Semi-permanent 2D Porous Code for Embedding into Various Building Materials

A semi-permanent two-dimensional (2D) code with ultra-small holes that can be embedded in various building materials and read with smartphones was developed. The code, called Porous code, is made of high-frequency cells of “1" data composed of shadows created from ultra-small holes corresponding to black cells in a Quick Response (QR) code and low-frequency cells of “0" data without holes, and thus no shadows, corresponding to white cells in a QR code. To read the code, it is first processed as follows: conversion to grayscale, detection of edges, binarization, morphological dilation and erosion, and smoothing of brightness. Since a conventional 2D code is obtained, it can be decoded using a conventional 2D code reader. We clarified the structure of “1" cells to achieve high reading accuracy through reading experiments involving Porous codes embedded in wooden and metal plates with various numbers of holes per “1" cell and hole depths. We succeeded in reading Porous codes under an illuminance of 12.5–7600 lux and verified that it is robust against illumination changes. In particular, Porous codes with four ultra-small holes in “1" cells were stably read the fastest under various illuminances. We also succeeded in reading Porous codes embedded in cement paste and marble under illuminances of 100 and 6090 lux.