# A Multi-Feature Automatic Evaluation of the Aesthetics of 3D Printed Surfaces
## 1 Introduction  
An increasing interest in additive manufacturing technologies, observed in recent years, makes it possible to enhance the rapid prototyping, customization, and small series production of various parts and elements. Starting from the RepRep project, initiated in 2005, through the expiration of the patent for additive printing in 2009, to further rapid development of this branch of technology, the most popular and widely available technology is based on Fused Deposition Modeling (FDM) utilizing polymer filaments, such as PLA (polyactic acid) and ABS (acrylonitrile butadiene styrene) [1]. Some other important types of 3D printing technology are SLA (stereolithography) and mSLA (masked stereolithography). The development of additive manufacturing technology is particularly important in the era of Industry 4.0 due to the possibility of using various materials, not only the popular plastic filaments, but also metal or concrete, for 3D printing. Therefore, various types of 3D printers available on the market require more or less precise calibration and, depending on the materials used for their construction as well as the type and quality of filaments, provide varied quality of final products [2]. Three-dimensional printing is a complex process, the quality of which is influenced by many factors such as nozzle diameter, head temperature, layer height, printing speed,and filament diameter [3]. The above parameters are related to the maximum volumetric flow, expressed in mm\({}^{3}\) per second, which determines how much material the extruder and head system is able to extrude. Nowadays, slicer programs have the ability to set a limit on this parameter, which leads to a reduction in the printing speed, depending on the layer height and layer's line width. However, even for correctly selected parameters, the printing process may be temporarily disrupted or completely interrupted. An example of such problems may be dirt or a foreign body in the filament, which usually leads to the nozzle being partially or completely clogged. The main problems with 3D print quality come from the lack of selection of appropriate printing parameters--filament calibration for the process. Head temperature that is too low leads to under-extrusion due to the nozzle being blocked by unmelted material. On the other hand, too high temperature usually leads to stringing and the filament's warping on the edges of the printed object. Additionally, poor quality filament with varying diameter differing from the required one (typically 1.75 mm) may lead to similar quality problems. The extruder motor rotates at a given angle and speed to extrude the appropriate amount of material. If the filament diameter deviates from the required one, then for a larger diameter the extruder will feed more filament, leading to over-extrusion. Nevertheless, an under-extrusion is also possible when the stepper motor is unable to push out the excess material and misses some steps. This will cause the material not to be supplied in the appropriate amount. For the samples used in the paper, the printing defects were obtained by interfering with the extruder's operation. Although in many applications, especially those involving the use of relatively cheap and widely available 3D printers, the presence of even minor distortions or cracks on the surface of a 3D printed object results in disqualification of the manufactured item, in some cases it is possible to correct minor surface defects after printing is complete. From an aesthetic point of view, the presence of such minor defects might be acceptable; however, the detection of some more significant issues during the printing process should result in its termination. Hence, it is possible to save material, energy, and time, preventing the production of objects that would be thrown away.