Abstract
Decorative paper and wood veneer have been widely used in the surface decoration of wood-based panels. These surface decoration methods require two-dimensional image acquisition of natural wood grain to obtain the digital grain. However, optically scanned images sometimes produce noise during the process of image acquisition and transmission. In this situation, scanned images cannot be used directly in wood grain reproductions. To reduce noise and retain or strengthen the image sharpness, studies are mostly aimed at the improvement of classic denoising algorithms and edge width-based sharpness evaluation algorithms. To enhance accessibility for common users, four kinds of wood grain images with distinct colors were chosen, and the noise filter (Dust & Scratches) and sharpen filter (Unsharp Mask, USM) were used to denoise and sharpen the images. According to the properties of the two filters, image definition in this study was considered from two aspects: detail retention and sharpness retention. To have an objective evaluation on the definition of denoised and sharpened images, two types of evaluation functions Roberts gradient function (RGF) and modulation transfer function (MTF) were introduced. The purpose of this study was to estimate the image definition by exploring the relationships between the evaluation functions and the commonly used filters in order to allow the required wood grain images to be quickly and accurately processed by common users. The results showed that RGF was only applicable to the case where the two parameters in Dust & Scratches were changed individually, while MTF was not suitable in any case. When both parameters were changed, the required denoising scheme could be obtained through PSNR and SSIM. For the images with distinct colors, even if they were acquired in the same way, denoising them with the same parameter setting was not recommended. For sharpness retention, the values of Radius and Amount in USM were given, and when the Threshold value was set to 20 (levels), the sharpness of the wood grain images barely changed. In this case, both RGF and MTF were suitable to evaluate the sharpness of the wood grain images sharpened by USM.
Highlights
The unique appearance of natural wood grain, which is characterized by beautiful colors, mellow gloss, and rich texture, leaves an excellent visual impression
To sum up, when only the Threshold value was changed in Unsharp Mask (USM), both modulation transfer function (MTF) and Roberts gradient function (RGF) were suitable to evaluate the sharpness of the wood grain images sharpened by USM
To estimate the denoising effects of the original scanned images processed by Dust & Scratches, objective evaluation functions MTF and RGF were used to help with subjective evaluation
Summary
The unique appearance of natural wood grain, which is characterized by beautiful colors, mellow gloss, and rich texture, leaves an excellent visual impression. In other reports [3,4], a UV-curable inkjet technology approach to print 3D wood grain surface layers on wood-based panels was presented. These surface decoration methods all require image acquisition of the natural wood grain to obtain the digital grain, and they are applied to mass reproductions. Much of the image noise comes from the scanner’s sampling grid, while reducing the noise weakens the image edges and blurs the image. In this situation, scanned images cannot be used directly in wood grain reproductions. To smooth the noise and enhance the edges and textures in the image, the authors developed new denoising algorithms to strengthen the contrast of image edges [8,9], which is seen as sharpness [10]
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