Study of the quality of wood texture patterns in digital image correlation

Wood texture (WT) is sometimes used as speckle patterns in digital image correlation (DIC) method. However, the various WT due to orthotropic behavior of wood cause it vary difficult to assess the quality of WT and the accuracy of DIC analysis. In this paper, the mean directional intensity derivative (MID) was used to qualify the quality of the WT patterns for digital image correlation. To verify the effectiveness and correctness of this parameter, numerical displacement experiments in x and y directions were performed. The results show that the measurement errors of WT patterns in x and y directions are related to the mean directional intensity derivatives. The WT pattern with a lower MID in x or y direction has a larger measurement error when the mean intensity gradient are nearly equal to each other, and a higher values of the mean directional intensity derivative in one direction leads to the standard deviation error decrease. Furthermore, to produce equivalent measurement errors in the two directions, an elliptic (or rectangular) subset whose ratio of the radius between x and y directions should be approximately equal to the inverse ratio between MIDx and MIDy may be more appropriate for WT patterns.