Studio of Objects: post-processing (Guest Post: Adam Goddard)

Sunday, 15 February, 2015

With the scan complete for the Paolozzi Studio of Objects project, our starting point was the 41.7GB of compressed data from the P20 in the form of .bin files which required post-processing. The tasks required sounded fairly simple; import the data into Cyclone, import and attach the cube map images using the ‘texture map browser’, register the scanworlds to produce one point cloud, export and send to Touchpress as a PTX file (a fairly standard pointcloud format). However for a number of reasons it was not as simple as we had envisaged!

The first problem related to the multiple returns issue noted earlier. Information on optimal import settings and post processing techniques to help remove these points had been requested from Leica before the Edinburgh scan but it was a while before they were able to respond.

With post-processing starting, it was discovered that Cyclone v8 (installed on the network at Kingston) couldn’t import the P20 data and v9 was required. This had not been an issue during testing as post-processing had been carried out with a single install on v9, however this was not possible following the Edinburgh scan due to the amount of data collected. Cyclone was finally up and running just after New Year!

More problems were to follow… after numerous unsuccessful attempts to import the data, it was discovered that the existing workstations within the university did not meet the system requirements for Cyclone 9.0 and a new workstation was built. Originally this was accessed remotely but the amount of data being transferred over the network made this impossible and so a new desk was arranged in order to access the workstation locally however the problems continued.

During this time we were in constant contact with Leica’s helpdesk who provided faultless assistance for what must have seemed like an endless series of issues! Leica offered to import the data themselves and return it to us as an IMP file ready to be accessed by Cyclone. This offer was duly accepted and a flash drive dispatched to Leica’s Milton Keynes offices. Leica’s second tier support team (in Germany) then became involved once again. After remotely accessing the workstation, they established that the import was failing as the Cyclone software had been installed by the IT department with the settings used for previous Cyclone versions. These allowed the log and data files to be mapped over a network. However in Cyclone 9.0 these files have to be mapped locally. With the settings altered the import was finally successful and the flash drive duly arrived from Leica just as the issue had been resolved!

Once the scans could be viewed it was evident that the multiple return issue was still a problem despite adopting the import settings recommended by Leica. Furthermore, the tools Leica suggested could potentially eliminate this issue had little effect. This was the advice received:

“As you have seen, applying some of the filters - have slightly improved the noise levels but when applying full filters - this removes too much information. Our second support have gone through the workflows and have confirmed that the only way to remove these type of extraneous data, is to do manual fencing. This obviously means more manual work. There is currently no automated functionality that could solve your issue.”

Following this confirmation, it was decided to produce a point cloud immediately due to the work required to manually remove the points and then review both potential automated solutions and assess the time required.

So mapping the cube map images began. The images are imported by right clicking the image folder found under the ‘Known Coordinates’ section of each project. In order to align the cube maps to the point clouds, matching points have to be manually picked from the image and point cloud. This process is carried out in the modelspace for each scan using the ‘texture map browser’ found under ‘Edit Object - Appearance’. Only three matching picks are required for each cube map but the more picks you have the better the alignment is likely to be. After selecting matching points, Cyclone computes the picks to provide an estimated pixel error for each pair of picks and an average error for all of the picks. Any pair of picks with a large error can then be removed and recomputed until there are a satisfactory number of picks with a low average pixel error. The texture map is then confirmed by selecting ‘create multi-image from cube map’ in the texture map browser. This then adds the images under the correct scanworld in the navigator window. Right clicking on the multiImage folder within the scanworld provides the option to ‘Apply MultiImage’ which burns the texture map to the point cloud. This can also be completed in batch mode by right clicking on the project folder and selecting ‘Batch Apply MultiImages.’ The original images can then be deleted from the ‘Known Coordinates’ folder so the new cube map can be imported in order to texture map the next point cloud. This is a time consuming process but worth the effort to produce well aligned result.

Once all of the point clouds had been texture mapped, they were ‘stitched’ or registered together to produce one point cloud containing the data from all the scans. This is a process made easy by our use of targets during the scanning process in Edinburgh. Once the required scanworlds had been selected, the ‘Auto Add Constraints’ function was used which produced a registered point cloud with only a 2mm RMS error.

The registered and texture mapped point cloud was then exported as a PTX file, which includes the RGB data as well as XYZ coordinates and intensity for each point. The export process is a long one (allowing time to write blog entries!) with the resulting file 140GB uncompressed and 27GB compressed, containing around 2 billion points! Posted via USB to Touchpress….


in close association with hijack and Dacapo

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