## 2 Literature Review
### In-Line Dimensional Inspection  
All the previously presented methods only execute off-line measurements on cooled-down workpieces. Measurements directly performed on warm and hot workpieces have been rarely reported [1]. Their main shortcoming is that high temperatures affect the measurements of contact-based methods while the strong radiation affects the optical equipment, thus reducing the quality of the captured images [19]. The spectrum selective method presented in [20] filters specific wavelengths from the captured images to allow the reconstruction of the hot parts. Ref. [21] integrates a specific wavelength and power laser beam with surface fitting to measure the length and diameter of hot cylindrical workpieces. Refs. [22; 23] presents an in-line measurement system which uses two-dimensional laser range sensors (TLRS) coupled with servo motors for 3D reconstruction of hot cylindric workpieces. As an alternative to optical scanners, ref. [24] presents a vision system with two cameras that capture and process the hot workpiece without requiring any laser beams. These in-line approaches for hot dimensional assessment have been developed for workpieces with non-complex geometries (such as cylindrical workpieces).  
It is worth mentioning in-line dimensional inspection approaches for cooled-down workpieces. Point cloud filtering [11] and accelerated mesh registration algorithms [12] have been developed to allow real-time inspection using 3D optical scanners and mesh reconstruction. Applications of these algorithms for in-line dimensional inspection of cooled-down workpieces include flatness inspection of rolled parts [25], inspection of large parts [26], and inspection of generic parts using geometric features [9; 27].  
\begin{table}
\begin{tabular}{p{142.3 pt} p{142.3 pt}} \hline \hline
**CMM** & **Optical Scanner** \\ \hline Highly accurate measurements [5]. & Less-accurate measurements [5]. \\ \hline Data collection relying on probe vs. piece contact. & Contact probe vs. piece not required. \\ \hline Technician assistance required for definition of piece feature coordinate systems [8]. & Technician assistant required for point sample vs. B-Rep (i.e., CAD model) registration [9; 10]. \\ \hline Time-consuming data acquisition protocol [8]. & Real-time data acquisition and post-processing of the digitized mesh (triangulation, mesh registration, feature extraction) [11; 12]. \\ \hline Inherently sparse point samples, conducted according to discrete trajectories. Analytic form fitting needed as a consequence [8]. & Dense point samples. Both mesh computation and analytic form fitting possible [9]. \\ \hline Competing equipment precision at the cost of off-line measurements [3]. & Accurate measurement systems for in-line dimensional assessment [3]. \\ \hline Requires specific clamps for each reference model, introducing additional complexity in the management of measuring resources. & Allows the use of fixed universal setups for many different workpiece references. \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison between standard contact-based Coordinate Measurement Machines (CMMs) vs optical scanners for dimensional assessment.