iTwin Capture Modeler – Processing stages

Introduction – hardware recommendations

This article aims to explain how to use computer resources during aerotriangulation and reconstruction. It also complements the hardware recommendations regularly published by the iTwin Capture product team.

Aerotriangulation phase

The aerotiangulation phase is the first and required process to create a reality mesh.

This process is divided into five phases:

1. Process initialization

 

This process will create proper project files and create tasks for other phases.

 

2. Keypoints extraction

 

This process will be aimed at analyzing the photographic material in order to detect characteristic points on its 2D plane and saving their coordinates for further analysis.

This process, depending on the size of the project, can be (and most often is) divided into tasks that last a predetermined time to search for Keypoints. The size of the project affects the number of tasks.

Dividing the task into smaller blocks therefore allows for their parallel search on several computers. The CPU is used for this process, and importantly, the scaling of the speed of all processes effectively ends at 32 processor threads - a processor with more of them will not affect the speed of calculations. Here, the computer's RAM is also important (we recommend 64GB).

It's worth knowing - Keypoints once found in photos are saved in the project memory and as long as we don't change the settings (for example from medium to high density), this process will be skipped, significantly speeding up subsequent Aerotriangulations. This also works if we re-import the same photos to a new block of the same project.

 

3. Pair selection

 

This phase of the AT process is responsible for finding photos that have common Keypoints. This is a process that does not scale well its speed and cannot be divided. It will run on the main computer. Here, the most important thing is to adjust the appropriate options such as min max view distance, change between default/sequential data structure. If we have several thousand photos without position metadata, we should consider dividing the project into smaller blocks. Important - if we want to use the "exhaustive" option for pair selection, we must always set the min max viewing distance, otherwise the program will take too many photos for analysis and the process will have trouble completing.

Again, the CPU and RAM will play a key role in this process.

 

4. Keypoints matching

 

Once the program knows the groups of images that share Keypoints, it is ready to divide the calculations into smaller subgroups again to find common constellations of points. Much like Keypoints selection, iTwin Capture Modeler can estimate the amount of work and divide it between subtasks. This process is also calculated on several computers in parallel, allowing for scalable calculation speed. Like in the previous stages, CPU and RAM memory is being used in this processing.

 

5. Main Aerotriangulation

 

The final phase of the calculation consists of collecting all the data collected so far and substituting them into the aerotriangulation equation, calculating the spatial coordinates of specific points based on their 2D coordinates in multiple photos as well as the camera parameters (exterior and interior). This process is calculated on the main computer, it is not divisible and relies mainly on the power of the CPU and RAM of the computer. In the case of choosing the "Compute poses" option, it will take longer than in the case of the "Adjust poses" option, which only adjusts the initial position of each pose. The final, but also optional phase of AT is the colour equalization and Splats creation process. Here, as an exception in the case of colour equalization, the graphics card of the main computer is used.

 

To sum up - AT process in iTwin Capture Modeler performs some calculations on parallel machines (in case of their common connection). This provides some scalability of calculation speed. However, it is still worth remembering that some of its calculations require time and must be calculated on a single computer - therefore let's not base the calculation time only on the parallel connection of powerful computers and calculation engines, but also on conscious settings of our aerotriangulation.