Finite-difference time-domain magnetic field. Illustration of a 2D finite difference time domain simulation at two different time steps, 700 top and 1300 bottom. The magnetic field H is represented with a red and blue scale for the positive and the negative field values respectively, the white being neutral. An impermeable, square shaped, black object is present to reflect the waves. The grid overlay corresponds to the ACR cell decomposition. Each cell is composed of several points of the simulation domain and is transparent for the software developer. A grey cell is in an inactive state, i.e. the electric field computation in that cell is not done. The cell grid is dynamically updated at each simulation step and only the cells that have a constant magnetic field are disabled. The ACR compiler directives were used to express the cell size, how to identify a grey cell and what approximation to use when we are in a grey cell.