5. The progress dialog box will appear displaying the current processing status. To cancel processing click Cancel button. 

Note: Points from the dense cloud can be removed with the help of selection tools and Delete/Crop instruments located on the Toolbar.


Build Mesh

After the dense point cloud has been reconstructed it is possible to generate a polygonal mesh model based on the dense cloud data or depth maps data. The second approach supports GPU acceleration and mostly provides better results for the same depth maps quality for objects and scenes with a big number of minor details.


1. Select Build Mesh command from the Workflow menu. Set the following recommended values for the parameters in the Build Mesh dialog: 


Note: if at the previous stage - Depth maps were built and kept in the project, we recommend using the parameter - Reuse depth maps to save the processing time.



2. Click OK button to start building geometry.




Measurements

You can take measurements on the model using the Ruler tool on the Toolbar in Metashape: