# Detection of Pest Feeding Traces on Industrial Wood Surfaces with 3D Imaging
## 1 Introduction  
Industries that use wood in various forms rely on material supplied from sawmills. One of the tasks associated with production automation is the control of parameters describing the quality of the provided material, and special attention is paid to detecting defects occurring on the material surface, which fall into three groups. The first group refers to the natural defects resulting from knots, colouring, or internal cracks. Another group results from incorrect mechanical processing and is identified by the presence of bark, machining marks, or dimensional flaws. The third group appears as visible traces of pests feeding, seen on the surface of the wood in the form of grooves and holes. These defects usually disqualify the wood from being processed in most industrial applications. The shape and dimensions of the wormholes depend on the pest species infesting the wood. Some pests that develop during tree growth are referred to in the literatureas physiological-technical pests [1; 2; 3]. Pests invading the wood in woodpiles can cause technical damage [4]. The shape and size of wormholes are crucial factors for selecting control system parameters to detect their presence. By analysing the depth of the holes made by pests, it is possible to distinguish between three subgroups of surface defects, described as surface wormholes. The first subgroup describes shallow wormholes that extend up to 2 mm into the wood. This damage is mainly caused by bark beetles, which are physiological pests that do not attack processed wood. If trees damaged by bark beetles are filled early, the resulting timber will retain its total value. An exception to this is the lesser pine shoot beetle (Latin: _Tomicus minor_). This species is responsible for causing the growth of a 'blue stain' fungus, which causes wood defects seen as black-grey patches in various tree species. The second subgroup describes shallow wormholes extending up to 50 mm into the wood. The sawing process smooths out some of the wormholes, but some remain in the plank. This type of machining results in a loss of width in the sawn material. With circumferential cutting, the pest damage is transferred to the veneer, extracted from the wood's peripheral parts. For this reason, wormholes are classified as a technical defect. The third subgroup contains defects referred to as deep wormholes. These defects are deeper than 50 mm and do not disappear during processing. They remain in the material made from round wood. Such signs of wood boring eliminate or drastically reduce the usability of the wood for technical purposes, including construction and furniture manufacture. Besides the depth of the wormholes, other crucial factors include their dimensions, i.e., the hole diameter or the length of the groove visible on the material surface. These dimensions also depend on the species of pest inhabiting the wood, making it possible to distinguish between two groups of holes. The first group consists of round holes up to 3 mm in diameter. An entire life cycle of small wood-boring pests can occur in wood stockpiles. These pests can also feed on finished products and sawn timber. This damage, characterised by the small size of the hole, occurs in large numbers and often leads to the total decay of the wood. The second group of holes are large round or oval holes with a diameter of more than 3 mm. The pests that leave these type of holes are mainly forest dwellers, and the number of holes they make is smaller than that of the first group. These large, deep, but mostly isolated wormholes are easily removed during woodworking. Because of the size and visibility of the wormholes, the most common way to remove the damaged material is during the sawing process. The analysis of the environmental conditions and factors that favour the development of different types of pests is not the subject of this paper. Instead, this paper focuses on analysing visible defects in the form of wormholes that occur on the surface of materials supplied by sawmills.