Evaluating the Quality of TLS Point Cloud Colorization

Abstract

Terrestrial laser scanning (TLS) enables the efficient production of high-density colored 3D point clouds of real-world environments. An increasing number of applications from visual and automated interpretation to photorealistic 3D visualizations and experiences rely on accurate and reliable color information. However, insufficient attention has been put into evaluating the colorization quality of the 3D point clouds produced applying TLS. We have developed a method for the evaluation of the point cloud colorization quality of TLS systems with integrated imaging sensors. Our method assesses the capability of several tested systems to reproduce colors and details of a scene by measuring objective image quality metrics from 2D images that were rendered from 3D scanned test charts. The results suggest that the detected problems related to color reproduction (i.e., measured differences in color, white balance, and exposure) could be mitigated in data processing while the issues related to detail reproduction (i.e., measured sharpness and noise) are less in the control of a scanner user. Despite being commendable 3D measuring instruments, improving the colorization tools and workflows, and automated image processing pipelines would potentially increase not only the quality and production efficiency but also the applicability of colored 3D point clouds.

Utilizing a Terrestrial Laser Scanner for 3D Luminance Measurement of Indoor Environments

“…Different TLS instruments employ various imaging sensor installations, such as completely separate camera systems operated from atop of the TLS instrument (e.g., Riegl [ 37 ]), integrated imaging sensors utilizing the same rotating mirror as the laser ranging sensor, or sets of cameras mounted in the instrument’s chassis, as in the applied Leica RTC360 scanner [ 3 ]. The realization of the imaging system affects the quality of produced panoramic images, e.g., through differences in parallax.…”

“…In the RTC360, HDR imaging is performed with a fixed exposure without any prior exposure measurements [ 3 ]. The imaging system of the RTC360 can therefore be calibrated to interpret the absolute luminance values of the measured environment.…”

“…The entire equirectangular panoramic image consists of 12 adjacent vertical images. The total scan time is 4 min 21 s, including HDR imaging with a scan resolution setting of 3.0 mm at 10 m. In the RTC360, HDR imaging is performed with a fixed exposure without any prior exposure measurements [3]. The imaging system of the RTC360 can therefore be calibrated to interpret the absolute luminance values of the measured environment.…”

“…In addition to visualization, the color information has been applied for registration [ 1 ] and segmentation [ 2 ]. However, the point cloud colorization quality varies, depending on the selected terrestrial laser scanning (TLS) instrument [ 3 ].…”

“…As stated, contemporary TLS instruments commonly contain imaging sensors for point cloud colorization. As the sensors are increasingly applicable for HDR imaging [ 3 ], the utilization of such HDR imaging-capable TLS instruments for producing a 3D point cloud with luminance information is a topical development issue. While the use of TLS for lighting design via luminance measuring has been suggested in earlier research [ 27 , 28 ], a solution employing the TLS images for luminance measuring is missing, since Rodrique et al [ 27 ] utilized a separate imaging luminance photometer and they did not register the luminance values into a 3D luminance point cloud.…”