figure 1 Let's do more adventure following yesterday's work. Highlight the rink to show the active play area as following: figure 2 The red highlight area is exactly the active play area from figure 2. In order to get it, firstly resize the rink image to match with play image. so we can use for pixel to pixel mapping. create black/white mask from resized rink image: figure 3 truncate the play image suing figure 3 and then create active red area figure 4 Finally weightly add figure 4 to the normalized to generate figure 1. Here is source code: