For each test, TOPINS requires two images: one image with a clear view of the grid board (Background Grid Picture), and one or more images with a view of the same board as seen through a transparency (Foreground Glass Picture). Because of this, all Foreground Glass Pictures used in a single test share the same Background Grid Picture.
Preprocessing of a Background Grid Picture needs to be done only once per test, but the procedure needs to be repeated separately for each transparency. In order to avoid any image imperfection, TOPINS has dedicated tools to correct it – Sensitivity Threshold and Minimum Length.
During the PREPROCESSING stage, TOPINS analyzes the image of the reference grid and an image of the reference grid as seen through the examined transparency, extracts the locations of the lines on the grid, and creates internal vectorization interpretations of them. With a clear view of the grid, those vectors will be almost straight lines. When the grid is viewed through a transparency, those vectors will usually be distorted in some zones.