In many grid-based cellular automata simulations, boundaries are treated as simple reflective (Neumann) or absorbing (Dirichlet) walls. However, to create organic visual effects like fire, smoke, and continuous ripples, the grid needs a constant source of new heat, energy, or noise.
A hot border is a boundary region of the simulation grid where cells are set to maximum value or injected with high-frequency noise on every single frame. For example, in a classic fire simulation, the bottom row is filled with random intense heat values. This heat then propagates upward, cools down, and diffuses, creating the natural licking flames visual.
If the hot border is directly visible on screen, it looks artificial: the user would see a solid, noisy line of maximum brightness at the very edge of the display.
To prevent this, ShoggothOx allocates a slightly larger simulation buffer than what is actually rendered to the screen. For example, for a visible screen width of 160 columns, the buffer might allocate 162 columns. The extra columns on the left and right (or rows on the top and bottom) serve as hidden borders. The hot border noise is injected into these hidden columns. When the graphics engine renders the grid, it clips out the hidden borders, showing only the natural, diffused waves and flames that have migrated into the visible region.